Over-cladding of existing buildings using light steel

P247: Over-cladding of Existing Buildings using Light Steel Discuss me ...

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Over-cladding of Existing Buildings using Light Steel

R M Lawson BSc(Eng), PhD, ACGI, CEng, MICE, MIStructE R Pedreschi BSc, PhD, CEng, MICE I Falkenfleth Architect (Denmark) S O Popo-Ola BSc(Eng), MEng, DIC, PhD

SCI PUBLICATION P247

Published by: The Steel Construction Institute Silwood Park, Ascot Berkshire SL5 7QN Telephone: 01344 623345 Fax: 01344 622944

P247: Over-cladding of Existing Buildings using Light Steel Discuss me ...

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© 1998 The Steel Construction Institute Apart from any fair dealing for the purposes of research or private study or criticism or review, as permitted under the Copyright Designs and Patents Act, 1988, this publication may not be reproduced, stored, or transmitted, in any form or by any means, without the prior permission in writing of the publishers, or in the case of reprographic reproduction only in accordance with the terms of the licences issued by the UK Copyright Licensing Agency, or in accordance with the terms of licences issued by the appropriate Reproduction Rights Organisation outside the UK. Enquiries concerning reproduction outside the terms stated here should be sent to the publishers, The Steel Construction Institute, at the address given on the title page. Although care has been taken to ensure, to the best of our knowledge, that all data and information contained herein are accurate to the extent that they relate to either matters of fact or accepted practice or matters of opinion at the time of publication, The Steel Construction Institute, the authors and the reviewers assume no responsibility for any errors in or misinterpretations of such data and/or information or any loss or damage arising from or related to their use. Publications supplied to the Members of the Institute at a discount are not for resale by them. Publication Number:

SCI-P-247

ISBN 1 85942 084 2 British Library Cataloguing-in-Publication Data. A catalogue record for this book is available from the British Library.

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FOREWORD The renovation of buildings is an important sector of activity, and opportunities exist for the greater use of light steel construction. This publication is aimed at designers and specifiers involved in building renovation, and concerns the use of light steel construction in over-cladding applications. The authors of this publication are Dr R M Lawson of The Steel Construction Institute and Dr R Pedreschi of the University of Edinburgh, assisted by Dr S Popo-Ola of SCI and architectural consultant Mr I Falkenfleth of Randers, Denmark (formerly Managing Director, H H Robertson Nordic). The work leading to this publication was funded by British Steel Strip Products.

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Dr M Gorgoloweski of The Steel Construction Institute and Dr R G Ogden of Oxford Brookes University provided additional information. The assistance of the following companies is also acknowledged: British Steel (Welsh Technology Centre) Canadian Sheet Steel Building Institute Edelstahl Rostfrei, Germany H H Robertson Ltd, UK and Denmark Imperial College, London Metsec Framing Ltd Rautaruukki Oy, Finland Terrapin Ltd University of Edinburgh As part of the background research, structural testing was carried out at Imperial College, London, in the Department of Civil and Environmental Engineering. The initial investigations were carried out under a DETR-Link funded project in the Construction Repair and Maintenance initiative, which was also supported by British Steel Strip Products. The current demonstration project is funded by the European Coal and Steel Community under contract 7215/CA/808, which is being carried out in collaboration with British Steel Welsh Technology Centre and Edinburgh University. This publication, and the companion SCI publication Over-roofing of existing buildings using light steel, are the first to be published under the general theme of the Building Envelope. Future activities in this area will be informed by the SCI-administered ‘Steel in the Building Envelope Group’ (SIBEG).

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P247: Over-cladding of Existing Buildings using Light Steel

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CONTENTS

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Page No. SUMMARY

vi

1

INTRODUCTION 1.1 Reasons for over-cladding of buildings 1.2 Principal features of over-cladding 1.3 Benefits of steel in over-cladding applications 1.4 Types of over-cladding systems

1 1 3 4 6

2

DESIGN ASPECTS OF LIGHT STEEL OVER-CLADDING SYSTEMS 2.1 Performance requirements 2.2 Sub-frames 2.3 Design for wind forces 2.4 Over-cladding panels and sheeting

8 8 9 12 13

3

USE OF STEEL IN OVER-CLADDING SYSTEMS 3.1 Steel as a material 3.2 Design life of steel components 3.3 Structural design 3.4 Fire resistance

15 15 15 16 16

4

CASE STUDIES ON THE USE OF STEEL IN OVER-CLADDING 4.1 Over-cladding projects in Denmark 4.2 Over-cladding projects in Finland 4.3 Over-cladding projects in Germany and Belgium 4.4 Over-cladding projects in Canada 4.5 Over-cladding projects in the UK

18 18 30 33 36 38

5

PROTOTYPE OVER-CLADDING SYSTEM USING STEEL COMPOSITE PANELS AND STEEL SUB-FRAMES 5.1 Description of prototype system 5.2 Monitoring of a prototype over-cladding panel 5.3 Thermal performance of prototype over-cladding 5.4 Measurements of zinc loss

41 41 44 44 46

6

ECONOMICS OF OVER-CLADDING

48

7

REFERENCES

50

8

CONTACTS

52

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SUMMARY Buildings are commonly over-clad to improve their appearance, to increase thermal insulation, and to reduce maintenance costs. This publication reviews the use of light steel construction in the over-cladding of existing concrete and masonry buildings as part of a renovation process. The over-cladding systems use sub-frame members that are connected to the existing structure or cladding. The new facade is attached directly to the subframe members. The sub-frame members are assembled from galvanized cold formed steel components, and a variety of different cladding materials may be used.

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The design aspects for over-cladding systems are reviewed, and the use of steel in over-cladding is discussed. A series of Case Studies from Belgium, Canada, Denmark, Finland, Germany and the United Kingdom are presented to show the range of applications of light steel in over-cladding. A prototype steel intensive over-cladding system is described, and interim results from the testing of this system are reported. It is demonstrated that in most cases the cost of the over-cladding will be re-couped within a 20 year period, if all the potential savings are considered. Vêture d’immeubles existants à l’aide de profils à froid en acier Résumé Des immeubles existants sont couramment revêtus d’un bardage, appelé nouvelle vêture, afin d’en améliorer l’aspect, d’en augmenter l’isolation thermique ou d’en réduire le coût de maintenance. Cette publication est consacrée à l’utilisation de profils à froid dans la vêture d’immeubles existants, à parois en béton ou en maçonnerie. La vêture utilise des éléments structuraux qui sont fixés sur la structure existante et la nouvelle façade est attachée directement sur ces éléments en acier galvanisé. Les aspects liés au dimensionnement de la vêture sont discutés et une série de cas réels, réalisés en Belgique, au Canada, Danemark, Finlande, Allemagne et Royaume-Uni sont analysés. Un prototype de système de vêture en acier est décrit et les premiers résultats des essais réalisés sur ce prototype sont donnés dans la brochure. Les économies qui peuvent être réalisées par une nouvelle vêture en acier permettent d’amortir son coût sur une période d’environ 20 ans. Dachsanierung bestehender Gebäude mittels Stahlleichtbau Zusammenfassung Diese Publikation gibt einen Überblick zum Einsatz des Stahlleichtbaus bei Dachsanierungen und zeigt die wichtigsten Tragsysteme auf. Sie zeigt, wie P:\CMP\Cmp657\pubs\P247\P247-Final.wpd

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bestehende Flachdächer “überdacht” werden können um die Wetterfestigkeit und Dämmung des bestehenden Gebäudes zu verbessern und beschreibt, wie der Stahlleichtbau für Dach-Aufstockungen (d.h. Satteldächer mit zusätzlichem Nutzraum) eingesetzt werden kann, ohne das vorhandene Tragwerk zu überlasten. Eine Reihe von Fallstudien belegt die verschiedenen Anwendungen des Stahlleichtbaus beim “Überdachen”, einschließlich der Verwendung von Modulbauweisen und Aufstockungen. Die Vorteile der “Überdachung” werden aufgezählt und es wird gezeigt, daß die Einsparungen bei den Energiekosten verbunden mit zusätzlichen Mieteinnahmen, die Kosten einer “Überdachung” rechtfertigen können. Hinweise zu anderen SCI-Publikationen, die sich mit dem Stahlleichtbau in Entwurf und Konstruktion befassen, sind enthalten. Ebenso werden Kontaktadressen von Firmen bereitgestellt, die Produkte und Systeme bei Dachsanierungen anbieten. Strutture leggere in acciaio per sovratamponamenti di edifici esistenti

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Sommario Gli edifici vengono rivestiti da sovratamponamenti generalmente per migliorare il loro aspetto, per incrementare l'isolamento termico e per ridurre i costi di manutenzione. Questa pubblicazione analizza in modo critico l'uso di costruzioni leggere in acciaio per il rivestimento di pareti di edifici esistenti in calcestruzzo o muratura come parte di un processo di adeguamento. I sistemi di sovratamponamento generalmente utilizzano telaietti che sono collegati alle esistenti strutture di tamponamento e sui quali è fissato il nuovo tamponamento di facciata. I telaietti sono realizzati con profili zincati sagomati a freddo mentre una gran varietà di materiali possono essere utilizzati per il tamponamento. Per quanto concerne i sovratamponamenti, sono esaminati gli aspetti sia progettuali sia legati all'utilizzo dell'acciaio. Con riferimento alla serie di esempi pratici considerati, realizzati in Belgio, Canada, Danimarca, Germania e Regno Unito sono presentati i settori di utilizzo delle strutture leggere in acciaio per sovra-coperture. Viene inoltre descritto un prototipo integrale relativo a un sistema leggero in acciaio di sovra-tamponamento e sono riportati i risultati intermedi relativi alla sperimentazione del sistema. Viene dimostrato come, in molti casi, il costo del sovra-tamponamento risulti ammortizzabile in un periodo di tempo non superiore ai 20 anni, considerando tutti i potenziali risparmi economici.

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Recubrimiento (over-cladding) de edificios existentes mediante el uso de acero ligero Resumen Los edificios se recubren generalmente para mejorar su aspecto, incrementar el aislamiento térmico y reducir los costes de mantenimiento. Esta publicación pasa revista al uso de construcciones ligeras de acero para recubrimiento de edificios existentes de hormigón y mampostería como parte del proceso de renovación. El sistema de recubrimiento usa piezas de subestructuras conectadas a la estructura de revestido existente. La nueva fachada se coloca directamente sobre éstas subestructuras que se ensamblan a partir de piezas de acero conformado en frío y galvanizado, pudiendo usarse variados tipos de materiales de revestimiento.

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Se revisan los aspectos de proyecto así como el uso de acero en recubrimientos. También se presentan una serie de casos construidos en Bélgica, Canadá, Dinamarca, Finlandia, Alemania y Estados Unidos, con lo que se pretende mostrar las posibilidades de uso del acero ligero en recubrimientos. Se describe el prototipo de un sistema de recubrimiento así como resultados provisionales de su ensayo. Se demuestra que si se tienen en cuenta todos los ahorros potenciales, en la mayoría de los caso el coste del recubrimiento es amortizado en un periodo de 20 años. Ombyggnad av befintliga byggnader med Lättbyggnad med stål Sammanfattning Ombyggnad av befintliga byggnader med Lättbyggnad används idag också för ombyggnad av befintliga betong- och tegelbyggnader för att ge en vackrare fasad och förbättra värmeisoleringen. Ombyggnad med Lättbyggnad med stål för fasader består av en tunnplåtsstomme som sätts mot den befintliga fasaden. Det nya fasadmaterialet sätts därefter utanpå stålstommen. Stommen tillverkas av förzinkad tunnplåt, och ett stort antal fasadmaterial kan användas. Vidare beskrivs dimensionering och val av olika system. Exempel från Belgien, Kanada, Danmark, Finland, Tyskland och Storbritannien visar ett stort antal olika tilläpningar. Publikationen persenterar också ett nytt ombyggnadssystem med stål, som även har provats. Det visar sig att i de flesta fall betalar sig investeringen i en ombyggnad med Lättbyggnad, inom en 20-årsperiod.

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1 INTRODUCTION ‘Over-cladding’ is a term used to describe the installation of a new facade over the existing facade of a building. It is often carried out whilst the building is occupied, making it an especially attractive technique for residential buildings. In contrast, ‘recladding’ means removal and replacement of the existing cladding, and is usually associated with commercial buildings. Over-cladding is often a preferred solution for achieving energy savings, extending the life of the building, and for improving the building’s appearance.

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Over-cladding may be combined with ‘over-roofing’ when improvements are also required to the roof of the existing building. (For a description of overroofing techniques, see the companion SCI publication Over-roofing of existing (1) buildings using light steel .) There are opportunities for the steel industry in both over-cladding and overroofing, because proven products and systems currently in use in new building construction can be extended to this new application. Light steel construction is an ideal solution, as it utilises cold formed steel components that are strong, light in weight, durable, available as standard products, easily adaptable and recyclable. This publication describes the general features of over-cladding, the benefits offered by light steel, and draws on case studies from a number of countries. It shows how steel components and cladding may be used effectively and economically for a range of building forms. In Section 6, the development and long term monitoring of a prototype over-cladding system which maximises the benefits of steel are described. Further information on the use of steel in over-cladding and other applications may be obtained from the companies listed on page 52.

1.1

Reasons for over-cladding of buildings

Renovation of existing concrete and masonry buildings is an important social and technical issue, particularly for medium and high-rise residential buildings. The poor thermal and physical performance of prefabricated concrete panel or masonry buildings, built originally in the 1950s and 1960s, has led to demand by the occupants for an improvement in the quality of their living environment. The reduction of high heating costs is often the main concern, but problems are also experienced in the decay of the building fabric, condensation, frost damage, and the generally poor appearance of such buildings, which add to the overall degradation of the urban environment. The potential demand for over-cladding of medium or high-rise residential buildings of concrete panel or masonry construction is large. There is a large stock of ‘system’ buildings constructed prior to the energy crisis of the mid (2) 1970s . Most were constructed by public authorities to offer cheap housing in the post-war period. Many continue to be built in the former ‘Eastern block’ countries. In the UK, there are more than 1,000 residential buildings over six P:\CMP\Cmp657\pubs\P247\P247-Final.wpd

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(1)

storeys high, and a significant proportion of them will need major renovation . Throughout the European Union, it is estimated that over 10,000 high-rise residential buildings may need attention within the next ten years, putting a major burden on public housing budgets. The most common reasons for over-cladding a building include: 1. Improvement in thermal insulation, by increasing the amount of external insulation and reducing air leakage, leading to: C reduced heating costs C reduced risk of condensation C drying out of the building fabric C smaller thermal movements of the structure. The main economic argument for over-cladding is often related to savings in heating costs. 2. Elimination of water penetration into the building fabric, by preventing: C further movements or deterioration of the structure or cladding

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C air or water leakage through the inadequate existing cladding and its joints C further deterioration of the joint sealants due to weathering. 3. Reduction of condensation risk, by providing: C a good level of insulation C an adequate vapour barrier at a correct location to avoid interstitial condensation C no thermal bridges in the existing fabric e.g. at exposed slabs C modest air movement behind the new facade. 4. Arresting deterioration of existing facade materials, which may have occurred due to: C carbonation of concrete surface C corrosion of the reinforcement causing spalling of the concrete C expansion and cracking, or frost attack of masonry and concrete C deterioration of the joint seals. 5. Improved appearance of the facade, including: C a modern architectural style C use of brighter, more varied and interesting colours C protection against weathering or discolouration of the existing surface C concealing of existing ‘panel-type’ construction C creating new building shape and texture externally C new windows and ground floor entrance, etc.

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6. Part of a general refurbishment of the building, which might include: C new windows and balconies C new lifts, stairs and access points C additional habitable floors (when over-roofing is also carried out) C new toilet units C enclosed balconies, creating a better internal environment. The cost of renovation work is significant, and can vary from £100 to £300 per square metre of facade area, depending on the complexity of the building and the quality of the new facade. Clearly, the economics of over-cladding must be judged against the savings in the cost of heating or, alternatively, against the additional capital cost of re-building of low rise replacement buildings (including the cost of temporary re-housing, demolition, and the remaining debt of the original construction). The broad economic arguments for over-cladding are considered in Section 6.

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1.2

Principal features of over-cladding

There are four main components in an over-cladding system; the new cladding, the insulation material, the sub-frame supporting the cladding and insulation, and the attachment system to the existing structure. These components are designed as part of the over-cladding system, and are required to: C

accommodate inaccuracies in the existing facade

C

achieve suitably strong and reliable fixings to the original structure

C

be reasonably fast to construct, often in difficult working conditions

C

provide for ‘breathing’ of the existing building

C

achieve a high level of thermal insulation

C

be durable, and achieve a good design life

C

allow for architectural details.

Over-cladding is often carried out in conjunction with the installation of new windows or covered balconies. In some cases, over-cladding is carried out at the same time as over-roofing, where a new pitched roof, or even a new occupied floor, is added above an existing flat roof. The technology of many over-cladding systems is not new. However, the challenge remains to develop an approach to the use of materials and construction that is economic and also achieves the required performance characteristics in terms of structural robustness, thermal insulation, durability and quality of appearance. In additional to the over-cladding system, modular components such as lifts and toilet units may be pre-fabricated and installed as part of the renovation package. In the project shown in Figure 1.1, modular roof units were lifted into place to speed the construction process. P:\CMP\Cmp657\pubs\P247\P247-Final.wpd

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Figure 1.1

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1.3

A 4-storey building after over-cladding and over-roofing to create a new habitable floor

Benefits of steel in over-cladding applications

The use of steel as over-cladding can offer significant benefits over other materials. For example, steel components are stronger and cheaper than aluminium and can span further, thus making the use of storey-high sub-frames feasible. Light steel construction, comprising galvanized cold formed steel sections and coated steel sheeting, is often faster than traditional forms of construction and, usually, avoids the need to strengthen or alter the existing building structure. The particular benefits of light steel construction when used in over-cladding applications are listed below. C

Sub-frames or cladding materials may be designed to span between floors.

C

Attachment of the new cladding to the support structure is independent of the existing wall. This is a major advantage where the existing wall may be subject to continued deterioration and may have insufficient strength to support the new fixings. The amount of thermal bridging is also reduced.

C

Sub-frames and panels are relatively light in weight and may therefore be moved and positioned easily.

C

Damaged components may be replaced easily by use of appropriate fixing and jointing techniques.

C

Steel is a quality assured material, i.e. steel sections have a guaranteed minimum yield strength, reliable section properties and are manufactured to specified tolerances ensuring accurate geometry for detailing.

C

A range of standard components, including galvanized sections, pre-coated sheeting and composite panels, is widely available. Most over-cladding systems may be assembled using conventional technology.

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C

Detailing of over-cladding systems is usually straightforward. Coated steel sheeting or panels may be used as external cladding in a variety of colours.

C

Steel is more resistant to fire than alternative lightweight systems such as aluminium or plastic.

C

The long span capabilities of steel mean that a small number of stronger fixings may be more economic and reliable than a large number of smaller weaker fixings. The complexity and installation cost of the sub-structure is reduced compared to other systems.

C

Steel cladding has been widely used in a variety of building types and there are many examples of successful over-cladding projects.

C

Steel may be pre-fabricated into modular units, which facilitates rapid installation from the outside of the building.

C

Balconies and other structural additions to the building may be created using the same type of structural components.

C

Light steel framing is part of a proven infrastructure of design, manufacture and installation, that is well supported by British and European Standards and publications.

Steel components and cladding may be used separately or in combination in over-cladding applications. A variety of materials and finishes may be used in the over-cladding system. The properties of steel as a material are reviewed in Section 3.1. Examples of over-cladding using steel sub-frames and various metal claddings are illustrated in Figure 1.1, Figure 1.2 and Figure 1.3.

Figure 1.2

Use of steel cassettes in over-cladding, and light steel supports for new balconies

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Figure 1.3

1.4

Enclosed balconies as part of an over-cladding project

Types of over-cladding systems

In general, cladding systems are classified by the way in which they provide resistance to rain penetration, which is itself influenced by the types of materials used. Two broad classes of cladding systems have been defined by Anderson (3) and Gill : face-sealed systems and rainscreen systems.

1.4.1

Face-sealed systems

Face-sealed systems are common in new construction and in re-cladding but are not normally used in over-cladding. These systems require that the facade is impervious to rain and that all joints are air-tight and water-tight. Face-sealed systems rely on a single barrier to provide complete weather-tightness. ‘Curtain-walling’ as a lightweight facade to a commercial building is assumed to be ‘face-sealed’. Industrial buildings are usually designed to be “weather-proof” (i.e. the interior of the building will remain dry under all but the most extreme conditions), but the cladding does not act in itself as an air-tight barrier. Therefore, they are not truly face-sealed. Over-cladding systems may also be designed to be watertight, but such systems are not usually air-tight, as they should allow the existing building to ‘breathe’.

1.4.2

Rainscreen systems

‘Rainscreen’ systems form a multi-layer wall in which the basic components are an external rainscreen, a cavity and an air-tight internal barrier. Rainscreen systems permit water or moisture to enter the ‘cavity’, but not to cross the inner skin or barrier. Internal drainage routes ensure that water penetrating the cavity is able to drain to the outside.

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There are two types of rainscreen system, classified according to the detailing (3) employed : a. Drained and ventilated systems. b. Pressure-equalized systems.

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Both systems comprise impervious panels with joints that allow air movement; the passage of water across the joints may be reduced by overlapping the panels, or by including baffles. Where porous insulation is placed in the cavity, it should incorporate a breather membrane. Provision is made to drain any water that enters the cavity either at the bottom of the wall or at intermediate positions, depending on whether cavity closures are used. A masonry cavity wall with air-bricks or open perpends is an example of a traditional drained and ventilated wall. Pressure-equalised or more exactly, ‘pressure moderated’ systems, may be seen as a development of drained and ventilated systems. One of the principal mechanisms for water penetration is the pressure differential across the wall generated by wind. If the pressure on the outer surface of the rainscreen is close to the pressure within the air cavity then there will be very little pressure drop across the joints, and hence less likelihood of water penetration. By careful design of the open joint in the rainscreen, and by effective sealing of the internal air barrier and compartment of the cavity, rapid pressure equalization of the air pressure in the cavity can be achieved, reducing the likelihood of water entering the cavity. Rainscreen systems are generally preferred for over-cladding applications. Whether or not they achieve pressure equalization depends on their precise design. Furthermore, the positioning of the insulating layer is crucial to achieving the required thermal performance. Some form of weather barrier is required at the joints to prevent the insulation becoming damp.

1.4.3

Prototype over-cladding system

It is possible to create a hybrid over-cladding system by using conventional steel cladding over the majority of the elevation of the facade, but providing additional ventilation at certain joints (by using wider joints). This additional ventilation reduces the risk of condensation and avoids excessive moisture penetration within the cavity, without greatly increasing the permeability of the over-cladding. Such a system may be said to provide ‘trickle ventilation’. A hybrid system of this type has been investigated as part of a prototype overcladding system that has been the subject of a research project. The prototype system uses composite panels supported by light steel sub-frame members that are attached to the existing floors; the composite panels span vertically between sub-frames. Some ventilation occurs behind the panels to avoid condensation, but this air movement does not greatly affect the insulation provided by the composite panels. The over-cladding system has been extensively monitored to assess its performance, leading to predictions of design life. The system and its measured performance are described more fully in Section 5.

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2 DESIGN ASPECTS OF LIGHT STEEL OVER-CLADDING SYSTEMS 2.1

Performance requirements

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The performance requirements of the new over-cladding system, including its sub-frame and attachments, may be defined under the following headings. 1.

Structural: The over-cladding system should be able to support its own weight, and to resist the wind loads that vary around the building. High wind suctions occur at the corners of tall buildings. The points of attachment to the existing building should be designed to resist these forces.

2.

‘Robustness’: In the event of one component or attachment being weak as a result of inadequacy in the existing building fabric, the system should be ‘fail-safe’ by incorporating a degree of structural indeterminacy.

3.

Rapid installation: Construction should be rapid and should take account of geometrical inaccuracies of the existing facade. There are consequent savings in scaffolding and access costs, and minimum disruption to the occupants.

4.

Insulation: An improved level of thermal insulation to the standard of modern buildings should be provided, leading to reduced heating costs, and a reduction in CO2 and other emissions.

5.

Weather-tightness: Prevention of water penetration within the building can be achieved by ‘rainscreen’ cladding, with suitable detailing at the joints.

6.

‘Breathing’: Ventilation or drainage of any condensation that might occur in the cavity behind the new cladding. Rooms with excessive moisture e.g. bathrooms should be separately ventilated.

7.

Durability: A design life of at least 30 years should be provided by the over-cladding system and its supports. The sub-frame may be designed for a longer design life, so that cladding panels can be replaced.

8.

Fire Resistance: The support structure should be fire resistant, and should prevent the spread of fire by suitable compartmentation. The cladding should not be combustible. Foam-type insulation should not be used where it may be directly exposed to a fire. A cavity barrier provides for compartmentation to prevent smoke and flames from passing through the cavity. External fires may also be a concern, and often the cladding to the ground floor of a building is constructed in a more robust material than the upper floors.

9.

Impact resistance: The effects of external damage at the lower levels (see also 8 above) should be minimised in order to prevent damage to the rest of the over-cladding.

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10. Movements: Thermally-induced movements and stresses should be accommodated. These movements are much less in steel sub-frames than in aluminium, for example. 11. Aesthetics: Good external appearance should be achieved, since it is an important part of the design of modern over-cladding systems. This can be created by use of colour and detail in the building facade. 12. Repair and maintenance: Access for cleaning should be provided, and it should be easy to renew or replace panels and windows. These are important considerations in the maintenance process.

2.2

Sub-frames

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The sub-frames that support the new cladding may be arranged in one of two ways: System 1.

As short-span members attached to the existing facade or cladding at relatively close intervals.

System 2.

As longer-span members attached only to the primary structure (e.g. floors or columns).

In system 1, the sub-frame elements supporting the new cladding need only be relatively light, as they are attached to the existing cladding or facade at relatively frequent positions. Vertically or horizontally spanning members may be used. In system 2, the sub-frame elements are stiffer and stronger in order to span directly between main structural supports, which are usually floors or columns. In both systems, the spacing of the sub-frame members and their attachments is dependent on the spanning capabilities of the new cladding. However, in system 2, if the cladding can span vertically between floors, the number of sub-frame members that are needed is considerably reduced. Figure 2.1(a) shows arrangements typically used for attaching sub-frame in system 1, and Figure 2.1(b) shows an arrangement in system 2. The advantage of system 2 is that no reliance is put on the transfer of loads through the existing facade, which may be subject to further long term deterioration, and is therefore unsuitable for reliable new attachments. A disadvantage is the increased size and cost of the sub-frame elements, and the reduced flexibility in layout of the cladding panels. However, this form of construction is well suited to light steel, on account of its good strength and stiffness characteristics, which are superior to those of alternative lightweight systems such as aluminium. In Figure 2.1(a) two orientations of sub-frames are shown: C

Horizontal members that support vertically spanning panels. An example of this application is shown in Figure 2.2.

C

Vertical members that support horizontally spanning panels. An example of this application is shown in Figure 2.3. The slotted holes reduce the ‘cold

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bridge’ heat loss through the steel and permit adjustment during assembly. The vertical members may themselves be attached to horizontal members that are in turn attached to the existing structure. Horizontal sub-frame member

Attachment via bracket to slab edge Slab edge Sub-frame member

Window

Vertically spanning cladding panel attached to sub-frame

Additional attachments to the existing facade Horizontally spanning cladding panel attached to sub-frame

a) Alternative arrangements of sub-frame members attached to existing wall Sub-frame attached via bracket to slab edge

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Sub-frame around window

Vertically spanning cladding panel

Window

Horizontally spanning cladding panel attached to sub-frame

b) Alternative arrangements of sub-frame members attached directly to floors

Figure 2.1

Different arrangements of steel sub-frames and their attachments

In Figure 2.1(b), two orientations of sub-frame members are also shown. Vertically spanning panels reduce the number of sub-frame members, but the panels are designed to span between floors. Vertically spanning sub-frame members permit use of a wider range of cladding types, as the spacing between the members can be adjusted to suit the cladding material and form. In both systems, additional vertical and horizontal members will be required around windows in order to permit attachment of the window units and also to control deflections. Special arrangements are required for horizontal spandrel panels and balconies, which are often enclosed in over-cladding schemes. Diagonally orientated sub-frame members and cladding panels may be used for architectural effect, particularly on flank walls (see Figure 2.4).

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Use of cassettes supported by horizontally spanning members 3260.vcd

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Figure 2.2

Additional screws for final fixing

Fixings every metre horizontally

Bolt in slotted holes

Vertical sub-frame member (Z section) Cassette panels fixed to sub-frame members Insulation between sub-frame members

45° slotted hole (Slotted hole at 45° gives flexibility in attachment) Wind barrier (impervious paper)

Air gap

Figure 2.3

Use of vertically spanning slotted Z sections supporting cassettes

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Figure 2.4

2.3

Use of cassettes supported by diagonally orientated sub-frame members

Design for wind forces

Wind forces vary greatly around tall buildings. Positive pressure occurs on the front (windward) face, whereas negative (suction) pressure occurs on the rear (leeward) and side faces of a rectangular-shaped building. The corners of the building are subject to the highest intensity loading (normally suction), and the over-cladding elements may require more closely spaced supports in these regions. Similarly, wind forces increase with elevation or exposure. Buildings that are closely spaced may also cause wind forces to be increased locally. It would be normal practice to detail 2 or 3 separate over-cladding zones in a tall building to optimize on the variation in wind pressure over the facade. (4)

Design wind pressures may be determined from BS 6399-2 , which replaces CP3 Chapter 5 Part 2. Wind pressures are now calculated from the mean wind speed and are multiplied by a factor representing the duration of the wind gust over the facade. For face-sealed systems (see Section 1.4.1), all wind loads are applied directly via the new cladding. For drained and ventilated systems (see Section 1.4.2), the cavity may attain a similar pressure to the external air, thereby partly relieving the pressure on the new cladding. However, the panel is generally designed for the full external wind pressure, unless pressure equalization can be shown to occur quickly. The recently published Standard for walls with (5) ventilated rainscreens recommends that pressure equalized systems may be designed to resist a pressure equivalent to two-thirds of the external wind pressure. To ensure effective pressure equalization, the permeability of the rainscreen should be at least ten times that of the air-barrier (i.e. the internal skin), and the cavity should be compartmented.

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Fixings should be designed for a high factor of safety in tension and shear. The fixings into concrete are particularly affected by any deterioration of the concrete surface. A horizontal steel sub-frame member with multiple fixings along its length will assist in distributing forces if one fixing is weaker than its neighbours (see Section 5.1). In this case, a lower factor of safety may be appropriate. It may be assumed that all loads on the over-cladding are transferred by the sub-frames to the existing wall or structural frame.

2.4

Over-cladding panels and sheeting

There are three generic forms of over-cladding using standard steel components.

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1. Profiled sheeting fixed to horizontal members, with insulation directly attached to the existing wall. A waterproof (but vapour permeable) wind barrier layer is attached to the insulation to avoid the direct incidence of any rain that penetrates the joints and to prevent air movement from reducing the effectiveness of the insulation. Some air movement may occur through the joints between the sheets. This system was used on some early overclad buildings, but may be visually less acceptable, unless carefully detailed (see Figure 2.5). 2. Composite panels, comprising two skins of flat or lightly profiled steel sheets with polyurethane or other core material. These panels are usually produced continuously in standard widths. It is possible to produce nonstandard widths by batch production to accommodate different facade dimensions. The composite panels can be designed to span 3 m or so between sub-frame members. They possess a high degree of thermal insulation (as low as 0.2 W/mEC). The panels would normally be designed as face-sealed in new buildings. However, in over-cladding applications a cavity is formed between the new and existing wall. A limited degree of ventilation can be designed into the system so that the risk of condensation is minimized and the thermal performance of the composite panel is not reduced. Composite panels should not be perforated, because the foam insulation may present a fire risk. Steel does not melt in fire, unlike aluminium, and therefore steel-faced panels are preferred. A prototype design using vertically-spanning composite panels is presented in Section 5 (see Figure 2.6). 3. Cassette systems, comprising basically flat steel panels with folded edges, which are attached to the support members at their edges. The size of the cassette panels is limited by the thickness of the sheets required to maintain flatness of the panels and by available sheet sizes. The panels may be ‘rigidised’ by bonding to another material on their rear face. Cassettes are mainly manufactured using coated steel and aluminium strip. They are generally designed as ‘rainscreens’. Cassettes can be designed in various rectangular proportions and orientations (see Figures 1.2, 2.2, 2.3 and 2.4). They offer an excellent opportunity for colour modelling of the facade.

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Additional insulation layers are used, as described in 1 above. Insulation that consists of mineral wool slabs having a water-tight and air-tight surface is now available, the use of which can negate the need for a separate wind-barrier.

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The case studies presented in Section 4 show how these techniques and materials have been used in over-cladding projects in various countries, notably Denmark and Finland.

Figure 2.5

Use of profiled sheeting and panels in over-cladding

Figure 2.6

Use of vertically spanning composite panels in a prototype over-cladding system (testing carried out at the University of Edinburgh - see Section 5)

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3 USE OF STEEL IN OVER-CLADDING SYSTEMS 3.1

Steel as a material

Light steel components may be used in both the cladding and the sub-frames of over-cladding systems. Coated sheet steel is widely used for cladding, and the different types of cladding panels and sheeting suitable for use in over-cladding are reviewed in Section 2.4. The sub-frames can be made from cold formed steel sections that are produced from galvanized (zinc coated) strip that gives corrosion protection. Strip steel can be roll formed or pressed into a variety of section shapes. C and Z-shaped sections are common in applications such as purlins and side rails in industrial buildings. Section thicknesses for structural uses are in the range of 1.2 to 3.2 mm. Section depths of up to 350 mm are produced as standard components by a number of manufacturers in the UK.

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(6)

Galvanized strip steel is supplied to BS EN 10147 , which replaces BS 2989, in standard designations of S280 GD and S350 GD that correspond to 2 guaranteed minimum yield strengths of 280 and 350 N/mm respectively. The 2 standard galvanizing designation is 275 grams/m summed over both faces of the strip steel (corresponding to a total thickness of zinc of approximately 0.04 mm).

3.2

Design life of steel components

The degree of exposure of the elements in the cavity of an over-cladding system is such that the galvanized steel sections are not subject to excessive or prolonged moisture, and therefore the risk of corrosion is small. The standard thickness of galvanizing provides sufficient corrosion protection in this type of cavity application. The minimum design life for over-cladding systems is 30 years, but frequently the sub-frame elements are designed for a longer life on the assumption that it may be necessary to replace damaged cladding panels during their design life. Furthermore, it is less easy to replace the sub-frame elements and their fixings. The exposure trials reviewed in Section 5.2 to 5.4 show that the design life of the galvanized steel sub-frame components is at least 60 years in applications with suitable ventilation. This is consistent with the long use of galvanized steel in industrial buildings, where the potential corrosion risk is similar. Where it is necessary to enhance the corrosion protection to steel components, 2 then thicker hot dip galvanizing (up to 600 grams/m ), or powder coating may be considered. The application of galvanized steel in lintels in masonry construction often requires a high level of corrosion protection, and these techniques have been used successfully in those circumstances. P:\CMP\Cmp657\pubs\P247\P247-Final.wpd

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Zinc-aluminium coatings may be appropriate in some applications, but they are not widely used for over-cladding. The design life of coated steel products depends on the coating material and colour. Guidance on British Steel products is given in their publication The (7) Colorcoat Building . The performance of coated products is defined in terms of the length of time before which some repainting of the surface needs to be considered. This time is considered to be when flaking of the paint surface occurs over more than 5% of the exposed area, although at this stage no corrosion of the steel substrate has occured. The time to repainting is typically 30 years for inland sites using lighter coated materials. Further guidance on the use of coated steel products may be obtained from British Steel Strip Products. Unlike normal fully face-sealed wall claddings (see Section 1.4.1), it may be necessary to provide a higher degree of protection on the inside of the overcladding panels, where moisture could run down.

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3.3

Structural design

The structural design of the light steel sections subject to bending and other (8) (9) forces may be carried out to BS 5950-5 or to Eurocode 3-1.3 . Often, control of deflections is more important than resistance; deflections under wind loading should normally not exceed span/360 (or a maximum of 15 mm). Guidance on (10) detailing in general building practice is given in a recent SCI publication . Section properties and member resistances can be obtained from manufacturers’ data and member resistances of generic sections are given in an SCI (11) publication . For scheme design, the span : depth ratio of the structural members should not exceed 25 in order to ensure that the members are sufficiently stiff.

3.4

Fire resistance

Over-cladding systems are not required to achieve a particular fire rating, but should be designed to take account of the possibility of external fires, or fire penetrating the cavity between the new and existing facade. The measures that should be adopted are: C

use of non-combustible cladding or insulating materials

C

fire breaks or compartmentation in the cavity to prevent the spread of fire

C

use of materials that are sufficiently strong at elevated temperatures for any structurally important components.

Steel retains at least half its original strength at 600EC, and still retains 10% of its strength at over 800EC, which gives it sufficient structural integrity even in severe fires. It is not necessary to consider the combined effects of wind load and fire, but clearly the self weight of the over-cladding should be considered in fire conditions. P:\CMP\Cmp657\pubs\P247\P247-Final.wpd

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Guidance on thickness of fire protection materials to steel members in fire resistant applications is given in Building design using cold formed steel (12) sections: Fire protection .

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Research into the risk of penetration of fires in the cavity of over-cladding systems has recently been carried out in the United Kingdom by the Building Research Establishment.

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4 CASE STUDIES ON THE USE OF STEEL IN OVER-CLADDING A substantial amount of information on the use of steel in the over-cladding of buildings is now available. The principal countries with experience of this technology are those in Western Europe and North America, which are economically capable of investing in extending the life of their building stock, and which have the weather conditions that affect thermal performance and long term deterioration. Scandinavia has proportionately the largest number of over-cladding projects, and many of the following case studies refer to projects in Denmark and Finland. Some over-cladding projects used steel for both the sub-frame and cladding, whereas others used steel only for the sub-frame elements. The three main over-cladding systems, described in Section 2.4, are featured.

4.1

Over-cladding projects in Denmark

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This section was written by Mr Ib Falkenfleth, a Danish architect with considerable experience of over-cladding. In the late 1950s Denmark was in desperate need of new dwellings, and the development of precast concrete elements was seen as a an opportunity to solve these needs. A great number of building materials and techniques were used to accelerate re-housing but, unfortunately, the buildings produced were not all able to withstand the country’s severe climate. Denmark is surrounded by the sea, and the rapid and frequent changes in climate make it very difficult for buildings to withstand the combination of moisture, thermal movements, wind, condensation, and frost. As a consequence, a growing number of buildings from the 1950s, 60s, and even 70s show major problems. This, together with the growing demand for better thermal insulation, meant that something had to be done to improve the building stock. The use of over-cladding, combined with additional insulation to reduce energy bills, was first adopted in the beginning of the 1970s by H. H. Robertson Limited. It is estimated that, to date, an area in excess of approximately 6 million square metres has been over-clad in Denmark. Most of these projects have used additional mineral fibre insulation, as foam insulation is considered to act as a fire risk externally. The following case studies illustrate a range of over-cladding schemes completed in Denmark using galvanized steel sub-frames and cladding panels. They show the architectural opportunities that are created, and illustrate some of the important technical points to be considered, including those related to the attachments to the existing structure, the enclosing of balconies, and the provision of new lifts.

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4.1.1

“HN Nje Gladsaxe” Copenhagen

In the suburb of Gladsaxe in Copenhagen, three of Denmark’s most prominent architectural firms designed some of the first major buildings in concrete in the early 1960s, based on le Corbusier’s ideas and the then new construction techniques. The first families moved in during the spring of 1964. The concrete apartment buildings consist of five blocks of sixteen storeys, two blocks of nine storeys, and ten blocks of two or three storeys, and comprise 2 approximately 2,000 flats in total (164,000 m floor area). The owners of the buildings are five different Building Associations, and the flats are rented by their members or by the public as social welfare flats.

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In the tall buildings, one of the design features is a balcony access block. The only entrance is by a lift in each of the five stair towers. A staircase acting as a fire-escape is located at each end of the building, and the balconies on one side of the building are used to reach this escape route. This layout became an important issue some 25 years later when the concrete elements began to lose their integrity as a result of calcination in the concrete, corrosion of the reinforcement, frost attack, and thermal movements of and between the elements. To keep the access open for fire escape from the building, the architect and the consultant had to re-design the north and the south facades of the new over-cladding scheme differently. On the north side, where the entrances to the flats are located, the architect maintained the open balconies as a fire-escape. Consequently, the most damaged elements were renewed and the others were given the best possible traditional repair. The entire facade was then protected against future corrosion with the help of a permanent cathodic protection of the reinforcement in all the concrete elements. On the south side, the existing balconies were closed-off to provide a self-contained balcony for each flat. After repairing the concrete elements to a lesser degree than on the north side, the entire south facade was covered with new glazed units incorporating openable windows. The tenants thus gained a new living / balcony area, and retained the ability to provide sufficient light and ventilation for the remainder of the flat. To support and to be able to align the new facade and absorb inaccuracies and thermal movements, a system of steel sub-frames was designed. The steel construction was prefabricated with bolt-holes, fish-plates, etc. and then hot-dip galvanised for corrosion protection. No cutting or drilling was allowed on site. The sub-frames comprised RHS tubes (120 × 60 × 3 mm thick) surrounding each balcony, that were hung on two cleats, one on each side, and connected to the frame above and below with sliding joints. The cleat itself was fixed into the main concrete structure. Teflon or a similar insulating material was used to facilitate movements, to prevent noise under wind vibration and temperature movements, and to avoid galvanic corrosion between the dissimilar metals.

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Figure 4.1 shows two frames supported on one cleat, and also shows the method of individual adjustment. Figure 4.2 shows four frames in place and the new window-frame for the sliding windows inserted. Single-glazed and permanently ventilated aluminium sliding windows were fitted in the steel frames and the area between the frames was insulated, fire protected and closed with flashings, giving the new facade the desired aesthetic expression. This project was completed in 1993, and the new building facade is shown in Figure 4.3. In this project, the urgent need to address the deterioration of the existing concrete was more important than reducing energy losses. Nevertheless, the 2 entire facade (24,000 m ), acting as a solar panel, gives an energy saving of up to 20% or approximately 2,300,000 kWh per annum for all the buildings in the Gladsaxe project. Building owner: Architect:

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Consultant: Windows: Steel: Cladding:

Figure 4.1

The Building Associations AB Gladsaxe, FSB, FB, AAB and Lejerbo, all in Copenhagen. Project Manager Anders Dragheim arch.m.a.a., NOVA 5 arkitekter A/S, Copenhagen. Project Manager Niels Ellegaard ing. M.IDA.F.R.I., Birch & Krogboe A/S, Copenhagen. Marius Hansen A/S, Aarhus (main contractor). Ejnar Kornerup A/S, Copenhagen. Grønbæk Construction A/S, Copenhagen.

Method of adjustment of galvanized steel sub-frames in the HNje Gladsaxe project, Copenhagen

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Figure 4.2

Storey-high frames used to support the new glazed facade during installation

Figure 4.3

Completed new facade at HNje Gladsaxe

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4.1.2

“Bybjerget” Copenhagen

In the late 1950s, the Building Association “Rødovre Almennyttige Boligselskab” built one 8 storey and two 4 storey blocks containing 400 flats in “Rødovre”, a suburb of Copenhagen. The project was one of the first social welfare projects, and as such the cost and construction time were important considerations. The choice was an ‘industrialised’ design based on standard sandwich-elements with 5 to 10 cm insulation for the walls and gables, and with concrete elements as the main structure.

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One of the disadvantages of a sandwich-element, apart from the thin layer of insulation in it, is the great number of thermal bridges, especially in the perimeter and at the joints. This became a major issue in the late 1970s when the heating bills became excessive. The end gables had no windows and the joints were completely worn out and leaked, so, in the 1980s, the gables were over-clad with sheeting and additional insulation. A sub-frame of 1.25 mm thick cold formed galvanized steel Z-sections was erected on galvanized steel angles bolted to the frame of the concrete structure. A 100 mm Rockwool layer was placed between the Z-sections and covered with a wind-barrier. On top of this was attached a 45 mm deep profiled 0.6 mm thick steel sheet coated with the “Versacor” coating system (consisting of a thick epoxy layer on both sides of the sheet and a silicone-polyester layer on top of the epoxy layer on the outside of the sheet). The sheets spanned between floors, and each sheet was connected to the upper sheet with a flashing, giving the possibility of ventilating and draining any condensation present on the backside of the sheets. This two-sided coating for the sheeting was chosen because the normal coating on the rear face of the sheet may not be sufficient where condensation may occur. Figure 4.4 shows the original facade, with the new over-cladding of the end gables. The colour of the over-cladding system was chosen to complement the existing facade. In 1987, severe calcination, frost-cracking and corrosion of the reinforcement of the exposed main concrete structure and the sandwich elements on the facades was discovered. As the doors and windows in wood were in good condition, it was decided only to over-clad the remaining concrete structure and renew the small balcony floor with a 60 cm wider floor to improve the use of the existing balconies. The new construction was designed in a way that would make it possible to renew the windows at a later time without damaging the over-cladding system. The buildings are situated in an ordinary suburban area next to a main road, and the owner and the tenants wanted to change the architectural appearance by using a very colourful over-cladding system, making use of a combination of standard profiles and cassettes manufactured to order. The reinforcing steel was first cleaned and the damaged concrete cut away. A traditional concrete repair was carried out before the insulation and overcladding were installed. Galvanized steel Z sections were then attached to angles connected to the original structure. The Z-sections are only attached P:\CMP\Cmp657\pubs\P247\P247-Final.wpd

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periodically to minimize the thermal bridging. One layer of insulation was placed behind the Z-sections. The next layer of insulation was installed between the Z-sections. The insulation was attached to the sandwich elements so that no air movement could take place between the concrete and the insulation. As the tenants were living in the flats, a temporary front to the balconies was installed during the over-cladding work in order to maintain security. Before the over-cladding system was selected, a brochure explaining the reasons for the renovation and the solutions and the costs of the project was sent out to each tenant. The projected increase in rent was approximately 30% per flat. It was agreed that the work should go ahead, and a committee representing the owner, the tenants and the architect was elected to decide on the colour scheme.

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A few years later, the roof was also in need of major repair. However, rather than repairing the roof, it was decided that installing a new floor on top of each of the buildings was a better long-term investment. The extra costs involved were met by public subsidy and by the greater income resulting from the increase in the number of flats from 400 to 466.

Figure 4.4

Original facade at Bybjerget, Copenhagen, showing the overclad end gables

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Figure 4.5

Modular roof unit being installed

The new flats on the roof were designed as modular steel units, that would be built in the garden area next to the existing buildings and then lifted into place completely finished with installations, doors, windows etc., in place. By doing this, the building period and the costs were minimized (no scaffolding and easy access during the building process), and the tenants in the existing buildings were not greatly inconvenienced. Figure 4.5 shows the installation of a modular roof unit. On the existing roof, three steel beams (HE 240 sections) were placed 600 mm above the roof on top of steel columns supported by the existing concrete construction. The beams and columns were protected against corrosion by painting, and fireprotected with Rockwool. The modular units were designed as frameworks spanning between the existing cross-walls. The floor and roof members comprised 200 mm deep C sections spaced at 600 mm intervals. The roof construction (from the outside) is flat stainless steel on a sliding layer and plywood. A 50 × 100 mm lath, with 50 mm mineral fibre insulation between these laths, was placed on top of the purlins, and 2 × 100 mm mineral fibre insulation was placed between the purlins. Finally, a vapour-barrier and 2 × 13 mm gypsum boards were placed on the inside. This construction has 30 minutes fire certificate. The roof overhangs are supported by 60 mm diameter galvanized steel tubes fixed to the balconies. The walkways were erected on site and used as working platforms to protect the existing roof. The balconies and the walkways, the subframes, the floors and the railings are all made from galvanized steel.

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The underside of the overhang has a cladding of perforated profiled steel sheets with a sound absorbing insulation. The floor is also of sandwich-construction, with 2 × 13 mm gypsum board on 45 mm laths outside, 45 mm and 2 × 100 mm layers of mineral fibre insulation layers on and between the purlins, 20 mm profiled steel sheets, 2 × 13 mm gypsum board, and a parquet floor. This construction also has 30 minutes fire certificate. The external walls are 2 × 13 mm gypsum board on both sides of the steel framework with the appropriate vapour barrier and mineral fibre insulation, and a cladding of profiled steel sheeting with a PVF 2 coating. The partition walls are all 13 mm gypsum boards on a light steel framework with additional mineral fibre insulation placed between the studs. The new floor is connected to the existing staircases which act as fire escapes. Next to the existing building, a single glazed steel construction with a hydraulic lift was installed, which serviced the top floor only.

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The final building facade showing the new roof on the previous 4 storey building is shown in Figure 4.6, and the new facade of the taller building is shown in Figure 4.7. Building owner: Architect: Consultant:

Main Contractor: Unit-steel and cladding:

4.1.3

Rødovre Almennyttige Boligselskab, Copenhagen. Ralf Smidt, Architect m.a.a., Copenhagen. B.B. Bendtsen, rådg. Ing., Copenhagen. (The over-cladding project). Birch & Krogboe A/S, Copenhagen. (The additional top floor project). Enemærk & Petersen A/S, Copenhagen. E. & B. Nilson A/S, Copenhagen.

“Vestparken” Varde

On the west coast of Jutland, approximately 20 km from the North Sea, is a small town called Varde. In 1970, the Cooperative Housing Society “Borgerligt Socialt Boligselskab” built 104 flats in 2, 3 and 4 storey blocks in an area known as “Vestparken”. The main construction was in masonry, and the 30 cm thick gable walls are insulated with standard 7 cm mineral fibre insulation in the middle. The facades and the balcony rails were pre-cast concrete elements. In the late 1980s, the concrete elements had already suffered a severe calcination and corrosion. The masonry was in a reasonably good condition, but the insulation level was not up to modern standards, and thermal bridging was a serious problem. In 1990, it was decided to repair the concrete and to give the building an additional 100 mm of insulation. It was also decided to upgrade the architecture to maintain a certain uniformity of appearance of the buildings in the area. The choice of insulation thickness was made after examining the different combinations of insulation values, construction costs, energy prices and the potential savings. On top of the existing and repaired construction, a sub-construction of impregnated laths was erected on galvanized steel angles and filled with 100 mm insulation. A special 9 mm thick gypsum board was used, primarily as a wind-barrier, but also because it has good fire resistance and its surfaces P:\CMP\Cmp657\pubs\P247\P247-Final.wpd

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will repel water whilst at the same time allowing air to permeate. Thin impregnated laths were used to create a ventilation cavity behind the external cladding, which comprised specially designed steel-cassettes of 0.6 mm galvanized steel with a 200 µm Plastisol coating. Figure 4.8 shows the end result of over-cladding one of the blocks in the project.

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Building owner: Architect and Consultant: Contractor:

Figure 4.6

Borgerligt Socialt Boligselskab, Varde. Grønne, Thorup og Jessen, arkitekter m.a.a., Varde. Rasmussen & Schiøtz Vest A/S, Kolding. Konstruktøren, Varde.

Completed facade of smaller building in the Bybjerget project, Copenhagen

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Figure 4.7

Completed facade of the taller buildings of the Bybjerget project, Copenhagen

Figure 4.8

Completed over-cladding project at Varde, Denmark

4.1.4

“Toftebo” Vejle

In the centre of a small town called Vejle on the east coast of Jutland, 36 flats were built in 1955 under a multi-ownership scheme. The building was originally constructed in concrete with the main construction and the infill walls all cast on site and without any insulation. Although there was no loss of performance of the concrete, the lack of insulation resulted in very high heating costs and internal maintenance costs due to the cold walls. In mid-1996, it was decided to renovate the building by a total over-cladding scheme. P:\CMP\Cmp657\pubs\P247\P247-Final.wpd

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In the 1980s, a Swedish manufacturer had designed a Z-section with a series of slots in the body of the member in order to minimize the thermal bridging effect when used as the sub-frame in over-cladding. A Danish manufacturer adapted this system by turning the slots through 45 degrees and then attaching the section to a pressed galvanized steel angle with one 45 degree slot in each leg. By doing so, a reduced thermal bridging effect was achieved, and also flexibility in aligning and fastening the Z-sections to the angles, and the angles to the existing wall, was obtained. Figure 4.9 shows the pressed and slotted angles bolted to the infill wall and the mainframe, and the slotted Z-sections. Insulation is then placed between the sections. The method of attachment is illustrated in Figure 2.3.

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For the recladding of the flats in Vejle, the vertical spanning Z-sections were attached at a few points to the existing structure in order to minimize thermal bridging. It was decided to place the insulation so that it aligned with the front of the Z-sections, creating a cavity between the insulation and the infill walls, and at the same time minimizing the number of joints between the insulation batts. This cavity improves the insulation of the new wall, provided air movement is hindered in the cavity by closing the perimeter of every cavity with insulation. Another advantage of the system is that by using a driven nail through the slots into the edge of the insulation batts, it is secured in place, and the nail itself is secured by the neighbouring batts being pressed against the head of the nail. The chosen over-cladding profile was a relatively open cassette panel which is placed horizontally and close to the surface of the insulation. A further windbarrier is placed between the purlins and the over-cladding to close the joints between the insulation batts and between the insulation and the purlins. This wind-barrier is a laminated foil of paper/mesh/plastic, which is micro-perforated to make it permeable but still water repellant. The cassette panel and the flashings around windows are made from galvanized steel with a PVF2 coating. The darker flashings, being a non-standard colour, are Polyester powder-painted.

Figure 4.9

Detail of slotted sections and their attachments

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Figure 4.10 shows the careful detailing of the cassette cladding panels around the windows. The sill is bent up behind the window side, and under the window. The joint is then sealed. This allows for the windows to be replaced at a later date without affecting the cladding. Figure 4.11 shows the completed building.

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Building owner: Architect: Consultant: Contractor:

Toftebo 1, Vejle. Søholm Arkitekter, m.a.a., Vejle. Samfundsteknik, Vejle. Nisgård & Kristoffersen, Vejle.

Figure 4.10

Detailing of cassette panels around the windows

Figure 4.11

Completed building at Vejle, Denmark

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4.2

Over-cladding projects in Finland

In Finland, it is estimated that more than 2,000 medium rise and high rise buildings are currently in need of renovation as a result of deterioration caused by the harsh climatic conditions. The insulation characteristics of these buildings also need to be improved to reduce heat losses. The economic argument for over-cladding is based on life-cycle analyses which take into account the savings in energy costs and reduced maintenance costs, which usually outweigh the long term repair costs. The Finnish steel company Rautaruukki and its subsidiary, Mäkelä Metals, have developed over-cladding systems that allow for the creation of new balconies, (13) toilets, stairways and lifts . These additional elements are often produced as modular units that are fully serviced before installation. The cladding is generally in the form of steel cassettes. Three cassette systems have been developed, which have the following features:

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1. Cassettes with exposed fixings 2. Cassettes with hidden fixings 3. Cassettes with hanging attachments. The first cassette system is the cheapest, but has relatively wide joints between the panels. The cassettes may be attached to either horizontally or verticallyspanning steel sub-frame members. The second cassette system has narrow joints and is generally the preferred system architecturally (illustrated in Figure 4.12). The system uses horizontally spanning sub-frame members. The third system permits easy removal of the panels for replacement, but is the most expensive. It uses vertically spanning sub-frame members.

Figure 4.12

Detail of cassette panel with hidden fixings

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The following three projects illustrate the range of over-cladding schemes that have been carried out in Finland.

4.2.1

Water tower, Raahe

A tall stone water tower originally built in 1956 in the small town of Raahe in northern Finland underwent a complete transformation in a little more than 6 months in 1993. A series of 11 new floors was added around the tower to create new apartments. The terrace on top of the tower was replaced by a restaurant constructed in steel framing. A new staircase, a lift shaft, and balconies were added externally. The completed building is shown in Figure 4.13.

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The external cladding consists of steel cassettes on top of 80 mm of insulation and attached to horizontal steel sub-frames. Surface irregularities in the existing construction were eliminated by means of adjustable attachments. The detail of this cassette construction is illustrated in Figure 2.2.

Figure 4.13

Renovation of water tower at Raahe, Finland

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4.2.2

Apartment building, Forssa

In Forssa, near Tampere, a 6 storey 30-year old student dormitory was transformed into a modern apartment building. In addition to changing the ‘box-like’ appearance of the building, the apartments were equipped with new modern sanitary facilities. This was achieved using pre-fabricated light steel modular units stacked one on top of another external to the original building. The appearance was improved by adding steel balconies, also using pre-fabricated elements. The exterior cladding consists of pale blue steel cassettes with additional insulation behind the cassettes which are supported on galvanized steel members. The installation of the modular units is shown in Figure 4.14.

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The project was started in the summer of 1994 and the residents moved in six months later. The completed building is shown in Figure 4.15. It is proposed to extend this renovation technique to other buildings in the area.

Figure 4.14

Installation of modular toilet unit in the Forssa project

Figure 4.15

Completed over-clad building at Forssa, Finland

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4.2.3

Residences in Raahe

In the northern Finnish town of Raahe, the complete renovation of the facade of a series of 4 storey concrete buildings was carried out in 1993 using steel cassettes. New enclosed balconies in galvanized steel were attached to, and supported by, the existing concrete construction. The completed building is shown in Figure 1.2.

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A further project was carried out in Raahe which consisted of adding new pre-fabricated balconies and toilet units to a 4 storey building (see Figure 4.16). Other buildings in the Raahe area are being renovated using this technique.

Figure 4.16

4.3

Over-clad building in Raahe, Finland

Over-cladding projects in Germany and Belgium

Despite the large number of high-rise concrete buildings that have been constructed in Germany, the experience of over-cladding is relatively small. Two good examples are briefly reviewed here. The former administrative building of the Thyssen Stainless Steel Division in Krefeld was over-clad using stainless steel cladding and sub-frames attached to the original five storey concrete frame. The building consists of sprandel walls below the windows in the form of horizontally spanning corrugated panels. The panels are supported on U-shaped sub-frame members. Additional mineral wool insulation was provided behind the cladding. The completed building is shown in Figure 4.17. A three storey concrete building in Kotchen in former East Germany was overclad in cassette panels, and new modular toilet units were added externally to completely remodel the building. The building was also over-roofed to create new habitable space. The building before and after renovation is shown in Figures 4.18 and 4.19.

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(14)

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A major renovation project of an apartment building in Liège, Belgium , was carried out using various forms of steel panels, including curved corrugated stainless steel panels around the corners of the existing brickwork facade (see Figure 4.20).

Figure 4.17

Over-cladding of Thyssen building in Krefeld, Germany

Figure 4.18

Concrete building at Kotchen, Germany before renovation

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Figure 4.19

Completed building at Kotchen, Germany

Figure 4.20

Renovation of apartment building in Liège, Belgium

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4.4

Over-cladding projects in Canada

In Canada, the use of over-cladding is generally associated with improving the insulation characteristics of existing concrete and masonry buildings. Profiled steel sheeting on horizontal steel sub-frame members with additional insulation is the most common form of construction on the gable walls of these buildings. The renovation of some existing buildings in Canada is presented in a recent (15) brochure by the Canadian Sheet Steel Building Institute .

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A four storey student residence at Kingston University, Ontario, was over-clad using galvanized steel Z sections at 2 m centres, supporting insulation and profiled steel sheeting. The existing brickwork walls had deteriorated badly and major renovation was the only solution. The existing strip windows were 2 replaced by smaller 1.1 m windows, and the infill walls between them were made by using light steel framing and exterior grade gypsum board. The colours adopted for the external sheeting were grey and terra-cotta. The heavily profiled vertical ribs allude to ‘doric’ columns. The completed building is shown in Figure 4.21. A 15 storey apartment building at Victoria Park Road, Scarborough, Ontario was partially over-clad in mocha-coloured profiled sheeting to improve the insulation of the existing building (see Figure 4.22). In many other buildings, the end gables are simply insulated and over-clad using profiled sheeting to reduce heat losses, as shown in Figure 4.23.

Figure 4.21

Over-clad building at Kingston University, Canada

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Figure 4.22

Over-clad building at Scarborough, Canada

Figure 4.23

Use of profiled sheeting in over-cladding of gables of buildings

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4.5

Over-cladding projects in the UK

Although a number of buildings have been over-clad in London, Edinburgh and Glasgow, the experience of using steel in these applications is relatively limited. One system, developed by Terrapin Ltd, comprises a steel cassette system that is being used on a number of renovation projects. Recently, the Commercial Headquarters of British Steel Strip Products was completely renovated and extended by Matrex Design and Build, a division of Terrapin. The external cladding of the building, dating from the 1960s, was removed and replaced using British Steel’s colorcoat 200 in Merlin Grey, Jade, Hamlet and Heritage Green. A high level of insulation was incorporated. The cladding was supported on new horizontal sub-frame members attached to the original structure. A new pitched roof was also added. The buildings are being monitored over a 2 year period for environmental performance and user comfort. Figures 4.24 and 4.25 respectively show the building during construction and after completion.

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Composite panels were used in the renovation of the Inland Revenue building at Cumbernauld in Scotland. In this application, the composite panels were orientated horizontally and were attached to vertically spanning sub-frame members. The completed building is shown in Figure 4.26. The versatility of light steel when used in conjunction with other materials is demonstrated in the renovation and over-cladding of an existing concrete panel (16) building in Kingston University, London . A galvanized steel sub-frame was attached to the existing concrete panel structure. Insulation panels and boarding were attached to the sub-frame, and the final weather surface was completed by cementitious render. The completed building, which is now a student residence, is shown in Figure 4.27. In the UK, the over-cladding of a building is often carried out in conjunction with over-roofing. A seven storey building in Chingford, Essex, was over-clad and over-roofed using galvanized steel C section components and the Capella over-roofing system. Composite panels were attached to create the over-clad facade and roof. The completed building is shown in Figure 4.28.

Figure 4.24

Over-cladding of British Steel’s HQ in Newport, Wales

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Figure 4.25

Completed British Steel HQ in Newport, Wales

Figure 4.26

Over-cladding using composite panels at the Inland Revenue building, Cumbernauld, Scotland

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Figure 4.27

Over-cladding using galvanized steel sub-frames and cementitious render at Kingston University, London

Figure 4.28

Over-clad and over-roofed building at Chingford, Essex using the Capella system

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5 PROTOTYPE OVER-CLADDING SYSTEM USING STEEL COMPOSITE PANELS AND STEEL SUB-FRAMES 5.1

Description of prototype system

A prototype over-cladding system comprising horizontal sub-frame members and vertically spanning composite panels is being monitored continuously at the University of Edinburgh as part of a British Steel and European-funded demonstration project. The size of the prototype panel is representative of the over-cladding to two storeys of a concrete building, and is located on the north west face of the eight storey James Clark Maxwell building on the Edinburgh University Kings Building campus.

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The features of this system are listed below. 1. Horizontal steel sub-frames (modified C sections) are attached at intervals to the floors of the existing building by expansion fixings or resin anchors. Cleats are attached to the fixings and the horizontal sub-frame members are then positioned accurately at their ends before final tightening of the connecting bolts. Each horizontal member is attached by at least four fixings to the existing structure, thereby offering some redundancy in the event of one fixing failing or being weaker than the others. The members are approximately 5 m long with fixings every 1.5 m, reducing to 1 m in heavily loaded areas. 2. Rockwool insulation is attached to the inside of and above the horizontal sub-frame member to reduce cold-bridging. A damp-proof layer is attached to the upper surface of the member. These elements also serve to compartmentalise the cavity as a rainscreen and in fire. 3. Vertical members are provided at window locations and at the corners of the building to provide locally stiff points for attachment of the windows. Their outer surfaces are placed level with those of the horizontal members. The vertical members at the corners of the building also help to reduce air movement behind the over-cladding panels (see 2). 4. Composite panels (40 to 60 mm thick approximately) are lifted into place and attached to the sub-frame members at floor levels using self-drilling selftapping screws. The screws are hidden by a light flashing at the butt joint between the panels. A small plastic insert permits the necessary ‘trickle ventilation’ behind the panels to help combat condensation. The composite panels are able to span between the horizontal members and provide the necessary improvement in insulation to the facade. The ends of the composite panels are taped to prevent moisture or the effects of a cavity fire reaching the core material.

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5. All the elements are easily lifted and manhandled into place. The maximum weight of a component is 30 kg. 6. Separate sub-frame members are required around windows, at corners and at roof level (as noted in 3). 7. Masonry, or heavy duty panels, may be needed at ground floor level to avoid the risk of local impact damage. The detail at the horizontal joint in the prototype over-cladding system is shown in Figure 5.1.

Composite panel Cavity closer and damp-proof layer

Hanger bracket

Gasket

Self-drilling self-tapping screw

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Cover flashing

Steel brackets (100mm wide) Plastic spacer Fixings every metre Internal flashing Plastic spacing

Additional insulation behind the C section to avoid cold bridging

Clip

Horizontal sub-frame member

Figure 5.1

Joint detail in prototype steel over-cladding system

This system maximizes the spanning characteristics of all the steel components and uses the minimum number of separate components. The modest trickle ventilation permits ‘breathing’ of the existing fabric and avoids prolonged buildup of moisture, without affecting the insulation characteristics of the panels significantly. Moisture can drain away in the gap created between the composite panels and the horizontal member. The construction of the prototype test panel at the University of Edinburgh is shown in Figure 5.2.

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Figure 5.2

Construction of prototype over-cladding panel at Edinburgh University

The adequacy of the structural system was assessed in tests subject to simulated wind suction using air bags, as shown in Figure 5.3.

Figure 5.3

Load test on a 2.5 m x 5 m panel subject to simulated wind suction using air bags placed below the panels (testing carried out at Imperial College, London)

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5.2

Monitoring of a prototype over-cladding panel

The prototype panel is being monitored over a 5 year period (1994 -1999). Measurements that are being recorded by data-logger are: C

external, cavity and internal temperatures, including both faces of the composite panel

C

relative humidity externally and within the cavity

C

external and differential pressure across the panel

C

stresses in the skins of the panel

C

time of wetness within the cavity.

Meteorological data is also being recorded separately. A number of galvanized steel coupons were installed in the cavity with the intention of measuring the loss of zinc over the 5 year period of the tests, thereby predicting the design life of the sub-frame components.

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5.3

Thermal performance of prototype over-cladding

The U-value through the centre of the composite over-cladding panel has been 2o calculated to be 0.54 W/m C. However, thermal bridges in the vicinity of the panel joints result in a decrease in overall thermal resistance, leading to a 2o calculated overall U-value of 0.57 W/m C for the over-cladding system. When installed on the building at Edinburgh, the over-cladding panels have the effect of reducing the overall calculated U-value of the over-clad wall from an existing 2o 2o 1.57 W/m C to 0.42 W/m C. Monitoring of the thermal performance has provided temperature measurements of the over-clad wall, and of a control wall with no over-cladding. These measurements are presented in Figure 5.4 for a week in April 1995. The effect of the over-cladding has been to increase the internal surface temperatures by o o about 1 C to 2 C, and to increase the temperature on the external surface of o o the existing wall (behind the composite panel) by 5 C to 7 C. Since the heat loss through the wall is proportional to the temperature difference across it, the net result is a considerable reduction in heat loss that occurs through the wall. It is also noticeable that the temperature within the cavity behind the composite panel is significantly higher and is fairly constant compared with the temperature changes outside. This indicates that the over-cladding panel acts as a thermal ‘buffer’ to the outside air. Figure 5.4 also shows that the external surface of the over-cladding heats up when exposed to direct or indirect solar gains and reaches temperatures higher than the external surface of the existing wall. However, it cools down more quickly because of its lack of thermal mass. Preliminary analysis of the monitored temperatures of the over-clad wall during a typical period in April 1995 and in December 1995 suggests that reductions in heat loss through the wall of between 50% and 57% result from the new over-cladding panels. Slightly lower reductions occurred in December relative to April. These savings are lower than expected when considering the reduction in U-value, but can be explained by the effect of air movement in the P:\CMP\Cmp657\pubs\P247\P247-Final.wpd

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Temperature plot through the prototype panel for one week in April 1995

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Figure 5.4

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ventilated cavity. This has a noticeable effect in removing heat from the space behind the over-cladding panels. When internal temperatures are greater than external, heat will pass from inside the building through the existing wall into the cavity between the wall and the over-cladding. This heat is removed from the cavity either by conduction through the over-cladding panel, or by air movements through the cavity due to wind pressures or the “stack effect” (that is caused by temperature differences between the air within the cavity and the outside air, resulting in a pressure difference across the separating surface).

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The data indicates that, in addition to air movement in the cavity due to wind, which seems to be small, significant air change rates between the cavity and the outside air are thermally induced by the stack effect. Overall air change rates of over 5 air changes per hour (ach) are achieved in April and reach 12 to 16 ach in December. Measurements suggest that this air movement may be responsible for 30% of the heat loss through the wall in April and as much as 60% in December. Other measurements show that the time of wetness within the cavity was restricted to one 15 minute interval over the period. The differential air pressure results indicated that pressure equalization occurs relatively quickly in this prototype over-cladding panel. The air movement is sufficient to control condensation. The relative humidity of the cavity air also varies during the day, largely due to variations in the relative external air and cavity air temperatures. These preliminary results confirm the predicted performance of the overcladding system. The effective thermal insulation of the prototype overcladding system meets the requirements for new construction satisfying current Building Regulations. From the three year information analysed to date, it appears that the risk of condensation and water ingress is also very low, ensuring a good design life for the galvanized steel components. However, the prototype panel is not attached to a part of a building wall with a high output of warm moist air, as occurs from kitchen or bathroom walls. It is possible that the condensation risk may be higher in these cases than in the prototype panel.

5.4

Measurements of zinc loss

A series of zinc coated steel plates were suspended in the cavity between the composite panel and the existing wall. The zinc coating was in two forms: chromated and non-chromated. Three of the plates were removed every six months, and later every 12 months, to establish the weight of zinc that is lost due to oxidation and moisture effects. The measurements indicate a loss of 0.4 2 and 2.0 grams/m /year respectively for the two types of zinc coating, which is very low and indicates that a relatively benign environment exists in the cavity. On the basis of these measurements, the loss of 50% of the total zinc coating would constitute a life of over 200 years for chromated zinc specimens. Therefore a design life of at least 60 years can be confidently predicted for galvanized steel sub-frames in over-cladding applications, provided that the components are subject to only periodic wetting and drying. This is consistent P:\CMP\Cmp657\pubs\P247\P247-Final.wpd

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with the temperature conditions existing in the cavity, as described in Section 5.3.

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Further information on the durability of cold formed steel in residential (17) applications is given in a forthcoming SCI publication .

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6 ECONOMICS OF OVER-CLADDING A calculation of the benefits of an over-cladding scheme must take into account a wide range of economic and social factors, as described earlier. The quantifiable benefits are: C

Savings in heating bills for the occupants.

C

Increased rental charges made possible due to an improved internal environment.

C

Reduced maintenance and repair bills for the existing facade.

C

Reduced management costs due to the improved quality of the building.

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Other demonstrable savings of over-cladding existing buildings, as opposed to demolition and replacement, are: C

Not having to re-house the occupants temporarily.

C

The cost of demolition and new building is considerably greater than renovation of the existing building.

The benefits of over-cladding occur over a long time span, and an economic assessment should take into account the present value of savings over an assumed “pay back” period. An assessment of the benefits of over-cladding has been carried out for a typical high rise residential building in West London, consisting of 96 two bedroom flats. It considered the savings in heating costs, reductions in management and maintenance costs, and increased rental charges. The following conclusions can be drawn from the assessment. C

The assessment predicts that the total savings due to over-cladding and fitting new double-glazed windows to this typical housing block may be up to £600 per dwelling per year, leading to a total of over £48,800 per year for the whole block.

C

The occupants of the dwellings in the block will benefit from lower heating costs and improved comfort, but may in turn pay higher rental charges. The landlord of the block will spend less on maintenance of the building, and have the opportunity to increase his rental revenue.

C

Considering a 20 year pay back period, and assuming that there will be no increase in the real cost of energy, a life cycle cost calculation indicates that the present value of savings (using a 4% discount ratio) are up to £8,000 per flat, and in excess of £663,000 for the whole block. The equivalent 2 discounted saving is calculated as £166 per m , when expressed per unit area of facade of the building.

C

If energy costs are assumed to increase by 2% per year in real terms, the discounted savings over a 20 year period are predicted to be in excess of 2 £709,000 for the whole block, and £177 per m when expressed per unit area of the facade.

C

The above calculations suggest that an investment in over-cladding within 2 the range of £166 to £177 per m of the facade is likely to pay for itself over a 20 year lifetime. Therefore, the target installation cost for an over-

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cladding system, including new glazing, should be less than these savings if the discounted economic benefits over the life of the over-cladding system are to exceed the costs of the renovation work.

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The following implications for the design of over-cladding systems may be made regarding these conclusions: C

A light steel over-cladding system such as that described in this publication may be shown to be constructable within this cost target. It offers an attractive and speedy solution to the problem of high energy use and increasing maintenance costs in residential blocks, with a design life of at least 30 years. After 30 years, some repainting and repairs may be required, but the facade will still be serviceable and effective in terms of good thermal insulation and in preventing rain penetration. The galvanized steel subframe has a design life of over 60 years.

C

The cost-benefits described here are only part of the real benefits that an over-cladding system can generate. These range from the improvements in the comfort, lifestyle and health of individual occupants, to the global environmental benefits of reduced emissions of carbon dioxide and other pollutants into the atmosphere. Installation of an over-cladding and glazing system on the example block would lead to predicted savings in carbon dioxide emissions of about 259 tonnes per year.

C

As part of an over-cladding improvement scheme, it may be possible to provide a new pitched roof above an existing flat roof. In addition to reducing heat losses and so further reducing energy use, this can provide new habitable space which may be used for communal facilities, or as new dwelling units with additional revenue.

C

The common alternative to over-cladding an existing block is to demolish it and replace the dwellings. The initial cost of this is unlikely to be repaid over the lifetime of the dwellings by the future savings as compared to the over-cladding option. Furthermore, it will usually be necessary to re-house the occupants temporarily during the construction of the new buildings.

C

Ultimately, the light steel components can be re-used or recycled at the end of the useful life of the building.

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7 REFERENCES 1.

Hillier, M., Lawson, R.M., and Gorgolewski, M. Over-roofing of existing buildings using light steel (SCI-P246) The Steel Construction Institute, 1998

2.

Harrison, H.W. Over-cladding: External walls of large panel system dwellings Building Research Establishment, 1986

3.

Anderson, J.M. and Gill, J.R. Rainscreen cladding: a guide to design principles and practice CIRIA Building and Structural Design Report - Walls Construction Industry Research & Information Association, 1988

4.

British Standards Institution BS 6399: Design loading for buildings BS 6399-2: 1997, Code of practice for wind loads

5.

Standard for walls with ventilated rainscreens Centre for Window and Cladding Technology (CWCT), University of Bath, 1998

6.

British Standards Institution BS EN 10147: 1992, Specification for continuously hot-dip coated structural sheet steel and strip: technical delivery conditions

7.

British Steel Strip Products The Colorcoat Building British Steel, 1997

8.

British Standards Institution BS 5950: Structural use of steelwork in building BS 5950-1:1990, Code of practice for design in simple and continuous construction: hot rolled sections BS 5950-5:1987, Code of practice for design of cold formed sections BS 5950-6:1995, Code of practice for design of light gauge profiled steel sheeting

9.

European committee for standardisation (CEN) Eurocode 3 : Design of steel structures EC3 : Part 1.3 Cold formed thin-gauge members and sheeting Document reference ENV 1993-1-3 : 1996 CEN, 1996 (copies available from the British Standards Institution)

10.

Grubb, J.G. and Lawson, R.M. Building design using cold formed steel sections: Construction detailing and practice (SCI-P165) The Steel Construction Institute, 1997

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Discuss me ...

11.

Rhodes, J. and Lawson, R.M. Design of structures using cold formed steel sections (SCI-P089) The Steel Construction Institute, 1992

12.

Lawson, R.M. Building design using cold formed steel sections: Fire protection (SCI-P129) The Steel Construction Institute, 1993

13.

Using steel in house renovation Mäkelä Metals, a division of Rautaruuki Oy (Finland) Translation from Finnish, 1996

14.

Innovations in steel - Roofs and facades around the world International Iron and Steel Institute, Brussels, Belgium, 1995

15.

Retrofit the building envelope (brochure) Canadian Sheet Steel Building Institute, Ontario, Canada

16.

Case studies on light steel framing (SCI-P176) The Steel Construction Institute, 1997

17.

Popo-Ola, S. and Gray, R. Durability of cold formed steel sections in housing and other applications The Steel Construction Institute (to be published in 1998)

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8 CONTACTS The following companies offer light steel products and systems for use in overcladding: Ash & Lacy Building Products Limited, PO Box 58, Alma Street, Smethwick, Warley, West Midlands, B66 2RP Tel: 0121 5588921, Fax: 0121 5589645 Ayrshire Steel Framing Limited, Irvine, Ayrshire, KA12 8PH, Scotland Tel: 01294 274171, Fax: 01294 275447 British Steel Framing, PO Box 28, Mendalgief Road, Newport, Gwent, NP9 2WX (incorporating SureBuild) Tel: 01633 273642, Fax: 01633 211231

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Metsec Framing Limited, Broadwell Road, Oldbury, West Midlands, B69 4HE (incorporating Metframe and Gypframe) Tel: 0121 552 1541, Fax: 0121 544 6779 Steel Framing Systems (refer to Metsec Framing Limited) Terrapin Limited, Bond Avenue, Bletchley, Milton Keynes, MK1 1JJ Tel: 01908 270900, Fax: 01908 270052 Ward Building Components Limited, Sherburn, Malton, North Yorkshire, YO17 8PQ Tel: 01944 710591, Fax: 01944 710555 Other light steel components and cladding systems may be obtained from a range of manufacturers. For more information, contact: British Steel Strip Products, Commercial Department, P O Box 10, Newport, Gwent, NP9 0XN Tel: (01633) 290022, Fax: (01633) 464087

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