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Theme: cladding

As a practice that designs many different building types, from indoor ski slopes to the new Airbus factory, FaulknerBrowns is acutely aware of the numerous issues that need to be considered when specifying cladding and curtain walling. But keeping up with new developments has enabled it to continually find effective and innovative solutions There are so many building types and so many ways of covering buildings that it is impossible here to cover all the issues that should be considered in the specifi cation of cladding and curtain walling. Factors that should be investigated include: thermal performance; structural requirements; acoustic performance; fire resistance; durability and maintenance requirements; environmental sustainability; costs and value engineering; client requirements; and, not least, aesthetic considerations.

FaulknerBrowns designs many building types, and so must develop innovative cladding and curtain walling solutions to problems that arise through consideration of these factors. This article provides examples of solutions it has used in some of its recently completed buildings.

Warm and green The introduction of Approved Document L of the Building Regulations has increased U-value requirements and placed a greater demand on the construction industry to achieve a high level of building airtightness and reductions in solar gain.

Riverside House, one of the office buildings on the Newburn Riverside Industry Park, west of Newcastle, was the first building in the North East to be tested for airtightness under Part L. The client and design team had high environmental aspirations, ensuring the building achieved a BREEAM rating of excellent despite construction costs of only about ú850/m 2. Environmental aspirations were high throughout the regeneration and development of the site, and were vindicated by the fact that Newburn Riverside, the second largest brownfi eld regeneration project in England, won a number of major environmental awards during 2003 for reclamation work.

To achieve the environmental aspirations, FaulknerBrowns considered a number of options for cladding and curtain walling.

Products fabricated off site with factoryformed joints generally provide the best solutions to achieve airtightness. Such forms of construction include composite panels (like those produced by Kingspan, Ward, Hoesch or Corus) and unitised curtain walling systems (manufacturers include Permasteelisa, Schmidlin and Schuco).

Conversely, forms of construction dependent on a number of component parts brought together on site and erected in a series of operations (for example, twin-skin metal sheeting) can be less satisfactory. However, properly constructed stick-system curtain walling, or rain-screens with structural liner-tray back-up walls, can perform as well as prefabricated systems if the construction is supervised properly.

Both prefabricated and site-fabricated systems will only perform satisfactorily if the interface junctions with other elements are designed and constructed to ensure continuous insulation and air-barrier layers.

The need to limit solar gain and heat loss thorough glazed cladding must be balanced with the benefi ts of providing natural daylight for users of the buildings.

Solutions to this problem include specifi ation of external solar shading devices (manufactures include Colt, Schuco, CBS, and Levolux), reflecting and absorbing glasses or light-diffusing glazed panels (such as those produced by Saint-Gobain, Pilkington and Okalux). There are many glasses that combine a very good shading co-effi ient with a natural appearance, meaning they do not look highly refl ective.

On Riverside House a Schuco FW50+ glazed curtain wall system was chosen for the major elevations, with Pilkington 6mm HP Suncool clear 63/45 glass as the outer glazed surface. Schuco RS50 frames held the glass in curved elevations around the central atrium. All vision areas were shaded by a horizontal and vertical louvred brise soleil. The inner leaf of the spandrel double glazing in non-vision areas was covered with a ceramic coating and backed with 50mm of foil insulation. These features ensured a good balance between natural daylight and a low level of solar gain.

The end elevations of Riverside House were constructed with Baggeridge Etruscan smooth red porous clay bricks, with the dry-lining inner leaf extended up to the soffit and down to the structural floor.

For airtightness particular importance was placed on sealing the junctions of the inner leaf. Where structural movement was expected, for example where the inner leaf joined structural steelwork, butyl tapes and sealants were used.

The blockwork was sealed with suitable paints. Coordination of subcontractors' work and continual site supervision helped ensure that interfaces were both airtight and watertight, by checking that membranes were suitably lapped and service penetrations sealed.

Performance goes downhill FaulknerBrowns has designed a number of indoor ski slopes with real snow, which use cold-store composite panels as cladding.

These Xscape buildings are mixed use with multi-screen cinemas, extreme sports facilities, restaurants and shops, as well as the ski slope.

Condensation within the ski box construction has been a major consideration for these buildings, since it would result in freezing and possible delamination of the panel facing. It is imperative to avoid penetration of warm moist air from elsewhere in the building into the ski box via the joints of the composite panels.

The ski slope in Xscape Milton Keynes is housed within an insulated external skin.

The very large roof void was modelled using CFD analysis to assess the risk of condensation within the roof space.

Controlled natural ventilation was provided.

The composite panels enclosing the ski box not only interfaced with this void but also needed to provide fi re compartmentation between the ski slope and the other spaces. FiSEC developed a fire strategy for the building in conjunction with building control and the fire officer.

This strategy required all the panels to be non-combustible, and panels with a mineral-rock-fi bre core were identifi ed as the most suitable. The successful tenderer to the performance specifi cation, McVeigh Insulations, chose Ranilla panels for the project. To prevent freezing within the panel joints, a fully airtight seal was installed in the production factory. Full compression of the seal was achieved by the weight of the 200mm-thick rock-fi bre panels when they were laid horizontally.

Preventing freezing in vertical joints and any interface details was more diffi cult.

Generally the panels were through-fixed back onto structural steel so that two rows of sealant tape could be applied to the steel prior to fixing, and the gap between panels filed with expanding fire-rated canister foam. The installer covered all screw fi xing points penetrating the structure with a strip of Bitite tape to prevent air leakage.

Cut panels and junctions were treated in a similar manner, and all visible vertical and interface joints covered with a flashing.

A void behind the panels allows visual inspection from the warm side on an annual basis, and allows thermographic surveys and airtightness tests to be carried out.

The ski box in the second-generation Xscape building, Xscape Castleford, which opened in September 2003, was slightly different. Only part of the ski slope was surrounded by spaces used for other purposes (cinema, fitness suite), with the remainder of the box projecting above the surrounding roofs. As at Milton Keynes, the composite panels were fixed on the inside of the steel frame. In this instance, however, the external flange of the steel frame was covered with a single skin of stucco-embossed aluminium standing seam. The void created is quite small in comparison with Xscape Milton Keynes and the risk of condensation is correspondingly much lower. Even though the risk has been reduced, all surfaces within the void are treated with coatings suitable for external environments.

The panels forming compartmentation still consist of mineral-rock fibre. But because the ski box projects above the surrounding areas, the fire strategy deemed that PIR-cored panels with LPS1181 Grade B rating were acceptable on the projecting area. These lighter and thinner panels saved on costs. The fixing screws were treated with a coating that melts and seals the threads when driven into the panel. This removes the need for the Bitite layer over the screw points. All panels for Xscape Castleford were supplied by Isoclad and installed by McVeigh Insulations.

For the next in the series, the structure for the ski box may be placed within the box itself, and the composite panels may be used as both the insulating structure and the external skin of the building. There are major structural and thermal issues to consider. The panels will be subject to wind loads, so spanning capacity will reduce.

Thermal bridging of the structure may be increased, which will require additional trace heating and insulation of the steel.

At a stretch The imposed wind loads, service loads, snow loads, height of the building and exposure all influence the choice of cladding and curtain walling. Typically, composite cladding panels require support at 3-3.5m centres.

Long-span panels have been introduced, with the achievable span determined by consideration of the height of the building and the imposed wind loads.

Built-up systems will generally be fixed either horizontally or vertically to cladding rails. The spanning capability of the system will vary depending on the spanning capability of the outer sheet. The loads imposed on the outer sheets are transferred directly through the construction to the supporting structure via a spacer system, with a light-gauge liner forming the inner skin. Structural trays can replace the liner and, due to their ability to span and transfer loads, a saving in the number of cladding rails can be achieved.

FaulknerBrowns had reason to consider the structural and spanning capabilities of roof cladding when designing the English Institute of Sport (EIS) High Performance Centre in Sheffield. A clear span of 120m x 70m was required to provide the large internal space for the indoor athletics track and spectators, suggesting a roof-deck span of 20.4m between support girders.

Our design aspiration was for an economical spanning northlight roofing system where the natural curve of the roof would confer a light and elegant appearance.

The chosen cladding sheets consisted of two steel skins, both with 120mm trapezoidal profiles and 2.5mm-thick top-hat spacers - the skin-spacer system acts as a stressedskin construction. Between the skins were layers of acoustic insulation, polyethylene vapour-control membrane and mineralrock-fibre thermal insulation. Rautaruukki, the manufacturer of the chosen system, normally produces roof-deck spans of less than 12m, and its design data analysis could not easily be extrapolated to understand the bearing properties of a 20.4m span.

FaulknerBrowns and the structural engineers joined with the manufacturer to examine whether a 20.4m roof span was feasible. Rautaruukki engineers constructed a full-size prototype of the roof span in its factory in Finland. The testing regime - which among other tests required piling 4kN/m 2 of snow on the roof with a mechanical digger - proved the suitability of the spanning system for the building and enabled the design team to refine the detailing of the roof's supporting structure.

EIS Sheffield now has the world's largest-spanning twin-skin perforated steel roof deck. But there are many additional advantages of the roof system. Suspending the steel trusses of the structural frame from 45m-high masts proved to be economical when compared with several other more traditional approaches of clear-span trusses supported on columns. Many of the services - radiant gas heaters, inlet and outlet fans and industrial-type lights - are suspended from the main trusses, leaving clear uninterrupted roof vaults between trusses. And as a further bonus, the glorious eight-masted fully rigged and braced superstructure is an advertisement for the building from miles around.

Stay on your toes

The specification of cladding or curtain walling is complex and many issues need to be considered in parallel to ensure the best choice of material for the building.

Legislation and standards are constantly under review, illustrating the speed of changes within our industry. These factors, as well as changes in aesthetic fashion and client's requirements, ensure that specifiers must remain vigilant in maintaining their knowledge of new developments.

David Whitfield is a senior director at FaulknerBrowns

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