Facades and Interfaces

Facades and interfaces Façade systems comprise the structural elements that provide lateral and vertical resistance to wind and other actions, and the building envelope elements that provide the weather resistance and thermal, acoustic and fire resisting properties. The types of façade system that are used depends on the type and scale of the building and on local planning requirements that may affect the building’s appearance in relation to its neighbours. For example, brickwork is often specified as the external façade material, but the modern way of constructing the inner leaf consists of light steel wall elements (called infill walling) that have effectively replaced more traditional block-work. Other types of façade materials may be attached to light steel walling, such as insulated render, large boards, metallic panels and terracotta tiles. A wide variety of facade treatments and shapes may be created using light steel wall including large ribbon windows, curved and inclined walls, and with projections such as solar shading or balconies. Façade materials may be mixed to enhance the aesthetics of the building. It is also possible to pre-fabricate light steel wall panels with their cladding pre-attached. In multi-storey buildings, unitised curtain walling systems have been developed that are attached to the floors or edge beams of the primary steel structure. Steel and glass are also widely used in façade and roofing systems, and the local attachments are in the form of stainless steel brackets. Other interfaces that affect the design of the façade include the attachment of brickwork to steel edge beams, the design of projecting balconies, solar shading and attachments of parapets.

Installation of a unitised curtain walling system (Image courtesy of Arup Facades)

Installation of lightweight facade system attached to a modular building through a mast climbing system. (Image courtesy of Futureform)

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1 Types of façade systems o 1.1 Benefits of steel façade systems o 1.2 Design requirements 2 Solutions using light steel infill walls o 2.1 Benefits of light steel infill walls o 2.2 Design of infill walls o 2.3 Thermal performance o 2.4 Construction process 3 Curtain walling o 3.1 Building performance o 3.2 Unitised curtain walling o 3.3 Support conditions 4 Support to brickwork o 4.1 Stainless steel support systems o 4.2 Brick slip systems o 4.3 Facade retention in building renovation 5 Steel and glass facades

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5.1 Building performance § 5.1.1 Double façade systems § 5.1.2 Solar shading systems o 5.2 Glazing support systems o 5.3 Steel in atria and canopies 6 Interfaces o 6.1 Curtain walling support details o 6.2 External steelwork o 6.3 Louvres and canopies 7 References 8 Resources 9 See also

[top] Types of façade systems A wide variety of façade systems may be used in modern multi-storey buildings, which are: • • • • • • •

Brickwork and stonework (masonry) Precast concrete panels with various types of finishes Insulated render Metallic cladding Tiles and stone veneer panels Large boards consisting of an aesthetic and weather tight veneer Glass and steel façade systems

Large colour coated steel cassette panels supported on vertical rails The choice of facade system is dependent on the scale and use of the multi-storey building, and on its local environment and neighbours. A variety of steel components may be used in modern facade systems, such as: • • • • • •

Steel profiled sheets and composite (sandwich) panels Flat and rigidised cassette panels with folded edges Light steel infill walls using C sections Hollow steel sections (often circular) for facades and roofs, particularly used for visual effect in atria and in entrance areas Stainless steel glazing support systems Metallic elements in unitised curtain walling

Light steel infill walls have largely replaced the block-work inner leaf in both steel and concrete framed buildings. A variety of facade systems may be attached to the infill walls. Some examples are illustrated below.

Large colour coated steel cassette panels supported on vertical rails

[top] Benefits of steel façade systems The benefits of steel façade systems may be presented in terms of their functional and aesthetic requirements, as follows: • • • • • • •

A variety of colours and surface textures is possible Lightweight facades minimise the loads on the supporting structure Light steel infill walls using C sections can be used to support a wide range of cladding systems Facades can be highly pre-fabricated for speed of installation Steel glazing systems can be used for visual effect in tall entrance areas and atria Steel is non-combustible and robust to damage in façade panels A high level of thermal and acoustic insulation can be provided.

Use of composite (sandwich)panels to support tiles. (Image courtesy of Kingspan Panels and Profiles)

Use of large metallic panles in over-cladding of an existing office building. (Image courtesy of Tata Steel Panels and Profiles)

[top] Design requirements Planning requirements are likely to define the overall building form and its external appearance, including the cladding materials that may be used. In many medium rise office buildings in suburban locations, brickwork is the preferred cladding system, whereas for city centre offices, modern curtain walling systems are often used. The principal design requirements for the façade system are its ability to: • • • • • • •

Resistance wind loading and high local wind pressures on the corners of the building, which are a function of the building height and the proximity of adjacent buildings. Support a variety of external finishes, including masonry Permit relative movement where the deflection of the primary frame would impair the performance of the cladding Be sufficiently stiff that there is no risk of damage to the glazing or cladding in long windows or in tall walls Provide the required level of thermal insulation and air-tightness Provide weather-tightness, including during construction In some cases, provide acoustic attenuation to external noise sources.

[top] Solutions using light steel infill walls

Typical light steel infill walls in a steel framed building Light steel walls may be of two types: • •

Light steel infill walls that span between the floors or between the floor and edge beam Panelised systems that are placed outside the slab edge and are attached at discrete locations.

Light steel infill walls are more widely used because of the simplicity of the installation process and the ability to deliver cut-to-length C sections for the particular as-built dimensions of the project. The development of light steel infill walls has been one of the major innovations in the last 10 years. Light steel infill walls consists of C sections that span 2.4 to 5m between floors, and are designed to resist the wind pressures applied to the building façade and also to support the weight of the particular type of cladding system that is attached to them.

[top] Benefits of light steel infill walls The benefits of light steel infill walls are: • • • • • • • • •

Rapid construction system with an installation rate of over 50m² per day Less materials handling on site than for brick and block-work Tall walls up to 5m and high wind pressures up to 2kN/m² Ability to create large windows without wind posts Minimum material use (less than 5kg/m² of steel in the façade) No on–site waste when C sections are delivered cut to length Light weight, which reduces the loads on the supporting structure Can be used for a wide range of cladding systems Can be dismantled in building extensions etc. and re-used

[top] Design of infill walls

Metsec’s SFS system used on the external infill walls to a 4-storey composite frame at Colchester Hospital. (Image courtesy of Metsec) The design of light steel infill walls is dependent on the wall height and wind pressures acting on the façade. Normally the C sections are 100 to 150mm deep with steel thicknesses of 1.2 to 1.6mm. The C sections are placed at 400 or 600mm centres, which is compatible with the attachments to the internal plasterboard and external cladding. Large openings can be created by placing pairs of C sections vertically next to the openings, and sometimes pairs of C sections above and below the openings. The steel thickness can also be varied across the façade without changing the section size. For example, wind pressures are higher at the corners of the buildings and also increase with height. The deflection limits that are specified in design depend on the types of cladding that is attached.

[top] Thermal performance Thermal insulation is attached externally to the wall and mineral wool is often placed between the C sections to achieve the required thermal insulation (U-value). For insulated render or rain screen cladding systems, an external sheathing board is often used to provide local support to the external cladding.

A U-value of 0.15 W/m²K can be achieved by approximately 100mm of closed cell insulation board fixed to the C sections or sheathing board supplemented by 100mm of mineral wool between the Cs. The same wall build-up may be used for all types of cladding systems. Air-tightness is also important in modern building design, and it can be improved by use of a sheathing board fixed to the C sections.

[top] Construction process Light steel infill walls are generally installed as individual C sections that are cut to length and are placed between the floors or edge beams. The C sections are attached to a U shaped bottom track which is attached to the floor slab. At the top of the wall, the C sections slide in a U shaped top track that is fixed to the underside of the edge beam or floor slab permits relative movement without compressing the wall. The general guidance is to provide a minimum of 20mm relative movement in a concrete framed building and 10mm in a steel framed building. Pairs of C sections are often placed either side of window or door openings to resist the loads transferred across the window. The U tracks are connected to the concrete floor slab using powder actuated pins. The construction process is very rapid and does not require external scaffolding until the façade is attached externally. Alternatively, the walls may be prefabricated and installed as large panels, often with the cladding pre-attached - see photograph below. In this case, the cladding panel is placed outside the edge of the primary structure, and supports the cladding fascia. The cladding around the edges of the panel is then attached on-site.

Installing light steel infill walls. (Image courtesy of Metsec)

[top] Curtain walling

Lightweight prefabricated panel attached to a steel framed building (Image courtesy of Kingspan Panels and profiles)

Curtain walling system attached to a steel framed building in Spinningfileds, Manchester Curtain walling is the generic name given to metallic lightweight cladding or glazed cladding systems that are directly supported by a structural frame. In some cases, a stone veneer or large tiled fascia may be attached to give the appearance of a more monolithic cladding system.

[top] Building performance The curtain walling system is designed to provide the necessary functions of weather-tightness, natural lighting and shading, and thermal insulation. The joints between the elements of the curtain walling are therefore very important to these functions. In unitised systems, the panels are manufactured so that they are highly sealed and insulated, and the joints between the large panels are made by rubber gaskets and silicone sealants. The fascia may be designed to act as a rain screen by creating a cavity behind the fascia material and providing wider joints around the perimeter of the cladding panels. Therefore, under the wind action, pressure equalisation occurs between the cavity and external air so that wind driven rain is not forced into the cavity, thereby reducing the risk of water ingress through the joints. In highly glazed curtain walling systems, triple glazing is often provided in which the gap between two of the glazed sheets can incorporate shading devices. Generally windows are sealed in modern offices and therefore control of ventilation by other means is important. A high level of acoustic attenuation can be achieved which is important in city centre buildings.

[top] Unitised curtain walling

Fully glazed curtain walling system used in a multi-storey steel structure Modern curtain walling often consists of unitised systems that provide all the building physics and structural functions and also forms the external and internal faces of the facade. The size of the unitised panels is dictated by the floor to floor height and a sensible width for transportation and installation and should be compatible with the planning dimensions of the façade (normally a multiple of 300mm). Panels up to 1.5m wide and 4.2m high are typical. There are relatively few suppliers of unitised curtain walling systems in Europe and most have dedicated design teams who can provide detailed design and detailing for particular projects.

[top] Support conditions Curtain walling systems are generally supported vertically and laterally on the floor below and are laterally supported by the edge beam above. The support details should permit relative vertical movement between the upper beam and the cladding system, depending on the length and hence potential deflection of the beam. For this reason, the edge beams should be sufficiently stiff to prevent any damage to the cladding system, particularly if it is highly glazed. The span of a steel edge beam is typically 5 to 8m (6m and 7.5m are common dimensions), and the span of a concrete edge beam or slab is typically 5 to 6m. A total deflection limit of span/500 under imposed loading is normally specified for the edge beams for more brittle cladding systems. The installation of the panels should also allow for dimensional tolerances at the slab edge by use of packers or levelling devices. Some curtain walling systems are designed with steel ’strong backs’ so that they can span directly between the perimeter columns and therefore do not require vertical support from the slab edge although they may require lateral support to resist wind action on the panel. The ability to transport and lift these large panels is the critical design consideration. Strongback cladding system

[top] Support to brickwork

Brick slips used for the upper levels of a building. (Image courtesy of Unite Modular Solutions) Brickwork can be attached to a steel framed building by one of two methods: •

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It can be supported on the ground or an intermediate structure and laterally supported by the steel framework and infill wall. This approach is permitted for walls up to about 3 storeys high It is supported every floor or in some cases, alternate floors by stainless steel support angles that are attached to the edge beams of the primary steel structure or to the edge of the floor slab.

Brick tiles or brick slips have also been developed which give an external appearance of brickwork but which are bonded to a sheathing board of supported on horizontal rails or sheeting. A good example of this is shown on the right. The method of fixing brickwork to steel frames

[top] Stainless steel support systems Stainless steel support angles may be used to support brickwork at floor levels. The key design parameters are the wall height and the eccentricity of the brickwork from the supporting structure. The Stainless steel angles are typically 10mm thick so that they can be placed in the horizontal brick courses, and their position is adjustable to allow for geometric deviations in level of the coursing by attachment to stainless steel support brackets. Two generic support systems for the stainless steel brackets may be used: •

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Connection to the steel edge beams, which are generally made through steel plates that are welded to the flange tips of the beams to which the support brackets are attached. These plates are attached in 200 to 300mm lengths and allow the brackets to be bolted to them every 400 or 600mm. An example of this type of detail is shown in the figure below. Connection to the slab edge generally though a pre-formed steel edge trim to the floor slab, which has horizontal dovetail slots in which the connecting bolts are placed. This form of attachment is made every floor as it is not capable of supporting such heavy loads as the above system. An example of this type of detail is shown in the figure below.

Generic support systems for stainless steel brackets

Brickwork support system at a steel edge beam. (Image courtesy of Halfen Deha)

Brickwork support system at a s (Image courtesy of Halfen Deha

The eccentricity of the brickwork from its support is important because it determines the bending effect on the attachment points. The eccentricity is also dependent on the thickness of the insulation in the cavity space between the brickwork and the internal light steel walling. This maximum value is 120 to 150mm depending on the wall height. The brickwork is laterally supported by wall ties that attached to the infill walls at a density of about 4.4 ties per m² of the facade area.

[top] Brick slip systems Modern brickwork can be manufactured in the form of brick slips that are attached to a supporting steel sheet or composite panel. The advantage of this system is that it is lightweight and can be installed rapidly as mortar is not necessarily required. Brick slips can also be stacked vertically, and ribbon or unusual shaped windows can be created for architectural effect. Examples are shown in the photograph below. In this system, the brick slips are not considered to be weather-tight, and so the wind and weather resistance is provided by the backing material. Composite (or sandwich) panels provide both excellent structural and thermal characteristics for use as the backing system. Use of brick slips attached to steel backing system, such as a composite panel

Corium (Image courtesy of Wienerberger)

(Image courtesy of Kingspan Panels and profiles)

[top] Facade retention in building renovation

Existing brickwork supported by a temporary steel structure

In many building renovation projects, the existing brick or stone façade is preserved and is supported temporarily by a steel structure, whilst the rest of the building is demolished. A new steel permanent structure is constructed behind the existing façade which is then integrated into the new building. In this way, the appearance of the building is not changed but its functional use is much improved. A good example of the support to an existing brick façade by an external temporary steel structure is shown below. The framework at ground level allows for pedestrian access.

[top] Steel and glass facades

Steel and glass are synergistic materials and are often used in facades and roofs of multi-storey buildings. The glass panels are generally supported by separate vertical steel elements to the main structural frame of the building that may be internal or external to the building. Stainless steel and hollow steel sections are often used in combination with glass. Fixing of glazed facade systems to steel frames

[top] Building performance

Solar shading with bonded photovoltaic cells attached to a curtain walling system The glazed walling system is designed to provide the necessary functions of weather-tightness, natural lighting and shading, and thermal insulation. The silicone joints between the glazing panels are therefore very important to these functions. The main issue in the design of glazing systems is the avoidance of high solar gain, particularly on south facing facades, and also the heat loss due to the relatively high U-value of double or even triple glazing systems that adds to heat loss. A modern argon filled double glazing system (combined with low emssivity glass) has a U-value of 1.6 to 1.8 W/m², and this can reduce to 0.8 to 0.9 W/m² for high quality triple glazing systems. Large glazing panels are usually supported by vertical rails or in some cases, glass fins. The glass is designed to accommodate the movement of its support system due to the wind and other forces acting on it. Typical deflection limits under the design wind loads are defined by the Institution of Structural Engineers[1] Because of these requirements and for safety in cleaning, double skin glazing systems have been developed. The glass elements may also be combined with louvres and bonded photovoltaic panels, as shown.

[top] Double façade systems Double façade systems comprise an internal glazed façade which provides the thermal and air tightness functions and an external glazed facade separated by a zone that provides a thermal buffer and also incorporates devices such as solar shading. These systems are often used where the double facade extends outside the building line at the lower levels and often provides a canopy at street level. The space between the double facades can also be accessed for maintenance. The benefit of double facades is mainly on south facing walls where solar gain is highest. The space between the double facades can be ventilated in hot conditions or can be enclosed in colder conditions. A good example of a double façade system is shown below.

Double façade steel glazing system used in a multi-storey steel framed office building, 1 Angel Square, Manchester (Image courtesy of Fisher Engineering Ltd.)

[top] Solar shading systems

Solar shading using projecting roof with external tubular columns, Heelis building, Swindon (Image courtesy of Simon Doling/Feilden Clegg Bradley Architects. Copyright Simon Doling/Feilden Clegg Bradley Architects) There is a wide variety of solar shading systems that may be used and incorporated as part of the building façade. There are: • • • •

Oval shaped horizontal steel elements that span horizontally between external columns and their size and spacing is designed to reduce the intensity of solar gain. Projecting roof or canopy , often supported by an external steel structure as shown. Glazed or metallic louvres. Metallic perforated screens that allow natural light to penetrate but also provide a high degree of shading.

[top] Glazing support systems Modern glazing support systems are based on attachments to 2 or 4 separate glass panels using stainless steel brackets, also known as ‘spiders’ because of their multiple legs. The attachments to the glass panels are generally made by stainless steel brackets with neoprene gaskets through the glass, as shown below. These attachments permit articulation due to thermal and structural movements so that local stresses on the glass are minimised. Glazing support structures can be of various forms:

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External or internal tubular columns that may be inclined Horizontal tubular or lattice members that span between widely spaced columns. Cable tied systems, as shown below, using stainless steel external couplers, arms and struts. External glazing support system using stainless steel connectors

Corning Musem of Art, Corning, New York (Images courtesy of TMR Consulting)

The Manchester Justice Centre shown below is a good example of the vertical and horizontal support by an internal tubular steel structure to a fully glazed façade over 8 storeys. Cable tied systems can be external or internal and use the cables to resist tension forces due to wind action on the façade and the tubular sections to resist compression. For minimum visual impact, the tubes should be of small diameter.

Mixed use of glazed façade system and weathering steel at the Manchester Justice centre

[top] Steel in atria and canopies

Use of curved tubular steelwork to support an atrium roof Modern multi-storey buildings often contain atria which can be supported by tubular steelwork for architectural effect. An example of curved tubular steelwork with inclined arms to support the glazed roof on a regular grid is shown below. The same techniques may be used for vertically supported glass in multi-storey entrance areas and for canopies, as shown below. Loads include wind pressure, snow and access loads. These systems are designed to minimise movement of the supported glazing.

[top] Interfaces Interfaces between steel frames and cladding systems may take various forms as follows: • • • • • •

Brickwork support systems by Stainless steel angles and brackets. Attachment to curtain walling systems for both vertical and lateral support by the structure or the edge of the floor slab Attachment of steel hollow sections and cables in glazed cladding systems Projections for louvres or canopies, etc. Support to external steelwork Support to the atrium or featured steelwork.

These interface details are designed to take account of:

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Forces in the vertical and horizontal directions often combined with bending effects when used in louvers, etc. Allowance for relative movement with the support structure Allowance for installation tolerances in the alignment of the façade.

[top] Curtain walling support details Curtain walling is generally placed outside the line of the building structure and is supported on the floor slab or sometimes from the beams above. Therefore the attachments to the floor slab are designed to resist both horizontal loads due to wind action on the façade and tension and compression effect due to the eccentric vertical loads of the weight of the faced panel. Depending on the eccentricity of the panel to the edge of the building, the brackets can be quite large, and are also designed so that the panels can be carefully aligned by fine adjustments. The attachment of the panel to the floor above is designed to resist only horizontal loads and to allow for relative movement with respect to the edge beam.

[top] External steelwork An external steel structure can be designed to be part of the primary structure or to support canopies or bracing. Often the external steelwork can be designed as unprotected against fire by considering the intensity and direction of potential fire plume emanating from the façade. Also, the external steelwork is designed to be part of the architectural concept, as shown below in Exchange Square, which straddles the railway lines to Liverpool station. In this project, the beams projected outside the façade line, and so penetrated the façade. To avoid cold bridging, the beams in the floor zone were insulated over a length of about 1.5m on the inside of the building.

[top] Louvres and canopies Louvres and canopies are generally attached to the primary steel structure. To avoid cold bridging through the steel members passing through the insulation, special thermal break details have been developed, as shown below. Canopies are often highly glazed as shown below and can be supported by a separate structure or suspended from the internal structure. Curved steel members (particularly hollow sections) are often used in canopies for visual effect. Steel interface details

External steelwork used in Exchange Square, Broadgate, London

Attachment points for external canopies using thermal break bolted details

Use of glass canopy supported by curved steelwork

[top] References 1.

^ Structural use of glass in buildings (1999). Institution of Structural Engineers

[top] Resources • • • • • • • •

SCI P101 Interfaces: Curtain wall connections to steel frames SCI P102 Interfaces: Connections between steel and other materials SCI P103 Interfaces: Electrical lift installations in steel framed buildings SCI P166 Interfaces: Design of steel framed buildings for service integration SCI P193 Steel supported glazing systems SCI P298 Stainless steel masonry support systems- best practice information sheet for specifiers SCI P396 New Beijing Poly Plaza Cable-Net Wall SCI IE P2 Services coordination with structural beams; Guidance on defect free interface

[top] See also • • • • •

Infill walling Design codes and standards Acoustics Fire and steel construction Steel construction products

Category: Multi-storey office buildings