Wall claddings – commercial
Unlike residential buildings, for low-rise commercial and industrial buildings constructed using concrete, it is common for at least part of the wall structure to double as the wall cladding. Tilt-up slab construction, which provides high in-plane stiffness and strength, uses this approach.
Taller structures and other low-rise buildings that use an enclosed frame design have a wide variety of cladding options, including many of those commonly used in residential houses. Again, they may be categorised as lightweight and heavy claddings.
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In multi-storey structures, lightweight claddings are generally designed to move with the structure during an earthquake and must be able to accommodate inter-storey drift and stresses on cladding joints.
This is normally achieved by using movement control joints and overlaps, which allow individual components of the cladding to move independently without cracking or tearing or adjacent sections experiencing impact damage.
Large multi-storey commercial and industrial buildings often use a series of heavy panels called spandrel panels as a wall cladding. Spandrel panels are often constructed using precast concrete or steel and glass, and typically span between two floors of a building. Spandrel panel claddings may be used with concrete or steel-framed structures, and it is not unusual for them to weigh several tonnes each in some cases.
During an earthquake, deformation and inter-storey drift can create considerable forces in the panels if they are rigidly connected between storeys of a building. This can result in considerable damage to the panel connection system, which, in extreme cases, may fail completely, causing the panel to fall to the ground.
The connection system must therefore permit differential movement of the structure between storeys without transferring seismic forces to the panels or the connection system itself. This is usually achieved by using connectors that provide a simple and reliable method of decoupling the panel from the structure during an earthquake. In other words, the connector transfers the vertical gravity load of the panel to the frame but permits the frame and the panel to move independently under horizontal loads.
Several proprietary connection systems are available to achieve this objective, but most systems accommodate frame movement using either ductile or sliding connections.
In a ductile connection arrangement, the connector system consists of anchors to the frame at the top and bottom of the spandrel panel. The first ones at floor level support the panel’s weight with a fixed bearing connection. The second ones use a ductile tie connection below the floor level above to hold the panel in place. Under lateral load , the tie is designed to deform and prevent forces being transferred into the panel.
A sliding connection relies on a bearing connection at the bottom of the panel and a slot or oversized hole at the top. A correctly torqued bolt provides a friction or slip connection that allows the non-bearing connection to the panel to move when sufficient force is applied.
Connection systems like these work well for panels that are closely arranged in the same plane. However, most buildings have walls and features that create intersections where planes of panels meet at opposing angles.
During an earthquake, each of these planes will move relative to each other. If the structure and connectors are not correctly designed to accommodate this movement, this can cause spandrel panels to contact each other, which is possible is cases of high drift.
Many commercial building use large areas of glazing or glass curtain walls as an architectural feature.
However, glass is very brittle, and it cannot deform within the plane of its surface. Without sufficient space around the perimeter of the panes, it cannot tolerate the deformations imposed on the frame by lateral seismic forces, and the glass shatters.
To accommodate the frame movement and protect the glazing, glass curtain walls generally incorporate gaps between the glass panes and the surrounding mullions, transoms and rubber gaskets.
This allows a certain amount of movement in the gaskets before the glass comes into contact with the frame.
Proprietary connectors are also available that allow the glass panels to articulate independently from the frame and from each other.