A building in an earthquake does not simply shake as a solid block. Its frame bends, and each floor moves sideways a little more or less than the one below it. A facade is fixed to those floors and spans between them, so it is pushed out of square every time the building sways. If the facade cannot follow that movement, glass cracks, panels loosen and pieces can fall onto streets and exits.
This article explains, in general terms, how facades are detailed to cope with earthquake movement. It does not cover how a building structure is designed, or the values set by codes for a particular site. Those come from the structural engineer and the applicable code. The facade's job is to accept the movement it is given, safely.
What the facade actually experiences
The key movement is inter-storey drift: one floor moving sideways relative to the next. A rectangle of facade fixed top and bottom is pushed into a parallelogram. Vertical movement also happens, as slabs bounce and cantilevers flex, and the building can twist, which affects corners in particular. On top of this, the movement repeats back and forth many times during an event.
A facade is not meant to stiffen the building or carry these forces. It is designed to move with the structure while staying attached and closed. The amount of movement it must accept is a design input, provided by the structural engineer and written into the facade specification. Without that figure, a facade designer can only guess, so ask for it early.
Racking: the frame goes out of square
When a frame is pushed into a parallelogram, its corners move. In a curtain wall, the vertical mullions tilt and the horizontal transoms try to follow. If the members are rigidly joined to each other and to the building, something has to give, and it is often the glass or the joints. Facade systems avoid this by building in places where parts can slide or rotate.
Typical places include the connection between mullion lengths, the joint between unitised panels, and the anchors at slab edges. Each is designed to allow a controlled amount of movement in a known direction while still carrying the facade's weight and the wind. The movement joints guide covers the everyday movements from temperature and loading. Earthquake movement uses the same idea, but it is larger, faster and repeated.
How the glass is protected
Glass itself cannot stretch, so it is protected by space. A pane sits in its frame with clearances around its edges, resting on setting blocks and held by gaskets or silicone. As the frame racks, the pane can rotate within that space before the frame touches the glass edge. Contact between metal and glass edge is what usually causes cracking, so the clearance is a key detail.
Flexible fixings help. Gaskets compress, and in structural glazing the silicone bond can deform a little while staying attached. Where glass is fixed rigidly, such as with bolts through holes, the fittings must allow rotation, or the glass must be sized and supported to cope. The spider glazing guide explains how point-fixed systems articulate.
If a pane does crack, the next question is whether it stays in place. Laminated glass tends to hold together after breakage, which is why it is often chosen where people below could be hurt. The laminated or toughened guide sets out that choice. Glazing beads and retention details also matter, since a cracked pane that is still firmly held is far safer than a loose one.
Stick and unitised systems behave differently
In a stick system, mullions run continuously past slabs and are joined at splices, and glass is fitted on site into the grid. Movement is taken at the splices, the anchors and the glass clearances. In a unitised system, each panel is a complete frame that interlocks with its neighbours along vertical and horizontal stack joints. As floors drift, panels slide relative to each other along those joints, which can accommodate significant movement without forcing the glass.
Neither type is automatically better. What matters is that the chosen system is detailed and shown to accept the stated movement, including at awkward points like corners, setbacks and junctions with other facade types. The stick versus unitised guide covers the wider comparison.
Anchors, slab edges and building joints
Anchors connect the facade to the structure. Some are fixed, carrying the weight; others are slotted or sliding, letting the facade move relative to the slab. The arrangement controls where movement can happen. Anchors also have to cope with the slab edge itself moving and with the deflection of the floor between columns. The anchors guide explains the detail.
Where a building is divided into separate structural blocks, the joints between them can open and close considerably in an earthquake. The facade must not bridge those joints rigidly. A movement joint in the facade, aligned with the structural joint, lets each block move independently. These joints need careful design so that they remain weathertight in normal use and do not throw off covers or panels when they move.
Cladding, corners and heavy elements
Opaque cladding, stone, metal panels and fins need the same thinking. Heavy panels fixed rigidly across a floor line can be pulled off as the floors move. Brackets usually allow some panels to slide or rotate, and fixings are arranged so that each panel is carried by one floor. Corners are a weak point, because the two faces move in different directions; they often need a joint or a flexible detail rather than a rigid wrap.
Elements that project, such as canopies, sunshades and large fins, deserve particular attention because they can swing and they sit above entrances. The brackets and fasteners guide covers what holds these elements in place.
How performance is shown
Designers show that a facade can accept movement through calculations, drawings and, on many projects, testing. Laboratory tests can rack a mock-up sideways and measure whether glass cracks, panels come loose or seals stay closed at the stated movement. Some test programmes also check air and water performance again after the racking. The testing page and the structural tests guide describe what these methods measure.
At drawing stage, ask the facade designer to show how each joint moves, where the clearances are and what happens at corners and building joints. The shop drawings guide lists what to look for. A detail that cannot be explained in movement terms on paper is unlikely to behave well in an earthquake.
Questions for the design team
Earthquake movement is easy to leave until late, because it does not show on an elevation drawing. Raising it at the start keeps the facade system, the glass and the anchors aligned with the structure. These questions help bring it into the brief.
- What movement must the facade accept, and who has stated it in writing?
- Where in each bay does the movement happen: at anchors, splices, stack joints or glass clearances?
- How much clearance does the glass have before the frame touches its edge?
- Which panes are laminated or otherwise retained, and why were those chosen?
- How are corners, setbacks and structural joints between building blocks detailed?
- Will a racking test be carried out, and does it include air and water checks afterwards?
- How will the facade be inspected and made safe after a significant event?
Clear answers to these questions usually mean the movement has been designed in, rather than assumed. Vague answers are a signal to involve the facade consultant more closely before shop drawings are approved.
After an earthquake
A facade that has been through a significant earthquake should be inspected, starting with any risk of falling material above streets, entrances and exits. Make those areas safe first. Then look for cracked or displaced glass, torn sealant, open or crushed joints, panels out of line and visible damage at anchors. Damage that is not visible from outside may need access to the slab edges.
Record what is found, elevation by elevation, and compare it with the drawings. A pattern of damage often points to a particular detail or joint. The maintenance checklist and the glass replacement guide help plan repairs.
General guidance for orientation, not a specification. Applicable standards, values and details are confirmed per project with the facade consultant.
