Building shape in plan and elevation

Building shape strongly influences how the structure behaves in an earthquake, so the shape is decided at the planning stage.

Shape in plan

• Best: square, rectangle or circle. These are symmetrical, load paths are direct, and there is no stress concentration.
• Poor: L, T, U, H, Y and cross shapes. The wings vibrate differently, and stress concentrates at the re-entrant (inside) corners, causing torsion and damage there.
• Remedy: divide complex plans into simple rectangular blocks separated by seismic joints (gaps), or add strong ties/diaphragms at the junctions.
• Aspect ratio: length/width preferably not more than 3 (2 is better).
• Projections such as cantilever balconies should be small (not more than about 1.2 m), and the centres of mass and rigidity should be close together.

 Good plans              Poor plans
 +-------+   (o)         +--+   +--+
 |       |               |  |   |  |
 +-------+               |  +---+  |   U-shape
                         +---------+

Shape in elevation

• Best: a uniform, regular box shape, or one whose size reduces gradually with height.
• Poor: sudden changes such as set-backs, large overhangs, an open (soft) ground storey, heavy top floors, floating columns, and short columns on a slope.
• Principles: mass, stiffness and strength should be uniform or reduce gradually with height; walls and columns should be continuous from the foundation; avoid a very tall, slender building on a small base (overturning).
• A soft storey (open ground floor for parking) concentrates damage on that floor; add shear walls or bracing.

 Good           Poor (set-back)      Poor (soft storey)
 +------+          +--+              +------+
 |      |          |  |              |      |
 |      |       +--+  +--+           |      |
 |      |       |        |           +------+
 |      |       |        |           | |  | |  open ground storey
 +------+       +--------+           +-+--+-+