Bridging between buildings

An extreme example of spanning a seismic gap between independent structures occurs when they are bridged. In urban environments bridges spanning between buildings are a common sight. Designers must ensure that when the buildings move out-of-phase with each other the bridges remain undamaged. The form of bridging may be a literal bridge, providing horizontal circulation from one building to another, or perhaps a glazed roof canopy that creates a sheltered courtyard. The primary design challenge to be overcome is to cope with out-of-phase horizontal movements between the buildings. How can a bridge span between two buildings without connecting them, and how should a bridge be secured to prevent it falling (Fig. 8.30)? If firmly connected at each end a bridge is likely to be torn apart or buckle when the buildings vibrate or drift horizontally relative to each other.

The conceptual starting point of the design is acknowledging that the buildings will move relative to each other during an earthquake. The bridge is therefore anchored or fixed in its longitudinal direction to one building and free to slide on the other. Each end of the bridge must also be restrained at right-angles to its length to prevent it moving sideways under transverse wind and earthquake inertia forces. In Fig. 8.31 the

y direction movement

(a) Plan showing y direction relative movement between buildings

--Sliding

(c) Elevation showing maximum relative movement of buildings towards each other

--Sliding

(c) Elevation showing maximum relative movement of buildings towards each other y direction movement

(a) Plan showing y direction relative movement between buildings

i

Fixed

Sliding

(b) Elevation showing maximum relative movement apart

Sliding joint, teflon and stainless steel surfaces. (Shear key to prevent movement normal to bridge not shown)

Detail B

Detail A

Detail B

▲ 8.31 The relative drifts between two separated buildings (a) to (c) and generic bridge seating details (d).

bridge is connected to the left-hand building and slides on the other in the direction of the bridge length. The seating width at the sliding end should be conservatively assessed to ensure that the bridge will never fall off. Quite sophisticated architectural flashings are required to weatherproof the right-hand bridge-to-building connection. They must be sufficiently flexible to prevent build-up of force when the bridge slides and survive small quakes without repair.

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