Parapets and appendages

Parapets, particularly of unreinforced masonry construction, often fall from buildings during earthquakes. Due to high horizontal accelerations at roof levels parapets are vulnerable to out-of-plane collapse, especially if they cantilever vertically and depend upon unreliable mortar tension bond between block units.

Out-of-plane support can be provided by a variety of means. Vertical reinforced concrete cantilever columns or steel brackets can ensure the seismic safety of parapets if connected strongly to both parapet and roof structure (Fig. 11.18). There is usually no need to strengthen a parapet wall for inertia forces in its plane because of its inherent in-plane strength.

Appendages can take many forms including signs, canopies, cornices, mock-columns and other elements attached to the exteriors of buildings. In every case, appendages need to be tied-back or braced to structural elements that are strong and stiff enough to resist the inertia forces. The strength of a tie depends upon the intensity of the horizontal inertia force acting on an appendage, and that is related to the weight

▲ 11.19 Weakly attached unreinforced columns are tied back at first floor level with stainless steel ties and vertical bearing rods. Restaurant, Wellington.

▲ 11.19 Weakly attached unreinforced columns are tied back at first floor level with stainless steel ties and vertical bearing rods. Restaurant, Wellington.

▲ 11.20 Framing for interior partition walls. The wall top plate is inserted into a sheet steel channel without being nailed to it. This detail allows relative movement in the plane of the wall between the wall and the floor above. Office building, Wellington.

of the appendage, its height up a building and the dynamic characteristics of both itself and the building (Fig. 11.19).

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