Horizontal and vertical systems

A seismic resistant structure necessitates integration of horizontal and vertical structure. Together, these two systems resist inertia forces and transfer them along force paths predetermined by designers to the

Plan

(a) Diaphragm deflecting under y direction inertia forces. Since its span is excessive for its depth it may be too weak or flexible

Plan

(a) Diaphragm deflecting under y direction inertia forces. Since its span is excessive for its depth it may be too weak or flexible

Diaphragm span

foundations. In most buildings, as explained in Chapter 4, suspended floors provide adequate diaphragm strength and stiffness without needing significant modification. But where the horizontal distance between vertical systems lengthens, the structural adequacy of a diaphragm warrants special attention (Fig. 6.10).

Where architects penetrate or perforate a diaphragm or vary its levels along the length of a building, these changes weaken it and reduce its ability to span horizontally. In these cases, the plan distribution of vertical structure needs to be carefully addressed. More information on how to deal with diaphragm irregularities is presented in Chapter 9, but in general terms, wherever the monolithic and planar character of a diaphragm is disturbed, architects need to provide more uniformly distributed or closely-spaced vertical structure.

(b) More even distribution of vertical structure reduces the diaphragm span

▲ 6.10 A long diaphragm needs more frequent support from vertical structure.

Perimeter shear wall

Services . distribution zone

One-bay moment frame

(a) One-way, one-bay frames allow distribution of under-floor services

▲ 6.10 A long diaphragm needs more frequent support from vertical structure.

Perimeter shear wall

Services . distribution zone

One-bay moment frame

(a) One-way, one-bay frames allow distribution of under-floor services

Perimeter moment frame

(b) Perimeter moment frames or shear walls allow maximum freedom for services distribution

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