From Metal Building Systems: Design and Specifications, Second Edition


The discussion in Chaps. 7 and 9 has already highlighted an importance of specifying correct design criteria for lateral drift and vertical deflections. This chapter is specifically devoted to this critical and controversial topic that occupies the minds of many structural engineers who specify metal building systems.

First, the definitions: Lateral (story) drift is the amount of sidesway between two adjacent stories of a building caused by lateral (wind and seismic) loads (Fig. 11.1). For a single-story building, lateral drift equals the amount of horizontal roof displacement. Horizontal deflection of a wall refers to its horizontal movement between supports under wind or earthquake loading. Vertical deflection of a floor or roof structural member is the amount of sag under gravity or other vertical loading.

FIGURE 11.1: Story drift caused by lateral loads.

Why is any of this controversial? Drift and deflection criteria, along with some other issues such as vibrations, deal with serviceability, or functional performance, of buildings under load. Model building codes have traditionally prescribed the desired levels of strength and safety, leaving the more nebulous topics of satisfying the occupants perceptions of comfort and solidity up to the designers. The designers criteria for achieving these goals are necessarily subjective, as the building which seems flimsy to one person may feel comfortable to another.

Various design firms tend to specify similar, although not identical, limits on horizontal and vertical building displacements for medium and high-rise structures. On the other hand,...

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