1959 Drift Limitations

It is important to control lateral drift in structures because excessive drift can result in extensive damage to cladding and other nonstructural building components. In addition, excessive lateral drift can result in the development of P-A instability and collapse.

Lateral drift is evaluated on a story by story basis. Story drift, d, is computed as the difference in lateral deflection at the top of a story and that at the bottom of the story, as predicted by the lateral analysis. If the lateral analysis was other than a nonlinear response history analysis, design story drift, A, is obtained from the computed story drift, d, by the equation:

where Cd is the design coefficient previously discussed. The design interstory drift computed from Equation 19.22 must be less than a permissible amount, dependent on the SUG and structural system as given in Table 19.8.

The provisions require evaluation of potential P-A instability through consideration of the quantity 6 given by the equation:

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