Figure 2121

Performance assessment of frame basec

on

LDP evaluation (see notation in Figure 21.18).

21.7.2.5 Method V: Probabilistic Assessment — LSP (FEMA-350)

The previous four methods provided a deterministic assessment of the building performance. The same design will now be evaluated in a probabilistic manner using the criteria in FEMA-350. The process itself is not probabilistic, though the final evaluation results in a nondeterministic evaluation of the computed demands. We begin with the LSP. The steps involved in predicting demands are similar to method I. The total lateral force is applied as an inverted triangular load using the distribution given by Equation 21.11, and the resulting moment demands in each component are tabulated. These demands are then factored by the uncertainty factors (see Table 21.8) to establish the factored demand value ggaD. Acceptable capacity values are then determined (see typical capacity limits given in Table 21.9) and modified by a capacity reduction factor (shown in Table 21.8), to account for the inherent uncertainty in establishing such limits. The factored demand-to-capacity ratio, l, is used to establish a confidence level. The computed confidence levels for global interstory drift are tabulated in Table 21.17. The acceptability of the estimated confidence level is specified in FEMA-350 as follows:

Recommended Minimum Confidence Levels (%)
0 0

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