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ASCE 7-05 Wind Load - Simplified Methods

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Fundamental Period equation is in ASCE7-05 equa 18.5-5.

The deflections are based on the assumed triangular distribution of acceleration, that the first mode shape is the only significant mode for each horizontal direction, assumes an elastic model, and it assumes a rigid foundation.

A second iteration can be performed by, assuming an acceleration distribution in the shape of the first calculated deflections. Better yet, add to the deflections, the affect of a flexible foundation. Do not contradict Chapter 19.

The period is a constant, with the following assumptions.
1. Any but equal deflections between floors. I found that the maximum allowed story drift is a most usable value. Use about ½”, or a little less, for plywood in allowable stress terms, sizing plywood walls for a constant ratio of demand to capacity.
2. Any but equal floor heights. Consider an 8’ plywood wall plus floor.
3. Any but equal weight at each floor.

The period for that set of assumptions can be considered a dynamic analysis for all buildings complying with those assumptions. To solve for that universal period, the code equation is reduced to a quadratic equation that has two solutions with one usable and the other impossible.

If those assumptions are true, then no calculation iteration is needed.

To apply this period, for design, I have found that the period is so high that the stiffest case of a single story plywood building is enough to reduce seismic forces to the minimum allowed. I have yet to apply this to the wind criteria. Maybe a period value is needed for each, number of stories, building type.

I will have to get on it soon. Please post results to compare.

David Merrick, SE
Merrick Group, Sacramento, CA

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