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Distribution of Wind Loads to diaphragm in Spreadsheets

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I’m seeking some advice or comments from those of you who have created tools (spreadsheets or any other type of programming is applicable). I am struggling with a decision as how best to model a lateral distribution of loads tributary to the diaphragms in a multistory light-framed structure that complies with flexible analysis.

The most conservative model would calculate the end zones of the block that I am modeling and simply apply the end zone wind force across the entire diaphragm width. The logic is that the location of interior resisting shear elements probably do not align with the location where 2a and the Interior zone meet which makes the calculation in a spreadsheet where lines of resistance change a bit difficult to calculate.

The second choice is to assume the building is broken into blocks and that each block is consistent in width and depth at each diaphragm level. In this choice the total load caused by both end zones (2a*wwind) and interior zones can be calculated for the entire tributary area at the edge of the diaphragm. Calculating the total diaphragm load due to wind would then be divided by the tributary area of the entire diaphragm width in the block and this load (in psf) would be applied to the entire edge of the diaphragm as a unit load (2a*wwind is combined and proportioned into the unit shear at the level under consideration).


The first choice is very conservative while the second can store the corner shears (2a*wwind) for use later when designing the wall cladding and components. The second is less conservative but easier to deal with when attempting to look at the entire structure in order to compare wind and seismic in each line of resistance. The second also makes it easier to turn shear resisting elements on and off and quickly rebalance the distribution of forces in each interior and exterior grid line where a shear resisting element occurs.


Finally, when designing each line of resistance for the worst case Wind vs. Seismic force at that grid line or line of resistance, the actual load becomes conservative by the nature of using the worst case reaction.


Do you:

1.       Apply the corner zone shear across the entire diaphragm (tributary height adjacent to diaphragm) and compare against seismic OR

2.       Do  you calculate the end zone at 2a*wwind and average this into the Interior zone loads to apply across the diaphragm and compare against seismic.

3.       Does it matter?



Dennis S. Wish, PE


Dennis S. Wish, PE

California Professional Engineer

Structural Engineering Consultant

La Quinta, CA 92253