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Static vs. Simplified Static design

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This next question is to settle a debate (or to start one). For the majority of plan check that I run across, engineers and architects are disregarding the full-compliance methods and are assuming a flexible diaphragm design on less than three story light-framed structures made of wood with a wood diaphragm. However, the majority of submittals use the Static design methods per UBC or CBC Section 1630.2. While most calculate Rho, the results are almost always less than 1.0 so the default Rho value is taken as 1.0.

Then there are those who have chosen (for some reason) to use the Simplified static design which adds approximately 20% to the base shear value. I happen to fall into this category, but my rationale is that the conservative nature of the Simplified static design compensates for construction flaws.


My understanding is that the engineer can only use the Static design of 1630.2 if he first determines by analysis that the diaphragm is flexible. If not, he must follow the full-compliance method and balance or chose the worst case wind / seismic; (flexible/rigid) for each line of shear.


Now if he wants to use the Simplified Static design of 1630.2.3 he must first check the structure for irregularities and that the engineer must use the default values for soil properties for SD in zone 4 and a near source factor not exceeding 1.3. The comparative formulas that tend to yield the lowest shear for each type is;


V=(2.5*I*Ca/(1.4*R))*W (Static) Formula 30-5

V=(3.0*I*Ca/(1.4*R))*W (Simplified Static) 30-11


I typically beef up the near source values and place my site closer to the active fault in order to have a more conservative base shear if I don’t know where the site location is or a soils report is not provided or required by the city.


The question is; What is the correct interpretation for the use of Static Methods and when should one consider using the Simplified Static design? The obvious outcome of this is to determine what the existing standard of professional practice is regardless of whether or not it complies with the letter of the code which is not (in 100% of the 35 designs I’ve reviewed) being followed.


Your help in this matter would be greatly appreciated.





Dennis S. Wish, PE

California Professional Engineer

Structural Engineering Consultant



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