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Re: Static vs. Simplified Static design

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I am not sure where you got the idea that the static method is only for flexible diaphragms. The guidelines for using the static method are spelled out in section 1629.8.3. For all structures less than 65 feet or 5 stories one can use the static method.

The use of simplified method is more restrictive and the criteria is spelled out in 1629.8.2. Basically I see the advantage in mainly not having to calculate the drift, which is tedious for wood shear walls. You also don't have to distribute the forces along the height by the triangular method. The 20% conservatism is sufficient for making up any inaccuracies that may occur as a result these simplifications. 

Ben Yousefi, SE
Santa Monica, CA

>>> dennis.wish(--nospam--at) 07/08/04 07:28PM >>>
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

HYPERLINK "mailto:dennis.wish(--nospam--at)"dennis.wish(--nospam--at) 



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