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RE:Need Help for the Seismically Impaired

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There are code supported methods.

When concrete basements are significantly more rigid, relative to the wood structures above, for the lateral force distribution, the base is at the top of the basement wall.

The basement is significantly more rigid if it passes the code test. 1997 UBC 1629.8.3 or 1617.6.3.1 exception 3 "2 stage static analysis procedure"

The highly rigid basement is commonly assumed for a 2-stage analysis. It is common to demonstrate a period to use a 2-stage analysis for wood framed stories above a rigid single story (or multiple stories) of concrete shear walls.

This requires a dynamic period. Someone has already condoned a dynamic period for wood framing!

Moreover, onto another subject...

I believe it is possible to set dynamic assumptions of a wood design, by code, to conclude a significant reduction factor for seismic base shear. Possibly significant enough to cover for deficiencies created by not considering a rigid diaphragm. All legal and per code, buy one theorem and proof, poof no more rigid diaphragm concerns.

If a calculated period of a wood frame is allowed to be used to qualify for a 2-stage analysis, then a calculated period can also be used to attempt to reduce the base shear of a wood frame. This is most valuable when your wood design is being questioned, such as in litigation.

It is argued, the non-shear resistance of gypsum must first be released to allow the flexibility of the plywood system. I argue, if the non-shear-gypsum is rigid, at that floor level and wall line, shear walls are not being overloaded.

An argument follows that if the shear wall is within one of several levels (or stories), then non-shear walls of other levels could remain rigid and still have the shear wall be overstressed. That would disqualify a period value that assumes all levels flexible. This is physically true, but the same concept holds for a steel-frame dynamic-analysis where all levels are flexible. Recorded steel structural seismic responses rarely reflected those design analyses that ignore the non-structural resistance.

Suppose no one accepts the above argument. I have found significant force reducing periods letting all but one floor be infinitely rigid.

A constant simplifies by enveloping the complicated. Assume that most plywood panels, loaded to capacity, deflect about 0.4 inches. A constant deflection leaves only the weight as the variable for a single degree of freedom period calculation. This results in a quadratic equation allowing to solve for a constant factor, reducing the base shear acceleration, for any weight. Remembering this is for the case where all other levels are infinitely rigid. If any weight generates the same constant and any level has the same design deflection then that constant works for the whole base shear, any level.

If the plywood is stiffer as when over-designed, It is not overloaded and of no concern. If the plywood deflects more than the design capacity, as when overloaded, the shear is significantly being reduced and may result in a wall that is not overloaded and over deflected.


David Merrick, SE