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Wood Analysis Programs

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We might, some day, need a rigid analysis program for a 
calculation submittal. Today, we need these programs to 
understand the parameters of a rigid diaphragm. Rigid diaphragm-
analysis-studies can be made to insure a safe design, but not 
necessarily for a design submittal.

If a rigid diaphragm is how the building responds, if the code 
check demonstrates that it is rigid, and if the flexible analysis 
misses a large chunk of demand on an element, then, good 
judgement should prevail. Good judgement, to compensate a 
flexible analysis to envelope possible larger demands per a rigid 
diaphragm analysis.

Can one can empower a design for, or add rulings, that allow force 
demands less than the code and less than what is agreed to be 
physically more accurate? The simple flexible diaphragm does not 
envelop possible larger responses of a building that has a rigid 

There are conditions when a flexible analysis will satisfy the rigid 
diaphragm demands. Understanding when a rigid analysis is 
significant can easily be understood with using rigid diaphragm 
programs to test parameters. Rigid analysis may not be required by 
a Plan Checker but might be part of good engineering judgement. 

The LA city counter top has a publication that might help. Does it 
offer rules that would protect a simple flexible analysis to represent 
a rigid diaphragm response? Alternatively, is it attempting to 
undermine the code?

Good rule of thumbs might be, but not limited toà 

Increase loads to longer walls where there is less deflection due to 
HDÆs. Per unit length, the longer wall is going to be stiffer than a 
shorter wall. Use the ratio of the longest wallÆs stiffness to the 
shortest wallÆs, or use the length squared, to factor the loads 
distributed by the flex anal. (Could be big.)

Design HDs for a capacity greater than that of the nailing, not for 
the flexible-analysis-design demand. When using HDs, extra 
nailing may show failure in a rigid anal. and not in a flexible anal..

Increase outside wall loads that have a significant increase due to 
torsion, or increase everything due to an unknown torsion. Torsion 
should be low if walls are similar in size and uniformly designed 
(i.e. without over-designs).

If skewed walls occur; remember that HD demands will increase at 
all wall intersections (all walls). Skewed positions are difficult but 
the 30% perpendicular force rule can be used. See UBC97 section 
1633.1, 8th paragraph. Reduce complexity by taking the max of the 
simple analysis HD demands and increase all HD demands 30%. 
This is in addition to a HD having the capacity of one wallÆs 
plywood capacity.

Cantilever rigid diaphragms may not be resisted in torsion by the 
sidewalls. This will increase the back wall demand. Big time, if the 
next one back is near and stiff.

A good rigid diaphragm program can help to study the parameters.

These are comments for discussion only, not to be considered as 
opinions or recommendations.

David B. Merrick, SE