From: merrick group <merrickgroup(--nospam--at)compuserve.com>
Date: Fri, 25 Feb 2000 19:19:22 -0500
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
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
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