From: merrick group <merrickgroup(--nospam--at)compuserve.com>
Date: Tue, 4 Jan 2000 15:58:51 -0500
RE: Report on Wood Diaphragm
Every house that I know of, has glued floors, how is a glued
floor going to perform as a flexible diaphragm?
The response of a rigid diaphragm may be similar to a flexible
diaphragm. This may occur if the walls are designed for a
flexible diaphragm case and the walls are being loaded to the
designed strength. The distribution by analysis may be about
that of a flexible diaphragm case.
For each wall, at one level, the deflection is near to a constant
at design shear. Some variations could occur due to à
*Tall or narrow shear walls and proprietary walls (columns)
where hold-downs are a major contributor to the deflection.
*When shear capacity is not near to that required by analysis.
The code can not
say that all floors are flexible. Maybe we can say that we can
ignore the rigid portion of the structure in our analysis as a
partially conventionally constructed house.
The code does allow using the natural period to reduce loads.
Ultimate strength designs can also show relief. It is not that
hard of a calculation to greatly reduce wood shear wall loads.
We cant disobey the physics of a rigid diaphragm and not add
other code special restrictions to analysis methods. I hear they
cant revise the legal code but they are going to do it anyway.
That is anarchy. This complexity is avoided by recognizing the
conventional construction provision wisdom.
There must be a legal way to do this. Use conventional
construction provisions, allowing portions to be engineered.
Dont consider the rigid diaphragm portion of the building as
part of the shear wall distribution. This may be problematic
because we still need to decide the load to design the shear
wall. Further study of existing definitions is needed.
A legal defense is possible, by a more detailed and costly
analysis or possibly by considering the conventional
construction provisions. The convention construction provision
element can be considered only if relevant statements by local
jurisdictions are made. If they disallow the conventional
construction provisions for two story houses then they can also
allow the rigid diaphragm portion as not part of the engineering
I see that the Dennis Wish program allows the designer to fully
respect the code provisions. The code wood sections appear
accurate and represent the physical response of a wood
structure and the need for redundancy. Reductions can be
found in a detailed analysis but if one is in trouble, the cheapest
way out is to look to the local jurisdictions where they have
disallowed all of the conventional construction provisions.
I would think recommended wording could be developed for
David Merrick, SE