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Bruce Resnick's Question

• To: seaint(--nospam--at)seaint.org
• Subject: Bruce Resnick's Question
• From: Seaintonln(--nospam--at)aol.com
• Date: Thu, 1 Jul 1999 13:13:34 EDT

```Bruce,
It is my understanding that the Simplified approach referenced in 1629.8.2 is
only valid in structures without irregularities - basically, conventionally
framed structures that don't really need to be engineered (a dicotomy?).

I considered the same line of reasoning that you have taken. For example,
rather than calculating the CM and CR for a diaphragm of various depths
(which moves the CM farther or closer to the CR) I would take the worst case
load per foot and appy it to the entire width of the sturcture. This would
simplify the calculation to finding the center of the load only rather than
the center of a series of loads as indicated in the design examples.
The problem is that this might increase the shear to the structure by as much
or more than 40% as indicated in the comparisons in the examples (the seminar
notes from the SEAOSC February 1998 Wood Seminar and the draft of the ICBO
Design problem Volume 2).

There are many other problems that exist which the code does not address or
give guidance to that occurs by the geometry of the model. Some of the code
is so restrictive that it creates a liability if judgement dictates a lesser
standard. Some of the math is simply not practical to solve and some is not
possible since there is no rational methodology (like flexible diaphragm
analysis) to support it.  If models have not been tested, then no emperical
information exists to justify the code restrictions.

Others have suggested creating a FEM model. I am no expert in this area as my
experience is limited to simple moment frames. However, from the discussions
of others, certain assumptions are made that rationalize the diaphragm as
acting homogeniously and therefore able to be modeled. I would disagree -
from the very basic argument that if the blocking between joists is not
"tight" joist rotation will influence diaphragm lateral movement or
deflection. If nails split wood or fall too close to the edge of a member,
they become less than effective and their shear value suffers. APA requires
OSB panels at a roof diaphragm to have an 1/8" gap when installed - does this
affect deflection? Is it considered in the methodology?

Alternative Suggested methods:
I received a call from another engineer, not on this list, who is making
similar arguments that many of us have made on the list - a Simplified
Emperical Approach with Exceptions.

The idea is to apply scaling factors rather than go through the gyrations of
the methods suggested. He suggests writting up the proposal and submitting a
petition  to the practicing professional community for signitures. The
resulting document is to be submitted to SEAOC or to SEAOC Seismology
Committee for evaluation.

My feeling is that most historic designs requiring the type of analysis
suggested has been typically easier to do because of the uniformity of the
building - generally rectangular.  Custom Residential structures pose a much
greater challenge as there is greater diversity in the geometry of the model,
variations in roof construction, discontinuities occuring concurrently with
continuous diaphragms in the same geometric block etc. This creates a
nightmare in the design model that is not addressed in the codes or followup
design examples.

I would be interested in opinions as to dramatically simplifying the approach
as Bruce has recommended (and a great many others).

Dennis Wish PE

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