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Bruce Resnick's Question

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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