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Re: ICBO Seminar for 1997 UBC Earthquake Regulations

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-----Original Message-----
From: Dennis S. Wish <wish(--nospam--at)>
To: seaoc(--nospam--at) <seaoc(--nospam--at)>
Date: Thursday, April 30, 1998 11:20 AM
Subject: RE: ICBO Seminar for 1997 UBC Earthquake Regulations

>Bill, if you introduce a spring constant in the center of the beam,
>that be the same as introducing a flexible support? Wouldn't you then
>some reaction at the middle spring support?

[Bill Allen]
Of course. It would be a continuous beam with three supports. The reactions
would vary depending on the relative stiffnesses of the beam and supports.

>I've been out of school a long time so I'm really rusty on this one. My
>assumption was that the wood diaphragm and the steel beam deflect and are
>simply supported. The idea of deflection is used only as a control for the
>stiffness of the member. I realized that the rules of statics governs in
>determinant systems, but I don't see how a roof diaphragm spanning between
>two supports of different rigidity is anything other than determinant.

[Bill Allen]
This assumes a totally flexible diaphragm. If the diaphragm was rigid, you
would distribute the load proportion to the stiffness. If the diaphragm were
somewhere in between (i.e., reality), the results would be somewhere in

>What do others think on this issue?
>Also - does this change when the diaphragms converge on an intermediate
>shear element at different elevations? Is the shear transfer from high roof
>to low through a sheathed cripple wall assumed as stiff as the element
>Bill, I've stopped arguing this one since I am only interested in
>understanding how the code requirement accepting the lower R value for the
>complete structure originated. Was this consideration intended for wood
>construction with a wood diaphragm?

[Bill Allen]
You need to ask the code authors that one. Maybe you should contact Bill
Nelson and ask him that question.


Bill Allen