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Re: Effects of the New Code on Wood structures - good or bad?????

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		Lynn Howard wrote:

		<  Prior to our starting on the project, I met with the DSA
		<  and told them I was considering designing the diaphragms
		<  as rigid, because that is what the Code clearly said.

		<  They basically were not going to require the rigid
		<  analysis, and in fact WOULD NOT ACCEPT that type of 
		<  analysis alone.

		<  I find it very interesting that DSA would not accept 
		<  the Code required analysis.

		<  These people are aware of the Code requirements, 
		<  and do not accept it.   It is not because the are
		<  of the Code.

Mark Swingle responds:

Lynn, that is not what the code clearly says.  What I am saying is that the
code requires an accurate analysis of the ACTUAL stiffness of the diaphragm,
but since we don't know how to do that, we can be conservative and cover all
possible forces.  Doing ONLY the INFINITELY STIFF diaphragm analysis is not
sufficient (and by the way is not sufficient for many buildings other than

My point was that since there is no rational way to model the diaphragm
accurately, then by doing both a flexible analysis and a rigid analysis, you
always cover the worst cases for all walls, and so as I said, NO

In a "flexible diaphragm" analysis, all diaphragms are considered simple
spans from wall to wall.  The forces to the walls come from a simple span
reaction, which does not depend on the stiffness of the diaphragm (reactions
are independent of EI in simple spans).

In a "rigid diaphragm" analysis, the diaphragm is considered infinitely
stiff, and the forces to the wall depend ONLY upon the stiffness of each
WALL, and their relative spacing.  The resulting forces to the walls are
independent of the diaphragm in this case also.

Taking the highest load for each wall from the two cases will cover all
possible forces from any possible diaphragm system.  Some walls will have a
higher force under the flexible diaphragm analysis, and some walls will have
a higher force under the rigid diaphragm analysis, and that is most likely
why the plan checker does not want the flexible analysis thrown away.

My previous posts pointed out that there still remain serious problems with
determining the stiffness of plywood shear walls, so we are still left
without a rational way to comply with what I am proposing.  I will not
repeat that stuff in this post.

Mark Swingle, SE
Oakland, CA

These are my views and not necessarily those of my employer.

		"Swingle, Mark" wrote:

		> What is happening is that the professional community is
only now catching up
		> to what has been in the code for over 30 years.  The
design examples you
		> cite are just the first step in that catch-up process, but
it is not a
		> result of any new code provision.  As I understand it,
these new examples
		> simply perform both a rigid diaphragm and flexible
diaphragm analysis, and
		> simply use the worst case for each, so no consideration of
		> stiffness is required.  For the flexible case this is
simple, and is the way
		> it has been done typically in the past.