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Re: Rigid Diaph. Analysis

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In addition, if memory serves me correctly...when doing a mixed lateral
systems that contains moment frame, most (if not all codes...I know it is
in the BOCA) codes require that the moment frames be design to take at
least 25% of the lateral load even if a "stiffness" analysis indicates a
smaller value.

I think the suggestion to look at what load the diaphragm-to-brace
connection can actually carry is a good one.  It might be that the
diaphram connection to the brace cannot even develop the full load that a
rigid analysis "says" will be distributed to the brace frame.


On Wed, 10 Jan 2001, Dan Goodrich wrote:

> I doubt that this helps, but...
> I once reviewed a multistory building with a long concrete shear wall at the back, and a flexible moment frame at the front.  With a concrete diaphragm, and using a rigid diaphragm approach, the concrete shear wall took nearly 97% of the force.  The designer applied 25% of the lateral load to the moment
> frame anyway, and limited it's deflection to the code limits.  At the time, I thought that this seemed 
> reasonable.  Now I'm wondering if it would have been more appropriate to base the force applied to
> moment frame more on a flexible diaphragm rational.  For that building, the moment frame would have
> to take a percentage of the base shear for the building to remain standing during an earthquake.
> In your case, you might look at the actual force that can be applied to the braced frame by the diaphragm, and compare that to your rigid diaphragm results.  Is it reasonable?  Another thought
> would be to try and model the ridigity of the diaphragm.  This might be very tough though.
> Dan Goodrich, P.E.
>   ----- Original Message ----- 
>   From: Larry Hauer 
>   To: seaint 
>   Cc: Larry Hauer 
>   Sent: Wednesday, January 10, 2001 8:32 AM
>   Subject: Rigid Diaph. Analysis
>   I am presently designing a two story Type V, (steel/wood), commercial building which has blocked plywood roof and floor diaphragms and a combination of lateral resistive elements; plywood shear walls, OMRF's, and at the center of the building in one direction, a two story chevron braced frame, (2 bays). I have used an R of 4.5 due to the mixed use of lateral resistive elements, and of course, the requirements of the '97 UBC.
>   My problem is, in using an "envelope" solution for the lateral forces, the braced frame due to it's stiffness, takes almost the entire lateral load in one direction when the diaphragms are assumed rigid. I know, from a practical stand point, that due to the rigidity of the braced frame in comparison to the OMRF's and plywood shear walls it will take more load, but taking almost the entire load causes all sorts of problems such as increasing the rho, overturning, collector forces, etc. In relationship to the stiffness of the braced frame, the diaphragm should be considered flexible, but in relationship to the other elements, rigid. Is there some rationalization, code exception, or modeling, that would allow me to be more practical in how much load this braced frame will take?
>   Thanks in advance,
>   Larry Hauer SE