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Re: Rigid vs. Flexible Diaphragm

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Gautaum,
 
The shear wall program we wrote, when done executing the floor level being analyzed, generates the input file for the floor level below.  For a two story building, when the wall below is longer than the wall above at the second floor, we go into the input file and adjust the length of the shear wall before we run the first floor level below.  We do not modify the overturning moment or resisting moment from the floor level above, and they are additive to the overturning moment and resisting moments of the first floor wall.  When we detail the wall on the structural drawings, we continue the holdowns for the shorter second floor wall down to the foundation, and provide holdowns at the ends of the longer first floor shear wall.
 
If the second floor shear wall sits partially on a beam, we detail the beam to transfer the uplift/downward forces to the post and down to the foundation.  We do consider the stiffness of the beam that the wall sits on and the impact on the shear wall deflection.  To simply matters for beam stiffness we generally assume the beam will deflect 1/4", which really isn't very much movement when you think about it. We can calculate the exact beam deflection if we want to, but have generally found it doesn't greatly change the final design of the shear wall on the beam.  The 1/4" deflection greatly decreases the shear walls stiffness, just as would smaller 1/16", 1/8", 3/16" beam deflections to a slightly lessor degree.  The result is that walls sitting on beams take alot less shear due to the flexibilty of the beam they are sitting on, meaning other shear walls on that floor level must take more shear force.
 
Mike Cochran S.E.
 
In a message dated 5/10/2004 4:21:17 PM Pacific Standard Time, newabhaju(--nospam--at)hotmail.com writes:
Mike & Bill:

Thank you for your input regarding rigid and flexible diaphragm.

Mike:

How do you handle overturning requirements in your program for a two story
building where shear walls along any given line do not stack up - lets say
the edge of the second story shear wall lands somewhere other than the ends
of the shear wall below.  The uplift requirement from the story above will
affect the holddown requirement for the wall below and therefore the
rigidity.

Gautam