huangyhg

rigid distribution of wood shear walls
would someone please recommend a text book, or give an explanation of how to distribute loads to wood shearwalls, given a rigid diaphragm assumption? i know it could be assumed flexible, but in reality, a small footprint with a nearly square diaphragm would react more as a rigid than a flexible system. also, when shearwalls are offset only a couple feet on one side, the flexible analysis distributes most of the tributary load to the inner most wall on that side, and very little to the exterior walls just a couple feet away. any help would be appreciated.
"design of wood structures" by breyer is the book for wood design.
the only difference for wood shearwalls is accounting for nail slip and possibly some other small things in the relative stiffness calcs. once you have the relative stiffnesses, it's just like any other rigid diaphragm.
the rigid diaphragm assumption for a hand calculation would involve calculating the stiffness of each wall and the story stiffess of the entire floor.
then you can cup up we a rotational stiffness of the floor.
then you can calculate the centroid of the applied load lateral and apply it to centroid of your floor with an equivalent torque to account for the difference between center of rigidity and center of stiffness.
after that it's just a matter of calculating the deflections at each location in the diaphragm.... the deflection can then be used to determine the shear force in each wall.
something of a pain to do by hand, but not all that difficult to do with an analysis program.
forgot to mention:
the seaoc structrual/seismic design manual (volume 2) used to have an example on this. but, looking at the 2006 version it has been removed. take a look at the 2000 ibc vesion and will have a really good example.
any reasonable assumption will do. this is not something you can pin down precisely. if in doubt, beef up the shear walls a tad.
ba
there are cases supporting both flexible and rigid... i.e. if walls are spaced close together to assume rigid and distribute based on stiffness.
a very common example being shearwalls spaced at 15' max should flexible or rigid be assumed?
this varies from engineer to engineer. anyway, here is a great link with some useful formulas...
wallenengineering,
the 2006 ibc seismic design manual vol. 2 has solved examples for rigid diaphragm analysis for wood buildings.
when walls are close, we normally combine them into one shear line (equal shear/lf). you then design drag straps (or any drag mechanism) to transfer the load at points of misalignment (to transfer force to diaphragm then to next wall).
there is no guarantee that rigid diaphragm analysis is more accurate than flexible in wood buildings. the conservative approach is to design for the envelope value for both flexible and rigid solution (if you are doing a condominium project for example and can't take chances).
regards,
m.s. genidy, pe
structural soft, llc

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