huangyhg

shear wall diapham transfer?
my understanding of code is that shear walls will take load depending on orientation on the beam "diaphram". a little wall in the middle would be required to take half of the load, while strong outside walls take 25%. so the solution is to leave the little wall out "or walls" to eliminate heavy holddown requirements & high undesireable stress. the monkey in me tells me that the "more the better" when it comes to shear walls / that at low & reasonable loads the diaphram works well to transfer loads / that the smaller walls will yield and the larger walls will pick up load as a funtion of their capacity ????? this would allow a design that used all walls in a given direction in proportion to their capacities? can't do / but help me understand why. i'm trying to learn / thanks
first off, the walls will not take load depending upon their strength, but rather, their stiffness. from your post, it sounds like you are talking about wood framing shearwalls but the concept is applicable to all types.
the diaphragm has a set amount of stiffness. many engineers will assume either a fully rigid or a fully flexible diaphragm. in truth, the diaphragm is somewhere in between the two.
for a fully rigid diaphragm, the lateral load on the system will be transferred to the shearwalls in proportion to their stiffnesses and also with regards to their position and orientation. thus, there is a center of load, and a center of rigidity that can be calculated, and the load proportioned to the individual walls sort of like an indedeterminate continuous beam on flexible supports.
for a flexible diaphragm, the engineer will typically apply load to the shearwalls based on the tributary width of lateral load. this is independent of wall stiffness. so if you had a building with only three shearwalls, one on each end and one in the middle, and one end wall was twice the length of the other two, theoretically, each end wall would take the same load - simply the portion of lateral load attributed to half the distance to the center wall (1/4 of the total). and the center wall would take 1/2 of the total due to its greater tributary width.
i have found the following link to be useful in determining both the type of diaphragm and hence the distribution of shear forces to the various lateral load resisting elements.
generally wood sheating and metal decking (without concrete) is considered flexible.

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