RAM Frame - Diaphragm Constraints and Gravity Forces in Shear Walls

 Product(s):RAM Structural System
 Environment: N/A
 Area: Analysis

While shear walls are primarily a lateral force resisting system they also carry gravity loads including their self-weight. Gravity loads typically produce axial loads as the main force components for walls. However, asymmetric gravity loading and frames could produce shears and bending moments of significant magnitude in shear walls. This is due to the diaphragm action in most typical shear wall buildings. Any differential vertical displacement in a shear wall system causes the diaphragm to displace in the vertical and rotational direction resulting in re-distribution of both vertical and horizontal loads. The net lateral load under gravity load will, of course, remain zero.

First, take the case of a single wall, 10’ wide with 2 levels of 10’ in height, but with no rigid diaphragm. At one corner, a vertical load of 10 kips is applied at each level. The resulting overturning moment is equal to 10k x 5’ eccentricity = 50k’. Since there are no other elements connected to this free body (and since the wall element is not meshed into smaller parts), the net shear at the base is zero. There is, however, some lateral displacement due to this asymmetric loading. In other words, this shear wall will lean to the right.

Now consider two identical walls connected by a rigid diaphragm (or a stiff beam). The presence of the diaphragm inhibits the lateral deformation and internal shear forces can now occur. The wall moments are reduced by this effect, but the shear forces can be significant. The forces and reactions in the opposing walls counteract each other, so there is still no net shear (only the left wall reactions are depicted here). It is also of note that when the walls are meshed the relative corner will change.

See Also


Ram Frame - Rigid Diaphragm Constraints and Frame Shear

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