When specifying a pre-manufactured shearwall for a project, there are several factors that need to be considered, such as load values, seismic/wind requirements, wall width and height, wall placement, etc. There is also another critical factor that is often overlooked in multi-story applications — cumulative overturning.
Designers are accustomed to accounting for cumulative overturning when specifying multi-story, site-built plywood shearwalls — which is done by adding the overturning and resisting moments, and calculating tension and compression loads at each level. However, when specifying pre-manufactured shearwalls, designers typically calculate shear loads based on the building geometry and code loading requirements. A wall is then selected based on its ability to meet or exceed the required shear load using manufacturer provided allowable shear load tables.
What can get lost when considering shear capacity only is that the shearwall is not only governed by shear, but also by a combination of other limit states, including drift, tension and compression, flexure, anchor rod tension, and concrete or wood bearing stress. For single-story walls, the allowable shear given in the load tables is the lowest value of the various limit states. However, additional care must be taken in the analysis of multi-story shearwalls to account for the way the loads are distributed over the height of the building.
In multi-story structures, shear and the associated overturning forces due to seismic/wind requirements, must be carried down to the foundation by the building's lateral force resisting system. These forces are cumulative over the height of the building, and shear forces applied at the second or third levels of a structure will generate much larger base overturning moments than the same shears applied at the first story. If cumulative overturning is not considered, the design may result in forces several times higher than the capacity of the lower wall, anchor bolts and foundation anchorage.
When specifying stacked shearwall applications, it's important to consider cumulative overturning. The load values for Simpson Strong-Tie® stacked Steel Strong-Wall applications reflect the impact of cumulative overturning and thus, appear significantly different than other shearwall manufacturers.