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Vertical Load Distribution Cases
Since the distribution of dead and wind uplift load depends on the magnitude of shear overturning forces, there can be different distributions of dead and wind uplift loads and forces corresponding to all combinations of the following following cases:
Therefore the program tracks 16 separate dead and wind uplift loads load paths down the structure.
Dead Load Factor
As described in Dead Load Component, a larger load combination factor is used for the dead load component of compression forces than is used for tensile hold-down forces. However, the lower factor is used to determine whether the dead load completely resists overturning, and to calculate the excess load that is passed through to the level below rather that concentrated at the segment ends. More load is concentrated at the ends and less is passed through when using the lower load factor. This results in a larger compression end force than if the factor used for compression design was also used to calculate the amount of load available at the compression end.
This approach is used for several reasons. The reason for the 0.6 load factor is for conservatism in the case that the dead loads counteract shorter-term loads i.e. make their effects less severe. It therefore can be argued that the amount of load that is available to contribute to the compressive force should be maximized (made more severe) by choosing the lower, counteracting load factor at the tension end.
If instead the same factor is used to determine whether the dead load completely resists overturning, then it would be possible for it do so for compression-end design but not for tension-end design, leading to a possibly radically different distribution of dead load through the structure for these design cases. The additional design complexity implied by this is not merited by the advantage gained.
It can be shown that the difference in dead force at the compression end with these two approaches is equal to the difference in load combination factors multiplied by the vertically accumulated dead force at the segment. For seismic design, this represents a 40% increase; for wind design, a 25% increase.
Direction of Vertical Earthquake Load
A similar conundrum to that of the choice of dead load factor exists for the vertical earthquake load Ev. At the tension end we apply an upwards Ev, at the compression end a downwards Ev However, the upward Ev is used to determine whether the dead load completely resists overturning, and to calculate the excess load that is passed through to the level below rather that concentrated at the segment ends and used for compression design.
It can be shown that the difference in force at the compression end between these two approaches is equal to 2 Ev Shearwalls uses the simpler and more conservative approach of using the upward Ev in both cases. The factored Ev = 0.14 SDS D, where D is also factored, and SDS ranges from roughly 0.1 to 1.0, so this represents a roughly 3%-30% increase, depending on SDS.
Calculation of D Required for to Counteract Overturning for Seismic Design
The determination of the amount of dead load needed to completely counteract overturning is complicated by the fact that the vertical earthquake load Ev load depends on the unfactored dead load D. If T and U are the factored overturning and wind uplift force components from all sources, then the factored dead load D' required is equal to T + U + Ev ,where the factored Ev ,is
Ev ,= 0.7 (0.12 SDS D' / 0.6) = 0.14 SDS
0.7 is the load combination factor for Ev and 0.6 is the factor for D. The unfactored dead load D from the top of the wall that is required to resist overturning is therefore
D = (T + U) / 0.6 ( 1 - 0.14 SDS)