ASCE 7-16 Anchorage of Structural Walls @ Grade

[driftLimiter]

Section 12.11.2.1 Of ASCE7-16 address anchorage of structural walls in diaphragms and other supporting structural elements.

The second paragraph of this section suggests that anchorage that is not at roof level (or to rigid diaphragms) can be adjusted by this factor (1+2 z/h)/3. With a minimum of 0.2Wp (thats how I'm reading it but it seems worded strange reg'd the minimum).

Anyway I am looking for the out of plane reaction at the base of the wall. In this example Sds = 1.275g, Ie = 1.0, Ka = 2.0. So for wall anchorage at the roof level I get (0.4)(1.275)(1)(2) = 1.02 Wp.

The wall is being designed for 0.4 Sds Ie Wp (per section 12.11.1). = 0.51 Wp

Now if you go and take Z/h = 0 and plug that into the adjustment factor you get 1.02 * (1 + 2*0)/3 = 0.340 Wp.

In order to be sensible I am just going to use the reactions that correspond to the 0.51 Wp load that the wall is designed for.

Has anyone else ever wrestled with this? Seems to me that 12.11.2.1 doesn't properly address anchorage at base level. The commentary on this section suggests that the reduction factor when applied to walls with large parapets may be inappropriate, but as far as I can tell its silent on anchorage at the base.

 

[TLHS]

I'm not familiar enough with ASCE 7 on a day to day basis to know what's happening with this clause specifically or if it's applicable to what you're doing, but the height factor term is used in the Canadian codes as well. It's fundamentally approaching the problem differently than the 12.11.1 formula you've quoted. The height factor is basically scaling from ground to floor level to approximate a variable floor spectrum for design. It's apparently based on statistical analysis of instrumented buildings but it's been a while since I've dug into it. A 3:1 factor, though, makes some sense from peak spectral of a design spectrum often being in the 2.5xPGA range.