Wood Shearwalls on Podium Slab

Recently received a new comment from Seattle DPD plans reviewer Roussi Roussev. 

"The transfer slab at level 2 supports discontinued wood shear walls, and therefore is required to comply with ASCE 7.12.3.3.3.  Please provide verification of the slab for the maximum force (uplift or compression) that can develop in accordance with the seismic load combinations with overstrength factor."

Were the heck did this come from?  We've been doing two-stage analysis for years on this type of structure and have not had to consider wood shearwalls as discontinuous.  If we look back to the 2003 IBC section 1620.1.2 modified ASCE 7 and specifically exempted light-framed walls supported on concrete slabs from this type of irregularity.  Then the 2006 IBC saw most of the seismic design cut from the code and referenced ASCE 7 without modifications.

My initial response to Roussi was that since a two-stage analysis (ASCE 7 12.2.3.1) was used, only the amplified forces from the upper portion need to be designed for and that no irregularity exists.   This response was not well received.

Charlie spoke with Roussi concerning this comment and he was not really sympathetic to our way of thinking.  In fact Roussi mentioned that both John Siu and Steve Pfeiffer are taking a more rigid definition of shearwalls supported by concrete slabs and that they are indeed an irregularity that needs to be designed for.  He stated that there has been a project where the reinforcement did increase due to the seismic loading. 

In my second response to DPD I modified the PT slab model to include load combinations for seismic.  Below is a list of steps taken to modify the model.

• Added loads "Seismic Loading N-S" and "Seismic Loading E-W".
• Added the four load combinations:
• "Factored LC (N-S):  (1.2 + .2Sds)D + Omega QE +L + .2S"

• "Factored LC (E-W): (1.2 + .2Sds)D + Omega QE +L + .2S"
• "Factored LC (N-S):  (.9 - .2Sds)D + Omega QE

• "Factored LC (E-W): (.9 - .2Sds)D + Omega QE
• The seismic factor of 4.65 accounts for Omega*Rw/Rc*LRFD/ASD.  For all four of the load combinations there is an Alt. Envelope Factor.  Use the same factors except the seismic should be negative.  This has the effect of reversing the seismic load direction.
• Then for each N-S and E-W directions the holdown loads were applied at each end of the shearwall.  Sign convention used was that North and East are compression (down) loads, South and West are tension (up) loads.  Loads used are the allowable tension load from the threaded rod holdown schedule on sheet S1.4.

N-S Shearwalls



E-W Shearwalls

Modifications finished!  Run the model!

Reinforcement requirements did NOT change due to included seismic load combinations.

Provide response to comment along with PT calculations back to DPD.  If we let DPD know that the seismic load combinations don't control then maybe one day they will stop asking for it.