Missing Collector Straps!!

[Martin_O]

We have a light gauge bearing wall project in New Jersey (seismic Design Category B) with floor joists at 24" oc and 9/16" x 24 gauge decking with 1.5" light weight concrete topping.

We are the EOR, but the contractor hired a separate engineer to design the bearing walls for panelized shop construction that were shipped to the site. Everyone has agreed that the lateral system remains per our drawings. We assumed that the floor diaphragm carries lateral loads horizontally to the shearwall lines and various elements along those lines (wall top tracks, steel beams, ledgers etc) "collect" and deliver the loads to the individual shearwalls. In addition, we detailed continuous coil straps to ensure continuity at various transitions. Unfortunately the contractor (and special inspectors) missed the straps when they should have been installed!!

The panel engineer's position is that the floor deck likely has the strength and stiffness to act as both a diaphragm and a collector and, given the concrete topping, the most natural load path is for the diaphragm to deliver the lateral loads directly to the shearwalls without relying on the straps. As a result, they believe the focus should be on verifying the deck capacity followed by checking the local connections at the floor to shearwall transfer and not necessarily forcing the contractor to install the coil straps.

The (untopped)deck diaphragm shear capacity is 180 plf (ASD) which is higher than any of the shear wall loads from an individual level. So it appears at first glance that this approach may have merit?

 

[KootK]

The (untopped)deck diaphragm shear capacity is 180 plf (ASD) which is higher than any of the shear wall loads from an individual level. So it appears at first glance that this approach may have merit?

Perhaps. And, frankly, throughout most of the history of structural engineering, this is probably exactly what would have been done with little to no explicit attention to collectors.

As you know, however, using modern analysis design techniques, it is very difficult to tell a complete story of diaphragm action this way while still sticking the normal assumption of your deck being a "shear panel" (only shear resistance, no in-plane axial/bending). Once you take away the explicit chords and collectors, you find yourself having to move loads around from their points of origins (masses, exterior walls etc) without ways to get that done that are either robust or readily quantifiable.

In a low seismic jurisdiction like New Jersey, I would be inclined to see if the "do nothing" approach can reasonably work. That said, I know next to nothing about your project and you certainly done want 1/2 of your building P-delta racking out into space because it was relying on a tension connection to the diaphragm/shear walls that was based on the capacity of an unreinforced concrete topping and metal deck side lap connections.

How tall is this building?

What's the nature of your shear wall system? I'd like this a lot better if it's CFM shear walls distributed all over the plan rather than trying to move everything back to an elevator shaft and a couple of stairwells.