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Designing collectors based on shear wall capacity

Designing collectors based on shear wall capacity

Designing collectors based on shear wall capacity

Framing system: Light-frame bearing walls with wood sheathing.

Is it acceptable to design collectors based on the capacity of the shear wall, in lieu of using omega, rho, or the other required loads per ASCE 7? While I have not seen anything explicitly or implicitly saying so, this is a common practice in steel and, IMHO, meets the intent of the code.

A related question: ASCE 7-10 § exception 2 states that we only need to design using rho*Fpx when using exclusively light-frame shear walls. Is that true?

RE: Designing collectors based on shear wall capacity

For a single story structure, this seems to me it would be a conservative approach as long as you definitely knew all the properties of the shear wall material.

However, for as multi story system, the analysis could be come extremely conservative, as the total shear in the wall would be also driven by shears coming from above as well as the immediate attached floor diaphragm, where the drag collectors (alias collectors in drag) would be distributing forces from that immediate diaphragm to the wall.

Mike McCann, PE, SE (WA)

RE: Designing collectors based on shear wall capacity

I agree for the multi-story system, so let's just stick to single story where there is only one diaphragm loading the shear wall. This example could also be considered as the top story of a multi-story building.

Is there anywhere that this is stated as acceptable? Both AISC and AISI allow this for steel, but I could not find anything like it on wood. It seems reasonable, but plan checkers want to see the letter of the law.

RE: Designing collectors based on shear wall capacity

I feel that there is a subtle difference between wood and steel. With steel, there's a fairly well defined yield mechanism in the lateral system (usually steel or concrete bracing systems). That yield capacity, evaluated at over-strength, makes sense as an upper limit for collectors.

As far as I know, wood shear walls don't really have well defined yield mechanisms or easily calculable over-strength capacities. So while it makes sense to cap your collector demand at your over-strength wall capacity, determining that capacity reliably is tough. This is exacerbated by the fact that the 'intangibles' in light framed wood could significantly affect the lateral capacity of the bracing system. Stuff like drywall and return walls etc.

Does AISI allow this strategy when sheathed shear walls are used? Or only with strap bracing systems? If they allow it for sheathed walls, I would take that as strong support for you strategy.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

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