Friday, February 22, 2008

Perforate shear wall design and combined shear and uplift on edge nails

Hi list,

I am working on a retrofit of an existing apartment building built around 1970's. It has existing plywood shear walls on the exterior and a few interior shear walls with no holdowns and plywood lapped over the rim block/joist for shear transfer at the exterior. As was customary at the time, it has a soft story that we are proposing to retrofit.

I am trying to determine how much uplift/overturning can the existing shear walls above deliver to the structure at the soft story level. It would be way too conservative in my opinion that it can transfer the forces as per my design criteria and even based on the shear wall capacity because of the lack of holdowns. That is why I started looking at the perforated shear wall design procedure new to the IBC 2006 and also reviewing an APA technical note H335 "Structural Panel sheathing or siding used to resist combined shear and uplift".

The essence of what I am looking for is how much shear and uplift can a wall nailed to a rim joist transfer (as the worst case scenario). The APA document says that " wind uplift loads occurring at windows or door headers, even in perforated shear wall, must be distributed around the openings and into the structure below by the way of hardware specifically designed for such application". In my case these opening are existing and there is no hardware except for a few toe nails.
Here's the question: does that statement also applies to horizontal seismic loads? I think that this requirement has to do with the uplift from the roof structure above and it is not applicable to the seismic loads. Are there any references about that? (Buddy Showalter are you reading this?) If I am right, then one can assess both shear and uplift capacity of the plywood shear wall without any holdowns or other hardware around the window openings.

Another question is somewhat unrelated to my situation but is regarding the general application of the Perforated shear wall method: If one has to attach each stud with strap to the structure below for the unit uplift force equal to the shear unit force, is that a viable method of designing and constructing shear walls. My first reaction is that it would be cheaper and easier to strap and block around window and door openings while still providing for uplift hardware at the ends of the entire shear wall.


Sasha Itsekson, SE
Enginious Structures, Inc.
Oakland, CA

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