Allowable shearwall tollerance

[stickmotion]

I recently reviewed a single family residence that was near or within the path of a tornado. The door frames were out of plumb by as much as 3/8" in the in-plane direction of the shear wall. I was just wondering in anyone knew the allowable tollerance for a wall being out of plumb in the in plane direction. I looked in the IRC and was unable to find anything.

Also the roof was constructed with 2x6 rafters and ceiling joists at 16" oc and collar ties at 54" oc. There were a couple locations where the rafters appear to have raised. Can this be caused by the storm or did it likely occur during construction? Is is possible the panel edges aligned at that rafter and the sheathing had lifted?

Any help would be appreciated.

 

[MiketheEngineer]

In a tornado - just about anything can or does happen.
3/8'' seems a bit high unless you knew what it was before. Does the door still close?? What about drywall cracks? Do the windows still open, close and seal? If none of this is working correctly - that wall may be "wrecked"

 

[stickmotion]

I agree it seems high to me as well. There were a few drywall cracks, however most of the walls were covered with paneling so I wasn't able to view most of the walls. A few of the doors and windows did bind slightly.

I would like to include in my report an allowable tollerance if possible.

 

[msquared48]

A couple of points here:

1. The lifting of the rafters was more likely than not due to the Tornado. Make sure there are holddown clips at the ends of each rafter.

2. If the shearwalls are all drywall, as with seismic, after one event, the effectiveness of the drywall is significantly reduced. Thus, for vibrational seismic forces, the allowable forces are halved. Having been that close to a Tornado and seen the vibrational forces, I would consider applying the same criteria and recommending the installation of plywood shear wall sheathing. Not really required by the code, but I would consider it good engineering judgement.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[msquared48]

An additional thought or two here. Look for powdering of the sheetrock at the nails that are at the periphery of the shearwalls. If this is evident, the shearwall is ineffective for future similar foeces and should be either re-nailed or replaced, depending on the condition of the rock.

I do not think that the relative deflection or distortion is the key here, but the structural integrity of the shearwall as evidenced by any damage to the nailing or rock is.

Is there any evidence that the walls tried to lift off the sill plate? If so, additional reinforcing needs to be added there too - in essence a good retrofit is warranted.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[msquared48]

FYI, just ran across a Technical Bulletin from March of 2006 (2003 Codes) for an Structural Insulated Panel system with the following notes to the values presented for the shear wall and diaphragm values:

"...In jurisdictions that enforce the IBC and IRC, the allowable loads are applicable for shear walls that resist wind and seismic loads in in Seismic Design Categories A, B, and C. As per AC04, Section 4.5.2, the allowable racking shear load was determined as the lowest value based upon the following criteria:

1. load at which a net horizontal deflection of 1/8" occurred;
2. ...
3. ..."

In that the 3/8" lateral distortion is permanent, the integrity and structural usefulness of the shear walls has been compromised.

Your call.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[stickmotion]

The walls are likely sheathed with plywood or osb for the shear walls. I agree that the most important factor is the fastening of the sheathing. I am recommending the wall sheathing be refastened. I was just looking for a tolerance at which to require the framing be plumbed and leveled.