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RE: Wood / light frame shear walls without shear walls

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One other issue has to do with the plan irregularity – if none exists and the residential building can conform to the International Building Code, then the use of hold downs on braced panels is not required to be evaluated so long as all other requirements of the Conventional Construction provisions are satisfied. In the past I did a comparison of numbers based on various roof dead loads and the resulting lateral loads that govern based on wind or seismic. I did this only for one story and found that in many cases for a home with a tile roof constructed on a slab on grade and an 8 or 10 plate height on with a geometric plan of 40-feet wide by about 60-feet in depth, there is an uplift calculated that the IRC seems to ignore. If this were treated as a full-compliance structure (prescriptive design ignored) then there would be sufficient uplift on the panel based on the 2:1 1997 UBC 2320 requirement for H/b. With the spacing of the panels at 25-feet and a tributary width of 20-feet there was an uplift over 1000-pounds that I calculated on a less than conservative dead load. Furthermore, the interior braced panels that required to be placed so that the spacing did not exceed 50-feet was not required by the code to be set on a foundation and the wall was still required to be anchored to the slab by at least 3-inch embedment into a 3-1/2” thick slab – which I found unusual in regions with high water tables that allowed some moisture to rise to the bottom of the slab or leak through the moisture vapor barrier.

 

When I presented this to the building Officials, the position was that if the code was wrong and missed the uplift or the thickness of the slab did not require a continuous foundation, he was not going to change it as he would have to guess at what the code writer had in mind to justify the issue. He was afraid that if he added the requirements for hold downs or for foundations on the interior braced panels he may be changing the performance of the structure and causing harm that was beyond his ability to understand. Therefore, he took the word of the code without question.

 

Another issue to consider is the age of the home and what the roof was originally designed for. Generally if it was designed for  a heavy clay tile (for example) and then changed to a light weight tile (5.5 psf vs. 12.0 psf) there may be less resistance to uplift. In this case, during the re-roof he ignored this topic in the prior code but will not demand engineering based on the new IBC requirements. His rational was that the old code only considered gravity load on prescriptive design, but that more consideration should be given to lateral load in the new code.

 

Just a few thoughts on the subject.

 

Dennis

 

From: ECVAl3(--nospam--at)aol.com [mailto:ECVAl3(--nospam--at)aol.com]
Sent: Tuesday, July 08, 2008 12:01 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: Wood / lightframe shear walls without shear walls

 

I see your point. If it is a nailed sill plate to the floor, I think the rocking of the shear wall would result mostly as partial plate nailing withdrawal (or are you referring to splitting along the edge at the shear ply nailing?). Splitting of the plate would be more likely with anchor bolts. Doesn't the requirement of the large square washers help in that case? Maybe a minimum requirement for the nailed plate to have a light strap or a plate at each end of the wall attaching to the rim joist would help.

SHM

 

In a message dated 7/8/2008 11:07:02 A.M. Pacific Daylight Time, Suresh.Acharya(--nospam--at)ci.concord.ca.us writes:

SHM & erik_g,

 

Thanks for the input, but I was expecting discussion on minimizing non-ductile mode of failure due to splitting of sill plates which, if happens, would not qualify for R=6.5 (no yielding of nails here). CUREE-Caltech tests have shown much better performance of sill plates when holdowns were present. Rocking of wood shear walls are not welcome unlike concrete or CMU shear walls.

 

Regarding dead loads, tributary widths for resisting forces and seismic forces may not be necessarily same, but roof and ceiling dead loads are mostly lumped together in calculations. For example, ceiling joists and rafters spanning in different directions, or ceiling joists/rafters being supported by interior walls which are not shear walls, or due to presence of ceiling beams/ purlins/king posts etc.

 

Suresh Acharya, S.E.

 

 




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