Wood shear wall uplift load combinations 1

[patamd]

When we design hold downs for wood shear walls, we use the load combination (0.6-0.14SDS)D+0.7pE (seismic) and 0.6D+W (wind) to calculate the uplift. In an other word, the formula we use are T=(Mot-(0.64-0.14SDS)Mr)/L (seismic) and T=(Mot-0.6Mr)/L. We got many projects approved using these equations. In other people's spreadsheets, we found that they use safty factors instead of those ASCE load combinations. For example, in Daniel Li's spreadsheet, he uses safty factors 0.9 for seismic and 2/3 for wind. In an other word, he uses T=(Mot-Mr)/(0.9L) (seismic) and T=(Mot-Mr)/(2/3)/L. In his method, the dead load helps more to reduce the uplift tension. Does any one have any idea which method is right, or if you think neither is good, what do you use?
An other question is that in the book 2006 IBC STRUCTURAL/SEISMIC DESIGN MANUAL, the author uses the overstrength factor, omega, to evaluate the overturning moment. That results a gread tension value and is not practicable. We never use it and no plan checker questioned that so far. Any idea reguarding this issue? Any information will be appreciated.
Thank you very much.

 

[swm25]

I use the ASCE combinations like you use, I use the overstrength factor to factor up the load for drag connections with seismic loading.

 

[msquared48]

Don't you also have to apply the overstrength factor to discontinuous shearwall holddowns that create a soft story condition?

Mike McCann
MMC Engineering

 

[patamd]

According to the ASCE, I think we need to apply the omega to discontinuous shearwall holddwons. Please tell me if I am wrong. I discuss this with some other PE's, most of them do not use overstrength factor to fator up the overturning momnent when designing shearwall holddwons. If the omega is applied to the holddwon design, the uplift seems too large to be pratical, especially for residential houses. We did not fator it up in some projects in different cities and no plan checkers asked for that overstrength factor in designing holddowns. I feel very confused. I'll appreciate if anyone can educate me with this question.
Thank you.

 

[bdruehl]

yes, this is a very good question and i have found several different viewpoints, none of which i consider to be the ultimate authority. i have found a few referecnes that say you can avoid the overstrength by increasing embed depth so that the steel fails instead of concrete.