ASD Strength Design Discrepancy

[SteelPE]

While performing some calculations on an exterior masonry wall I have seem to find a discrepancy in the masonry code.

Previous versions of IBC allowed a load reduction for two or more variable loads in a load combination. IBC has removed this portion of the code in favor of including the provision directly into the load combinations in IBC 2006. In the analysis of a member only affected by wind load and its own self weight this reduction is not allowed.

I was having trouble getting an exterior wall to work using the Allowable Stress Design method when someone from this forum suggested looking at the wall using Strength Design. I was shocked to find that there was an increase in the capacity of the wall by nearly 50%. I attribute this to the IBC not allowing the 1/3 stress increase that is located in 2.1.2.3 of ACI 530-02 (my ACI530-05 is in the mail).

Here are some quick numbers:

For ASD #5 @ 24 provide Ma = 11,831 in-kips.

For Strength Design #5 @ 24 provide phi-Mn = 28,329 in-kips.

Divide this by 1.6 to go from strength design to allowable design in the load combinations (for comparison only) and you get 17,706 in-kips. 11706/11831 = 1.497

I was wondering if anyone has run across this discrepancy before?

 

[JoshPlumSE]

I have also found that the difference between WSD and USD is more severe in Masonry than in other codes.

The key for me is that the masonry design procedures are totally different for the two methods. AISI, AISC and NDS all use essentially the same formulas for capacities or allowables. They just use different methods of applying safety factors. Therefore, they almost always come up similar designs.

For Masonry, it feels more like WSD is the "simplified" method which will almost always be conservative.