Concrete-Strength Design-Load Factors

[mrengineer]

1997 UBC and 2000 IBC started using lower load factors for DL and LL (1.2 and 1.6). ACI 318-99 kept the higher load factors (1.4 and 1.7). Am I missing something? I don't see anything in UBC or IBC to compensate for the lower values, such as lower phi factors.

 

[JAE]

For the lower (1.2 and 1.6) load factors, there are lower phi factors - check the new ACI 318-02. I believe that the IBC allows you to choose between the traditional (1.4 and 1.7) load factors and phi factors, or use their "standard" load factors (1.2 and 1.6) with adjusted phi factors.

For now, you can choose, but eventually ACI will totally convert to the newer factors to be consistent with the IBC.

 

[mrengineer]

JAE:
I looked all thru IBC 2000 yesterday, and could not find any reference to adjusted phi factors that correspond to the lower load factors. If you can find any such reference when you have access to the IBC I would appreciate it.

 

[mrengineer]

The light bulb finally came on and I am going to answer my own question. IBC 1605.2.1 Exception No. 1 says "For concrete structures where load combinations do not include seismic forces, the factored load combinations of ACI 318 Section 9.2 shall be used" I was thinking that this was an all or nothing deal, where if you were designing for seismic, you would use only the IBC equations, and if no seismic, use only the ACI equations. It turns out that you use a combination of IBC and ACI equations, depending on whether the SPECIFIC equation includes seismic or not. This is confirmed on page 8 of the "2000 IBC Structural Application Guide", which lists the load combinations to be used for concrete structures.