ACI 10.11.1- Lateral Analysis

[CENAEN]

I wanted to make sure I understand this correctly:

For a lateral deflection analysis use service loads; columns/walls Ig=1; beams Ig=0.5. (ACI 10.11.1 commentary)

For a frame analysis/design use factored loads; columns/walls Ig=0.7; beams Ig=0.35.If wall cracks with Ig=0.7 rerun with Ig=0.35. (ACI 10.11.1 code)

For a second-order analysis use factored loads with service deflections; columns/walls Ig=0.7; beams Ig=0.35.
(ACI 10.11.1 commentary)

Any clarification is appreciated.

 

[WillisV]

I agree with the first two points in theory however it should be made clear that these "crack factors" are not code mandated for general analysis. That is they only appear in the code under section 10.11 "magnified moments" to be used in the approximate moment magnified procedures provided in sections 10.12 and 10.13. If the code meant to mandate these factors for every analysis they would have put them up in section 10.3. In the absence of other data many engineers use columns at 1.0 and beams at 0.5 even for an ultimate analysis. I tend to use the code values in 10.11 due to their more conservative nature.

As for the third point the loads used in a second order analysis should be in line with the reason for that analysis being performed. For instance a second order analysis for earthquake loads is generally an ultimate type analysis for strength design and factored loads and crack factors should be used. A second order analysis to check say wind deflections (which in my opinion is rarely warranted) should be based on service loads and crack factors.