Correct design loads for RC columns from FEA

[0.125pLL]

Hello!

How do I get correct axial loads for RC column design from a flat slab (irregular column layout) FEA model? I've come to a conclusion that for the design of the slab itself, in ULS (Ultimate Limit State - EC), it's acceptable to get slab internal forces using uncracked slab stiffness. But to get the correct column axial loads I should crack the flat slab?

 

[milkshakelake]

I use 0.25 stiffness modifier for slabs in FEM to get column and foundation loads. It's approximate, but it gets pretty close to hand methods. The number comes from ACI code as a suggestion.

Edit: I don't bother with cracking/uncracking the slab for column loads. That's for long term deflection analysis. The traditional hand method is to assign a certain percentage of load to interior/exterior columns because of slab continuity, and I don't think that method goes so far into cracking analysis. It's a tried and true method so I don't see the need to get too detailed.

 

[0.125pLL]

But do you agree that taking column loads from uncracked model is not correct?
I’ve heard of instances where this has lead to under design of certain columns and foundations.

P.S. can anyone suggest some literature on the subject aside ACI concrete manual?

 

[milkshakelake]

Using a 0.25 stiffness modifier for flexural stiffness is cracking it, and is the correct way to do it for column axial loads and foundations. I agree that it should be done.

Sorry, I think I worded things incorrectly. I meant that nonlinear cracked analysis is for long term deflection, and doesn't need to be done for column axial loading.

I don't have anything other than ACI as a reference. Maybe someone else can chime in.

 

[0.125pLL]

Thank you for answering!

 

[Celt83]

No method will give 100% correct axial load.

For critical foundation design an envelope approach may make sense run the model with gross section properties and again with reduced section properties. My understanding of the intent of the stiffness modifications noted by ACI is to more accurately predict the building stiffness at ultimate load levels in comparison to frame tests. It should be noted that for service load analysis which would be typical of foundation design ACI permits the reduction factors to be multiplied by 1.4 so even closer to a model with all elements modelled without reduction.

I think an import thing to note is that whichever direction is used the column design is consistent with that method don't use column moments from a gross moment of inertia analysis with axial load from a reduced moment of inertia analysis. Many would recommend enveloping the design treating the gross section analysis and the reduced section analysis as more or less a load combination.