LRFD Strength Load Case vs Service Load Case

[bnjneer]

Here is a conundrum. In doing an analysis on a reinforced concrete frame type (multi-column) pier, we are using AAASHTO LRFD code and looking at the strength and service load cases. In so doing, we are finding the service load case is producing larger demand as compared to the strength load cases. The reason for this is because we are calculating the forces due to internal thermal and shrinkage based on the stiffness of the frame (by means of the structural analysis program). In the strength load case, the load factor is 0.5 and in the service load case it is 1.0, so you can see the resulting forces from this will be higher with the service load case.

So, again, in summary, the service load case is higher than the strength load case. Based on this approach, the service load case will control over strength, which seems odd to this old guy that started with the Standard Specification way back when. How have you guys been handling this?

Designing the columns for this service load case will even be more interesting due to the interaction of axial plus bending. How has this been handled for service load cases?

Thank you.

 

[VoyageofDiscovery]

I have found that the shrinkage force that the Code specifies gets quite large. It also is not always conservative to assume a large force. I have not seen evidence of such large forces in most concrete bridges here in Canada. The Canadian Code has picked up on this where differential shrinkage is concerned and a reference in this section suggests using the tensile Modulus of Elasticity instead of the compressive.

HTH