SAP2000 Reinforced Concrete Analysis with Layered Shell Elements

[kjhaiar]

I am modeling a reinforced concrete pile cap with layered shell elements. The output stresses for the reinforcing and concrete have me convinced that I either don't have the model constructed properly or there is some other problem. The output moments and shears appear to be accurate, but when I calculate steel stress by hand based on the output moments M11+M12 or M22+M12 it does not match the stress output, S11 or S22 for the reinforcing layer in the model. I have tried non-linear material behavior, non-linear load cases, and adjusting the stiffness modifiers. Another indicator of a problem is that the shell stress at the bottom of the concrete is well beyond the modulus of rupture thus indicating that cracking should be occurring. I believe the concrete shell layer is incorrectly carrying tension force at the bottom of the section and thus reducing the tension in the reinforcing steel.

As a test model I re-built the section, but with two layers of concrete. The top layer simulates the whitney stress block and the bottom layer the rest of the section below. I assigned a very low elastic modulus for the lower layer. This produced results very close to what I would expect as far as stresses and behavior of the section.

I have also modeled the problem with more conventional and simpler design software to verify the expected results.

Has anyone else experienced this type of behavior with SAP and if so were you able to determine a solution?

 

[johnbridge231]

In order to design concrete are elements, I obtain their forces per length and design a unit-width section using

http://engissol.com/cross-section-analysis-design.html