Hi,
I'm running a ASCE 7-10 Response Spectra load case in RAM Frame for a concrete moment frame project. When designing the moment frame beams in RAM Concrete I've noticed that a very small number of them seem to have very atypical moment diagrams, see image below. The beams with this problem seem to be located randomly around the project, and these beams are similar to the beams around them and should have a similar moment diagram.
After a lot of messing around with options, I figured out that it was the dynamic seismic case that was causing this issue and that it was caused by having the program consider sign for the dynamic analysis results. When this option is turned off, this is the moment diagram, see image below. This is what I would expect to see, and matches the frame beams above and below this beam.
Some questions regarding the above information for planar special moment frames.
1. Can concrete moment frame beams be designed with the option to consider sign for the dynamic analysis results off? I think the answer should be yes since the beams should only be designed for the forces from the RAM Frame dynamic analysis without interaction from other beam forces.
2. Can concrete moment frame columns be designed with the option to consider sign for the analysis results off? This one I am not sure of, it seems like it should be okay since the columns are still designed for the forces from the RAM Frame dynamic analysis. The special provisions for SMF should also be okay since those checks do not consider the original signs of the forces.
3. Can concrete moment frame joints be designed with the option to consider sign for the analysis results off? I think the answer should be yes since the joint capacity calculations are based on the probable moment forces and do not consider the original sign.
Thanks for your help,
Jake
It is generally fine to use dynamic RSP load cases for concrete member design, but the issue of sign that you noted could come up. We calculate the sign based on the dominant mode, which works well for member end forces, reactions, etc. but tends to break down for meshed slab deflections or "smaller" results (for more see: https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/4029/ram-frame---dynamic-analysis-faq). For concrete beams we need the forces and many points along the length of the beam which adds to the complexity, and while I have not seen a case like this it does not surprise me.
Not considering sign is a good solution. Since there are combinations using + and - terms on the Dynamic load case results the forces are all reversible anyway.
Alternatively, using a static seismic load with equivalent story shear to the dynamic case is an option.
Curious, is this a beam in a meshed slab, i.e. with several finite element nodes along the length?
Good to know. So the concrete beams have a slab, but it's not meshed together since the slab is set up as a rigid diaphragm.
Do you think there would be an issue with the column design to not consider the sign? I wasn't sure if it could cause a miscalculation of the Cm value as part of the moment magnification procedure.
Not considering sign is always fine when it comes to member design. Considering Sign is useful for checking the "big picture", things like drift for example, but not a requirement.
Answer Verified By: Jacob Pulfer