RAM Frame dynamic analysis and sign consideration option causing odd moment diagrams in RAM Concrete

RE: RAM Frame dynamic analysis and sign consideration option causing odd moment diagrams in RAM Concrete

Seth Guthrie — Fri, 14 Jan 2022 20:46:05 GMT

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.

RE: RAM Frame dynamic analysis and sign consideration option causing odd moment diagrams in RAM Concrete

Jacob Pulfer — Fri, 14 Jan 2022 20:09:17 GMT

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.

RE: RAM Frame dynamic analysis and sign consideration option causing odd moment diagrams in RAM Concrete

Seth Guthrie — Fri, 14 Jan 2022 18:39:58 GMT

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?