I've created a model of a rectangular structure that's sitting on a concrete pad. The structure has 4 pinned supports 18" inboard of the 4 corners. The 4 corners have compression springs to simulate the structure sitting on the concrete pad (downward forces only). The model appears to be working except that some load cases are showing (-) forces on the compression springs. I'm curious why that's happening.
Attached please find the .std file.
Main Skid HE mod w links rails braces.std
thx
Jeff
Based on a quick look at the model here are some things that you need to address. You should use REPEAT LOAD as opposed to LOAD COMBINATIONs for an analysis with compression only ( or tension only ) springs.
More details on this topic are available at
https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/3441/member-tension-and-combination-load-cases-tn
In fact you only need to analyze the load case 7 onwards ( should be defined using REPEAT LOADs) with the compression only springs. So put a PERFORM ANALYSIS and CHANGE after load case 6 and then you can define the supports with the compression only attribute.
An unrelated observation, the ACC should be 0.05 as opposed to 1.
Thx for the response. I can easily change to repeat load for case 12 and onward. However, how should I handle load case 8-11, since they need to include SRSS solution?
Jeff: We did have a conversation regarding this but for others who may be looking at this post in the future, when SRSS combinations are used to combine load cases, you have no choice but to use LOAD COMBINATION. You cannot ascertain the nature of force in the springs in such cases because
1. the responses obtained for a load combination is a linear algebraic summation of results which is not valid for a non linear support condition like compression only
2. the responses are all positive due to the combination method used