I am modeling a small stand for an experiment at a research reactor facility. It is required to withstand seismic forces. The base of the structure is a stainless steel right-triangle shape plate. There are three small HSS "columns" at each corner and another SST plate on top of the columns. There is an "experiment" on the top plate which I will not describe for brevity. The base plate grid has 4 anchors and those are modeled as simple supports. The balance of the base plate FE grid nodes are modeled as compression only with the statements below. the problem is that I am getting X and Z shears (Y is gravity axis) and even some small tensions in these node reactions. I am at a loss...suggestions?
SPRING COMPRESSION only 31 TO 36 38 53 TO 391 393 TO 686 688 TO 842 -
844 TO 968 970 971 TO 979 KFY
SUPPORTS
31 TO 36 38 53 TO 391 393 TO 686 688 TO 842 844 TO 968 970 TO 978 -
979 ELASTIC FOOTING 5 5 DIRECT Y SUBGRADE 1000
When you use DIRECT Y, then lateral restraints in X and Z are applied automatically. If your intention is to have the supports in vertical directions only at the base plate FE grid nodes, use DIRECT YONLY for those and then the lateral forces would only be taken up by the supports you have defined for the anchors. Also ensure that you are using REPEAT LOADs for combining the cases as otherwise you may end up getting tension in the compression springs.
The same is mentioned in the wiki below although the wiki itself relates to a different topic
http://communities.bentley.com/products/structural/structural_analysis___design/w/structural_analysis_and_design__wiki/26838.compression-force-developed-in-tension-members