![]() RE: Tension-only members in RISA 3D msquared48 (Structural) 18 Jun 14 16:07Įric's suggestion (adding a small lateral load) is akin to a "notional" load such as those used by AISC for their direct analysis method. For rods or cables (like you have in your model) I would suggest adding in a negative thermal load to give the X bracing some pre-tension because they will likely have pre-tension in reality. ![]() There are a number of ways to deal with this. Hence the program reports an instability. only now it doesn't have any lateral force resisting system. What is likely happening for your gravity only solution is that all braces go into compression (and get turned off) then the model is re-solved. If the program experiences an instability for an iteration, then it deals with it in the normal way (by locking certain degrees of freedom). The program determines convergence when there are no load reversals in these tension only members. ![]() At the end of the next iteration, the program may turn some members off and other members back on. Based on the results, it decides which tension only members are in compression and turns them off for the next iteration. By that I mean that the program first assumes all members are active, then solves the model. Rather it is iterating a linear solution. Remember that RISA is not doing a true non-linear solution for those tension only members.
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