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I don't know why this is instability .. not this time .. there's many problems with instability when i analysing beams ?? why is this ? is there's some wrong with supports ?
Hi,
I know this is a little hard to visualize but I believe what is going on here is that you have an RX instability caused by using two pure pin supports. Neither pin support provides any rotational fixity about about the project X axis (happens to be aligned with the member local x axis too). The member, in this case, mathmatically speaking (in terms of the stiffness matrix), is free to spin about it's own local x axis. The way to fix this problem is to provide rotational fixity at one of the supports as I've shown in the attached image. If you try this and dont' find that if fixes the issue, please attach your model so we can take a closer look.
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