Rod X-Bracing In RISA 3D
Rod X-Bracing In RISA 3D
(OP)
I am trying to model a structure with steel rod x-bracing in RISA 3D but having some problems. I have made the members tension only and made them weightless (density=0) (as recommended in the help manual). When I run the analysis and show the deflection diagrams the rods look like bungee cords. Can someone help me out? Thanks






RE: Rod X-Bracing In RISA 3D
Never, but never question engineer's judgement
RE: Rod X-Bracing In RISA 3D
RE: Rod X-Bracing In RISA 3D
RE: Rod X-Bracing In RISA 3D
RE: Rod X-Bracing In RISA 3D
Mike McCann
MMC Engineering
RE: Rod X-Bracing In RISA 3D
RE: Rod X-Bracing In RISA 3D
Try clicking on the member and looking it's moment diagram. If it is truly a tension-only member and the self weight is 0, then there should be zero moment. I would look at this under the dead load only case (assuming there is no lateral dead load that would put any stress on the x-brace.
RE: Rod X-Bracing In RISA 3D
I would check the stability of the surrounding members as well; In some structural analysis packages if you have nearby columns deflecting in under some load combination the software will ignore your "tension only" member setting and show the element as buckling. I would suggest trying adding a restraint at each side (one at a time) to see if this corrects the issue.
Good luck, and remember this problem when you review your next set of output sheets prior to desiging members from the results.
Cheers,
YS
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...
RE: Rod X-Bracing In RISA 3D
In both RISA programs, as with any matrix based solution to pinned and fixed structures, each and every joint must have at least one degree of freedom fixed in ordr to solve the joint forces. otherwise the joint will spin in space, giving you infinite deflections, i.e., being unsolveable.
Check your member end pin/fixed window.
Mike McCann
MMC Engineering
RE: Rod X-Bracing In RISA 3D
DaveAtkins
RE: Rod X-Bracing In RISA 3D
That solved my problem and led me to the real reason for the deflection. I created a "rod" material with 36ksi steel properties but made it weightless. I messed up by assigning my rod section set material as 36ksi steel instead of the weightless "rod" material I had created. Thanks for everyone's assistance. There are many opportunities to design junk with these programs if you are not very careful.
RE: Rod X-Bracing In RISA 3D
I'm late to the table here, but you obviously found the flaw.
I did just want to point out the proper thing that you did in this issue -that is to always...always....look at the deflected shape of a new model, when you've run it for the first time. Checking the deflections first, before diving into the forces and stresses, is the proper first step.
RE: Rod X-Bracing In RISA 3D
Yes, these programs solve the problem you tell them to solve. It's the engineer's job to make sure the correct problem is being solved.