Rod X-Bracing In RISA 3D 1

[McQSE]

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

 

[COEngineeer]

what do you mean by look like bungee cords? Did you put the right material?

Never, but never question engineer's judgement

 

[McQSE]

They look like they are deflecting into infinity. I have them entered as 36ksi steel.

 

[Lion06]

what are the forces in the rods and what size rods are you using? If you are using small rods and have high axial forces, it could be distorting your deflected shape.

 

[McQSE]

I am using 3/4" rods but I have increased the size to check if that was the problem and it does not seem to be. It seems to be treating the rods as beams. I don't have it assigned as a beam.

 

[msquared48]

I would look to units, units, units.

Mike McCann
MMC Engineering

 

[McQSE]

I am not exactly sure how units would be causing this problem. I believe it is something I am missing in how I am assigning the members in RISA. I figured someone would know something to do besides make them tension only members and weightless. Thanks for everyone's input and please keep it coming.

 

[Lion06]

Is it both rods in the x that have this shape? If it is only one of them, it might be the one that is trying to take compression (but obviously can't), and is showing its buckled shape.
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.

 

[youngstructural]

Only slightly off-topic: These types of problems come up from time to time, and I have only once had to completely restart a model from scratch, HOWEVER, these problems are important to remind us of the limitations of the software: We must be certain that the answer is in the probably correct range, rather than just the possibly correct realm.

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...

 

[msquared48]

youngstructural beings up a good point.

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

 

[DaveAtkins]

Did you enter "-1" under Y GRAVITY for your DL case? If so, delete that and enter the selfweight of the structure by hand.

DaveAtkins

 

[McQSE]

DaveAtkins,

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.

 

[JAE]

McQSE -
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.

 

[stack]

So it was an input error...

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.