steel truss question
steel truss question
(OP)
I'm designing a 100' span steel truss in RISA 3-D. According to the RISA-3D general reference guide they sent me, you can model the top and bottom chords two ways and you should get the exact same result:
1. one long single member
2. individual members between the bracing members
I have tried both ways, and they give me different solutions. When I model the chord as one long single member, the compressive stress is huge. The fa value greatly exceeds the Fe value because it is such a long member. When I model it as individual members(with fixed ends), there is no problem with the compressive stress, and actually the model is throwing less axial force into it. Which way should I be modeling it? In a truss, can you assume the compressive length of the chord to be the distance between the bracing? I have called RISA, but they have yet to call me back. Any help would be much appreciated. Thanks.
1. one long single member
2. individual members between the bracing members
I have tried both ways, and they give me different solutions. When I model the chord as one long single member, the compressive stress is huge. The fa value greatly exceeds the Fe value because it is such a long member. When I model it as individual members(with fixed ends), there is no problem with the compressive stress, and actually the model is throwing less axial force into it. Which way should I be modeling it? In a truss, can you assume the compressive length of the chord to be the distance between the bracing? I have called RISA, but they have yet to call me back. Any help would be much appreciated. Thanks.






RE: steel truss question
RE: steel truss question
RE: steel truss question
RE: steel truss question
RE: steel truss question
RE: steel truss question
RE: steel truss question
RE: steel truss question
RE: steel truss question
RE: steel truss question
It also concerns me that you said your alternate model has "fixed ends" at the end of each member in the model. You're not putting supports at each node are you?
DaveAtkins
RE: steel truss question
RE: steel truss question
RE: steel truss question
As you seem to have discovered, you have to be very careful about how you define the bracing of "one long member" (RISA's Physical member) vs. individual short members.
In both cases you are modeling the top and bottom chord as a continuous, connected member with no pins, right? So the matrix solution is the same both ways. The intermediate nodes don't affect the behavior unless you do a PDelta analysis.