steel truss question

[structuralnerd]

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.

 

[jike]

My advice is to model it as you are going to build it. This implies that I would model it as one continuous member. You may be forgetting a vital piece of input and that is the out of plane bracing. You must be bracing it at regular intervals someplace aren't you?

 

[civilperson]

jike is correct. Bridging or bracing is required for the bottom chord as well as the top/compressive chord.

 

[samdamon]

Are you sure there is no mistake in the model? Check deflection plot and do a model merge to make sure all your nodes are attached to the chords. Regards

 

[structuralnerd]

I do have bracing at the bottom and top chords, but that isn't helping my compressive stress check. I tried a model merge as well, and it still isn't working.

 

[miecz]

In the spreadsheet for Hot Rolled Steel Design Parameters, under the Hot Rolled Tab, are your unbraced lengths specified as "Segment"?

 

[structuralnerd]

The yy direction is...should I do that in the zz direction as well?... as long as I have bracing in that direction..

 

[miecz]

I expect that's where your problem is. Check the help file for unbraced lengths of Hot Rolled Shapes. For physical members, it defaults to the length of the member.

 

[MiketheEngineer]

99% sure it is a bracing problem and the suggestions offered are correct. I use Risa quite often and if the model is right - the results are right. Remember GIGO - garbage in - garbage out.

 

[structuralnerd]

I got it to work properly. I didn't have the unbraced lengths modeled correctly. Thanks for all the assistance!

 

[DaveAtkins]

The RISA-3D general reference guide is wrong, if that is what it says. As jike said, it should be modeled as a continuous member, because that is how it will be built.

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

 

[haynewp]

If the segmented chords are internally fixed to one another at each end, then it should give the same forces (analysis results) if it were instead modeled as a continuous member. The unbraced lengths affect the design allowable compressive stress.

 

[structuralnerd]

haynewp - That makes the most sense to me as long as the member is braced laterally in both directions.

 

[JAE]

I use RISA all the time - I think if you check your deflections for both cases, you should get the exact same results. That will tell you that the matrix is the same and being solved the same.

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.