RISA Joist Model Troubleshooting 2

[waytsh]

I am trying to do a detailed analysis of a 30' 24k7 joist for some custom loading in RISA 3D. I started by analyzing the joist for the SJI allowable uniform loading. Unfortunately right off the bat things are failing badly which has me questioning my model. I am hoping you guys may be able to help me troubleshoot it (pics below) Let me know if there is other information you would like to see. Really appreciate your help on this one, look forward to hearing your thoughts.

 

[jayrod12]

What do you have your top chord unbraced lengths set as?

Also, and this may be incorrect on my end, but I don't believe the joist suppliers design the top chord in bending at the support the way your model shows. Right now, your top chord is in a whole bunch of bending where your last diagonal connects away from the support. I believe they have the diagonal and top chord centroids align directly over the support to eliminate this bending.

 

[EZBuilding]

Joist design often assumes that the loading is provided as point loads at panel points - neglecting the effects of bending on your top chord. It is hard to identify if you have specified any bracing for your top chord. The flexural demands and the unbraced length of your top chord are likely the biggest contributors to those elements not working. I can't tell if the web member is failing at 1.07 or 1.97 D/C ratio. If the formed, it may be a difference in the loading assumptions. The latter would be odd to see a single member that over demand with neighboring web members not failing.

 

[JoshPlumSE]

I probably would not model the little "joist seat" length. By that I mean that little length of top chord that goes into extreme bending. Instead, I would model it as a pinned-roller support (as you have) where the first diagonals frame right into that support.

During my time as as the manager of the RISA Support group, I saw a number of people who were doing similar things with modeling joists like this. It's been a few years now, but my recollection is that there are often differences between what you can get with RISA (which is based on AISC codes) and what the SJI tables show. My thoughts are the following:
a) The way the bracing of the top chord is entered is important.
b) SJI may not consider torsional buckling or flexural torsional buckling limit states of the chord. There may now be a way to force RISA to ignore these checks (setting the L_torque to zero). But, I'm not certain that works. During much of my time at RISA, this would not have been an option.

Caveat:
Obviously, I used to work for RISA (for 16 years). However, my employment with RISA ended somewhat contentiously when they were purchased by a large multi-national company (Nementschuck...sp?). But, I now work for one of their competitors. Therefore, anything I say about RISA or opinions I give on the program should be viewed with the understanding that I have some bias on the subject.

 

[waytsh]

Thank you all for the input. I will rework the model for point loads only and bring the supports back to where the end web connects to see what happens. Here is how I currently have the properties for the top chord defined.

 

[EZBuilding]

I have not used RISA since my internship days, but one way you could cheat the model to consider the reduced unbraced length is to provide lateral support at your panel points.

Note that once you consider this joist in uplift - the bottom chord unbraced length becomes a function of the spacing of your joist lateral bracing.

 

[waytsh]

provide lateral support at your panel points

I should have shown a better view of the restraints.

 

[sbisteel]

Take a look at the SJI standard specifications table 4.3-1 for appropriate K values on chord and web members.