Beam Framing Analysis - RISA 3D 1

[FEM4Structures]

Analyzing steel framing system in RISA 3D. You encounter a beam with “Max Deflection Ratio” greater than L/240 {for example Max Deflection Ratio is L/120} such that max deflection is at the end which frames into the girder. Load on beam is negligible. Is it correct to say that under this condition, higher deflection ratio can be ignored.

Also, during the analysis, there are some beams which have KL/r ration greater than 200 (such as KL/r = 300). AISC 360-05, Chapter E “recommends” the ratio less than or equal to 200 for compression beam. For WF-beams I don’t see any issue with KL/r >200 recommended values provided that difference is not great. Any experience with higher ratios or pitfalls?


Thanks,

FEM4Structures

 

[JoshPlumSE]

Easier question 1st: KL/r ratio
There is a setting under Tools - Preferences - Solution and results which allows you to turn on the KL/r > 200, or L/r > 300 limit. After all, this is a code recommendation not a requirement.

For beams, this limit usually only comes into play because the beam is seeing a nominal amount of compression (due to framing continuity in the 3D model) and the user hasn't entered in an Lbyy value for the beam. So, while you can turn it off using that Tools - Preferences setting, I would usually prefer to handle the situation by specifying a more realistic Lbyy for the beam member.

Deflection Ratio:
RISA calculates the deflection ratio in one of two ways.
a) For a normally framed beam the deflection ration is measured as the relative deflection from the deflected end joints of the beam. By that I mean that a joist supported by girders that each deflect some amount would have it's deflection ratio measured from the straight line between the deflected positiong of its end joints.

b) If the deflection diagram for the beam appears to resemble a cantilever beam, then the deflection ratio will be measured based on the supported end.

What happens for cases which don't neatly fit into this situation. This could be a column or a continuous beam. Well, they will be interpreted as one of those two cases based on the deflected shape. This interpretation is not always correct.

In your case, the fact that the deflection ratio is happening at the end of the beam suggests that the deflection diagram for that beam is getting interpreted as a cantilever. If it is not a cantilever, then you might want to completely discard the RISA deflection ratio for this beam and calculate one yourself.

At some point in the not too distant future, I expect RISA-3D will overhaul it's deflection calculations to be a bit more advanced. It will probably never get as sophisticated as what we do in RISAFloor, but there is a lot of information that we now have for support locations of continuous beams or cantilevers or such that we did not have back when these deflection calculations were originally implemented. Therefore, I know we could do a significantly better job now. We just need to find the time to re-write this feature.