Unbraced Lengths

[eit09]

An S shape beam has its top flange bolted to the bottom of a wide flange beam. I used a rigid link (fully fixed end conditions) to connect the under hung S beam to the wide flange. The total length of the S beam is 48'. It is supported from its top flange at 0',20',& 40' with the remaining 8' cantilevered out. From a Risa standpoint how would you model this and what values would you choose for your unbraced lengths? I ran three separate models and not sure which the correct way to model in Risa.

Model #1 I have a 48' long beam and for the unbraced lengths I left Lbyy & Lcomp bot Blank while I put segment for Lbzz & Lcomp top.

Model #2 is the same as model #1 except I put segment for all four unbraced length conditions.

Model #3 has three separate beams (two 20' beams and one 8' beam totaling the 48'). The beams have fully fixed end conditions.

Model #3 Risa results matched up with my hand calculation but I keep going back to model #1 because it gives the true Lcomp bot unbraced length of 48'. Could someone point me in the correct direction?

 

[JoshPlumSE]

This is really a question about what is the appropriate unbraced length to use for your structure.

Top Flange:
I would tend to think that the top flage should have an unbraced length of 20 feet. By that I mean that I would probably consider the connection to the wide flange as being sufficient to brace it. Provided that the wide flange is perpendicular to the S beam.

However, this is certainly open for debate because I believe the AISC spec wants to see both translational and rotational restraint at the flange in order to consider it to be braced.

Bottom Flange:
I'm not sure that this is braced at all. Though you could probably cause it to be braced at the beam locations if there were a strut or stiffener which connected it to the supporting wide flange beam.

Weak Axis buckling (Lbyy):
I'm not sure whether the connection to the wide flange is really sufficient to call this braced at the beam connections. At least not without thost struts that I mentioned. Granted, this probably doesn't matter much because there isn't much axial force in the beam. But, still.

 

[Jatfuentes]

eit09,

Your problem looks like a problem of monorail, where the compression flange is not braced. You see at the supports where the moments are higher, you bottom flange is not braced. You can go to AISC article :
" Allowable bending stresses for overhanging monorails", by Stephen Tanner. AISC Journal Third quarter /1985.
jatfuentes

 

[JAE]

I'm being "stupid on Saturday" but if your S shape is loaded on the bottom flange can LTB occur? The load would always straighten the beam out right? But I could be stupid here.

 

[WillisV]

JAE - yes, it can. The LTB equations in the steel manual actually assume centroidal loading, so you are correct that load on the bottom flange actually gives more strength than what the manual would predict due to the tipping restraint (though it still can occur), but conversely the condition most often seen with load on the top flange is actually a little unconservative with the standard LTB equations.