Is support condition same for both flexure and torsion?
Is support condition same for both flexure and torsion?
(OP)
I'm analyzing a channel supporting a uniform load. i've assumed a C8x11.5 spanning 11.5'. the channel supports a metal deck. the channel is the end support and the span to the adjacent beam is 4'. I'm designing the channel as a simply supported member in flexure. The channel will be connected to the in-place columns with L3x3x1/4 angles welded to both the channel and column web. My question is, is the end support condition the same for torsion. I'm thinking, the angle and welds carry only shear and are flexible enough to model a ss end condition. When putting torsion on the three sided weld between the angle column web, isn't the connection fixed? This is a pretty big difference since the theta prime factor is much larger for simply suppored spans. Also if the channel is unbraced in the 11.5' span, only pure torsion would occur, correct? Another question I have regarding the tables in the back of AISC design guide 9 - the torsion at the end of the fixed span is 0 and the torsion at the end of a simply supported span has a value. Shouldn't this be the opposite? Why does a fixed condition have 0 torsion at the ends (See case 7 for theta prime).






RE: Is support condition same for both flexure and torsion?
Also, when designing your channel, be sure to use the eccentricty to the shear center and not to the centroid of the web.
RE: Is support condition same for both flexure and torsion?
With a channel, the support is generally a single angle (not two angles like with a wide flange). The angle should not be welded to the column on three sides, but only along the bottom and vertical leg. Leaving the top of the angle "unwelded" allows the angle to flex, which means the assumption of a simple support is correct.
DaveAtkins
RE: Is support condition same for both flexure and torsion?
RE: Is support condition same for both flexure and torsion?
What do you mean exactly by "close to the shear center". Is e0 (distance from face to shear center considered close? If the deck/slab bears on the entire flange, the distance to the shear center would be half the flange width plus e0, right? Is this considered negligible?
You said not to weld the top of the angle to the column to approximate a simple shear connection and allow some flexibility. Is this also the case for attaching other members designed as simply supported to columns/beams. I've never seen this done. From what I remember, I always saw 3 sided welds for WF members connected to a supporting member. Correct me if I'm wrong?
asixth: Are you saying that if J is a low value then the member won't attract torsion? If the member has a low J, doesn't that mean it can't resist torsion as well as another member might be able to and yield/fail faster than a member with a higher J might?
RE: Is support condition same for both flexure and torsion?
About the 3-sided weld on the angle. You should weld 3 sides of the angle to the supported member, but not to the supporting member. That connection relies on the flexibility of the angle to act as a simple shear connection. If you lock up the angle by welding all 3 sides, then you don't have a simple shear connection anymore. That being said, I would not leave the top of the angle completely unwelded. I would return the weld on the top for a length of 2*weld leg. The max allowable per AISC is 4*weld leg.
RE: Is support condition same for both flexure and torsion?
DaveAtkins
RE: Is support condition same for both flexure and torsion?
sorry if some of these questions are repeated, I'm just trying to grasp the concept.
RE: Is support condition same for both flexure and torsion?
I would explain it this way. If you have a single beam framing to a spandrel that happens to have a J of 0 (just for demonstration purposes), the spandrel will provide no torsional restraint and the beam will be free to have a simple end rotation.
If the beam is framing to a very torsionally stiff member (say a HSS 16x16), then that spandrel will provide end restraint for the beam (meaning the beam will have an end moment) that must be resisted through torsion in the spandrel.