Moment at Cantlevered Beam over HSS Column?

[JasonMcCool]

I'm doing some connection design, and have a case of a beam extending 5' over an HSS column, with a beam splice to the next beam there. At the column, the beam is attached via a 3/4" thick cap plate with 4-3/4" bolts. I'm trying to confirm the adequacy of the beam to column connection. Is it legitimate to consider this a pinned connection that doesn't take any moment? It seems like the bolts at the corners of the cap plate and the plate thickness itself would act to restrain rotation about the theorteical column-beam "pin". But then, would my analysis depend on how rigid a connection I had? Would I have to do an itereative check to determine whether it was pinned or fixed or somewhere in between first, then actually determine final capacity? I haven't seen much about this type of connection other than an example in the AISC design guide on HSS that gives a moment on the connection and then shows checking the cap plate for prying action, the bolts for tension, and the HSS walls and beam web for local yielding or crippling. My software that I typically use (RAM Connection) considers a cap plate connection to be a moment connection. My boss and another engineer here have always viewed it as a pinned connection that is too flexible to develop any moment. At some point though, it seems like the pinned assumption breaks down as you use thicker plates and more/bigger bolts. Also, the moment I have is considerably larger than the example in DG24 that works out cleanly. Mine completely blows out the connection when I run it in RAM. Is there any guidance as to when these assumptions would be valid and when they would be questionable? Much thanks for any responses.

 

[dhengr]

Jason:
There certainly isn’t anything wrong with coming to E-Tips for help and further insight, but you would be much better off asking your boss or a senior engineer and/or finding a good, experienced, local mentor. You can both be looking at the same plans and details; you can both see his or your sketches as you are discussing the problem; you can also see the expression on his/her face when they see you’re not getting it, or vise versa; or you can stop them right there when you don’t follow something. As these relationships develop they can be long lasting and really rewarding for both parties. They are more likely to really care about your success or maybe even have a vested interest in it.

Look again at my last post, last para. to you, and think about it a little more when you get time and can study this further. The idea of the Girber Beam framing method is to optimize the size of the beam, so the moments over the columns and the center moment on the beam should be about the same, within a small range. You should look at skip loading, snow drift loading, unbalanced loading to the extent that it might cause a max. moment at the center of the longer beams. Study the slope of the beam with long beam with max. load and smallest canti. moment. This will give you an indication of how the cap pl. can be cranked by the beam, and will cause prying and bolt tension on the canti. side.; thus some potential moment into the col. Otherwise, the beam tends to press down on both sides of the cap pl. and there isn’t much potential moment to transfer to the col. Also, ask yourself how much moment can the cap pl. plus the beam bearing on one side of the col. produce, that’s still a fairly flexible connection. I do think your .75" cap pl. is plenty thick for that col. and beam. I’d rather bend (yield) the cap pl. than the beam flg. I don’t mean neglect the potential of col. moment, but it usually isn’t very significant. More importantly you have to watch the lateral support of the beam and of the col. at the top of the col. Also, when you extend the bot. chord of a stl. jst. to the col. or beam, you should probably make the stl. jst. designer aware of those potential loads into the jsts. Coordinate your canti. splice location so it doesn’t fall at a stl. jst. bearing.

 

[haynewp]

BA, you are putting words in my mouth.

 

[hokie66]

He's just technologically challenged. Me, I don't even try.