Moment at Cantlevered Beam over HSS Column?
Moment at Cantlevered Beam over HSS Column?
(OP)
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.






RE: Moment at Cantlevered Beam over HSS Column?
Is your cantilever moment greater or less than the fixed end moment on the span portion of the beam? If greater then you could reduce the 5' cantilever to bring them a little closer.
BA
RE: Moment at Cantlevered Beam over HSS Column?
RE: Moment at Cantlevered Beam over HSS Column?
RE: Moment at Cantlevered Beam over HSS Column?
RE: Moment at Cantlevered Beam over HSS Column?
RE: Moment at Cantlevered Beam over HSS Column?
RE: Moment at Cantlevered Beam over HSS Column?
RE: Moment at Cantlevered Beam over HSS Column?
If there is any argument, provide the loads and geometry of the structure.
BA
RE: Moment at Cantlevered Beam over HSS Column?
Make sure there is a stiffener on the beam to brace the top of the column. Please post the size of the column and beam.
It’s no trick to get the answers when you have all the data. The trick is to get the answers when you only have half the data and half that is wrong and you don’t know which half - LORD KELVIN
RE: Moment at Cantlevered Beam over HSS Column?
RE: Moment at Cantlevered Beam over HSS Column?
RE: Moment at Cantlevered Beam over HSS Column?
Also, the beam should be laterally braced at the end of the cantilevers, at the columns and near the inflection points.
BA
RE: Moment at Cantlevered Beam over HSS Column?
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Moment at Cantlevered Beam over HSS Column?
And, I’m not real sure I see a solution to our problem until we have design guides which show the detail and calcs. for every possible (every conceivable) detail and loading in the world. It will be helpful when they finally come out with apps. for i-phones, where you say “I need a beam,” it’ll read your mind knowing you mean steel, 40' long; it’ll calc. the loads so as not to overtax the designer who is multi-tasking with tweets to friends and lovers. This will all be connected to the cloud because the codes are changing so fast the ink no longer has a chance to dry. And, this will all be connected to one of those instant prototyping printing machines right at the site. And, some of us older guys will be able to say to a great grand child, “I designed that structure, it’s still standing, and we actually used a thing called a pencil to do calcs. and drawings of the structure.”
Jason:
Isn’t the moment in the beam the same on both sides of the column? So, what moment goes to the column? Isn’t that the general concept of this cantilever framing design method? What would cause the moments to be different, and how much is that range of different moments on each side of the column? What is the slope and/or rotation of the beam on either side of the column? How much of that moment can the cap pl. and bolts transmit to the column? Would that moment distribution be in proportion to the relative stiffness of the beam and the column? Are the beam and column laterally supported at the top of the column?
RE: Moment at Cantlevered Beam over HSS Column?
If that is not correct, perhaps the OP could post a sketch of the actual condition.
BA
RE: Moment at Cantlevered Beam over HSS Column?
RE: Moment at Cantlevered Beam over HSS Column?
Some answers first:
BARetired - Yes this is what I've heard called a Gerber girder system (primarily Candada, I think?). This is my first exposure to it, and I haven't found a lot of guidance on it other than an older post on this forum detailing a large failure in Canada due to not providing adequate lateral bracing. Beam top and bottom flanges are braced by OWSJ at the column and at the end of the cantilever.
Slickdeals - Beams are typically W21x44 continuous over HSS6"x6"x1/4" columns.
Paddingtongreen - The beams are spliced with a single plate shear splice at the end of the cantilevers to form a series of alternating beam lengths in the 40' bays such as: 40' backspan with 5' cantilevers on each end, 30' suspended span, another 40' backspan with 5' cantilevers each end, another 30' suspended span, and so on. Then the end bays are a suspended span - no cantilever. Might be easier to google "gerber girder" if this paragraph was as clear as mud. There are 2 places where this system is part of the LWFRS and the beam spices are bolted extended moment end plates.
Now for my reasoning for asking:
I agree with much of what dhengr said. Sometimes I wonder why an EOR has delegated responsibility for connection design to begin with. Rarely do they actually provide enough information to fully design any complex connections. They seem to fall into 2 groups - 1) those who do their own complex connections to "make sure they're right" and delegate the piddly stuff that would've taken them half the time it takes me to get familiar with the job and document everything sufficiently for their review, and 2) those who design the typical shear tabs and whatnot themselves and delegate the stuff they have clue where to even start on it (or they got stuck and ran out of time and slapped a "design by fabricator" note on the drawings). However, I think our profession will continue on safely if younger engineers like myself recognize when we're out of our expertise and remain conscientious to seek input from more experienced engineers, which was my intent in coming here. I do realize this is a very common connection with no problems, but I wanted to understand why it's not an issue. It seems like as the cantilever deflects, there would be a beam rotation exerting a prying action on the bolts. Maybe I'm not thinking about this right, but that's why I wanted more experienced input. Thanks!
RE: Moment at Cantlevered Beam over HSS Column?
Not necessarily equivalent, but I considered these pinned but I haven't done too many of these.
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Moment at Cantlevered Beam over HSS Column?
BA
RE: Moment at Cantlevered Beam over HSS Column?
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.
RE: Moment at Cantlevered Beam over HSS Column?
RE: Moment at Cantlevered Beam over HSS Column?