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.

 

[BAretired]

The connection will transfer some moment. There is no escaping that fact. The consequences of assuming a pin connection is that the column will be designed for zero moment at the top.

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

 

[SethGuthrie]

I agree with BA, assuming the beam is fairly narrow, the bolts will have a wide spread and the cap plate will resist moments. They key is (probably) to adjust the cantilever length to better balance the moments and reduce the demand.

 

[hokie66]

Why worry about this? Of course, the connection is not truly pinned, but most connections aren't. I hope there isn't a problem with this connection, as I have done thousands that way without a second thought. Steel is a wonderful material...when yielding occurs, it still carries the load until the next member picks it up. Teamwork.

 

[WillisV]

@hokie - agreed.

 

[JasonMcCool]

I've actually been hired by the fabricator to do connection design on any connections not designed by the EOR on a project they're providing the steel for. So asking him to change his typical cantilever layout that he used on 7 buildings this late in the game is probably out of the question unless it's just some unworkable setup that he specified. This was one that he detailed out on the structurals, so it's really not in our scope, but I was curious about how I would go about designing it. Always looking to learn new stuff that might come in useful later Anyway, he had a max 50k reaction out 5'-4" from the column at one cantilever, and a 35k reaction at the end of a 5' cantilever on the other end, with a 40' span between columns. Going off the AISC Table 3-22b for cantilevered beams, I come up with a beam moment at the column of roughly 199 k-ft. This seems like an impossible number to design for using the the DG24 procedure, so surely I'm looking at this wrong. This is a relatively common configuration that I've seen elsewhere, but I just want to understand a rational basis for it working like it obviously does on many other buildings. Thanks!

 

[TLHS]

If the beam's fairly stiff and the span between supports isn't too long I've designed this as a pin before because I'm comfortable that the couple forces will be stiff enough to make that a reasonable load path. If the beam's significantly more flexible than the column I'd probably start to assume that there's moment transfer at the connection because I'd worry that the deflection in the beam would make the couple force moment resisting load path somewhat questionable.

 

[hokie66]

The beam moment is what it is. That has little to do with the connection to the column, as the beam resists the moment on both sides. 50k x 5'-4" is 267 ft-kips, but the column doesn't see that moment.

 

[BAretired]

Sounds to me like the OP is considering a column moment which does not exist. It simply cannot be as high as he claims.
If there is any argument, provide the loads and geometry of the structure.

BA

 

[slickdeals]

Agree with what has been posted above. The beam is continuous over the column. For a 40' span, the beam is probably going to be fairly stiff compared to the column. The column is assumed not to see any moment. The moment you are calculating is not all transferred to the column. First off, it will pick up its share based on stiffness. Second, the maximum moment it will ever see if based on the couple between the bolts. You won't overload the bolts to failure prior to the "other side" of the beam picking up the loads.

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

 

[dcarr82775]

I would normally ignore the moment in the column.

 

[Ron]

Agree with above. Yes, there's little moment at the seat, but check beam crippling at the outside edge of the plate (cantilever side) and move on.

 

[BAretired]

I would use split HSS stiffeners welded to the beam web and bottom flange and extending to, say three quarters of the height of the WF.
Also, the beam should be laterally braced at the end of the cantilevers, at the columns and near the inflection points.

BA

 

[paddingtongreen]

I thought the OP said the cantilever was spliced to another such beam/cantilever, but he didn't say what type of splice. If it is a full moment splice, it becomes a beam, not two cantilevers.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[dhengr]

Wow... Structural Engineering is really getting complicated, and very scary. The EOR doesn’t design connections, as likely as not he/she isn’t sure how to do that and doesn’t want to waste the time, and/or isn’t getting paid enough to do that. So, he delegates that to the fabricator, who hires and engineer who isn’t too sure either. And the AHJ probably wants all the calcs., inter office memos and computer files for two years on either side of the job, to cover his ass. And, we call this Professional Engineering and keep signin stuff, including higher premium checks to our E&O insurers. This is an incredibly scary way to be doing business, and it is getting scarier to walk under any structure that hasn’t already been standing for at least thirty years.

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?

 

[BAretired]

paddingtongreen, I am visualizing a 40' span with a 5'-4" cantilever at one end and a 5'-0" cantilever at the opposite end, i.e. an interior span of a Gerber system.
If that is not correct, perhaps the OP could post a sketch of the actual condition.

BA

 

[haynewp]

Back to BA's first response of the moment induced on the column itself might be my only concern. But a tremendous amount of these cantilevered connections have gone up with a pinned assumption at the top of the column and I yet to hear of a failure of the column due to this approach. You can calculate the stiffness and the resulting moment transfer to the column if you really wanted to using FE. Then use the effective K for the column including the connecting beam at the top for the column design. You might see thar with a low stiffness connection that the moment into the column can be ignored.

 

[JasonMcCool]

Thanks for all the responses. I had to pull away to work on another project and hadn't checked back on this thread yet.
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!

 

[paddingtongreen]

What you lose on the swings you gain on the roundabouts. The fixity that induces moment also reduces the effective length in the direction of the induced moment.

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.

 

[BAretired]

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!

The moment applied to the connection to the column is the imbalanced moment, i.e. the difference between the cantilever moment and the beam fixed end moment. If the connection is unyielding, it will transfer some of that imbalance to the column, the amount depending on the stiffness of the column vs. the beam. Normal practice is to keep the moment imbalance to a minimum, but with alternate spans loaded, an imbalance is quite possible and should be considered.

BA