Beam Cover Plate Moment Splice Check
Beam Cover Plate Moment Splice Check
(OP)
See attached. This is from a shop drawing, the detailer's proposed moment splice in a beam. He is doing bolted at the top flange and a welded cover plate at the bottom (due to clearance/height issues). The plate is wider than the beam, I'm guessing that this is to avoid overhead welding. He is not welding the end, only the sides.
When checking this I feel like there should be some kind of double shear lag check to get the force up to the connection plane and then out to the corners. Am I imagining this? Any special checks for this beyond the usual? Something about this rubs me the wrong way. I'd like to weld the end but that would force him to weld overhead or flip the beam once spliced.
When checking this I feel like there should be some kind of double shear lag check to get the force up to the connection plane and then out to the corners. Am I imagining this? Any special checks for this beyond the usual? Something about this rubs me the wrong way. I'd like to weld the end but that would force him to weld overhead or flip the beam once spliced.






RE: Beam Cover Plate Moment Splice Check
I don't like mixing the bolts and welds like that. I feel like the stiffer welds will take a disproportionate amount of the force. Why not weld the top too? Then I'd be a lot more comfortable with it.
Not saying that it can't be done this way. Just that I don't like it.
RE: Beam Cover Plate Moment Splice Check
Assuming they're not slip critical bolts, I think you'd have issues with mixing welds and bolts. Bolts need a little room to slip into bearing and the welded connection doesn't give you that. You'd get disproportionate force in the bottom flange until the bolts kick in. Maybe it can handle it, maybe not.
I would just have him weld the top and bottom. You can make the top plate narrower than the flange so it's all welded overhead. If you need to thicken it, lengthen it, or weld the ends of the top plates you can do so.
I don't believe you need a shear lag check because the length of your weld appears to be greater than 2x the width of the bottom flange. (I think this is Chapter D in AISC 360)
RE: Beam Cover Plate Moment Splice Check
I'm not sure that I understand what you're proposing as the second stage lag issue here. Is it the vertical offset between the centroid of the flange plate and the centroid of the flange? If so, that's valid but would amount to next to nothing. I see this as straight up tension lag between flange and flange plate. Just the "out to the corners" bit.
I don't share this sentiment. I essentially see the upper and lower flange connections as two independent connections, neither mixing bolts and weld. I might have concerns if this were a designated seismic hinge or a fatigue situation bit I don't think that's the case here. I'm guessing NY historic reno.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Beam Cover Plate Moment Splice Check
Then lets say the beam finally experiences it's design loads, will that plate or weld hold up if they were designed without that "initial" tension in mind?
That might be stretching it but this is what I'm envisioning at the moment.
RE: Beam Cover Plate Moment Splice Check
RE: Beam Cover Plate Moment Splice Check
Mixing Welds and Bolts, Part 1
Mixing Welds and Bolts, Part 2
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RE: Beam Cover Plate Moment Splice Check
RE: Beam Cover Plate Moment Splice Check
RE: Beam Cover Plate Moment Splice Check
As I see it, this scenario would violate statics. T_flange = C_flange ~ M/d. Always and no matter what, right?
As best I can tell, none of those documents cover this particular case. Certainly, I agree that bolts and welds should not be used in concert to resist a common force. I still contend, however, that here the top flange and the bottom flange rationally represent two separate connections, neither mixing bolts and welds.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Beam Cover Plate Moment Splice Check
RE: Beam Cover Plate Moment Splice Check
Correct me if I'm wrong: It would not violate statics the same way a prestressed concrete beam does not violate statics. The plate is initially tensioned "before" it becomes part of the beam.
TC bolts only give you a "snug tight" connection. The bolts must go into bearing before they provide any resistance. If this were a slip critical joint, I'd have no issue.
RE: Beam Cover Plate Moment Splice Check
Well yes, if you are looking to develop the plastic moment capacity of the entire section including the web, then I would agree 100%. Based on the proportions of the detail, I was assuming that you were developing tension and compression forces less than bf x tf x Fy.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Beam Cover Plate Moment Splice Check
I'm afraid that I just don't see it DETstru. If you FBD through the centre of the splice under an applied moment, I see no way for the tension and compression forces not to be equal. Even if you used longitudinally short slotted holes in the top flange, this should still be the case. Granted, bookowski wouldn't much like the deflection...
I forgot to ask, what scenario leads to this detail? I've yet to encounter it. Perhaps the CJP is less disruptive to the deck above?
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Beam Cover Plate Moment Splice Check
DET - I don't the see bolt/weld issue here. The flanges each must carry T/C and be in equilibrium. Where is the C offsetting the initial T, unless you are thinking of the interaction of the web initially, or some catenary/hanging action initially?
RE: Beam Cover Plate Moment Splice Check
Overall, there isn't anything that makes me say, "no way, this is flat out wrong". Especially if they're using slip critical bolts.
In that sense, I agree with KootK and the others. But, at the same time, I wouldn't put my stamp on this connection unless there is a compelling reason for them to resist welding both the top and bottom flanges. Not sure what the difference in cost would be...
RE: Beam Cover Plate Moment Splice Check
I suppose that you might get some flexure in the lower plate prior to bolt engagement. At ultimate, you'd still be able to plastify the entire plate in tension however.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Beam Cover Plate Moment Splice Check
RE: Beam Cover Plate Moment Splice Check
However, there is still a strain compatibility issue. The connection will deform more at the top before the bolts engage. You'll get movement at the top, but little movement at the bottom. My feeling is that this will put more stress on the weld, though I don't know exactly how. Because I don't know exactly how it will work, I'm a bit uncomfortable with it.
I keep thinking back on the pre-Northridge connections where they found the flange welds were taking the shear forces because they were more rigid than the bolted shear tabs that were assumed to take all the shear. Not exactly the same situation here. But, similar enough to raise some concerns. I'm confident that (at least at seismic level loads) we couldn't get this connection anywhere near the probably plastic moment of the beam.
That's probably too stringent of a criteria to use for the OP's project, of course. And, for lower moment demand and less ductile requirements, it would likely be fine..... just not with my seal.
RE: Beam Cover Plate Moment Splice Check
For welded moment splices, both flanges webs of the beams are butted together.
In the original illustration there is a gap at the bolted flange, the bolted web, and the welded flange.
In the second illustration there is a gap at the bolted flange and the bolted web, but not at the welded flange.
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RE: Beam Cover Plate Moment Splice Check
I think this is how:
That plate moment would imply weld tension forces perpendicular to the beam axis. The effect would be localized near the connection centreline I think.
I once asked a super, super, super senior engineer about bolt slop in a bracing connection. He said that any connection with six or more bolts would effectively have no slip. At least one bolt would be engaged from the start. No science behind that that I'm aware of however. Just an intuitive sense of random variable probability distribution.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Beam Cover Plate Moment Splice Check
But I do wonder about why one would splice the beam in that way. Like the bottom picture of SRE's post, the top plate and bolts make installation of the deck messy.
But one for SRE...surely it doesn't take that many bolts to splice a W16x26 flange?
RE: Beam Cover Plate Moment Splice Check
1. Any shear transfer at the splice will allow a vertical shift between beam ends due to bolt slop. This would put an unzipping force on the welds. Not too concerned though. If unzipping starts, it will stop when the lower plate can flex enough to cover the minor amount of vertical shift.
2. If the contractor is set up to field weld, he might as well finish the job. I'd detail a skinny plate on top for a top down weld and do an all weld splice. To me, bolts only make sense when you can prevent field welding or can allow the crane to move to the next pick. This detail works, but doesn't make sense to me.
RE: Beam Cover Plate Moment Splice Check