Composite Behavior of Adjacent Concrete Beams
Composite Behavior of Adjacent Concrete Beams
(OP)
Here is a hypothetical problem. I would appreciate any input to support or reject the concept.
There is an existing concrete beam (say 5'W x 5.5'D) supporting concentrated loads from both sides of the beam. Now we need to relocate/ divide these concentrated loads beyond either edge of the beam. In order to do this, we are adding 2.5'W x 5.5'D sections on either side of the existing beam. So in plan view this modified configuration will have 2.5'W new section, 5'W existing section & 2.5' wide new section. These new concrete sections will be connected to the existing section with dowels and transverse post-tensioning (straight). Please note that all the concentrated loads (previously on existing section) will entire fall on these new concrete sections. Also, another important criteria to note would be that we are not altering the support conditions. Thus, the new beams are connected to the existing and only the existing beam is directly supported by the columns.
So, the question is, can we consider this arrangement (3 sections stitched together with dowels & PT) as a composite section?
Please consider the fact that the existing section do not have adequate shear capacity to support the loading condition alone. The arrangement will work if we assume composite property and count the shear capacity of the new section near supports.
I will sincerely look forward to your responses.
Thanks.
There is an existing concrete beam (say 5'W x 5.5'D) supporting concentrated loads from both sides of the beam. Now we need to relocate/ divide these concentrated loads beyond either edge of the beam. In order to do this, we are adding 2.5'W x 5.5'D sections on either side of the existing beam. So in plan view this modified configuration will have 2.5'W new section, 5'W existing section & 2.5' wide new section. These new concrete sections will be connected to the existing section with dowels and transverse post-tensioning (straight). Please note that all the concentrated loads (previously on existing section) will entire fall on these new concrete sections. Also, another important criteria to note would be that we are not altering the support conditions. Thus, the new beams are connected to the existing and only the existing beam is directly supported by the columns.
So, the question is, can we consider this arrangement (3 sections stitched together with dowels & PT) as a composite section?
Please consider the fact that the existing section do not have adequate shear capacity to support the loading condition alone. The arrangement will work if we assume composite property and count the shear capacity of the new section near supports.
I will sincerely look forward to your responses.
Thanks.






RE: Composite Behavior of Adjacent Concrete Beams
Or, am I reading your question wrong?
RE: Composite Behavior of Adjacent Concrete Beams
I totally agree with you.. I too think that it will not be prudent to assume composite section just by connecting the new to the existing with dowels/ PT. Because ultimately the load path to the supports is through the existing section only.
However, it has been argued otherwise. I will definitely wait for any reply that can agree with this concept and would like to know the plausible explanation if any.
RE: Composite Behavior of Adjacent Concrete Beams
RE: Composite Behavior of Adjacent Concrete Beams
If somehow a punching shear failure mode is what controls then certainly the added concrete does not good.
Given the size of the members in question I imagine the loads are substantial. I probably wouldn't do it in my own design, but I think it could work because you can't break one without the other. Of course I could be wrong and if so I don't doubt someone will put me in my place
RE: Composite Behavior of Adjacent Concrete Beams
The idea of "composite" action doesn't really apply here - the three separate beams all can act to share the load if they are adequately tied together to transfer the outer load application into the center beam.
There is not "composite" action here in the traditional sense in that the beams are not adding to the flexural capacity through horizontal shear behavior...only transferring a portion of the load to the center beam in proportion to the individual beam stiffnesses.
The shear at the end sections might be mutually shared up to a point near the end, but as you approach the support, with only the center beam being supported, the outer sections must shed the shear into the center beam to deliver it to the support.
RE: Composite Behavior of Adjacent Concrete Beams
RE: Composite Behavior of Adjacent Concrete Beams
RE: Composite Behavior of Adjacent Concrete Beams
Attached sketch - if the two side beams are not attached to the support, they simply slip down with the shear-failed center beam.
RE: Composite Behavior of Adjacent Concrete Beams
"dcarr82775" - You have correctly assumed that these concentrated loads are substantially high similar to bridge beam bearing points on pier cap. I agree with you that the new concrete will reinforce the shear capacity of the existing beam. However considering that the existing beam has inadequate shear capacity, if the new pieces are not directly connected to the column it will encounter a global failure as sketched out by "JAE".
The junction of the existing beam and column is monolithic and is crisscrossed by several rebars in all directions. Nevertheless, we need to connect the new pieces at this beam-column junction to provide a load path directly to the column as an alternate to the existing beam only. We will have to use PT and/or steel dowel. Detailing this connection will be essential and critical as this will be the primary load path for this new arrangement. I believe we will have to use the concept of shear-friction to design the dowel and PT. I am also thinking of using stainless steel to avoid future corrosion issues.
Any thoughts on the type of analysis/detail of this connection will be greatly appreciated.
RE: Composite Behavior of Adjacent Concrete Beams
My assumption has always been with the caveat that some careful detailing goes into the connection between the old and new beams. If the connection is properly detailed than you can not get that crack in the old beam without getting a similar crack in the new beams. I am thinking something akin to a 'C' bar that in doweled into the bottom and top of the existing beam (crossing that inclined failure plane similar to a stirrup) and sticking into the new beam each side. These bar prevent the failure from occurring per your sketch. I wouldn't count on any shear strength from the new concrete, but I think you get to count on the new Vs or at least some respectable portion thereof with proper detailing. I think the detail is more complex than just epoxying in some #4 bars, but I think it could be done.
We have installed vertical threaded rods through concrete beams in the past to serve as additional shear steel. It would be similar to that.
RE: Composite Behavior of Adjacent Concrete Beams
Not sure I agree at all with you. My sketch shows that the three beams are completely tied together and if the center beam shears off the column the two side beams simply drop with the center beam because the OP indicated that there was NO connectivity between the side beams and the support.
Now you can argue that this is more like a shear friction failure down a vertical plane at the end of the center beam (vs. a diagonal arching shear failure), but it still can fail that way and the two side beams aren't taking any shear because they have no place to deliver it.
If you tie the three beams with an infinite glue you could still get the failure in my sketch.
RE: Composite Behavior of Adjacent Concrete Beams
RE: Composite Behavior of Adjacent Concrete Beams
We will have to agree to disagree. You simply can not get that crack without also failing the added steel in the new beam. The steel in the new beam delivers the shear load across the inclined crack in the old beam. The new concrete gets you nothing, but the steel can get you something if done right.
Again, I wouldn't do it this way. There are certainly more efficient better ways to do it with cleaner load paths.
RE: Composite Behavior of Adjacent Concrete Beams
I like your concept and I kind of agree that this detail will engage the new bars in its shear capacity. However, I am not sure how to quantify (100%, 75%, etc.) the contribution. Any thoughts? Also, I would modify the detail a little bit. I would drill a threaded bar @ top and bottom completely through the existing (and grout obviously) and will use mechanical anchors (to avoid lapping of bars) in the new sections at each ends to provide continuity between the three elements. So, it will be like a big stirrup enclosing the 3 pieces.
But still I feel that it will be difficult to overcome the deficiency of the existing (as per the calcs it will be more than 100% overstressed in shear). So, we definitely have to rely on the direct connection to the column junction, most likely with similar concept along with PT.
hokie66;
You mentioned galvanized stress bars instead of stainless steel. Usually galvanized bars have lesser capacity (about 5% less) than regular PT threaded bars. So, I am thinking if we grout the full length of PT and also the PT anchorage ends (which will be in a pocket), do we still need galvanized PT bars?
Thanks to all for the ideas.
RE: Composite Behavior of Adjacent Concrete Beams
RE: Composite Behavior of Adjacent Concrete Beams
hokie66 correctly identified the concern:
My sketch was responding to that issue - [blue]whether the shear delivered to the support/column is an issue in the OLD beam since it is the ONLY thing attached to the column.
The assumption in my sketch is that he supporting column extends up between the two new side beams. The two new side beams DO NOT attach to that column.
The rebar you show in your sketch does NOTHING to keep the two side beams from sliding down the sides of the column. Or....what if the two side beams don't even reach the column/support face but stop 1 inch away from it?
The center beam (the ONLY thing attached to the column) can crack through its width and then the center beam with the two attached side beams WILL be able to slide down the column.
The OP specifically said that the side beams are NOT attached to the support.
My sketch is a 3D view so perhaps difficult to visualize but the crack can occur through the center beam and the thing fail despite your Super-glue-infinitely-fastened-completely-attached side beams.
The side beams just go down with the center beam.
RE: Composite Behavior of Adjacent Concrete Beams
I also followed the OP's restriction that the new side beams are in no way connected to the column. I may have gone beyond the OP's quote in proposing a manner in which you could gain some additional shear capacity in the existing beam by utilizing the new beams. I assumed that was the ultimate end he was getting at. I took the leap of faith and assumed his question was really, "Is there a way to add to the shear strength to the system if the new beams are only attached to the existing beam and not the column?" So I looked at it from that end, and feel I was very clear on that point.
I don't propose any super-infinity-fastened-smartass-magic-glue as you claim. What I do propose is a plain old simple piece of reinforcing steel crossing an inclined crack functioning similar to every other beam stirrup in the known world. The new concrete gives you nothing, we all agree, but the new steel can give you something if it is anchored into the existing beam such that it crosses the crack. It would function in a similar way to carbon fiber wrapped around a beam and anchored on the top side of the beam to add shear strength (see Fyfe for examples of just this method). IMO, the added steel means the crack you drew can't happen at as low a load as it would if the added steel wasn't there.
Cheers
MSRRR77
My thought process was just kicking around an engineering idea that could work, but like I said I probably wouldn't do it that way. So my sketch was simplistic to just show a conceptual idea. I would probably core vertically through the beam and add vertical rods to act as additional shear steel if I needed a reasonable amount of additional strength. But since you are looking for 100% more than you should just try tying into the column since you may be bumping up against the shear limits.
RE: Composite Behavior of Adjacent Concrete Beams
You are talking about increasing the beam capacity in diagonal tension, while JAE and I were concerned about failure in more or less direct shear at the column face, which is another potential mode of failure. Extending the side beams past the column and clamping them to it (shear friction, some call it) may be a way of addressing that mode of failure.
msrrr77,
I didn't remember that galvanized PT bars had less capacity, but doubtless they would, as you have to allow for the galvanizing in the threads. But if the PT is fully grouted, I think that gives the corrosion protection you need.
RE: Composite Behavior of Adjacent Concrete Beams
dcarr - I agree with you completely that with the proper detailing you can get some improvement in the overall shear capacity of the beam along the span where the three sections are attached to each other. No complaints with that.
RE: Composite Behavior of Adjacent Concrete Beams
In JAE's sketch, there is a triangular wedge left attached to the column post-failure. With the right, perhaps impossible ply fastening regime, the failure on that triangular wedge could be a three sided affair similar to punching shear on an edge column. Presumably, the capacity of this mechanism would be greater than straight up diagonal tension.
Is that right Dcarr?
I know, this desperately needs a sketch. Unfortunately, I'm on my phone in traffic.
RE: Composite Behavior of Adjacent Concrete Beams
As detailed, I think that you'd have to deliver the entire shear in the side beams to some hanger stirrups concentrated over a short length (~d) at the end of the side beams. Then the tension in the hanger stirrups would have to make their way to the top of the middle beam by way of shear friction / dowel action. As Dcarr implied, it would be tough.
I've always been confused about shear friction in situations like these. Does one need to satisfy SF at the column face in addition to Vc + Vs? Or is it enough to satisfy just diagonal tension? SF isn't normally checked in monolithic connections but an FBD through the beam at the column face makes me feel as though it applies.
RE: Composite Behavior of Adjacent Concrete Beams
KootK's 2nd post more or less is what my thoughts are.
Certainly easier to connect directly to the column or add to the column, but sometimes the nicest solutions are not the ones we get to use for a myriad of reasons.
RE: Composite Behavior of Adjacent Concrete Beams
Again, I have not yet ran the numbers, which will certainly say whether this is achievable or not.
I agree with Dcarr82775 that providing a loop (modified stirrup) will definitely contribute to the vertical shear. However, direct connection at the beam-column junction will be necessary. This modified stirrup will act like a corbel detail at the junction. Also, I am a little skeptical with vertical drilling to add shear rebars as the spacing between the longitudinal bar in the existing beam is pretty tight (barely 6 inches).
In addition to the dowels+couplers+C bars arrangement, the PT force will provide additional shear strength through shear friction at the existing-new concrete interface. Also, cutting out a shear pocket and passing the PT through it will also engage the concrete aggregate interlock component of the shear friction.
I am not too bothered about the shear in the existing beam right at the column face. Up to a distance 'd' from the face the load will get transferred to the column through arching action of concrete. However beyond that the additional stirrup will hold the pieces across the plane of the potential shear crack. I will engage a junior engineer to crunch the numbers to see if the design will work based on this concept.
Please let me know if you have any suggestions or concerns.
Thanks once again for such an involved discussion.
RE: Composite Behavior of Adjacent Concrete Beams
Will you use ducts and grout tubes at the stress bars? What type couplers will work with the mild steel bars in that configuration?
RE: Composite Behavior of Adjacent Concrete Beams
RE: Composite Behavior of Adjacent Concrete Beams
RE: Composite Behavior of Adjacent Concrete Beams
We are not using mild steel.. It will be HYSD carbon steel bars.
JAE, you are right. This arrangement will provide an alternate load path to the existing column (other than thru the existing beam). It would have been best if we could provide independent support for this new pieces. Dcarr82775 stated it appropriately "...but sometimes the nicest solutions are not the ones we get to use for a myriad of reasons.."
RE: Composite Behavior of Adjacent Concrete Beams
RE: Composite Behavior of Adjacent Concrete Beams
You are absolutely right. In this case we cannot allow rotation of either bar (straight or the C). The bar ends need to be threaded in right orientation to allow positional lock configuration. The direction of the k vector (perpendicular to the plane of threaded rotation) at each end of the coupler will be the key factor in this situation. Turning only the coupler in one direction should result in tightening of the connection. These couplers are not so uncommon.
RE: Composite Behavior of Adjacent Concrete Beams
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