Composite Behavior of Adjacent Concrete Beams

[msrrr77]

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.

 

[KootK]

I believe that Dcarr's point is this:

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.

 

[KootK]

With diagonal tension improved, the "direct shear" failure referred to essentially comes down to the capacity of the compression strut (Vc_max), right? This would be a substantial improvement I would think.

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.

 

[dcarr82775]

Maybe it boils down to I look at JAE's sketch and see what I think of as beam shear (2sqrt F'c in ACI318 language) and nothing more fancy than that. I agree with JAE's sketch on the failure, with the caveat that if you detail things right you can have steel in the new beams that increases the shear strength of the existing beam. So the crack will have to go through the new concrete as well, but that new concrete cracks for 'free'. For the nth time, but in a little different language: no new Vc, but new Vs to contribute towards Vn of the existing beam.

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.

 

[msrrr77]

See attached concept detail that I am planning for this case.

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.

 

[hokie66]

Commiserations to your junior engineer. I remember that feeling. But I agree, your concept will work.

Will you use ducts and grout tubes at the stress bars? What type couplers will work with the mild steel bars in that configuration?

 

[JAE]

msrrr777, Do I see in your drawings that you are indeed now connecting the side beams into the column?

 

[hokie66]

Yep, his sections show that in concept. Now the young/junior engineer has to do some number crunching.

 

[msrrr77]

well said hokie66.. I am glad that I do not have to play with the numbers to crack down the puzzle...
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.."

 

[hokie66]

Sorry, I mispoke on the mild steel. I meant the C-bar and coupler arrangement, which I assume is regular reinforcement, as opposed to what I imagined as high tensile stressbars for the main through bars. The bars can't be turned, but there are some couplers available which will work. Never mind, your junior will figure it out.

 

[msrrr77]

hokie66;
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.

 

[jsha811]

The case you have is very common for bridge cap beam seismic retrofit. You definitely have to extend the new section through the column so that it will be effective for the shear. We used pretty much the same concept as you have. Using dowel bars and design based on shear friction and also have prestressing bars to tie them together.

Moment curvature analysis,reinforced concrete column design and prestressed concrete pile design at

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