Existing Beam - Added Shear Capacity

[PEinVA]

How do I add shear capacity to a reinforced concrete beam that looks what is shown on the attachment. I know you can use carbon-fiber on a typical rectangular beam, but I can't do it here due to existing geometry. I'd rather not add the cost of demo'ing the extra 4" overhang around the entire opening that is being infilled.

This beam is 20' long and is supported by beams on either end. The shear capacity is needed in the middle because the shear is greater than 50% of the Vc. The reinforcement is there but it is spaced at 10" oc. This does not meet the requirement of d/2 minimum spacing.

This area is getting infilled so that is why the beam does not have the required shear reinforcement, and I was wondering how we can reinforce this beam. Any suggestions? I don't have the head room to put a steel beam below this beam.

RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[apsix]

If you are infilling with concrete can you incorprate an edge beam to the infill with the required strength?

 

[msquared48]

If you can take a core sample of the concrete and test it, maybe the f'c attained is greater than what was spec'd in the oroginal structural drawings, and just maybe that's enough to avoid having to add the extraq shear reinforcing?

Mike McCann
MMC Engineering

 

[hokie66]

The simplest way may be to drill in from the top and insert double headed studs, set in epoxy mortar.

 

[csd72]

hokie66,

The problem with that is that they will want to diamond core it through something that deep and those sides of the hole will be as smooth as .. well as smooth as something really smooth.

RCraine,

Can you put a steel channel either side of the beam?

 

[JAE]

I would second some of the suggestions above - first nail down the f'c. Also consider the idea of two C8 channels - one on either side.

 

[Teguci]

Harped external PT strands may be an option.

 

[jike]

I agree with hokie66's suggestion. Another thought I had was similar except using single leg #2 or #3 rebar hooked on the tension end set in epoxy filled drilled holes. You could also use external vertical plate straping. Any external solution may require fireproofing to maintain the fire rating.

 

[PEinVA]

apsix,
We were planning on infilling with a series of small steel beams and some metal deck. The opening is only 10' wide by 18' long, so we weren't planning on using concrete in this application.

mssquared,
That could be an option, but I don't think we'll get much benefit because the shear is about 80% of the shear provided by the concrete. So the increase wouldn't get us below the 50% required to not require stirrups. If it was closer, then possibly, that could save us in this situation.

hokie66,
I have never seen this done, as I'm pretty new (~2yrs). Do you ahve information on this? I know I've read about it recently and seen mostly in PT slabs but any information you can pass along would be great.

csd72,
For the channels, would they be set with epoxy/expansion bolts as required to transfer enough shear to be below the 50% required? Then I'd be able to cut this short once I'm beyond the critical point where shear provided from (Vs + Vc) can be used due to the spacing?

Teguci,
The PT seems excessive for this application, we've [our firm, not me ] done this in the past at a PT garage we're constantly monitoring.

Jike,
Is the plate strapping doing the same thing as the C8, with regard to distributing the shear?


Thanks for all your responses! I truly appreciate this great forum and I look forward to your continued help!




RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[geoffdale]

What about wrapping the tee with a steel plate?

 

[ronster]

When you checked the design of the beam did you take into account the 8" slab on top and design it as a T beam or just check it as a 12"x16" beam? If you checked it as a T beam and it still does not work then I would add a steel channel for the full length on one or both sides as required. I would design the steel channel to take the added infill load by itself.

You said the shear capacity is needed in the middle. Assuming you mean the middle of the span, that is where the shear would be the lowest. Please explain.

 

[hokie66]

RC,

You have caught me out. I have never seen headed studs inserted either. There has been a lot of research into doing this to increase punching shear capacity in flat plates, but I don't know if anyone has actually done it in a building.

The channels each side as proposed by csd72 and JAE should work, but the drilling for that solution will be more difficult because it is horizontal.

Are you sure your stirrup spacing exceeds the maximum? I am not up with the ACI Code, but the Australian Standard, which has its roots in the ACI, allows spacing up to .75D in some conditions. It also allows assessing shear resistance by truss analogy.

 

[frv]

ronster-


I'm pretty sure the effective flange width does not contribute to shear capacity.

 

[youngstructural]

I have previously provided a narrow strip of steel with bolts through the full depth of the section. You can actually use the steel plate as additional tension reinforcement if you are willing to fasten frequently enough to satisfy Tau = VQ/It longitudinal shear.

If you are not interested in coring through the section, I would suggest you consider adding external loop stirrups fitted around the section and epoxy into the slab at the top, or sides. This is similar to one of the above suggestions. The loops can then be packed out with grout dry-packing, or you can jacket the section and pour Self-Compacting-Concrete treated with a superplasticizer.

Let us know which way you go in the end, it's an interesting problem!

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[PEinVA]

ronster,
The T beam does not allow any help for shear, as stated above. If I am wrong, please let me know, but I couldn't find where that would help.

The issue with shear at the center of the span is due to the added infill framing. We're framing across the short direction of the opening to keep beam sizes smaller, but this adds some load to the center of the beam, that wasn't there when the stair was in the opening. This increases the load to greater than 50% of the shear strength provided by concrete which in turn invokes ACI 11.5.5.1 - Minimum shear requirements. Then based on that, the spacing limits are controled by 11.5.4.1 - basically says "typical shear minimum spacing is d/2 in non-prestressed, and 0.75h in prestressed members, nor 24 in."

hokie,
HAHA, sorry I caught you. I know I've seen it and read about it, but I don't think my boss would want to do that. Where is the part about the truss analogy. The only waiver I see is through testing. Thanks.

youngstructural,
Thanks for the suggestions.



How do you design the channels as added shear reinforcement?



RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[JAE]

I wouldn't design the channels for "added" shear reinforcing but rather for 100% of the shear and ignore the existing concrete altogether.

 

[PEinVA]

Is there any connection between the concrete beam and the new channels? Or am I just designing this as a beam?

RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[youngstructural]

You should definately make a connection; Given that the existing beam is already taking the load, unless you make the channels VERY stiff they will not pick up load, until the concrete beam collapses that is!

You should think of the beam as the source of your load, so what you are in fact doing is bringing your load into the channels trough the beam and the fastening to the beam. Also, by fastening the channels (presuming they are facing opposite directions, typically both toes out), you are reducing the effective length for torsional failure of the channels.

Also, remember that the shear centre for a channel is behind the web, away from the toes, in free space. This is an additional reason why toes-out steel channels jacketing an existing section works so well.

Just as a point of interest, I am currently designing a series of toes-out channels on either face of some existing walls. Done correctly (with particular attention to installation procedure) doing this allows the contractor to simply cut the wall out!

Back to my own beam modifications...
Good luck!
Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[PEinVA]

youngstructural,
Thanks for getting back. I meant, do I make a connection at say, 1'-0" oc to transfer some shear to the steel of the channels, or do I just make end connections? I was only going to be using a C8 so it wouldn't be too stiff, but I have short edge distances, so the loads the bolts would transfer are low.
It just is confusing to me because at some point all the shear in the channel has to return to the beam because I can't span the entire beam on both faces.

I'm just confused on this the more I let my brain try to warp itself around it.



RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[ronster]

I'm not clear on two things.
Do you have drawings showing the stirrup spacing decreasing at the ends of the concrete beam. Reinforcing the mid span only of beam for shear when everything else works is not very common.

How is the infill framing for the proposed opening attached to the concrete beam. Why can't you have the infill framing secure to the concrete beam in a area where the stirrup spacing is adequate or better yet, why can't the infill framing frame into a new 8" beam alongside the existing concrete beam so that you do not add any additional load to the existing concrete beam?

Without seeing a framing plan it is difficult to know what is going on.