Shear flow in concrete beam

[jeffhed]

I have a concrete beam I need to reinforce with a plate or angle scabbed on the side. Most of my shear flow calculations have involved reinforcing with a new element entirely on the bottom and/or top. So when you calculated Q, your area and ybar of the element you were attaching is entirely below or above your point of interest. Now my question is this: Say I am attaching an 8" steel plate at the bottom of the concrete beam on the side with a row of bolts along the center line of the plate, so 4" up from the bottom of the beam. When I calculate my area and ybar, it should be for the entire plate that I am attaching, right? Not just the portion of the plate that is below my point of interest, in this case the bottom 4" of the plate? I have looked in a few strength of material books at the office and can't find any examples of reinforcing beams on the side, probably because it is the least efficient. Obviously with the significantly lower area and ybar, if I am incorrectly using the entire area of the plate, this could make a big difference in the calculated shear flow, in this case it is double. Has anyone else done this before, if so, did you calculate the shear flow as I have, or did you use only the area and ybar that is below the point of interest?

 

[ToadJones]

I'll state again that if the beam is existing, and you slap plates on the side, it will do absolutely nothing if the beam is currently loaded to its service load. The plates will only help with additional load.

 

[jeffhed]

ToadJones,
I agree. The building had never been occupied and has only been supporting dead loads, no live loads. We are reinforcing the beam the beam to support the dead + live loads. The building has already been temporarily shored. We will retrofit the beam and then remove the shoring.

 

[ToadJones]

Now another worry comes to mind....

Will the cracked concrete be able to transfer load to the plates?

As it is now, the beams is probably theoretically cracked and the tension reinforcing is carrying all the tensile load.

How to transfer that load to the steel plates is an issue.

 

[ron9876]

Might want to consider carbon fiber. Fire rating on the steel plate may be a problem.

 

[jeffhed]

ToadJones,
That is a concern, right now the cracking in the member is small and we were going to epoxy inject all the cracks prior to installing the plate. There are a lot of bolts, but the contractor specifically asked for this type of repair despite my warning of the amount of bolts. Another option would be to use two plates and cut the bolts in half, but the plate on the exterior will be exposed to soil, so corrosion protection is a concern in this scenario.

ron9876,
A concrete slab on grade will be cast up to and above the repair so it will not be exposed. If the contractor doesn't want to install the amount of bolts I have specified, then we will probably be forced into a carbon fiber type solution.

 

[hokie66]

Is carbon fiber any better than steel for fire rating?

 

[TXStructural]

Rfreund's top sketch will probably provide the action you desire, with a few problems and conditions:

1) the line of action of the plate (as a tension member) will follow the bolts, but the "U-factor" of the eccentrically-loaded plate will interfere with the purely axial tension of the plate. I would use a thicker plate/bar to reduce this problem.

2) the line of bolts will become a fracture plane, along which I would expect the beam to crack. To be sure you do not lose the cover and effectiveness of the added steel, the bolts will need to be above the longitudinal reinforcement and inside the stirrups.

3) I'd be concerned about the eccentricity of the new repair and the resulting torsion.

Why exactly can the repair/added steel not be put on the bottom of the beam? I would encourage you to cut a groove wide enough and deep enough into the bottom of the beam to let the steel into the bottom if needed. You can then distribute the anchors across the bottom, extending them into the confined core of the beam. Hammer drill the holes so as to not cut existing reinforcement. Install adhesive anchors in accordance with 318-11 App D (including qualified installers).

New cover can be placed over the repair for fire protection purposes.

 

[jeffhed]

TXStructural,
Thanks for your input. My replies are as follows:

1) Do I need to worry about the U-factor? My element is fully connected to the concrete beam as specified in case 1 of AISC Table D3.1. Load is transferred directly to the member. U comes into account if I was using an angle and I had to transfer tension force to the horizontal leg that is not directly bolted to the member.

2) Bolts will be located above the existing tension steel and within stirrups. Existing reinforcing will be located prior to drilling holes to ensure no existing rebar is drilled through.

3) Eccentricity will no longer be a concern because I showed the contractor the difference in bolting on one side versus bolting on both sides. He has elected to bolt on both sides and protect the steel on the outside from exposure to soil and/or weather.

Putting the steel on the bottom would be ideal, but since this is a basement wall/grade beam there would be a lot of excavation to make enough room for a guy to get in there and drill some holes.

 

[Teguci]

Q = the full A of the plate times the difference between the plates' Neutral Axis (NA) and the composite NA (concrete beam and steel plates).

Theoretically, if the NA of the steel plates matches the composite NA then VQ/I = 0. Of course you will still need to transfer the load to the steel plates with a nominal amount of anchors. But the curvature of the steel plates will match the curvature of the composite section.

I suggest thinking of the curvature of the plates vs the beam. If the plates are below the NA then the curvature of the plates will be under tension summed over their areas. Per statics, this tension must be balanced. This balancing comes from the shear transfer anchors which balance the plate stresses by imparting a compression on the plates. This balancing must happen regardless of the location of the anchors.