Concrete Beam Strengthening 4

[ali07]

We have a sitution where 60 ft span beam have to carry more live load and reinforcement becomes less than the required. I need to provide deficient reinforcement with steel plates anchored to soffit of the beam. How can i design the anchors? Beam is 24"x48" in X-sec.

 

[Lion06]

Calc the required shear flow for composite action, then determine the number of Hilti (or Powers) adhesive fasteners that you need to develop the required shear flow. Is the reinforcement needed for strength only or also for deflections? If it is needed for deflection also, I would do the same thing at the top of the beam (if it is accessible). This will help with long term deflections.

 

[Lion06]

I would also be very careful that adding the reinforcing plate on the bottom of the beam doesn't make the beam over reinforced. It may be hard to reinforce this enough (considering the reduced phi factor for the different strain levels of the lowest level of steel) without adding a plate to the top to help with the ductility.

 

[Lion06]

One last thing..... look at the local buckling of the top plate if you go that route.

 

[shin25]

Post installed anchors may cut thru the existing reinforcing. So, if not careful this may do more harm than good.

One option may be to verify the actual strength of the concrete in the beam. Quite often, the actual strength is more than the design strength. You may opt to have some cores taken to verify this.

If the higher strength does not give your beam sufficient capacity, you may try external post tensioning.

Sometimes, if possible, adding an extra beam (secondary) line on the side of the existing beam may help.

 

[shin25]

One more option may be to use FRP to strengthen the beam.

 

[yakpol]

Assuming that steel plate will be epoxy glued to the soffit the anchors will be required only at the end of the plate. Shear capacity of anchors shall be larger than tensile capacity of the plate.

 

[ali07]

Can someone show here approximate calcs to design Anchors.
I understand the beam is already loaded and we are fixing that plate to account for additional live loads. How can i determine force in anchor which might be longitudinal shear force in the beam for additional moments. As in some cases we cannot guarantee the hole size in plate exactly same as anchor so anchor looks like only holding the plate to concrete face.
(Beam is not going to be overreinforced and there is no deflection problem as such now).

 

[graybeach]

I wonder the same thing - the bolt holes in the plate will not necessarily be in contact with the bolt. Maybe you can grout between the plate and the concrete and then tension the bolts. The tension times the coefficient of friction would be your resistance per bolt. In any case, I would be nervous to drill a lot of holes in the girder, and there could be hole misalignment issues. There is a lot of info about FRP strengthening out there - it is really looking like a viable solution for flexural strengthening.

 

[minorchord2000]

The best way to strengthen existing concrete beams (and for that matter columns), is to wrap the beam or at least apply a carbon reinforcing fabric to the tension face. There are many companies that specialize in this type of application.


Check quakewrap.com for more information.

 

[UcfSE]

I would check out either external post-tensioning or FRP wrap.

 

[csd72]

Is the shear capacity okay?

A steel channel either side may do the trick(expensive, but probably cheaper than carbon fibers). Yuo need to look at strain compatability though.

 

[civilperson]

I agree with UcfSE suggestion of external post tensioning. A few outrigger members and anchors with 1/2" prestress cable is probably the cheapest fix.

 

[yakpol]

The strengthening completely relies on epoxy, bolts provide only extra security for ultimate capacity, so a little slip is OK.

 

[hokie66]

yakpol,

Can you suggest a standard or code to use for justifying the steel to concrete bond through the epoxy adhesive? What adhesive would you use? What are the creep characteristics of the adhesive? What shear stress would be used? Is a clamping force necessary? What about the fire rating of the beam? There may be answers for all this, but the justification is not well enough formulated for my liking.

I agree with others that external posttensioning is the way to go.

 

[ali07]

Shear capacity of beam is adequate. FRP is no doublt a good replacement but problem with FRP is fire rating, it does not comply with applicable fire resistance and behaviour of epoxy under elevated temperature result is bond failure of concrete and FRP adhesive. If it is necessary to provide FRP it requires additional fire resistance.

 

[shin25]

For additional fire resistance, you may try shotcrete on top of the FRP.

 

[yakpol]

There's a British Standard or other British code for this type of construction. It has all explicit requirements for anchors and for epoxy bond. I used to have it, but now even don't remember the title. Many, many reinforced concrete beams have been retrofited this way, nothing unusual here. Using fibrewrap will force general contractor to hire a specialised sub and spend more $$.

 

[hokie66]

Well, is there some other Brit out there who can refer me to what yakpol is talking about?

 

[apsix]

I'm not a Brit, but it's probably this;
"Design manual for roads and bridges. Vol. 3: Highway structures: inspection and maintenance. Section 3: Repair and strengthening. Part 1: Strengthening of concrete highway structures using externally bonded plates.
Corporate Author: Great Britain Department of Transport
Publisher: HMSO
DTp (now the Highways Agency) publication no. BA 30/94. Dated February 1994. "