Increasing the Moment Capacity of Existing Concrete Beam
Increasing the Moment Capacity of Existing Concrete Beam
(OP)
I have a concrete cantilever beam (See Elevation Below). The concrete cantilever will support a new steel beam (a new point load at its tip). The existing moment strength of the beam is that corresponding to 4#8's at the top, which is insufficient for the new load condition. I have thought of two solutions to increase the moment capacity of the beam for the new load.
The first solution consists of augmenting the cross section of the beam by adding a concrete pour to the bottom of the beam, such that the 4#8's would be sufficient. The new concrete pour would be attached to the existing beam through epoxy dowels simulating stirrups.
The second solution consists of augmenting the base of the concrete beam to add the required additional negative steel through the sides of the column to the other side. The new concrete pours on either side of the beam would be attached to the beam through epoxy dowels simulating stirrups.
I appreciate any thoughts on this.
The first solution consists of augmenting the cross section of the beam by adding a concrete pour to the bottom of the beam, such that the 4#8's would be sufficient. The new concrete pour would be attached to the existing beam through epoxy dowels simulating stirrups.
The second solution consists of augmenting the base of the concrete beam to add the required additional negative steel through the sides of the column to the other side. The new concrete pours on either side of the beam would be attached to the beam through epoxy dowels simulating stirrups.
I appreciate any thoughts on this.






RE: Increasing the Moment Capacity of Existing Concrete Beam
RE: Increasing the Moment Capacity of Existing Concrete Beam
At your detail with dowels be sure you check edge distance.
RE: Increasing the Moment Capacity of Existing Concrete Beam
I that I know of is Sika at www.sikausa.com
RE: Increasing the Moment Capacity of Existing Concrete Beam
Your proposed solution #2: I am suggesting a slight variation - I am assuming the existing rectangular section is adequate in beam shear, long term deflections, and compression due to flexure caused by the existing and the additional point load and also accessibility from the top......
You may saw cut channels in the slab on either side of the beam/column, wide and deep enough to accommodate rebars and grout/epoxy. Although the beam and the slab might have been poured monolithically, you may also have to check the adequacy of the existing stirrups for horizontal shear at the interface of the slab and the beam web. The new steel does not have to have hooks at the cantilever end, since your sketch shows existing top steel with hooks.
You may also check adequacy of tension reinforcement being developed by a standard hook at discontinuous ends of members - ACI 318-05, section 12.5.4.
RE: Increasing the Moment Capacity of Existing Concrete Beam
You could consider adding top external reinforcement in the form of a steel plate or angle each side of the beam fastened with Hilti anchors adequate to develop the required tension in the added steel.
I am assuming the face of column is flush with the face of beam. The plates or angles should extend from the tip of cantilever to the point inside the column where 4-#8 are adequate to do the job.
Alternatively, you might consider adding a steel channel each side of the beam extending each side of column and fastened to beam and column with Hilti anchors adequate to transfer the new reactions.
Finally, fireproof the new steel shape with sprayed on fireproofing material.
BA
RE: Increasing the Moment Capacity of Existing Concrete Beam
Considering your solution #2, of adding conc. cover to the two sides of the beam; but you haven't mentioned the magnitude of the new load or the location of the new steel beam, so the details are wide open, consider the following: (1) Looking at your elevation, draw a straight line from the lower tip end of the canti. (lower canti. beam corner at the tip), to the top edge of the conc. slab (outside face of the column) and on to the inside of the column. This line will intersect the inside face of the column 6-8" above the top of the beam. This line is prestressing on each side of the canti. beam. (2) In its simplest form, put at large angle iron under the lower corner/tip of the canti. beam to anchor the prestressing; vert. leg up and horiz. leg under bottom of the beam. With a similar saddle on the inside of the conc. column. (3) Obviously, there will be an advantage to having this prestressing line at the inside face of the column being at 2' or 3' above the top of the beam, if you can. (4) The conc. on the sides is really just cover or fire proofing, and almost any kind of stirrups will hold it in place.
RE: Increasing the Moment Capacity of Existing Concrete Beam
Thanks for your help.
RE: Increasing the Moment Capacity of Existing Concrete Beam
RE: Increasing the Moment Capacity of Existing Concrete Beam
There was a misunderstanding in the field because the elevation shows 7#8 bars but the section cut shows 4#8's. So,"logically", they went with 4#8's.LOL.
RE: Increasing the Moment Capacity of Existing Concrete Beam
My eyesight is pretty bad but I see 4-#8 bars in Section and no mention of top bars in Elevation.
BONILL:
1. Do you really want to cut off the additional reinforcement at the cross beam? I would prefer to see another channel inside the column attached to the plate and through bolts.
2. The through bolts are eccentric to the centroid of the channel. Have you considered combined bending and tension in the channels, plate and bolts?
3. The crossbeam on Gridline (4) complicates matters a bit. Would it fair to say that the existing 4-#8 bars are adequate to resist the full moment at a point half way between the column face and the end of cantilever? You need to add 2-1" dia. bars each side through the existing crossbeam. They could be 3.0m long and centered on the column. You then must find a way of anchoring them at each end. To me, this would be the better approach.
BA
RE: Increasing the Moment Capacity of Existing Concrete Beam
BA
RE: Increasing the Moment Capacity of Existing Concrete Beam
Look at his elevation with his post of 9FEB12, 16:40; 3-#8's from the back span and 4-#8's for the canti. and terminating approx. 6' into the back span.
RE: Increasing the Moment Capacity of Existing Concrete Beam
By the way, BA, get "em checked. :<)
RE: Increasing the Moment Capacity of Existing Concrete Beam
Yes, it is fair to say that the 4-#8 bars are adequate to resist the full moment at a point halfway between the column face and the end of the cantilever. But you lost me on centering the bars on the column and anchoring them at each end. Can you explain?
RE: Increasing the Moment Capacity of Existing Concrete Beam
See attached detail. Bar length = 1.2 + 1.2 + 0.6 = 3.0m. Middle of bars centered on Gridline 4. Anchor each end of bar to concrete beam (to be discussed).
Hokie: I have had 'em checked, but I still do not see 7-#8 bars at the top of the beam in question. Maybe I should find a new opthalmologist.
BA
RE: Increasing the Moment Capacity of Existing Concrete Beam
RE: Increasing the Moment Capacity of Existing Concrete Beam
BA
RE: Increasing the Moment Capacity of Existing Concrete Beam
Now I understand, after seeing your detail. I have drawn up a new detail using the same concept as yours but providing a small concrete pour on either side of the beam to develop my reinforcement in lieu of having to anchor the bars. Let me know what you think.
RE: Increasing the Moment Capacity of Existing Concrete Beam
Kieran
RE: Increasing the Moment Capacity of Existing Concrete Beam
You can't hook the bars if you intend to insert them through a hole in the existing crossbeam. So you have to use straight bars and for that reason, you should extend them enough to develop bond at each end.
You really are depending on developing bond to the face of the existing beam. You probably should roughen the surface of the existing beam with a chipping hammer.
I don't think I would use #3 bars lapped in the middle of the beam. I suggest a more robust bar, probably an all-thread rod with nut and washer at each end to develop shear-friction. It should be located midway between the 2-#8 added bars.
Should have confinement steel around the 2-#8 bars...#3 seems okay for that.
How do you intend to pour the concrete? Do you drill holes through the floor slab to allow the concrete to be poured from above or do you use pneumatically placed concrete (shotcrete) shot on from below?
BA
RE: Increasing the Moment Capacity of Existing Concrete Beam
If there are 7-#8 bars in the existing beam, would it be adequate without remedial measures?
BA
RE: Increasing the Moment Capacity of Existing Concrete Beam
It is feasible. But the cantilever span can't be reduced so much as to only require 4#8's.
RE: Increasing the Moment Capacity of Existing Concrete Beam
I intend to pour the concrete from above by drilling holes through the slab.
Good point about roughening the surface of the existing beam to increase shear friction.
I have replaced the #3 bars spliced at the middle of the beam with all thread #8 bars in the middle of the two bars.
It is true I cannot hook the bar I am drilling through the beam at both sides, but I can do so on the left end of the column. I am restricted in space there because after 0.45M.,I have double T's framing perpendicular to the concrete beam.
Am I missing anything else?
RE: Increasing the Moment Capacity of Existing Concrete Beam
Your latest section shows the top bars below the slab, while the original showed them within the slab depth. Which is it? Wasn't the slab cast monolithically with the beam?
RE: Increasing the Moment Capacity of Existing Concrete Beam
The reinforcement has been field verified, and confirmed to be only 4#8's. The reinforcement is actually within the slab depth but lower than d'(it is an error in my detail. I just assumed worst case scenario when checking the existing reinforcement and carried it on to the detail.)
RE: Increasing the Moment Capacity of Existing Concrete Beam