Increasing the Moment Capacity of Existing Concrete Beam 5

[BONILL]

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.

 

[BONILL]

Here is the elevation.

 

[ron9876]

Can you drill thru the column and add top steel and add a pour with dowels on top.

At your detail with dowels be sure you check edge distance.

 

[ash060]

You could always try fiber reinforcing. Call a manufacturer and they have engineers on staff that will tell you if it can be reinforced with the new load

I that I know of is Sika at

http://www.sikausa.com

 

[DST148]

@BONILL: Your proposed solution #1: Assuming concrete is added to deepen only the cantilever side of the beam, then the column will have to be checked for unbalanced moment due to different depths of the beams.
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.

 

[BAretired]

I would be concerned about getting good contact and bond between the new and existing concrete, particularly with Solution #1.

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

 

[dhengr]

BONILL:
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.

 

[BONILL]

Upon receiving additional information from the field, I now realize that there is another beam framing into the column perpendicular to the beam in question. I have modified my detail using channels at both sides of the concrete beam, anchored using hilti bolts to develop the full capacity of the channels. Upon reaching the secondary beam I am running rebar through the beam to the other side to develop the full moment capacity required. See modified attached detail.

Thanks for your help.

 

[hokie66]

The elevation of the beam shows 7-#8 bars top. Why do you think there are only 4?

 

[BONILL]

@hokie66,

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.

 

[BAretired]

hokie:

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

 

[BAretired]

My eyesight is even worse than I thought. I still don't see 7-#8 shown in elevation.

BA

 

[dhengr]

BA:
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.

 

[hokie66]

There is nothing logical or "logical" about assuming the section takes precedence over the elevation. When an elevation exists, it always rules over a section or a schedule. One thought--could some of the bars be spread in the flange? Have the number of bars been site verified?

By the way, BA, get "em checked. :<)

 

[BONILL]

@BAretired,

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?

 

[BAretired]

BONILL:

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

 

[hokie66]

3-#8 x 11050 and 4-#8 x 4850. That's 7-#8 shown.

 

[BAretired]

Okie dokie hokie, now I see it. I was looking at the wrong elevation. Maybe they should do an X-Ray to determine how much reinforcing they have. My guess would be 7-#8 top, but what do I know?

BA

 

[BONILL]

@BAretired,

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.

 

[kieran1]

Is it feasible to bolt a steel bracket to the beam colum junction underside the beam and thus reduce the cantilever span?

Kieran