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.

 

[BAretired]

@BONILL,

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

 

[BAretired]

BONILL,

If there are 7-#8 bars in the existing beam, would it be adequate without remedial measures?

BA

 

[BONILL]

@kieran1,

It is feasible. But the cantilever span can't be reduced so much as to only require 4#8's.

 

[BONILL]

@BAretired,

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?

 

[hokie66]

Are the three other bars definitely not there?

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?

 

[BONILL]

@hokie66,

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.)

 

[hokie66]

If the 3-#8 x 11050 do not exist, you may have problems in the backspan as well as in the cantilever.