Hello all, I hope we are all ok in 1

[pancho10]

Hello all, I hope we are all ok in these challenging times. This is my first post and would appreciate all of your expertise.

Mu = 110 k-ft
Vu = 23 k
fy = 60 ksi
f'c = 4 ksi

Span Length = 11.8 ft

The situation has arisen where I need to reduce the depth of a concrete beam from 35 inches to 24 inches in order increase an opening for a generator. This 35 inch beam supports a slab above it (poured monolithic with beam), a cmu wall and roof above it. In addition to these loads, this beam also supports near midspan, a beam that is 24 inches deep (also poured monolithic with slab).

My intent is to have the contractor chip away 15 inches off the bottom of the 35 inch beam. This will leave me 4 inches to install new reinforcement for flexure and to drill and epoxy hook bars into existing 20 inch 'beam'. Then simply form this new beam and pour concrete. First question. Provided this beam works for deflection; how much hook reinforcement do I need to add (size & spacing)?

If I can get deflection and flexural reinforcement to work, I run into the problem with shear. Since the beam will be chipped off, the stirrups (#3 @12) will be cut off and no longer serve any purpose for strength really. I just don't get enough shear capacity with concrete only. My Phi Vc is around 8.6 k (24 in X 8 in beam) While my Vu is 23 k. Second Question. What are my options to increase the shear capacity of this beam?

I'm thinking either expand (make beam wider) beam or add some steel channels on either side of the concrete beam. Only, if I do this option, the steel channels will not be continuous as it will be interrupted by that other concrete beam that is being supported by the concrete beam I am strengthening.


This brings more questions though. If I make beam wider so as to have more concrete area, how do I determine the number of bars I will need to connect new concrete to existing (ie. size of reinforcement/ embedment depth)? If I add channels, how can I go about analyzing this, I've never worked on composite beams before. How do I determine channel size, simple shear capacity calc of channel? Connection of channel to beam (ie bolts, anchors) sizing? etc?

I will be very much appreciated of any comments/ feedback. Thanks.

 

[humanengr]

Have you considered or will the situation allow for constructing a new beam(s) (steel or concrete) adjacent to the existing beam,
transfer loads to the new beam, then demo. or partially demo. the existing concrete beam?

 

[bridgebuster]

Have you considered a fiber reinforced polymer wrap (FRP)?

 

[pancho10]

Thanks for the reply. Well the beam in question is the beam supporting the opening in a wall. This beam is needed to attach door frame.

 

[pancho10]

I believe someone mentioned that here (FRP) but my superior said he will not be doing that, so... Here I am. Thanks!

 

[dhengr]

Pancho10:
Could you use posttensioning to advantage for the new tensile reinforcing? For the stirrups and their development, could you just bend the bot. 12”, or so, over/around a piece of 1” pipe so the bot. portion (old vert. side portions of the stirrups) was now sitting horiz., and just below the new tensile stl. and pointing down the length of the beam? Thus, still pretty viable stirrups, with some cutting and bending, but without drilling and epoxy. The shear demand should be less in the mid third of the beam, and that may be the place where you really need the new clearance too. This shorter length cut-out might reduce the amount of chipping and stl. removal really needed, and allow the high shear (end reaction lengths) to remain as is. The posttensioning could be harped, and on either side of the existing beam, and then covered with conc. protection, for the wider beam.

Edit: Anything new, side channels or new conc. and mild stl. is kinda passive, unless you unload the existing beam by jacking it up to unload it. Then, install the new channels and/or conc. and rebar. Then when you remove the jacks the existing and the new take the loads in proportion to their relative stiffnesses as the new composite beam takes on load or deflects. With the harped posttensioning, the mid length is a horiz. low elev. and the harp points are located right under the door jambs above, helping lift those concentrated loads. I’m making some favorable assumptions about the door location above.

 

[azcats]

Any chance you can reinforce from above?

 

[pancho10]

Thank all of you guys for the input but the contractor and I decided to go another way. Thanks all, very much appreciate it!!