Strengthening existing rc beam for shear 1

[asixth]

I would like to draw from another post attached.

I have an existing beam 2100 deep x 300 wide (7'x1') where I am demolishing the bottom 900 (3') of the beam leaving a 1200x300 (4'x1') beam. The shear reinforcement was two U-bars lapped to form a closed tie. When demolishing the beam, the bottom closer of the U-bar is cut off, leaving undeveloped and unanchored shear reinforcement which I'm sure violates all good engineering principles.

Because the beam shear is greater than 0.5*?*Vuc, I need to provide the minimum shear reinforcement. Firstly, I don't have any shear reinforcement cogged around longitudinal reinforcement so I think conceptually that is the first thing wrong with the design.

What is the best way to increase the shear strength of the concrete beam, I am thinking to install a wide-flanged beam underneath the rc beam, doweling into the beam with shear studs to ensure a transfer of load is possible.

Are there any other methods to increase the shear strength of the reduced concrete beam section?

Also, how would I go about propping the rc beam so demolition can take place and then ensuring that a full transfer of load occurs when the steel beam is installed.

Any help will be greatly appreciated and I will post a sketch to give a better idea of what I am trying to explain.

 

[asixth]

And attached sketch.

 

[asixth]

The first attachment is incorrect, the thread I am referring to is attached

 

[youngstructural]

There are multiple other methods commonly used, everything from dowel bars to hanger rods through to side plates... There was a detailed discussion of several of them in a thread about a year back. I'll see what I can dig up.

Cheers,

YS

B.Eng (Carleton), P.Eng (Ontario), MIPENZ (Structural-New Zealand)
Working in Canada, and missing my adoptive New Zealand family... at least I brought the little Kiwi with me!

 

[youngstructural]

Have a look at this thread:

http://www.eng-tips.com/viewthread.cfm?qid=221830&page=25

I think it should address your concerns.

Hope that helps,
Regards,

YS

B.Eng (Carleton), P.Eng (Ontario), MIPENZ (Structural-New Zealand)
Working in Canada, and missing my adoptive New Zealand family... at least I brought the little Kiwi with me!

 

[hokie66]

asixth,

Two questions:
1) Can you not just leave the concrete beam, removing only the slab?
2) Why not demolish the rest of the beam and recast it to the new size with new reinforcement to suit? Propping the beam for demolition and installation of a steel beam will be very tedious.

 

[asixth]

Answers to question

1) No, we are trying to provide clearance height underneath the beam.
2) We cannot demolish the slab because there is a corbel that supports a 4.5" brick wall. Also, the upstand itself is supporting a 6" block wall.

Structurally I think we may be going down the wrong path trying to demolish this slab.

I have run through the code which requires the following:

"Shear reinforcement shall be deemed to be adequatley anchored provided the following requirements are met:"

"Bends in bars used as fitments shall enclose a longitudinal bar with a diameter larger that the diameter of the fitmet bar. The enclosed bar shall be in contact with the fitment bend".

I cannot justify that the shear reinforcement as illustrated in the sketch is fully developed in tension so I think this beam with the reduced section will fail in shear.

Saying that, my solution is to epoxy a 380PFC (C15) to the side of the beam. I am not trying to make the beams act composite so the only shear on the anchors is vertical shear (no longitudinal shear). I then anchor the channel over the columns where a compression strut occurs and the load can then be transferred from the steel channel back to the column. I design this steel channel to take 100% of the load (no composite action).

Is there anyway to justify or does anyone think the shear reinforcing will be developed when I am cutting the steel as shown.

 

[BAretired]

I assume you would install the channel before any demolition takes place, contrary to sketch 1 of 3.

The channel will be loaded eccentrically. Are you going to rely on the upper part of the existing concrete beam to resist torsion?

Is the longitudinal steel at the bottom of the corbel adequate to provide bending resistance without the channel? If so, could you field weld steel anchors to each leg of each stirrup to develop them in tension?

BA

 

[youngstructural]

Personally I would add in a flat plate at the underside of the "new" beam shape, and use stitch beams to provide temporary support to the existing structure.

Despite the fact that your choice of a PFC is good for this application, given that the shear centre is inside the concrete and thus the eccentricity of the loading is greatly reduced, I do not think it would be easy to take out the torsion that will inevitably occur.

Your torsion is taken out by the shear stirrups, and I think you're planning to cut them out, correct?

The addition of steel plate at the bottom is highly efficient and just as simple as most other approaches. You just need to go through the more complicated calculations regarding the longitudinal shear.

Oh, and I do think you're STILL going to have to deal with some torsion and ensuring that your detailing permits the existing concrete you retain to take that load... Only now it should be low enough (since your newly upgraded beam is nearly concentric to the load) that simply folding the legs of the stirrups to form a new lap at the bottom will probably suffice.

This is an interesting stituation/problem; Please keep us up to date.

Cheers,

YS

B.Eng (Carleton), P.Eng (Ontario), MIPENZ (Structural-New Zealand)
Working in Canada, and missing my adoptive New Zealand family... at least I brought the little Kiwi with me!