Existing Concrete Beam Fix

[ash060]

I have an existing concrete beam, and it is over stressed in shear, but not moment for the new load.

I have a very limited depth, so I was going to attach to the bottom of the beam with a WT and say that the shear would all be carried by the WT.

Is this acceptable? I was going to try to make the section composite, but the shear flow I get is way to high for any kind of post-installed anchors.

I cannot connect to the sides of the existing beam because there are cast-in-place joists framing into it.

If I assume that the beams act separately than the load would all go to the concrete beam and it would not work. I was thinking about it from a limit state approach. i have more shear strength (using the two sections) than applied so it is ok.

 

[youngstructural]

I would not approach this in the manner you've indicated. If you don't make it composite, you aren't going to get the stress into the section below without the forces harming the concrete beam above.

There was a good tread on this subject recently. Please do a search, and if you can't find it, post again and I'll see if I can't dig it up.

Basically there are other options.

Good luck,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[apsix]

If the new beam can take all the shear and all the moment I don't see a problem, if there is adequate support at the ends.
Provide wedges along the length to ensure that transfer of load occurs.

 

[hokie66]

YS is correct. The two beams will not be compatible, so the concrete beam will suffer shear distress before the steel beam takes much load. It could be OK to prevent collapse, but is not the way we design structures.

 

[apsix]

But if you treat it the same way as inserting a new lintel into a brick wall say, it will be perfectly adequate.

 

[BAretired]

I think it would be helpful if you could provide more details...perhaps we could give you a more intelligent response.

BA

 

[apsix]

Yes. We do know that "have a very limited depth" which doesn't bode well.

 

[ash060]

I have about 8" to work with. The building is about 60 years old. The reason the beam needs to be reinforced is because the architect wants to move one of the existing columns about 4' along the beam span.

The concrete beam is 12" x 15" with 3 #8 top and bottom, but it only has #2 Hoops at 24" c/c, so there is practically no shear reinforcing. I checked the beam for the change in load, and it works for moment but not shear.

Also, the architect is only moving the column below not the column above, so now I have a transfer condition which makes matters even worse. I do not the deflection in the beam to adversely effect the roof beam above because one of the support columns is settling to much.

The new spans are 12 and 14.5 feet and the orginial spans are 16 and 10.5 feet. They are carrying an office load. the orginial design is supposed to carry the same load according to the copy of the building code i have from original construction.

Yes the whole thing does not bode well, but that is what I am stuck with.

 

[csd72]

I would suggest wrapping it in GRP to take the shear. The GRP then takes the place of the shear hoops.

I would also be concerned about where the laps of the top and bottom bars are in relation to your new column position.

 

[ash060]

It would be hard to wrap the beam due to the CIP joists that frame into it.

The joists are 10" deep and spaced at 2'8" c/c

 

[csd72]

Can you install a diagonal 'sash' between these joists?

 

[msquared48]

As the buiding is 60 years old, have you taken a core sample near the center of the beam to verify the f'c used in your shear calcs for the beam? You may have more capacity than you think.

Mike McCann
MMC Engineering

 

[Teguci]

wrap the beam with gage steel. Cut out the wings to fit around the joists. Through bolt or epoxy in anchors in the side for shear transfer. The gage steel will act as your shear reinforcement.

Just one of many options.

 

[BAretired]

If you can place a W8x? below the existing beam, I believe it will work just fine. The span will be only 14.5', so distress in the existing concrete beam is likely to be minimal. The new beam should be capable of carrying the full load without reliance on the existing.

BA

 

[ash060]

I am putting the W8 below the existing and going to assume it carries the whole load. I need all the extra flexural stiffness I can get to prevent the column above from deflecting too much. I don't want to have to tell the owner that the roof beam needs to be fixed as well.

 

[ron9876]

What is the magnitude of the transferred load. Is there enough room to connect the new W8 at ends. The concrete beam may have to fail before the load is transferred to the steel beam. The previous comment about the continuity of bars is valid since they may have stopped the bottom bars at the column.

 

[Tomfh]

The concrete beam may have to fail before the load is transferred to the steel beam

Prestressing the steel fixes that.

 

[hokie66]

ron9876 has a good point. How would you connect an 8" steel beam to the column to carry all that load?

I would worry about continuity of the top bars over the new column. The 12' span will still work as a concrete beam, unless you intend to reinforce it as well. Is the upper column supporting another floor?

If you can develop the reactions at the columns and predeflect the steel beam to carry at least the dead load, the steel beam solution should work.

 

[ash060]

The existing columns are going to have to be reinforced as required, but they are made of steel and will make them easier to reinforce.

I am not worried about splice too much, from what the drawings show there is no splice over the columns, I think that there is only one splice along the length of the beam and it does not occur near the area I am working in.

The load can be transfered but I have to bottom cope the beam and weld a plate to get the bolts I need.

 

[Einsteim]

Can you install the new steel beam below and jack/shim under the concrete beam and on top of the steel beam to be certain the dead load and any future live load will tranfer into the steel member?

This should insure that the steel beam will be the primary load carrying member and the concrete beam will only act as a filler.