Shear Reinforcement

[shrews34]

I have and existing foundation in a Chemical Plant that originally supported a flat bottom tank. The client wants to use this foundation to support a tank with a skirt. This new loading will reverse the bending moment of the original design. In order to get the required depth and be able to install the new tension reinforcing in the top, we need to add a one foot section to the top of the foundation. Connecting the new section to the old section will require horizontal shear reinforcing. I have never calculated reinforcing of this type and have been unable to find a good reference or example that shows the calculation process in detail.
Any help or examples would appreciated.

 

[Lion06]

I would drill and dowel U-shaped rebar into the existing footing (with the U part obviously up) making sure to provide enough embedment in the existing to develop the bars. Roughen the top of the existing footing to get a coefficien of friction equal to 1, and use those U-shaped bars for shear friction to meet the horizontal shear flow. You probably won't need much.

 

[delagina]

I also work inside a plant. Another engineer in my company just use concrete bonding agent sika or similar for the longitudinal shear.

 

[rb1957]

you can calc shear due to bending at a plane ...

 

[JAE]

delagina - you should never count on bonding agents to develop structural shear. There is too much variability in the actual bond created - no good way to test for it.

 

[paddingtongreen]

If you need to save the height, then dowels, but if height is not a problem, why not place a new complete slab over the existing slab, let the existing slab be structurally passive. I doubt the extra concrete and reinforcing will cost more than the preparation and doweling the old slab.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[delagina]

@JAE, noted. as i said it was another engineer to who did this.

@shrews34, i was also interested at this topic one time.
you can look at ACI318 chapter 17.
you may also use example 12.6 of PCA notes as a guide.
also reinforced concrete by wight and macgregor chapter 16 composite concrete beams

 

[hokie66]

shrews34,
Is your foundation a slab or a series of beams? If a slab, you may be able to achieve the horizontal shear required by just roughening. If beams, then the inserted stirrups would be required.

 

[rscassar]

I have done this in the past.

1. Specify the existing surface to be scabbled.
2. Drill and epoxy L-bars across the interface (or U-bars as per SEIT suggested). Because you have one foot to work with you should be able to achieve full development of the reinforcement across the joint.
3. Apply a bonding agent (see Parchem or Sika who will have products which can be used).
4. Run through the calculations for longitudinal shear to ensure the section can behave compositely.

Regards and all the best.

 

[delagina]

also based on what i read, you may want to add more dowels instead of just U bars. U bars will only be connected at the ends.

you may want to add more dowels between the ends and the maximum moment (somewhere in the middle) for a more composite action.

 

[hokie66]

Mention of bonding agents makes my skin crawl. Unless the timing is exactly right, these materials can act as debonding agents. And on construction sites, timing is never exactly right.

 

[BAretired]

I have little faith in bonding agents. Use roughened surfaces and shear friction reinforcement between new and existing concrete.

BA