Dowel lap length too short

[rabroke]

I have an issue on a job site where rebar dowels were fabricated too short, therefore not providing the required lap length with the vertical reinforcing. One situation occurs at a basement wall for the reinforcing at both faces, and another occurs at column piers (steel building with concrete piers on the footings). The testing agency inspecting the rebar missed that they were too short, and the footings have since been poured. So, I now need a fix prior to pouring the walls and piers to get the appropriate lap. Two solutions I thought of initially were to either provide mechanical splices for each of the rebars, or dowel and epoxy the vertical wall reinforcing into the footings. I think both are very expensive, and even though it's not my dollar, I still hate to propose such expensive fixes if not warrented. Any simple fix that I am missing?

 

[EngineeringAdam]

I am not an expert on the fixes for something like this, but my inclination would be inline with what you are suggesting.

Either use mechanical couplers or drill and epoxy.

However, how short are the laps? 90% of what was required? If it is close, then it might be worth check your demand to see if the reduced strength provided by the shorter lap is still acceptable, using As,req/As,pro.

 

[DST148]

I don't know which code you refer to. But as per ACI 318, the development length / splice length depends on concrete strength, clear cover to rebar, and the ties enclosing the bars being developed / spliced (there are other parameters as well). To reduce the development length / splice length, you could use a richer concrete mix, provide larger cover, or provide closely spaced ties to enclose the rebars being developed / spliced.

 

[Lion06]

I would double check the development length with the actual equation - Even a #6 bar with 1.5" of cover will get a cb/Ktr ratio of 2.5, which significantly reduces the required development length.

 

[rabroke]

I've never calculated development using section 12.2.3 (ACI 318 05) before. I always just used the equations on the table above that. Wow, that can make a huge difference! I got my wall reinforcing to work by a lot, and the pier reinforcing is right on to what it needs to be (33" for a #8 bar). And I'm confident that even if they screw it up a bit more, and are off by another couple inches, the concrete strength never ends of being only 3000 psi. Usually get at least 3500 or 4000. Thanks for the help.

 

[MiketheEngineer]

Weld them together??

 

[Lion06]

You can't weld unless the rebar is A706.

 

[Jc67roch]

Could you potentially bend the dowel projections into hooks, and the vertical bars ends to be added as well, to provide a shorter required development length?

 

[Lutfi]

You cannot weld gade 60 bars. That is out of question as was suggested above. I agree with Lion06.

I suggest the following:

1. Use mechanical joint coupling. They are available.
2. Consider, in lieu of lapping, to extend the vertical bar into the footing and epoxy it so it is developed.


Regards,
Lutfi

 

[grantstructure]

Unless your walls are retaining walls, it's unusual to need actual yield stress in the bars at the top of the footing. Is your wall designed to span simply between footing and the floor above? If so, dowels with full ld to every vertical wall bar at each face is probably overkill. All you really need is shear-friction steel to handle the shear forces at the footing/wall face.

Similar situation may apply with your piers. Is there any real lateral load being applied at the base of those steel columns (or moment?) If it's just vertical loads (and therefore no bending in the pier) then all you really need, potentially, is a compression lap, which is much smaller.

 

[Lion06]

I don't believe the reduction in development length for As,req'd/As,provided applies to shear friction because the shear friction provisions require development of fy on both sides of the shear plane and the As,req'd/As,provided doesn't apply where development of fy is required.

I think there is a rational basis to potentially reduce it, but I don't believe ACI allows it and I, personally, wouldn't do it (depending on how close it is, obviously).

I think the reduction from the cb/Dtr factor solved his problem, though.

 

[msquared48]

Cadwelds (Thermite welding) may be the only option, but getting the couplers in place could be a real problem if the steel is already placed, which it sounds like it is.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com