Cast in place corbel

[Lutfi]

I came across a continuous corbel detail that is being formed as part of concrete tilt-up wall. I always used ACI detail and requirements for corbels. However, the attached corbel detail is not detailed in the way I like to see them.

In my opinion, the detail may have enough shear capacity through shear friction. However, it may not have the tensile capacity. Additionally, using strut and tie method, the tension and compression zones will almost over lap based on this detail (ideally you want the compression block on the interior zone of the wall while the tensile load as far out towards the exterior face). The code does not explicitly indicate how deep to extend the tension steel. However, ACI pictorially show it as far back towards the exterior face of the wall/column. I also do not think ACI allows for transverse bars to carry tension.

Any thoughts about this detail are welcome?


Regards,
Lutfi

 

[ishvaaag]

I neither love this detail, particularly the lack of engagement of the corbel seat rebar with the vertical wall rebar; also, cover is mighty too big -maybe leading to fissuration problems- for the face in the side of the corbel.

Other than battling for a reform of the detail, you may model a composite slice of rebar and steel in 3D, pass it to some FEM package and see what the behaviour is under factored loads.

 

[JAE]

Would the #5 ties be developed into the wall past the shear failure plane?

 

[DST148]

The special provisions for brackets and corbels typically apply where the member behaviour is in one direction. For a continuous bracket with discreet point loads, the behaviour would be in two directions, some thing similar to a ledge. Assuming the bracket/ledge is loaded at beam locations only, then transverse reinforcing may be counted in carrying the loads along the length of the wall.

 

[jdgengineer]

I'm with JAE, it doesn't appear that the transverse reinforcing is developed into the wall enough. If its something that is already built you can check if it is ok by comparing the development length that is there with the full development length required and reduce the steel strength accordingly.

 

[msquared48]

I am not only uncomfortable with the corbel configuration and reinforcing pattern, but also the single layer of wall reinforcing considering the eccentric load from the corbel.

I believe that the #4 reference lines that look like extended rebar are merely leader lines to the bars, not the actual reinforcing. Right Lutfi?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[rapt]

I am with Mike on this one, both the corbel (awful) and the wall reinforcing are not adequate.

 

[dcarr82775]

I agree with DST that a continuous ledge is not a corbel. That being said no way the bars are sufficiently developed in 3.125-inches.

 

[shobroco]

No one likes the detail, myself included, however I don't think we can pass judgement on whether or not it works. What's the load on the corbel? As DST says, the ledge is continuous but the load isn't so there is more to consider. What is the concrete strength? From simple unreinforced shear analysis, the compression zone extends into the wall (4" eccentricity & 12" deep ledge) and may well carry the load. Nothing pretty, nothing elegant, plenty to worry about, but it could be one of those things that "we've always built this way" & it works by blind luck. That doesn't mean you have to put your name on it.

 

[Lutfi]

How about the bars do not meet ACI 12.5! Essentially, the bars do not meet the development.

I still ca this a cornel, a continuous one. Yes similar to ledger.

Regards,
Lutfi

 

[ztengguy]

The single layer in the wall might be ok, if counting on the floor to brace the panel. Whats the loads?

 

[Lutfi]

JAE, you are right on. I do not think the #5 bars are developed per ACI. In my opinion, that is the only calculation that needs to be done.

I typed my earlier response using my I-phone and some of the words got auto corrected. The last sentence should have read: "I still call this a corbel, a continuous one. Yes similar to ledger." Sorry about that.


Regards,
Lutfi