Corbel detailing

[Rainbowtrout]

This detailing practice may be perfect OK. But I am suspicious since ACI doesn't have this. I am not sure what is wrong and therefore putting it out here for people to comment. Thanks!

Basically, the corbel has a continuous primary and framing bar. Both ends of the bar extend well into the supporting wall. There is an anchor bar at the outer end, but not welded to anything. My concern is anchorage of the primary bar. ACI states:
At the front face of a bracket or corbel, primary
tension reinforcement shall be anchored by (a), (b), or (c):
(a) A weld to a transverse bar of at least equal size that is
designed to develop fy of primary tension reinforcement
(b) Bending the primary tension reinforcement back to
form a horizontal loop
(c) Other means of anchorage that develops fy

Is there any reason why ACI doesn't have "integral" primary bar as a suggestion?

Thanks!

 

[haynewp]

Do you have a sketch?

 

[KootK]

The continuous primary bar is legit but, to avoid spalling, the bend radius needs to hit tangency in front of the assumed bearing area. In practice, that usually means small primary bars and bearing pads set a fair ways in from the edge of the corbel.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[hokie66]

Not sure what this continuous or "integral" primary bar looks like. A sketch would help.

 

[Rainbowtrout]

Here is the sketch. I think KootK was on point.

 

[haynewp]

If the primary reinforcing is not welded to the anchor bar to develop Fy then it has to be long enough to develop Fy on each side of the face of the column. The continuous bar you have shown shouldn't have to be welded if the length provided into the corbel develops the bar, and the bearing location relative to the hook meets ACI 11.8.7.

 

[hokie66]

I think anchorage is the issue here, rather than development. I would think the provision of welding to the longitudinal bar is to insure that intimate steel contact is achieved, not transferred through a thin sliver of concrete.

 

[haynewp]

I thought he was trying to "anchor" the primary Asc bar into the corbel, achieving item (b) in the original post through development of Asc.

 

[hokie66]

In the picture, he doesn't show the primary reinforcement in a horizontal loop per (b), but rather in a vertically oriented bent bar. The welded longitudinal bar, or plate, is for the vertically oriented bar solution.

 

[dhengr]

When the red bar at the top of the corbel is loaded in tension, and is only bent around the anchor bar it tends to slip a bit as it brings the anchor bar into play (into bearing btwn. the two bars), and likely crushes some concrete at the juncture too. You do not want this kind of movement (false extension, bond failure, conc. crushing in bearing) before you really start to fully stress the red bar. As Hokie suggested, the welding has proven to bring the two bars into more intimate contact and bearing and really makes the anchor bar act as an immediate anchor for the red bar.

 

[KootK]

In this context, anchorage basically amounts to development of the primary steel across the compression strut assumed to be constrained by the primary steel. Semantics aside, the sketch below from PCA Notes makes is fairly clear what is expected of designers.

I would expect the arrangement shown below to perform better than a welded anchor bar. I'll take conventional development/anchorage over localized mechanical anchorage any day. It's just not a popular configuration because it requires a long corbel and/or rather small primary bars.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[hokie66]

With respect, I disagree that anchorage and development are the same thing, and would never use the PCA detail.

 

[KootK]

I disagree that anchorage and development are the same thing

No, they're not the same thing. But they are definitely related. Development alone does not guarantee anchorage. Hence the development of ACI Appendix D for post-installed rebar etc. However, development across a competent, restraining compression strut does guarantee anchorage and is, in fact, the most common form of rebar anchorage. That's how the bottom bars of simple span beams are anchored at the supports.

and would never use the PCA detail.

No inverted tee beams for you? Or cantilevered bridge bents, which are really just corbels in macro? In my experience, both of these forms utilize a version of the PCA method rather than mechanical anchorage.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[hokie66]

Nope. Never done one of those precast monstrosities like in your first picture, and since I am now retired, I won't have to. They are never tied together well enough to suit me, to the point that I avoid parking in them. Bridges, well, I admit to not inspecting most of the ones I drive over.

 

[KootK]

They are never tied together well enough to suit me, to the point that I avoid parking in them.

Excellent. Never, then, shall I accuse you of hypocrisy.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.