Bundled Bars

[tclat]

Hi,

I'm reviewing drawings prepared by another engineer and he has 6" wide beams with two 2 -3/4 inch bars top and bottom. Because the structure is close to the sea, the concrete cover is 1.5" which doesn't leave any space between the bars. I raised this with the design engineer and he indicated that the two bars are bundled bars so he need not satisfy the spacing requirements of ACI Cl7.6.

I've never seen a 6" wide beam before much less two bundled bars in the bottom and two in the top with a ties around them. Any thoughts or comments?

Thanks

 

[BAretired]

I guess it works okay, but I don't think he is saving any money using such a narrow web. I would prefer 8" minimum.

BA

 

[KootK]

I've done something similar with 8" grade beams before. The bundling doesn't bother me if no splicing is required. The two issues that I foresee:

1) I've never been sure how to interpret end anchorage requirements in this situation. I guess you could develop for the force required but, if you're going with "terminate with hook past support center-line", it's tough to know if that still applies with bundled hooks. Not even sure that there's such a thing as a bundled hook. I suspect not.

2) There's always a danger that this will lead to a severly over-reinforced beam. I guess that's not technically possible if the compression steel matches the tension but still...

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.

 

[dik]

Bad idea for a structure close to the sea...

Dik

 

[hokie66]

I wouldn't call a 6" wide concrete member a beam. More like a joist. I would either widen it, or if that width is really all that is required, would use a single larger bar.

 

[tclat]

Is there a requirement for the length of the top and bottom part of the ties for anchorage?

For a joist we would typically detail a 180 degree hook. For a 6” wide beam with 1.5” cover the length of the standard hook will be less than 3”. The tie size is 3/8”.

 

[KootK]

Yeah, that's the other trick. Can you even get neaningful ties in there? S-shaped single legs might be more sensible. I would hope that something proportioned like this wouldn't be working too hard in shear. I agreee with dik's concerns regarding durability in a coastal environment. Sounds like it's not your baby anyhow though.

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.

 

[slickdeals]

In a perfect world, a #3 tie would work with a "J" distance of 3". However, once you take off the tie diameter, you are only left with 2-1/4" in between and the 3/4" diameter bars can't be in the same horizontal plane. In this case, whether you use a 180 degree hook or a tie, the "J" value would still be 3".

What is this beam holding up? Is this just some sort of turned down edge (header) to connect the top of a window or door?

Would agree that you might be better off with a single bar or bars in parallel layer (even if you want to use bundled bars, provide them one on top of the other rather than one besides the other in a horizontal plane)

Commentary R7.7.6 recommends a cover of 2" for walls and slabs and 2.5" for other members. Not sure how many engineers follow this recommendation, but this is just FYI.

If you don't get proper consolidation and rock pockets in this congested beam, you are going to have big problems with spalling etc. in future.

 

[dik]

I've used single leg stirrups, alternate them... worked well. Added: Still a bad idea to use 6" width...

That may be a bit of a solution using one or two bars stacked/bundled vertically with single leg ties; this could maximise the concrete cover.

Dik

 

[rapt]

Why would you want to do it? 8" minimum and use proper ties.

 

[thaidavid40]

Are the bundled bars stacked vertically, or placed side-by-side? It would seem that if in the side-by-side condition, you would risk segregation or honeycombing under the bars in such a narrow joist.
Dave

Thaidavid