Minimum distance between reinforcing bar bends

[BridgeSmith]

We have a concrete parapet detail with a blockout where the detailing of the rebar would be greatly simplified if we could do a double bend to create a small offset or jog in the bars.

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1561038826/tips/parapet_reinforcing_nvypct.pdf[/url]

Is there is a limit on how small an offset can be made when bending the bars? We've always detailed ours with 2 times the bend radius as a minimum, but I'm wondering if we need to.

 

[JAE]

Not to answer your question directly here - but when that bar goes into tension it will try to straighten out and it will pop off the concrete at the re-entrant bend.

But I don't believe there is any code-mandated limit or CRSI recommendation on this.

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[BridgeSmith]

That's a good point to consider, JAE. In this case, I think we can live with that possibility, since this exists to connect a highway guardrail at the end of a bridge. The only time that bar would see any significant tension is if a vehicle hits near the end of the guardrail or hits the parapet itself, in which case we would expect there to be substantial damage to the parapet anyway.

 

[JAE]

Yep - makes sense.

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[MotorCity]

Don't have an answer to your question, but rather a suggestion to simplify the detail and eliminate the bend.

Can you make the stirrup straight and then put the horizontal bars on the outside of the stirrup at that location?

 

[BridgeSmith]

Considered that, MotorCity, but we don't want to deviate too much from the crash-tested configuration. Making the change from a stirrup that only comes down to the blockout, with a straight bar behind the blockout into the slab, may be a hard sell to the head honchos as it is, even though I think a continuous bar is an improvement. To me, it seems better to use a continuous bar with a couple of bends instead of 2 bars that are kinda, sorta, not really, lapped. Plus, at one point we have an interference problem between one of the straight vertical bars and the horizontal bars above the blockout, which they ignored in the crash-tested configuration.

 

[dauwerda]

As your original post alludes to, the constraint (if there is one) would lie more in the bend radius. You could use typical bend radii (3db) given by ACI and have whatever jog you wanted by changing the degree of the bend (i.e. two 15 degree bends, two 45 degree bends, two 90 degree bends, etc.). I suppose a rebar fabricator could tell you if they see any minimums being an issue for them.

 

[BridgeSmith]

Thanks dauwerda. When I drew up a couple of bars with different offsets, I was able to see that it is as you say. It wasn't as bad as I first thought, especially using the bend radius for stirrups and ties (1" inside for the #4 bars). Not knowing what the bending equipment looks like, I just wasn't sure if there would be a limitation on how close the 2 bends could be.

 

[Agent666]

I think I'd just live with the extra cover and take the stirrup through vertically if you can live with the additional cover, if you're only talking about 1" more cover it's, here nor there. Makes fabrication easier. You mentioned on the sketch that the offset varies, but didn't say by how much so hard to say.

 

[BridgeSmith]

For the particular detail under consideration, the offset would actually vary from 1/2" to 4-1/4" (in 1-1/4" increments). The location of the 1/2" offset is the one that's the most problematic (for us type A personalities in the bridge design world), because without it there's an interference between the vertical bar and the horizontal bars at either the top of the bottom.

 

[rapt]

If the offset varies that much, every bar wilkl be different.

I would use a straight tie with a horizontal U at the top that can gradually move sideways as the offset increases.

 

[Agent666]

I think something like rapt suggested:-
Think of the extra U bar as simply having something nearer the face with a few smaller longitudinal bars, but otherwise the other standard reinforcing just does the business behind. Add it when you can get the additional bars in depending on acceptable covers.

 

[BridgeSmith]

The crash-tested configuration has a stirrup that looks similar, except it just comes straight down the front face to the blockout, with a straight bar up out of the slab 2" clear of the blockout, extending behind the horizontal bars in the top. The blockout is tapered from 4-1/2" deep at the end to zero at 18" along the front face. There are 4 vertical bars (front) and 4 stirrups hooking over (full height in the rear face down to the blockout in the front) at 4-1/2" spacing. The detailing of the crash-tested configuration ignores the interference between the last vertical bar and the horizontal bars along the front face above the blockout, so we have to change it some just to make it work. However, we cannot make any substantial changes, even if it makes it 'better'. When it comes to vehicle barriers, stronger isn't always better - it is expected to absorb the impact, not necessarily withstand it.