Anchoring Multiple Layers of Beam Reinforcement 1

[pappyirl]

In designing a simply supported beam a scenario has arisen where there is a requirement to anchor 2 layers of bottom reinforcement at a column support with L bars, see attached sketch. It is probably similar to a situation which would arise on a deep beam close to a support, anchoring the longitudinal reinforcement without using mechanical anchors.

Is bearing stress inside the bend of a reinforcing bar an issue when there is another bar directly in front of it which exerting the same bearing stress although the bar bend location is slightly elevated due to spacers in the layers? Might be splitting hairs but any advice, code or otherwise is much appreciated.

 

[12345abc6ttyui67]

No sketch attached.

 

[jayrod12]

Would you need all of them to be anchored? It's unlikely that the bulk of the bottom steel is required right at the support.

 

[pappyirl]

Thanks for the quick replies.

Generally you would not need to anchor them all. However in this case the max BM moment is closer to the support than the required anchorage length, so they do end up being anchored into the support.

Sketch should be attached now.

 

[KootK]

1) If space permits, I'd run the beam past the column a bit and alleviate this with detailing,

2) I feel that a strut and tie model at he joint would be appropriate and would capture at least some of this.

3) my gut feel is that it's probably not an issue. I imagine that the outer bar bend stresses are s whole lot less local by the time that they get to the inner bars.

 

[pappyirl]

Thanks KootK,

There is space so I will be suggesting the beam is extended past the column.

I have looked at S&T but was coming up with a similar issue / non-issue in anchoring the tie.

There is a bar diameter gap between the bars so the stresses would reduce remote from the bend. There are clauses to calculate the stress within the bend in the code but not for anchoring in-line multiple bars. Deep beams would be a more common example of this arrangement, anchoring multiple layers.

 

[KootK]

Thanks KootK

You're most welcome. It's an interesting question that I'd not considered before.

There is space so I will be suggesting the beam is extended past the column.

To quote someone horribly unpopular, I think that's an excellent final solution as far as the detailing goes. You just can't beat convincing anchorage on a major transfer element.

I have looked at S&T but was coming up with a similar issue / non-issue in anchoring the tie.

Agreed. Unless you have gobs of bearing, the inner layer hooks tend to be difficult to anchor within the node. I take that as confirmation of the issue / non-issue that you've rightly identified.

There are clauses to calculate the stress within the bend in the code but not for anchoring in-line multiple bars.

My local, North American codes do not have such clauses to my knowledge. Are you working with Eurocode?

Deep beams would be a more common example of this arrangement, anchoring multiple layers.

When I use multiple layers in less demanding situations, I'll typically only hook the bottom layer. That, simply to avoid congestion and constructability issues. Like jayrod mentioned, in situations where it's more of a Bernoulli beam than a disturbed region joint, you can usually accomplish what you need to by anchoring just one of the layers.

 

[pappyirl]

Yes, working to EC2. There was a clause in the previous British Standard which had a similar requirement to check the stress at a bend in a reinforcing bar.

I'm open to correction but I think the check is generally not necessary because the minimum bend is sized not to crush the concrete when the bar is fully stressed. It might be a necessary check for non-standard bends or bars in low grade concrete.