External anchorage of reinforcement - is this feasible?

[BridgeEngineer21]

I have a situation that I can't seem to find any examples of in practice. During staged construction, a slab is loaded right to the edge, utilizing the primary longitudinal reinforcement. These bars will eventually be coupled and continue into the next stage of the slab (figure 1). But temporarily, they need to be anchored somehow. We've proposed various solutions like hairpin bars and additional headed bars, but the reinforcement density is quite dense, so for ease of constructability the contractor has asked about this alternate solution to anchor the bars externally, to a steel plate at the face of the slab (figure 2).

In principle it is very similar to headed bar anchors, but I guess the capacity of those is based on manufacturer testing that would include things like the effects of bond between head and concrete and confinement of adjacent concrete, which aren't present here. In theory, it seems possible and easy to just verify the bearing capacity of the concrete behind the plate, as well as the required torque on the nuts to ensure the plate is tight against the face of concrete. But I'm a bit unsure about using a solution which there is seemingly no precedent for, at least not in any code or reference about rebar anchorage I can find. It's more comparable to post-tensioning - but we're not using post-tensioning bars here.

Has anyone seen something like this before? And if not, can anyone think of any concerns or reasons this wouldn't work that I'm maybe not identifying?

Figure 1:


Figure 2:

 

[jhnblgr]

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[BridgeEngineer21]

Thanks, but that one is not for a structural application. This external plate needs to be able to transfer the anchorage force (tension in the bars) into the concrete through bearing on the surface.

 

[jhnblgr]

Your question is vague and lacks information, you are saying you need development length but that makes no sense, what calculation are you doing to get this? What is this for?

 

[jhnblgr]

slab implies bearing on grade correct?
I assume then you are putting traffic to the slab edge, how would this relate to anything beyond the edge shear capacity of the concrete? The rebar isn’t really doing anything in this case. I don’t understand why you need to support the edge of the pour.

 

[BridgeEngineer21]

Ahh, sorry for the confusion. No this is a slab in a complex building structure being built in stages, which will have construction vehicles and loading running on it prior to casting the adjacent portion of the slab. So its a cantilevered piece of slab, and there's a large demand on the top reinforcement right up to the edge. So in order for that reinforcement to be effective starting from the free edge, it needs to be anchored

Of course there's other alternatives such as temporarily supporting the slab, or restricting load close to the edge, and we're also in discussion about all of those. But right now my question is just about this specific alternative of anchorage outside of concrete - is it reasonable?

 

[jhnblgr]

I don’t really see the issue, how have you calculated the development length in the cantilever? What is it vs the available length. By normal safety standards you would keep equipment away from openings to start with.

 

[BridgeEngineer21]

Hopefully this can clarify. The structure is quite complex (It's a large project so I am being purposely vague to avoid giving away what the project is) with very complex loading conditions and geometry during this particular construction phase. So the demand at the tip of the reinforcement is not just a simple cantilever moment, it is determined by an FE model.

Basically, for the purpose of my question here, let's take for granted that this demand is accurate and the anchorage of the bar here is needed. Then my question is just, is the external plate a problematic way of achieving it?

 

[HTURKAK]

Your internet country code is DK. Just search the local market for threaded coupler for rebar . One of them ,

Threaded connection MAX FRANK Coupler – MAX FRANK

www.maxfrank.com

 

[jhnblgr]

I think you have the development length theory backwards.

 

[BridgeEngineer21]

Thanks HTURKAK, but the coupler is not the problem. We know the couplers we're going to use. The question is, can a rebar be anchored by being bolted to a plate bearing externally on the surface of the concrete (rather than traditional methods of bonding over a straight anchorage length, or bearing internally on concrete via hooks, legs, or headed bars)?

The bar needs to be anchored before the future slab (in dotted lines) is cast and there is anything to couple the bar into.

 

[BridgeEngineer21]

I think you have the development length theory backwards.

You can't count on the full capacity of the bar to be activated and acting compositely right at the tip of the bar. This is why we specify anchorage lengths, hooks, bends or headed bars.

 

[jhnblgr]

Draw a shear and bending diagram

 

[Enable]

Sounds like OP's case will develop tension in the bar that will exceed the bars ability to develop that tension capacity if it's not otherwise anchored. This occurs at free edges of balcony slabs and the like where we have a large line or point load (and bar area varying across the length). In this case, his construction joint is a cantilever prior to the second slab being poured and he is worried about the construction loading causing this issue. It's definitely a real thing.

I see no reason why your scheme won't work. Thumbs up.

 

[BridgeSmith]

Like jhnblgr, I'm not understanding the issue here. At the load position, the moment is zero, and increases linearly to the support location. The capacity of the bar with partial development can likewise be assumed to increase linearly from the face of the concrete (the increase in development is actually a curve that is a bit better than a linear increase). Unless the temporary cantilever is shorter than the development length of the bars, you should always have more capacity than demand at any point along the bar.

We have dealt with this on the cantilevers of pier caps, where we have concentrated girder bearing loads near the end of the cantilever. Obviously, on a much larger scale, but the principle is the same.

Of course, having equipment operating within a few cm of the edge of the slab is a major safety issue, anyway, so you may want to get clarification on whether that is actually going to be allowed.

 

[JAE]

Sounds like the issue would be just for the top bars if I'm understating this correctly.

Could you embed a steel angle along the top edge with weldable rebar connected to the vertical leg (horizontal leg on top and level with the top of slab)?
Then just install the angle/rebar assembly prior to concrete placement. The assemblies could be in short 5 ft. lengths perhaps to allow for two men to set them.

 

[jhnblgr]

You have a shear force and a moment developing as you get towards fixed end. I would think shear failure would occur prior to the rebar losing bond to the concrete. Precast pretension would fail. I guess provide some evidence of why you think it would be in failure. So you are saying the end concrete will have so much force that it would break and pull out.

 

[Aesur]

I came across these at a structural engineers convention recently, they looked interesting and may work for what you need.

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[Brad805]

The plate idea seems to work for this. Meadow Burke and others make form savers that use NC thread (tested to 1.25fy). You could simply drill your plate for bolts. I am sure someone in your country makes something similar in metric threads.

 

[BridgeEngineer21]

Thanks for the input everyone, much appreciated. The contractor has already identified some materials they'd like to use for this, though the threaded rebar gives me the idea to look into a possibility of rebar threaded beyond the, so I could fit a plate and nut plus a coupler behind it like I sketched.

As for the slab loading, I have been purposefully vague but this is a complex structure loaded in all directions (not just from construction equipment), and right up to the face of this opening we have a demand (based on an envelope of load cases, force effects, and design checks) for ø32 bars @ 150mm. The bars needed to be anchored so they are fully developed from the face of the slab to meet this demand.

I guess I'd like to try to ask the question in a different way because it seems people are getting hung up on trying to understand the loading situation. Imagine any situation where a rebar is needed at full capacity right near the face of concrete. Typically we would anchor that bar using a 90 or 180 degree hook, or a headed bar, embedded in concrete. In theory, is there any reason that anchorage couldn't be provided by an external plate? It seems so straightforward that I'm having doubts why I can't find any examples of it having been done in practice, for example at the edge of slabs over external columns with a large negative moment.