Use of ties & vertical rebar reinforcement to help with Concrete breakout strength of shear lug

[oengineer]

I have a shear lug/key that I am working on designing using Hilti Profis for a concrete pedestal. However, Profis is telling me that the shear lug is failing in concrete break out strength. My pedestal is 44"x44" & 40" tall, with (28)-#8 vertical rebars (see images). The tie reinf. is also shown in the images.

My shear forces acting on the pedestal are the following: Horizontal load on Pedestal (Hx) = 143.8 kips & Horizontal load on Pedestal (Hz) = 225.6 kips

If the pedestal itself has been designed to resist the shear forces based on the pedestal section, concrete strength, vertical rebar & all the ties....would it be safe to consider that the verti. reinf. & ties would be adequate to prevent concrete break out for the shear lug/key?

Suggestions/comments are appreciated.

 

[BridgeEngineer21]

Jumping in a bit late here, but thought I'd share my thoughts on breakout reinforcement, which I believe would benefit from a clear design guide by ACI as the guidance in ACI 318 is vague and to simplistic in my opinion.

For the shear reinforcement, when ties or stirrups are used, do you all believe that multiple rows of ties could be used? Based on Fig R17.5.2.9b, I only see one layer of reinforcement, and they mention for shear hairpins that the reinforcement is to be as close to the surface as possible, so multiple layers wouldn't be allowed.

The requirement for hairpins to be as close to the surface as possible originates from some experimental research which is reported in the reference book Anchorage in Concrete Construction (Eligehausen et al, I highly recommend getting this reference if you want to understand anchorage design better). Here's an excerpt:


Basically the requirement for hairpins is because they work through direct engagement in the anchor (which is why they don't need an embedment length in the breakout cone), so the further down you place them, the more the anchor needs to deform before it engages them.

Surface reinforcement, on the other hand, is working by tying back the failure cone of concrete, so the governing factor should be whether it has sufficient anchorage beyond the failure plane. So you can certainly use more layers of reinforcement, but you have to consider how much they're actually anchored. See the sketch below.

 

[HDStructural]

Thanks for the response. I understand the logic behind T = V(1+e/z).

I see that this equation comes from book that you referenced. Do you assume that the increase in force due to eccentricity (1+e/z) is covered by the phi factor, or do you still use phi = 0.75 and increase the required force by 1+e/z?

Do you typically assume that the vert dowels in the piers act as the edge reinforcement required by ACI 318?

Thanks,

 

[BridgeEngineer21]

That requirement is in Eurocode 1992-4.



The force would be checked against this anchorage length in the breakout body. fbd is the design bond strength which has already been applied with a material factor. The material factor is roughly comparable to the ACI phi factor, a bit different in application, but the principle that its accounting for uncertainties and tolerances in materials, construction, etc is the same. A known eccentricity in load application wouldn't be considered an uncertainty covered by it.



ACI 318 is less developed regarding the design of anchor/supplementary reinforcement (though they do have some good figures for detailing). Basically everything about it is in one section of the commentary, R17.5.2.1. This sentence here implies the same thing as Eurocode equation 6.6, that the shear force should be increased for eccentricity.

 

[BridgeEngineer21]

Regarding the edge reinforcement, it depends on how your vertical dowels are placed (are they L- or U-bars with a horizontal leg). The main requirement is that the edge reinforcement should be perpendicular to the shear force, across the breakout plane.



Eurocode has a bit more detail about the edge reinforcement as well, basically it should be designed for the tie force that develops if you do a STM to distribute the shear force from your anchor out to the surface bars you're relying on.