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.

 

[OffshoreWindStructures]

I dont think it's enough to stay the pedestal is strong enough to resist the total shear, otherwise you could end up with some strange designs with the shear lug less than 2 inches from the edge etc.

Check whatever code/recommended practice you are following to see if there is a comment on concrete breakout check not being required/a valid failure mode if correctly placed rebar is present. Many codes do discuss the effect of correctly placed rebar on breakout and your code might give a spacing or other requirement to satisfy that condition. then it might be OK by inspection.

Hilti profis also allows you to add rebar in the model for some situations (and so switches the concrete breakout failure mode check to rebar strut and tie checks) , i don't know if profis allows the simultaneous use of a shear lug and explicit rebar. If it doesnt then it's likely an engine limitation rather than a design limitation.


Hilti presentation on profis rebar checks

https://files-ask.hilti.com/original/u8/u8zu0nc1ml.pdf

 

[KootK]

There needs to be some check that covers whether or not your anticipated lug breakout frustum convincingly engages the shear reinforcement in the pier. In practice, this is analogous to doing a strut and tie design and checking that your strut angles are sensible / permissable.

The 3rd Edition of AISC's design guide on base plate anchorage now incorporates the latest research and is massive improvement over the previous two editions of the guide. I'd get your hands on a copy of that if you haven't already.

 

[oengineer]

I dont think it's enough to stay the pedestal is strong enough to resist the total shear, otherwise you could end up with some strange designs with the shear lug less than 2 inches from the edge etc.

Check whatever code/recommended practice you are following to see if there is a comment on concrete breakout check not being required/a valid failure mode if correctly placed rebar is present. Many codes do discuss the effect of correctly placed rebar on breakout and your code might give a spacing or other requirement to satisfy that condition. then it might be OK by inspection.

Hilti profis also allows you to add rebar in the model for some situations (and so switches the concrete breakout failure mode check to rebar strut and tie checks) , i don't know if profis allows the simultaneous use of a shear lug and explicit rebar. If it doesnt then it's likely an engine limitation rather than a design limitation.


Hilti presentation on profis rebar checks

https://files-ask.hilti.com/original/u8/u8zu0nc1ml.pdf

I am using ACI 318-19. I am looking in this code to find use of supplementary reinforcement to help with concrete breakout for shear lugs.

 

[Michel60]

The pedestal capacity aside, you may want to look more closely at the constructabilty of what is proposed. #8 hair-pins will have an inside bend radius of not less than 6Db. Couple that with the anchor bolts, shear lug, and all the ties and I think you'll have lots of pieces colliding with each other.

 

[CDLD]

Oengineer, supplementary reinforcement will help your breakout capacity by a couple %.
You should look into anchor reinforcement rather than supplementary reinforcement.

 

[CDLD]

As an alternative to a strut and tie model, you can count on the ties as anchor reinforcement if they are developed on both sides of the breakout cone:

I wouldn't count on any ties beyond the first three layers.
Also, as a rule of thumb I would only count 50% of the ties as effective given that some of them will have 135 degree hooks that limit the capacity in the tie.

 

[oengineer]

Oengineer, supplementary reinforcement will help your breakout capacity by a couple %.
You should look into anchor reinforcement rather than supplementary reinforcement.

What other anchor reinforcement should I look into besides the ties currently shown in the images I have provided?