Reinforcement Development Length

[MHEngineer]

I am having issues understanding why the concrete design codes ACI or AS3600 particularly allow shear ties to be fully developed with a cog/hook C8.2.3.4 while any other reinforcement is reduced by 50% by the hook/cog as per AS3600 chapter 13 C13.1.2.6.
AS3600 commentary states that the fitment relies on the transverse compressive pressure for anchorage, is there any reference for this?
The issue stems from the problem when doing strut and tie modelling and the bar needs to develop right at the edge, could I use the same logic as tie/fitment reinforcement if I have an inclined strut at the node such as the one found in shear tie truss model
PS what I usually do is provide a U-bar where I need direct development as per below


Which I generally reinforce for as such


My two questions would be then:
1. Why does AS3600 (and I think ACI) allow a shear tie to have full development directly from the cog and hook but any other reo can only have 50% due to cog and reo and remaining needs to be developed conventionally.
2. In an STM model as the ones above, can I use a U-bar to obtain full development when the node is directedly at the edge? surely a U-bar enclosed a transverse bar is better than a hooked/cogged bar where the code allows for development for shear.

Cheers

 

[bugbus]

My understanding is that the longitudinal bar in the corner of a fitment improves anchorage of the fitment in two main ways: (1) it acts as a mechanical anchorage to the fitment, distributing the concentrated force at the bend over a larger area; and (2) it controls splitting in the plane of the cog/hook. I think the cog/hook being in the compression part of the beam is of lesser importance - I can't remember the specific requirement but from memory I think there was a previous clause requiring the fitment spacing to be increased if the hooks were in the tension part of the beam. This may have been removed recently, but I'm sure someone can confirm.

For a strut-and-tie model, I think that as long as the tie is anchored in the same manner as a fitment (i.e., hooked around another bar with the same or larger diameter), you can assume it is fully anchored at the bend.

 

[MHEngineer]

Hi Bug us, thanks for your response.
I believe AS3600 requires both shear tie and a cogged bar to enclose a longitudinal bar of equals or larger size. However the distinction is that the shear tie would be considered fully developed while the cog/hook in any other case is considered only 50% developed.
The commentary states that the transverse compressive stress is what allows for the shear tie to be counted as fully developed and that is the code assumption.
I am trying to understand why that is and how it could apply to other situations other than beam shear ties. Also to make sure I do not pull the shear tie card in an non conservative situation.
Do you know of any research on the effect of the transverse compressive stress or quantitative effects of the longitudinal bars. Lately I am finding journal papers providing mechanical models for cog development, wondering if there are such model for this case as well
Much appreciated your help!

 

[bugbus]

I see your point now. I would say it is probably a matter of bar size then. It is not stated anywhere in the codes, but the requirements for fitment anchorage seem to be intended for smaller bars sizes only (up to N20 say). I had previously asked this same question to Ian Gilbert who confirmed this is the intention of the code. Noting that as bar size increases, the bar strength increases as a function of d_b², whereas its surface area (therefore, the effective bond strength) only increases as a function of d_b. Beyond a certain diameter, achieving 100% anchorage simply by bending the bar around a larger bar must stop being feasible.

If you look at ACI 318, which I believe is where much of the Australian Standards' anchorage requirements originate from, it introduces a penalty for #6, #7 and #8 fitments (19, 22, and 25 mm), whereby in addition to providing a longitudinal bar enclosed in the bend of the fitment, there is an additional requirement that the fitment needs to extend a certain distance beyond the bend before it can be considered fully anchored. For whatever reason, the Australian Standards don't put any upper limit or detailing requirements on fitments larger than a certain diameter.

This is just my opinion, but I would be comfortable assuming 100% anchorage for bars up to N20 if they are detailed like a fitment. For anything larger, then probably best to treat it like a standard hook/cog, or alternatively provide some kind of mechanical anchorage if the strength is needed.

 

[MHEngineer]

I see what you are saying, very interesting to know that ACI has a penality on larger bar sizes. As you said sounds very logical.
I still though can't find a reference that backs up What AS3600 commentary says regarding full anchorage being assumed due to transverse compressive field, as unlike ACI it doesn't make a reference to full anchorage being achieved only due to transverse corner bars.