Concrete Beam Stirrup Hooks at interior corners of beam - 90° or 135° 1

[ajk1]

Is there any reason to specify a 135° hook on a stirrup rather than a 90° hook when the hook is on the side of the beam that is contiguous with a slab (please see attached)? Wouldn't the 90° be preferable because it is easier to fabricate the beam rebar cage with stirrups with 90° hooks? I understand that 135° hook is required in seismic regions if the hook is on the exterior side of the beam, because the cover concrete in that case can spall off rendering the hook ineffective. I ask the question because I am reviewing/revising our office typical details and I think the detail that shows 135° hook on stirrup at interior corner of beam should be a 90° hook. But I see 135° hooks also on other than our own drawings. Seems to me not the best thing to do, and is just put on willy-nilly. Do you agree that 90° hook is ok at interior corners of beams,even in seismic regions?

 

[hokie66]

I can't remember ever seeing an outward turned hook, in the US or Australia, so I can't think of a reason to use them.

 

[KootK]

- I see a lot of beams in Canada detailed with the bars outward when torsion is nominal. Frankly, I'd always though it to be designer fear/ignorance that resulted in the bars being turned inwards when there's confining slab on both sides.

- I don't doubt for a second that bars turned inwards are better, for the reasons stated above. But I think that the issue is really less about what is best than it is about what is adequate. There has been an industry perception that the outward turned bars are more constructional when beam cages are constructed in place. I'm sad to report that I've never built a beam cage myself so I've little choice but to rely on the advice of others.

- The clip below is from the ACI detailing manual, 2004 edition. At this point, I'm unsure as to whether or not we consider this to be "modern".

- In the precast world, it is quite common to turn hooks such that they are aligned with the longitudinal axis of the member. Again, a nod to fabrication shop constructability. This is likely worse than both inward or outward conditions as you lose the wrap around bar.

 

[KootK]

CRSI doc circa 2015. Perhaps I'm reading too much into the graphical presentation. If so, that presentation is unfortunate.

 

[Settingsun]

I'm curious why people are so against the 135 degree hook? And why is it lazy/ignorant to specify inward cogs/hooks? What are the benefits of the alternatives? Are the benefits lost if you draw the slab reinforcement? Some of the details posted neglect that detail but perhaps it's not important to the discussion (not sure as I don't know what the purported benefits are).

I do have a thought on outward-turned cogs: Inward cogs/hooks would be better for transport of pre-fabricated cages. How do Canadian builders work: pre-fab or stick-build on site? Maybe this is a detail that is a coin-toss if you don't know which contractor is going to win the job.

The Australian code is nowhere near as concerned with seismic detailing as NZ (we refer to the NZ code for real seismic requirements) but even we aren't allowed to use 90-degree cogs to anchor stirrups unless the cover is > 50 mm. I just flicked through the 'Reinforcement Detailing Handbook - Recommended Practice' and the only place where bars were cogged outward into a slab was a band beam but this wasn't a complying stirrup shape. All 'real' beams had stirrups hooked (135 or 180 degrees) inwards. I suspect we'd get ridiculed for specifying what is apparently normal practice in Canada. So maybe best to ignore anyone who isn't Canadian.

Edit: from the Australian code commentary:

 

[ajk1]

Very interesting comments, particularly Agent66's comment about the strut and tie mechanism. I will speak to the engineers in the office to get their comment on that.
It appears that different geographic locations have different practices so it would be helpful to me if respondents said or reminded me of their approximate locations, if you have not already said so. I am in Ontario, Canada.

I have sent an email to the Reinforcing Steel Institute of Canada (RSIC) to get their comment. I will also ask our chief of field engineering who has 40 years of field experience inspecting rebar, for his comments.

I will try to post their answers when I get them.

I believe the beam cages are assembled on site in this area

 

[jayrod12]

Ours are closed stirrups with a 90 to be clear. So they lap around the corner.

 

[KootK]

I'm curious why people are so against the 135 degree hook?

I, for one, am not against it. In fact I do it, just pointed outward in situations that I deem appropriate. If there is a constructability advantage to 90 vs 135, I'll have to confess to not understanding it. That said, it took a field demonstration for me to fully grasp why 180's at slab edges are a problem.

And why is it lazy/ignorant to specify inward cogs/hooks?

"Ignorant" belongs to me but "lazy" does not. I'll handle both for sport though.

Both comments must be taken in the context of the markets (Canada, not AU) of the commentators where we take it as a given that outward hooks are both allowed and, in some cases, preferred by our construction teams. Given that context:

1) Ignorant. Some folks do certain things as opposed to other things because their understanding of other viable alternatives is not complete. That was pretty easy. I feel like Merriam Webster.

2) Lazy. Sometimes a designer will be working at speed trying to get something out the door and they'll pause and ask themselves "Maybe I could make this detail more constructable by turning the hooks outwards?". Then they follow that up by "screw it, I'm in a hurry and I know that inwards will work so I'll leave it". Anybody who hasn't seen this in action hasn't spent much time in a production shop. Same phenomenon gets us a lot of closed stirrups where open stirrups would do.

But, again, all this has to be taken in the context of the marketplace of the commentator. If AU codes tell AU engineers that outward hooks are bad, then I certainly wouldn't fault any Australians for turning all their hooks inwards.

 

[KootK]

Very interesting comments, particularly Agent66's comment about the strut and tie mechanism. I will speak to the engineers in the office to get their comment on that.

Since you're interested in this, and I largely disagree with it, I'll toss my thought on this into the pool for consideration.

Usually the stirrups are intended to be closed to confine the concrete so inwards hooks logically follows to achieve this requirement.

For the purpose of the open stirrup detail being considered, I think that we need to omit discussion of beams requiring serious confinement of internal concrete. The detail being considered would only be appropriate, even in Canada, in a member internal to a slab and not expected to be doing anything fancy durability wise.

Also given you are resolving the internal strut and tie mechanism with the hook anchorage it makes no sense to turn it outwards.

It also would make no sense to turn it inwards. At least not if the strut and tie mechanism that we're talking about is the classic truss model of beam shear resistance. For that purpose, we'd want to turn the hooks such that they're running parallel to the length of the beam in the direction of decreasing shear. And, to my knowledge, nobody anywhere is doing that. Not least because you loose the benefit of hooking around a longitudinal bar. As I mentioned, this is done in the precast world but it's done for constructability reasons and is, in my opinion, a bit sketchy.

 

[KootK]

In my opinion, for a low ductility, torsion rectified, interior beam, the only issue is as shown below. Effectively just an anchorage problem. While there is this mechanical imperfection, it is my feeling that this is one of the many cases in RC concrete where we simply accept the reality that much is empirical. If this can be shown to empirically not work, then that's another kettle of fish.

 

[KootK]

I knew I'd seen this someplace before... It's from a Gilbert presentation that I cherish. Gilbert has obviously exerted a lot of influence in the south pacific marketplace (that all important context). He's brilliant. And something of a purist hard-ass as those in academia and earthquake prone regions naturally tend to be. Notice that even an inward 180 open stirrup is undesirable.

 

[ajk1]

steveh49 - Does the Australian Code say why the hooked stirrup in the second diagram is incorrect?

to Kootk -

- Gilbert seems to be saying that only closed stirrups are satisfactory. There are a lot of non-edge beams in non-seismic areas that have performed entirely satisfactorily over 100 years or more, where the stirrups are not closed.
- when you say "durability" do you mean "ductility"?
- I don'like the lapped stirrup. Seems to introduce extra field work to tie the stirrup up vertically so it is lapped; potential for field errors. I have a feeling that laps should be avoided if possible.
- Like you, I never heard of break-out of concrete in the lower re-entrant corner as shown in your diagram

None of my comments should be taken as "Canadian" or even "Ontario" practice. Parts of Canada are in significant seismic areas. I don't know what general Canadian or local practice is. I believe one of our main competitors in Ontario uses open stirrups with inward turned 135° hooks for interior beams with slab on both sides. Another of our competitors uses closed stirrups for such beams. We currently show open stirrups with 135° hooks turned outward.

The question in my mind is should we change that to inward turned. I note that Gilbert does not show that as an acceptable stirrup. Why? Maybe he is thinking of seismic regions?

I wonder if anyone has ever done actual lab tests on the various configurations discussed, for interior beam with no torsion, to compare how they performed? Perhaps I should call ACI.

 

[KootK]

Gilbert seems to be saying that only closed stirrups are satisfactory. There are a lot of non-edge beams in non-seismic areas that have performed entirely satisfactorily over 100 years or more, where the stirrups are not closed.

You're preaching to the choir here. To clarify my understanding of Gilberts stuff, he's only saying that open stirrups are undesirable when ductility is required.

when you say "durability" do you mean "ductility"?

Yup. Obviously, there are no beams for which durability is not required.

I don'like the lapped stirrup. Seems to introduce extra field work to tie the stirrup up vertically so it is lapped; potential for field errors. I have a feeling that laps should be avoided if possible.

I don't have a problem with it for low ductility and have had contractors request this on numerous occasions, particularly for deep, narrow beam construction.

Like you, I never heard of break-out of concrete in the lower re-entrant corner as shown in your diagram

Ditto. I hate to use the contractor "I've always done it this way..." argument though. Makes me feel like a hypocrite. That said, anecdotal evidence is a valid, if weaker form of evidence than experimental evidence. Were one to consider this in more detail, additional beneficial factors would come to light:

1) The compression fields coming in from the hogged slab via flexure and shear will improve matters.

2) The outward hook detail requires a longitudinal hanger bar which changes the mechanism substantially. Less outward thrust and closer to to pushing straight down.

None of my comments should be taken as "Canadian" or even "Ontario" practice. Parts of Canada are in significant seismic areas.

In my opinion, discussion of this detail cannot be had meaningfully unless you tag it as aseismic/low-ductility.

1) If this is a gravity only beam in Vancouver, outward 90's are okay.

2) If this is a moment frame beam in Vancouver, outward 90's are absolutely NOT okay.

3) Really, if this is a moment frame beam anywhere, outward 90's are probably ill-advised as they will not offer buckling restraint to the top bars.

Why? Maybe he is thinking of seismic regions?

I would bet a thumb that it is for the reason shown below.

 

[ajk1]

ok. I will wait for a response from RSIC before making a final decision.

 

[hokie66]

I agree that Gilbert is a brilliant concrete guy. And much of our Antipodean arguments about beam stirrups use his stuff for reference. One thing he has not had success with, as far as field work is concerned, is the (c) Satisfactory placement of stirrups. Note that he shows the hooks on the compressive side, which would require them on the the bottom near supports, and on the top further out in the span. To my knowledge, that is not done, even for the most pedantic of engineers.

 

[ajk1]

Antipodean!!!

 

[KootK]

KootK vocabulary... successfully expanded. I can use this too. When the Antipodeans gang up on me, as is known to happen, I often want to refer to them en mass. Antipodean will play much better than my default "you people".

 

[hokie66]

You are welcome. Somehow, I thought that was common usage.

 

[Settingsun]

I wouldn't suggest a northerner use that name in vain, unless you want to see a real antipodean gang-up!

Gilbert's satisfactory stirrups are probably a case of finding what academics think is ideal and dialling it back a notch to get the real-world answer. Maybe also the case that minor constructability issues are matters of opinion. AFAIK basic slabs and beams are easy money for reo fixers.

 

[ajk1]

Not terms known here in Canada!

 

[ajk1]

I will ask my niece who lives in Australia if she or her husband know the term.