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?

 

[Agent666]

An important point to consider is you are battling the guy on site who doesn't care which way round your 90 degree hooks are actually installed, so in my mind they don't really belong on site. I don't really agree its easier to fabricate, you still need to bend the rebar either way, whether its to 135 vs 90 degrees doesn't really factor into the fabrication of the actual stirrup. Arguably its easier to tie in place/accommodate the longitudinal reinforcing though.

The 90 hook is allowed also in NZ (seismic region) where the hook is confined by an adjacent slab or equivalent (our code is also historically based on ACI), but I've never seen it used in practice because its simply a poorer performing detail in the minds of engineers.

Not aware if its performance is similar or degraded, but a 90 degree hook without any slab confining it is definitely a poor performer based on some research I read years ago. The potential in plastic hinge regions for loss of cover and therefore lack of confinement if the hook is not anchored into the core of the beam is very high.

 

[ajk1]

I never said the 90° hook was easier to fabricate; I said the beam cage was easier to fabricate. I thought I took pains to be clear about that.
I never suggested a 90° degree hook be used where it should not. What I did suggest is that 135° hooks seem frequently to be shown on drawings where 90° hooks could be used and still meet all good engineering practice and Code requirements.

 

[ajk1]

I do take your point about the installer on site could install the stirrup the wrong way if one leg of it has a 90° hook and the other leg of it has a 135° hook. Thanks for pointing this out.

 

[Agent666]

I suspect most engineers will be nervous about putting 90 degree hooks in the cover concrete even in the case where they are expressly allowed, so as best practice they simply don't get used or are not allowed. ACI used to allow (may still) intermediate links/legs to also have a 90 degree hook, these also perform poorly when compared to anchored 135-180 degree hooks.

Suspect whomever created your standard notes perhaps did so on this or a similar basis. You give the option to the contractor for selected locations and I almost guarantee you will start seeing them all over the place where its inappropriate to use them simply because it's seen to be easier, contractors love the path of least resistance for themselves even if it's at your expense!

 

[ajk1]

I get your point, namely that it is possible to do the wrong thing with 90° hooked stirrups. But to adopt a procedure that specifies 135° hooks on all stirrups would be to adopt a practice that is clearly and specifically recommended against, by the RSIC Manual of Good Practice (Reinforcing Steel Institute of Canada). That is a group representing the rebar industry in Canada. Their reason for saying to avoid the 135° hook where it is not needed by Code is clearly stated in the RSIC Manual. If I can figure out how, I will copy the relevant paragraph from the RSIC Manua and post it here shortly. So I am reluctant to adopt a non-recommended procedure with the possible negative effects that the RSIC notes.

As to why the details in our files shows 135° hooks all over the place, that is a good example of why I have been called out of retirement and tasked with reviewing them all. They were put together by various engineers of various experience, for specific projects, and it has been many years since this was all reviewed...much of it was unfortunately never reviewed.

As a final comment, I cannot help but recall my mentor from 50 years ago (who was the finest structural engineer I have met in my engineering career) tutoring me not to use 135° hooks where they were not needed. If I say any more people in this area will know to whom I refer.

Thanks for your comments and advice, and your positive contribution to this project. Much appreciated. I do accept your advice not to use a 90° hook on one leg of a stirrup and 135° hook on the other leg, because that can be placed the wrong way.

 

[Agent666]

I guess something else to consider is what are other engineering practices are typically doing in the area where you operate? Similarly what is the level of competence in the rebar fabricators in your area?
If they are used to 90 degree hooks being used and familiar with where they can be used, and you as the engineer are confident in their use being appropriate and its allowed by the code, then I guess there is reason to consider it. If standard practice is 135 degree hooks in your area then asking those on site to change their ways might cause more hassles than its worth for your staff and will ultimately result in additional remedial work on site.

In my geographic area even contemplating using 90 degree hooks would be a complete disaster based on typical onsite practices, the labour base is very low skilled, and historically even though its been allowed by the code we are taught at university to avoid them at all costs.

I can't help but consider that engineering knowledge has actually moved on in the last 50 years also, while on site practice/knowledge has actually decreases in quality over the last 50 years. Doing anything to improve that balance in your favour saves you time and money in the long run.

 

[hokie66]

I would have to look back a few years, but research has definitely shown that 135 or 180 degree hooks on stirrups provide much better anchorage. The construction led trend to use open stirrups rather than closed was the reason for the research. After that, there was a new understanding of the importance of stirrup anchorage, even to the point where some codes specified that stirrup hooks should not be in the tension side of the element. Because that would have required the hooks to be placed so much in conflict with existing practice, that provision never gained a lot of traction, at least where I am.

 

[ajk1]

I just re-read the RSIC page. Looks like I had read it wrong. I accept now what you say about 135° hook on stirrups. Indeed, I see that RSIC shows a 135° hook even at the interior corner. I will see if I can post it here. My apologies.

 

[ajk1]

Looks like I cannot copy it.

 

[ajk1]

Attached is the relevant page from RSIC dealing with stirrups.

 

[Agent666]

Thanks for that, I must say those other details in my opinion those are truly awful solutions from a detailing perspective, and in most practical cases I hope they wouldn't be allowed by any self respecting standard. I'd be interested to hear what the provisions are in the Canadian standard if they make an argument for allowing them, and the limitations imposed on their use.

Our NZ standard has restrictive rules for lapping stirrups in the cover concrete, it is related to torsion/direct shear stresses and is only satisfied in combination with other factors such as you must use deformed bars (usually not the case with stirrups), and must not do within potential plastic hinge regions or beam/column joints (often the whole beam can be deemed a potential plastic region dependent on the location of your hinges), and cannot use where you are reliant on confinement of the concrete. Even if you were to satisfy the requirements I don't think any self respecting designer round these parts would purposely lap in the cover concrete by one of the methods shown when they could simply use a full stirrup or a method like noted below. That type of detailing is totally inappropriate for any country subject to seismic.

If you must leave the top of the cage open, the best option in my mind is to have a main U shaped stirrup with two >135 degree hooks, and a closing link with two >135 degree hooks.

 

[jayrod12]

Where I practice, extremely low seismic, you would get crucified for specifying 135 hooks unless you're resisting torsion. 90 degree stirrups are the norm.

 

[hokie66]

That's disappointing, jayrod. I didn't realize that Canada was that far behind.

Agent666, I am surprised to hear you say that deformed bars are not usually used as stirrups. These days in Australia, deformed bars are the norm for stirrups, N10, N12...

 

[Agent666]

Interesting, there is nothing stopping you using deformed stirrups in NZ, just need to consider the larger bend diameters maybe making some of the detailing a bit harder. Its just not the norm for beams or columns here, one of those always been done this way type of things I guess, so why change. As I understand it the plain round bars can come in coil form here (in 10/12/16mm diameters at least) and are simply unwound straight off the coil into the fabricators cutting and bending processes (mind you you can also get deformed bar in coils but I only see it being used for spirals really in piles). Might be easier to process plain round bars into closed stirrups compared to deformed, I'm not sure?

 

[jayrod12]

Canada was that far behind.

I can't speak for all of Canada, but my local market I can tell you 135s are extremely rare.

 

[ajk1]

Very interesting comments, all, but now I am even more uncertain what to do. I will call the people at both the Cement Association of Canada, and the Reinforcing Steel Institute of Canada and see what they say. As I said earlier, I was told 50 years ago to use 90° hooks on normal stirrups, unless there was a Code reason to use 135° hooks, and it was sloppy practice or laziness to use 135° hooks everywhere. I am practising in an area of Canada that is in a low seismic area...unlike New Zealand. In my career since 1964 we have always used deformed rebar, but in an earlier era plain bars were used. Today we use hard metric bars and the smallest size is 10M or 100 mm². A bit unfortunate that the smallest Canadian metric rebar is larger than the old #3 imperial bar which was about 0.11 in² or 71 mm2.

 

[ajk1]

Another question: When is it appropriate to specify stirrups with the hooks turned inward (type S3 and S6), and when with the hooks turned outward (S1 and S4)? See attached for diagrams of the stirrup types. In my career I have always shown stirrups with the hooks turned outward.

 

[Agent666]

Unsure, I don't think I've ever seen outwards turning stirrup hooks for beams or columns in a modern structure. Usually the stirrups are intended to be closed to confine the concrete so inwards hooks logically follows to achieve this requirement. Also given you are resolving the internal strut and tie mechanism with the hook anchorage it makes no sense to turn it outwards (similar in a way to always turning hooks inwards for longitudinal beam bars in beam/column joints to anchor the strut/tie nodes).

Given you have only ever specified outwards hooks as per your question maybe you can tell us your thinking on always doing it that way?

I'd only consider outwards hooks for secondary reinforcement when it makes sense from a detailing perspective. Many older structures (100 years old type of stuff) do have outwards turning hooks though, but often don't have a top closing tie.

 

[jayrod12]

I'm with Agent666, for me it would make sense that typically it would be inward. One other option I could see for outward would be when you have an extremely wide beam and are putting in multiple stirrups but want to keep the centre of the beam top open for anchors, concrete etc.