Anchorage of shear reinforcement 2

bugbus · Nov 23, 2020

Just wondering how everyone feels about the two alternative shear reinforcement arrangements below:

The reason for not using the typical 135 degree hooks in this situation is to avoid clashing between the hooks and some spiral reinforcement behind a PT anchorage (not shown). Ideally the shear reinforcement would not intrude into the core of the beam.

Obviously the option on the left is generally not acceptable. In Australia the rule is: "Fitment cogs shall not be used when the anchorage is solely in the outer layer of reinforcement. In this case fitment hooks shall be used." I believe the rule is generally the same across the various codes.

Any thoughts on the arrangement on the right? I believe there must be a point at which the cogged part of the stirrup is long enough to avoid the stirrup losing anchorage when the cover is lost. For example, see below the standard prestressed bridge plank used by the state road authority in NSW, Australia. This strictly does not comply with the rule mentioned above ("Fitment cogs shall not be used when the anchorage is solely in the outer layer of reinforcement").

r13 · Nov 23, 2020

Obviously the option on the left is generally not acceptable.

Then the one on the right is unacceptable for the same reason. Use double stirrups and let the cogs on opposite ends.

hokie66 · Nov 23, 2020

I disagree with r13. The arrangement on the right would satisfy my interpretation of the code.

r13 · Nov 23, 2020

Yeah, I would agree.

Agent666 · Nov 24, 2020

Neither would be allowed round these parts (NZ).

r13 · Nov 24, 2020

Agent,

So when you can't bend the legs 135°, what is your option in NZ?

hetgen · Nov 24, 2020

If you are designing according to the Eurocode read the note below. However, if the beam is subjected to torsion the second detail you showed should be used, and if seismic is in play then the link should be properly tied around the beam steel, both open links you sketched are not permitted/recommended.

EC2 -Clause 9.2.2 said:
Open links may be used for beam and slab construction using L-hooks where the width of rib is 450mm or more. In such circumstances, a top locking link is also used (see Figure 6.21).

Agent666 · Nov 24, 2020

So when you can't bend the legs 135°, what is your option in NZ?

There is no alternative from a 135/180 hook apart from welded anxhorages. There is only one scenario where a 90 degree hook is accepted by our code, but no one uses it in practice because it is an inferior detail to have 90 degree hooks in the cover concrete in the seismic detailing realm. I shudder when I see this is still acceptable in other major international codes with no limitations on use. Guess it depends on what your 'normal' is.

This one time scenario is in beams where a slab is present and this slab stops the return hook from unravelling. Otherwise stirrups/ties/spirals require minimum of 135 dehree hooks in all members (walls/beams/columns/slabs) for every single stirrups.

It never seems to be an issue in practice, there is always a way. Flip the stirrups to locate hooks in alternative location, use a U shaped bucket stirrup with closing link, lap stirrups, weld stirrups, use smaller diameter stirrups that can be more easily be bent on sire, etc, etc, etc.

hardbutmild · Nov 24, 2020

Agent666 said:
apart from welded anxhorages

Is this permitted in seismic areas though? I always thought welding was not allowed in seismic areas at all.

Agent666 said:
It never seems to be an issue in practice, there is always a way.

If the contractor is willing to do it.

Tomfh · Nov 25, 2020

Agent666, what if you kept going and wrapped another side? Are 90 hooks still forbidden? Each time you turn a corner you add another 90 degrees to whatever bar you need to terminate. Do you not reach a point where you beat a single 135 hook?

KootK · Nov 25, 2020

I suppose that, in a reversible hinging event, it's conceivable that you could lose your cover all the way round.

Agent666 · Nov 25, 2020

Is this permitted in seismic areas though? I always thought welding was not allowed in seismic areas at all.

Our concrete code allows you to lap weld stirrups, provided the yield strength of the bar can be developed. For main bars we need to be able to break the bar away from the weld, details are limited to butt welds, no side-by-side lap welds. Obviously, appropriate inspections and NDT is required same as structural steel.

Agent666, what if you kept going and wrapped another side? Are 90 hooks still forbidden? Each time you turn a corner you add another 90 degrees to whatever bar you need to terminate. Do you not reach a point where you beat a single 135 hook?

As per kootk's answer, once the cover concretes gone, your anchorage has evaporated. A circular spiral is sort of like what you are describing, except a rectangular "spiral". A circular spiral requires a hooked anchorage into the core of the concrete section or welded anchorage. It's no different to what you describe when rectangular, you would always anchor your spirals wouldn't you (at least NZ code requires that)?

If the contractor is willing to do it.

They do what they're told.....

bugbus · Nov 25, 2020

"You would always anchor your spirals wouldn't you?"

In Australia, at least, not always.

"Helical reinforcement shall be be anchored at its end by one and one half extra turns of the helix." AS5100.5-2017 & AS3600-2018.

Then there are additional detailing requirements for seismic effects. If the pile or column is relied on to behave in a ductile fashion, there are certain detailing rules such as providing hooks into the core or extending the spiral reinforcement into the pile cap/beam/footing/etc by a certain distance.

For bridges at least, if it falls into the BEDC-1 (least onerous) earthquake category, for which there are no special seismic detailing rules, it is allowed by the code to have the spiral reinforcement terminate at the top and bottom of a pile or column and be anchored by the extra 1.5 turns, without continuing into the pile cap or footing.

Agent666 · Nov 25, 2020

It's interesting that in AS3600, they require splices in separate helices to have the hooks, but at the ends no hook, just this extra 1.5 turns. Seems to be a little mixed messages, let do it half right? If they are so sure the extra turn works, wonder why they don't just splice using similar logic.

But as long as the seismic detail requires the hooks, that's what really matters I guess when the cover concrete is potentially spalled.

I know in ACI you're allowed these 90 hooks in cover concrete, I remember going to a talk by Michael Wallace (least I think that was his name, some bigwig in terms of research in the states regarding performance based design) several years ago where he admitted it is one of the worse details in ACI that they just cannot seem to get rid of because it's just one of those we've always done it that way, despite a mountain of evidence that it is inferior to hooks into the core for compression members especially.

hardbutmild · Nov 25, 2020

Agent666 said:
They do what they're told.....

Agent666 said:
they just cannot seem to get rid of because it's just one of those we've always done it that way

That's exactly what I was aiming at when I wrote "If the contractor is willing to do it.

". In Europe, most of the western and northern countries don't have earthquakes, but a lot of contractors and structural engineers down south (in my country at least) learned in those countries so they never did it that way (they never bent the bars past 90°). And of course, since most of the northern and western countries are the more developed ones, they tend to trust "their way of doing it" more (they believe in the german engineering).
Another problem that I noticed, unrelated to this one, is that people rarely think about the ductility of the reinforcement and don't care if the ultimate strains are 2,5% , 5% or 7,5%.

Thanks for the explanation on the welding of stirrups, it makes sense.

rapt · Nov 25, 2020

For the beam stirrups, AS3600 Commentary shows acceptable options, and they have 135 degree kooks as per the wording of the code. 90degree are undesirable as they are in the cover concrete.

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