Non-Seismic Column Ties: 135 vs 90 Hooks

I am not aware of any reason the orange tie in the second example needed to or should have 135* hooks. 90* hooks would have been fine.

I'd have to see the code excerpt.

What I'm used to seeing on the interior (and this is not recent knowledge, say 2000, maybe 2011-2012 when I was studying for the SE-16 hour) is the 135-90 (what they call a crosstie in the figures you've posted) bars going up the middle are flipped at every other tie, so the vertical bars get a 90 degree hook, the next tie spacing up they get the 135 degree hook, etc. And every other vertical bar needs a tie, as well as the corners. That eight bar with the 'diamond' tie is not something I've seen. I'd point you at a recent CRSI that may have better visuals. Mind you, the SE-16 hour exam really couldn't care less about "normal" concrete structures and detailing.

To what Lomardil said, I think the 135 at alternating is required, so that's why there's a 135 on one end and 90 on the other. That or it's :just the way it's always been done:

What you draw up may come back as an RFI/detailed a little differently on the rebar shops (and perhaps not clouded), especially if I'm right about the 135/90 being typical and a 135/135 is harder to place and tie and they'd rather not.

Good thoughts both - thanks for the input.

One alternative idea is that the orange tie in diagram two is considered a crosstie - so therefore it needs a 135 on at least one end. If the other end was a 90 - it wall call for some kind of alternating which doesn't make much sense here compared to the normal crossties.

90 degree are not recommended due to spalling problems.

If the concrete cover spalls off, you have no bond. So definitely not recommended for higher strength, more brittle concretes.

Best to stick with 135 if it is not a problem.

This is fairly recent:

Recommended Details for Reinforced Concrete Construction Part 3: Columns, Fanella, Structure Magazine, August 2019

It's in ACI for column detailing independent of additional seismic requirements.

haynewp - specifically thinking about non-seismic cases here. I'm more familiar with seismic detailing so this is what I'm used to as well - just curious if the standard changes outside of seismic zones. Generally seems like most people use the 135's anyways!

There are a lot of changes in high seismic concrete, particularly in the detailing, and usually there's additional rebar in places where it counts to provide adequate confinement when the exterior cover gets damaged during earthquake induced movements. Ductile concrete is the strategy. Energy dissipation, acceptable structural damage, relatively low hazard to life safety, (more important structures are designed for higher loads, as you probably know, with the objective of being at least plausibly immediately occupiable, like hospitals, prisons, police and fire stations, etc.) You can't fully divorce Risk Category from Seismic Design Category, because they interact or the one (Risk Category, after 5 p.m. we call call it Occupancy Category....) influences the other (SDC).

Similar to steel, provided "R=3, not specially detailed for seismic resistance" is used, or rather, I suppose NOT, Steel has an option to be designed somewhat normally without a lot of the extra seismic reinforcement but it's either a) not allowed or b) only economical in lower seismic regions. Scratch that, let's start over.

What I'm getting at is if you detail a concrete structure for high seismic and it's in a low seismic region, the structure is safe. There are differences, and the seismic detailing and extra rebar or lap lengths and other changes are not necessary in low seismic regions.

Hypothetically you could get a shop drawing in a non-seismic region that's detailed by someone used to seismic detailing and it will have some extra rebar if they don't know the less stringent rules, but I would expect it's unlikely. Maybe if you had a Las Vegas rebar detailer and the project is in southern Arizona?

Luceid said:

haynewp - specifically thinking about non-seismic cases here. I'm more familiar with seismic detailing so this is what I'm used to as well - just curious if the standard changes outside of seismic zones. Generally seems like most people use the 135's anyways!

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That's what I posted from 318-19, that's for non-seismic columns.

haynewp - Ah sorry - misread the section numbers too quickly, thought I saw a section 18 at the front! I think what's confusing about that 135 degree line in my thinking is that the 135 degree requirement there is not referring to the angle of the hooks at the ends of the bars. This is supported by the images on the following page (see the images of the first post) which all have blue rectilinear ties without 135 degree hooks at the end despite coming right after the code reference you posted.

If you go back to ACI 318-11 - they have a different image in the commentary which I think is what they are trying to get across with 25.7.2.3(a) - that the angle between the legs of the tie must be less than 135, not specifically the hooks at the ends. (See below)



What do you think? Is ACI 318 just being confusing again?





lexpatrie - 100% agree, 135's are always going to work to satisfy requirements in nonseismic or seismic territory - that's why I'm exploring this more conceptually than to pose a real change in my detailing. Makes me think that ACI should just revise their imagesto show 135's at all of the rectangular ties to settle it and move on, especially since the hook length for 90 vs 135 is the same, so the only real difference is the rebar fabricator bending a little further...

Once we have 135 on both ends, it may be difficult to actually get the cross-tie around rebars on both ends. Just visualizing the process in my head, a 90 bend on one end will make the process much easier

Luceid, I think you're right but even the CRSI has 135 deg alternating hooks on their standard column detail. The details that come with the ACI detailing manual also show the 135 deg hooks at one end on the internal ties for typical columns. I think I have always seen it that way.

Luceid said:

Hi all - I'm more versed in the seismic world, so I'm very familiar with the 135 degree hooks at the corners of rectilinear ties. If you are not in a seismic zone - can you work with 90 degree bends instead?

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Since the diagrams you first posted have "crosstie" called out at the interior ties and a "crosstie" is defined by ACI below, I would go with the diagrams for non-seismic including where the 90 deg bends are shown. This also matches the other references I found.

But I think their intent should be clarified in 25.7.2.3 paragraphs (a) and (b) to match the diagrams use of a "crosstie" with the single ended 135 deg (seismic) hook at the interior ties. But I wouldn't change what appears to have always been used anyway even if the word "crosstie" does not appear explicitly in paragraphs (a) and (b).

Luceid said:

Could I just specify 135 and avoid the headache? Certainly. But for the fun of the exercise...let me know what you all think.

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If you mean all 135 deg hooks then there may be some complaining from the field if it is in a low seismic area and they aren't used to installing 135 deg hooks all around.

Right - that's more what I was trying to figure out - does everyone just specify 135's or are there parts of the country/world where this would be more of a surprise in the field. Appreciate the follow up here - good discussions

I think in SDC C and below it's typically a 135 on one end and a 90 on the other. I also wouldn't call them "seismic" 135 degree bends. They seem to require a 135 degree bend on the the one end in all situations.

haynewp said:

But I think their intent should be clarified in 25.7.2.3 paragraphs (a) and (b) to match the diagrams use of a "crosstie" with the single ended 135 deg (seismic) hook at the interior ties.

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Source: Fanella, Structure Magazine, 2019 (see link in previous post).

As side note, higher seismic the vertical bar splice locations shift from bottom of slab to middle of column height.

The word seismic maybe doesn't belong but it is coming from the ACI definition of a crosstie.

I'm not sure where the above came from, but the below is from the CRSI Design Guide to ACI 318. It looks like the detailing diagrams from ACI 318 in the original post with no 135 deg hooks at the corner longitudinal bars.