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24" x 24" precast columns with (4) #11 vert reinforcement
5

24" x 24" precast columns with (4) #11 vert reinforcement

24" x 24" precast columns with (4) #11 vert reinforcement

(OP)
For 24" square precast columns I would prefer to use (4) #11 bars (ρ = 1.08%) due to the simplicity of the cage with no need for crossties. When I run it in Bentley software with seismic provisions checked it comes out as a good design.

I mentioned that to another engineer and his response was "you can't use a 24" column with only 4 bars in seismic design (SDC D). When pressed he could not sight a reason but was adamant about not using (4) #11 bars.

Opinions? Yes / no and why.

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

Someplace in the latest 318, I'm pretty sure that I've seen a max requirement for bar spacing. 14" maybe?

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

KootK is on it, but I think it's even more restrictive than 14"... I want to say 8" off the top of my head.

Edit - with certain axial load limits, so you may not need it for a lightly loaded column.

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

(OP)
I know of a maximum spacing between ties around bars in a single plane so that if you have verts at 6" oc you only need to wrap a tie around every other vert.

Min number of vert bars Per ACI318-19 10.7.3.1(b)Four within rectangular or circular ties

Also relevant to this discussion is that these are precast columns and detailed per 18.14 "Members not designated as part of the seismic force-resisting system.

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

(OP)
In the PCI Design Aid 3.13.2. Column interaction curves, they show (4) #11 with øPn = 1450k and øMn max over 700k-ft.

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

(OP)
@jittles do you have a section reference for max spacing?

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

18.7.5.2

Please correct me if I'm wrong... I have this down but I don't actually have the code in front of me at the moment.

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

(OP)
Section 18.7 is titled Columns of special moment frames These columns are pin connected top and bottom and not a part of the seismic force resisting system. Section 14.14.4.1 does not refer to 18.7.5.2.
18.7.5.2 is titled "Transverse reinforcement shall be...
It deals with the max spacing between laterally restrained bars. The graphic shows Xi as between interior verts along the face of the column. I do not see anything that says that there needs to be more than just the corner bars. It could be argued that he 14" limit also applies to the distance between the corner bars but there is noting in figure R18.7.5.2 showing that. If that were the case, the figure should have a "14" max without added face bars" on the dimension of the corner bars.

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

Well it seems my confusion was equal to your colleague's - I would have said SDC D, but you may be correct that it's specially a lateral element in SDC D.

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

I see nothing wrong with a 24x24 column with 4-#11 bars, one at each corner. Bars are well supported against buckling when they are at the corners.

Maximum spacing requirement came up once before in thread507-280906: Max bar spacing in concrete columns.

BA

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

(OP)
@BAretired, Yep, spacing starts at 6" full height and gets tighter from there depending on loads. That is another reason to avoid the interior bars because they all need cross ties.

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

I'd be of the opposite opinion to BAretired and yourself,

What you're proposing is an awful detail for seismic coming from a seismically active country, the ductility and moment curvature you'll get out of it will be below what is expected/intended by codes. Real lack of confinement as well, they require the cross ties for a reason you know...

Even for non-seismic, most codes including ACI I think require at least 8 minimum longitudinal bars in a rectangular column to give some degree of confinement to the core concrete. Then as others have mentioned there are the spacing limits of bars along an edge which may add the requirement for further bars.




https://engineervsheep.com

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

(OP)
ACI318-19 10.7.3.1(b) requires a minimum of 4 longitudinal bars in a rectangular or square column. Precast columns are pinned at the top and bottom so the only moment in them is due to eccentricity of the vertical loads. Given that they are in SDC D, they need to be detailed per 18.14.4 which requires ties at 6" max spacing for the full height of the column section. Section 18.7 relates to Special Moment Frames which are an entirely different animal.

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

Sounds as though there's no requirement for extra bars, but I'd go for 8 bars with the diamond ties for the bars in the middle of the faces. Still easily built and better performance under overload.

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

Quote (Haydenwse

For 24" square precast columns I would prefer to use (4) #11 bars (ρ = 1.08%) due to the simplicity of the cage with no need for crossties. When I run it in Bentley software with seismic provisions checked it comes out as a good design.

....Opinions? Yes / no and why.)


My opinion is NO !!!..

I understand your approach .. When you look ACI 318, the use of (4) #11 bars (ρ = 1.08%) is O.K...

I will suggest the use of 8 no bars with cross ties.

The reasons are;

- The use of small dia longitudinal bar is better for seismic design,
- One of the seismic code limits the spacing btw. longitudinal bars 25x dia. of Transverse reinforcement.( think about the reason).

When you use 4 bars , in this case , the transversal reinf dia should be 21/25=0.84 in. ( #7 bars ).

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

The snip below was taken from the CRSI publication "Columns by Ultimate Strength Design", prepared by R. C. Reese, copyright 1967. Some might say it's a little out of date, but the same document is available on ebay today. At that time, 4-#11 in a 24" x 24" column was permitted.

Actually, the table goes even further, with the largest square column listed as 40" x 40", reinforced with 4-#18S bars.

BA

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

Quote (Agent666)

I'd be of the opposite opinion to BAretired and yourself,

What you're proposing is an awful detail for seismic coming from a seismically active country, the ductility and moment curvature you'll get out of it will be below what is expected/intended by codes. Real lack of confinement as well, they require the cross ties for a reason you know...

Even for non-seismic, most codes including ACI I think require at least 8 minimum longitudinal bars in a rectangular column to give some degree of confinement to the core concrete. Then as others have mentioned there are the spacing limits of bars along an edge which may add the requirement for further bars.

There may be some validity to that opinion. I come from an area with virtually no seismic activity. However, it seems to me that if that is a valid concern, one would think it would be spelled out a little more clearly in the ACI and NBC codes which, unless they have changed recently, do not have a requirement for more than 4 bars. If such a requirement now exists, it must be followed.

Edit: By the way, vertical bars do not provide confinement to the inner core of concrete. In fact, they need confinement themselves, which is provided by the ties.


BA

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

The Australian code also has no maximum spacing requirement for longitudinal bars. The commentary shows the situation though, so not clear to me why the maximum horizontal spacing of bends/hooks in ties to confine the core (and therefore spacing of longitudinal bars) isn't the same as the limit on tie spacing along the length of the column. If you have 4 bars in a giant column, is it just accepted that the proportion of unconfined concrete is the same as a smaller column although the actual unconfined area grows in proportion to the column size?

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

(OP)

Quote (HTURKAK)

- The use of small dia longitudinal bar is better for seismic design,
- One of the seismic code limits the spacing btw. longitudinal bars 25x dia. of Transverse reinforcement.( think about the reason).

When you use 4 bars , in this case , the transversal reinf dia should be 22/25=0.88 in. ( #11 bars ).

Can you provide a code reference to the 25X bd provision you mention. I am not aware of that one.

If all things were equal and design decisions had no cost implications I would might look at things differently. There is some beauty on a cage with a bunch of vert, interlocking hoops and cross ties. When I am designing the boundary elements on Special Reinforced Concrete Shearwalls, I have all of that because that is the requirement of the code.

In this case, these columns are as close to a pin connection at the top as can be designed. The connection is a flat top column with the beams sitting on top of bearing pads with the connection made by 1¼"ø threaded rods up through 3" diameter PT ducts with filled isolation sand for the first 3" to allow for translation at the beam/column interface. There is no induced moment at the top other than what is created by the minor difference in the vertical load on each side of the column.

But this is not a perfect world where design decisions have no cost implications. I work directly for a precaster so I am very aware of the difference in cost between a column with (4) #11 vs (8) #8. While there is only a 2% difference in the weight of the verts, there is 2 times the labor in handling the bars and 3x to 4x the labor in tying the transverse reinforcement. It also adds a significantly to the weight of transverse reinforcement with cross or diamond ties.

If more bars are required, I will not hesitate to add them but so far I can find no code requirement that drives me to that solution.

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

(OP)

Quote (steveh49)

The Australian code also has no maximum spacing requirement for longitudinal bars. The commentary shows the situation though, so not clear to me why the maximum horizontal spacing of bends/hooks in ties to confine the core (and therefore spacing of longitudinal bars) isn't the same as the limit on tie spacing along the length of the column. If you have 4 bars in a giant column, is it just accepted that the proportion of unconfined concrete is the same as a smaller column although the actual unconfined area grows in proportion to the column size?


Interesting graphic. We do not have anything like that in the ACI and it gives food for thought. In looking at the plan view of the effectively confined core (ECC) for the (4) bar column it shows significant reduction of area which would lend credence to using more bars.
But when you look at the perspective view which shows a (4) bar column, you see that the ECC reduces to minimum between the transvers bars but has less reduction at the tie. Given that my max tie spacing is 6" and 4" has not been uncommon for higher loads, the actual ECC will be significantly greater than what is shown on the plan section.

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

Those three cross sections on the left of the image are taken at the ties. Between ties is less confined. Though, for your 6" tie spacing, it would hardly change. What drives that tight spacing?

Edit: what's the length of the precast column? I want to compare to Aust code and we have extra requirements depending on column clear height.

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

Vertical bars are primarily stressed in compression and would buckle outward if not tied. Ties prevent vertical bars from buckling. Ties are stressed in tension. The concept of vertical bars providing confinement to the concrete is wrong! Without ties, the vertical bars would simply push the outer concrete cover away from the core, reducing the concrete area. The only confinement provided to the inner core is provided by ties, not vertical bars.

BA

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

Quote:

The only confinement provided to the inner core is provided by ties, not vertical bars

Confinement is provided by the bends in the ties, which have longitudinal bars in them. If there were a horizontal limitation on spacing of confinement tie-bends, that would implicitly limit the longitudinal bar spacing. There doesn't seem to be such a limit which seems odd to me.

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

Quote (steveh49)

Confinement is provided by the bends in the ties, which have longitudinal bars in them. If there were a horizontal limitation on spacing of confinement tie-bends, that would implicitly limit the longitudinal bar spacing. There doesn't seem to be such a limit which seems odd to me.

Concrete in a column is not significantly confined by reinforcement. When concrete is confined, its compressive strength goes up by a factor of 3 or more. Column ties prevent vertical bars from buckling. That is their purpose and that is all they are intended to do. They manage the job quite well in a square column with four corner bars.

BA

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

Confined concrete is more ductile, but I agree that minimum confinement such as being discussed does little but restrain the reinforcement (a minimum level of ductility). I'm just curious why there's a (seismic) requirement for close-spaced ties to restrain the reinforcement but no requirement in relation to the concrete which is carrying the majority of the load. I think the Aust code is similar. OTOH, Agent666 and Hturkak have said their codes are more stringent so maybe ACI/AS just lagging.

BTW that image I posted shows square columns. The confined part of the cross section would be smaller for rectangular columns and could be quite a small part of the total area.

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

Quote (Haydenwse
Quote (HTURKAK)
- The use of small dia longitudinal bar is better for seismic design,
- One of the seismic code limits the spacing btw. longitudinal bars 25x dia. of Transverse reinforcement.( think about the reason).

When you use 4 bars , in this case , the transversal reinf dia should be 21/25=0.84 in. ( #7 bars ).

Can you provide a code reference to the 25X bd provision you mention. I am not aware of that one.)


The spacing of longitudinal reinf. is limited at some codes and recommended practices but for seismic design . In your case if the subject columns are not part of SFRS , you may feel free and follow ACI 318 strictly.

Some code references;

- When you write IS 13920-2016 ( it is free ) you can see the following snip ;



- Another snip from other code ,



- Excerpt from Recommended practice NIST GCR 8-917-1,

As shown in Figure 5-16, ACI 318 permits the horizontal spacing between legs of hoops and crossties to be as large as 14 inches in columns. Confinement can be improved by reducing this spacing. It is recommended that longitudinal bars be spaced
around the perimeter no more than 6 or 8 inches apart. According to ACI 318 - 21.6.4.3, vertical spacing of hoop sets can be
increased from 4 inches to 6 inches as horizontal spacing of crosstie legs decreases from 14 inches to 8 inches.





- Excerpt from ( https://www.structuremag.org/?p=10415 )Confinement of Special Reinforced Concrete Moment Frame Columns


As importantly or perhaps more importantly, the center-to-center spacing between laterally supported bars is restricted to a short 8 inches. In the absence of high-strength concrete or high axial loading, the maximum spacing goes up to 14 inches. In ACI 318-11 and prior editions, the 14-inch limitation used to apply to the center-to-center spacing between legs of hoops and crossties.

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

2

Quote:

Edit: By the way, vertical bars do not provide confinement to the inner core of concrete. In fact, they need confinement themselves, which is provided by the ties.

Yeah I meant if you had an intermediate bar it would obviously require a tie and therefore would be a better confined section overall. Wasn't implying simply adding longitudinal bars enhanced the confinement.


How come ACI CL18.7.5.2 isn't applying here? Seems to make it pretty clear that you could be down to 8" spacing required for longitudinal bars when you're dealing with higher loaded columns (Pu>0.3Agf'c), or have higher strength concretes. Otherwise you're required to comply with the 14" [350mm] limit which a single bar in the corners would not comply with in a 24" [610mm] column. Not up to the play with all these SDC's though so maybe this does not apply. But this clause would be what I would expect to see based on NZ code (given it is based on ACI in the deep dark past).

Curious, how are you detailing these 'pins' in the columns that you refer to? Or is it you deciding they are a 'pin' vs it really not being a 'pin'?
Concrete is very rarely exhibits pin like behaviour in typical continuous construction, it's a fantasy to assume otherwise if it's built in a monolithic manner. Secondary structure like this still has to go along for the ride, and be detailed appropriately to the seismic provisions, it's just you might not be strictly reliant on it as part of the lateral resisting system.

We saw two collapses and significant loss of life in NZ in 2011 due to designers through the 1982-1995 period historically being able to treating the gravity system as a secondary system that did not require the same level of detailing as the actual lateral system, nor consideration of the compatibility in terms of deformations. Think in one structure that suffered total collapse with the largest loss of life (115 people) it had 400 diameter columns with 6x20 diameter bars with close to 50 cover with R6 [6mm] spiral at 250 ctrs. That type of detail does not stand a chance when subjected to inelastic demands (but at the time of the original design it was compliant with code due to some bright spark succeeding in substantially relaxing the rules for about 13 years). We spent the next decade after this code change was reverted back ignoring the problem that we created (she'll be right), and the next decade attempting to strengthen this building stock in an effort to reduce the risk of collapse due to this non-ductile column issue (she wasn't alright).

Remember codes are a minimum requirement, they have been wrong from time to time. I reiterate 4 bars in the corner of a column that size subject to seismic loads is an awful detail even if it is allowed by a code. You're allowed to educate yourself and use judgement to go beyond the minimum where best practice dictates otherwise.

FYI, we've required at least 8 minimum bars in a rectangular column since the 1970's when our codes embraced the capacity design philosophy, this requirement in our latest concrete code can be relaxed for low loads <10% of Agf'c and/or when the resulting bar spacing after reducing the number of bars spacing is still <150mm. So basically it really only practically applies to smaller columns in practice.

https://engineervsheep.com

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

(OP)

Quote (Agent666)

Curious, how are you detailing these 'pins' in the columns that you refer to? Or is it you deciding they are a 'pin' vs it really not being a 'pin'?
Concrete is very rarely exhibits pin like behaviour in typical continuous construction, it's a fantasy to assume otherwise if it's built in a monolithic manner. Secondary structure like this still has to go along for the ride, and be detailed appropriately to the seismic provisions, it's just you might not be strictly reliant on it as part of the lateral resisting system.
You cannot get much closer to a pin connection than this.

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

2
I'm looking at the ACI 318-14. ACI Section 18.14.3.2 requires columns not designed as part of the SFRS (in SDC D-F) meet the requirements of 18.7.5.2. 18.7.5.2e) says "Reinforcement shall be arranged such that the spacing of long. bars laterally supported by the corner of a crosstie or hoop shall not exceed 14 in". That's probably the code requirement for not having only (4) bars in a 24" column.

Go Bucks!

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

.1 Is a 90o hook deemed less effective in containing the concrete than a 135o hook?
.2 Does a 90o hook contain the main (exterior) stirrup?
.3 Is it usual practice to alternate 90/135 stirrups vertically?



BA

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

Quote (BA)

The concept of vertical bars providing confinement to the concrete is wrong!

Quote (BA)

Concrete in a column is not significantly confined by reinforcement.

I'd disagree, and I think ACI would also. As straub46 mentioned above, seismic detailing requirements for non-participating columns in SDC D+ specifically require closer TIED long bar spacing (depending on DCR of the column) and also closer tie spacing at the head and foot of the columns due to flexural demand during drift. There may be some exceptions if you can prove that rotation of the foot/head of column is under some value....can't remember off the top of my head.

The intent is to maintain the integrity of the 'confined core' (Ach) of the column in the event that the column is "rubble-ized". In my mind, during a design level earthquake, you might end up with some columns that are essentially a cage full of rocks. And a cage full of rocks with at least some remaining capacity is better than 4 #11 bars and ties at ~12"(?) without any rocks. Ties keep the long bars from buckling, yes, but the tied long bars also form a sort of mesh/cage with the outer hoops. With bigger long bars, the tie spacing can be increased as bar buckling stress goes up.

The same concept is utilized for boundary elements of special conc shearwalls. It is not uncommon to use 3-4" long bar spacing and 2.5-3" tie spacing, with every vertical bar tied with 90/135 hooks, for boundary elements of shearwalls in SDC D+. There's a lot of info in ACI 318 commentary and some of the NEHRP documents.

This is a big item that plan reviewers always needle us on.

But...for OP's precast column...it sure does look awfully pinned, as opposed to a cast in place situation. I've never dealt with precast.

OP, in RAM concrete column module, check your settings and make sure you have SDC set to the correct category and frame category (yes, in the column module, not ram frame). It usually freaks out about the tie spacing, but I'm not sure exactly if it looks at long bar configuration.

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

(OP)

Quote (dold)

OP, in RAM concrete column module, check your settings and make sure you have SDC set to the correct category and frame category (yes, in the column module, not ram frame). It usually freaks out about the tie spacing, but I'm not sure exactly if it looks at long bar configuration.
Your comments are all points that I have run through in my personal debate before I posted that question.

I used Bentley RAM column module with Intermediate seismic provisions. and it shows "OK". The building is system is Intermediate Precast Shear walls around the perimeter and several interior wall lines. The roof structure is precast double tee joists with cip topping. It is a stiff building.

If I was working within the IS 13290-2016 code as posted above and the SFRS was Special Reinforced Concrete Moment Frame, I would not have asked the question because the plan view A calls out hc and Bc to be 300mm max on a four bar column. Given that it is a intermediate shear wall structure with lots of shear capacity I would probably still be where I am on the four bar columns. There just is not a lot of drift in these columns and no significant induced moment.
ACI figure R18.7.5.2 shows xi as the dimension from corner bars to face bars OR face bars to face bars. Bc1 and Bc2 show dimensions from face of outer ties on both axis, but for the life of me I cannot find anything in the code that references those two variables. To my mind, there is a significant difference between corner bars and face bars and if the code intended to require a max dimension for corner bars without additional face bars it would have had language to that affect.

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

Quote (OP)

To my mind, there is a significant difference between corner bars and face bars...

What would you say that difference is given that the face bars in question would be tied?

Quote (OP)

...and if the code intended to require a max dimension for corner bars without additional face bars it would have had language to that affect.

I feel that you may be:

a) Giving the ACI code development team too much credit in assuming their work to be error free and;

b) Brining a measure of confirmation bias to bear upon the issue.

A precast "post" is an interesting animal in that, with common detailing such as yours, it is not possible to use the rebar in the column for either compression or bending at the column top. So any longitudinal reinforcement that you do have is primarily there to deal with the potential for buckling at mid-height.

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

(OP)

Quote (KootK)

I feel that you may be:

a) Giving the ACI code development team too much credit in assuming their work to be error free and;

b) Brining a measure of confirmation bias to bear upon the issue.

A precast "post" is an interesting animal in that, with common detailing such as yours, it is not possible to use the rebar in the column for either compression or bending at the column top. So any longitudinal reinforcement that you do have is primarily there to deal with the potential for buckling at mid-height.

I cannot argue with any of your points. The one that holds the most influence on my though process is the final one about precast posts being a different sort of animal. I have sent the question to both ACI and PCI technical to get a reading. It will be interesting to see if my membership dues will get me an answer.

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

Well, please do report back with your answer. I do a fair bit of precast work myself so I certainly sympathize with your predicament. My guys would even be clamoring for the outer hoop tie hooks to be a pair of 90's.

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

Are 90o stirrup hooks as effective as 135o hooks?

BA

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

I would suggest not, but don't have a reference. The 135 deg bend takes the leg into the area of confined compression shown for the AS3600 print above, so therefore should be much better, I would think.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

(OP)

Quote (BAretired)


.1 Is a 90o hook deemed less effective in containing the concrete than a 135o hook?
.2 Does a 90o hook contain the main (exterior) stirrup?
.3 Is it usual practice to alternate 90/135 stirrups vertically?
.1) In SDC D and above, we are required to use 135º hooks on ties and stirrups. Yes, I do believe that the 135º hook is more effective than a 90 because the tail stays anchored in the core wereas the cover is expected to crack and/or shed away in large deflections.
.2) The column hoop ties have 135º hooks so that they are anchored into the core.
.3) It is SOP for the crossties when used to be swapped end-for-end so that the 90º end is not aligned on the same bar.

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

If we are to believe the diagrams below, where cross hatching indicates confinement, it appears that untied vertical bars in the middle sketch contribute nothing. The right sketch indicates that straight ties with 135 degree hooks each end confine the concrete. Can we say the same for a tie with 90/135? Should 90 degree hooks be permitted in ties?

BA

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

BARetired,

ACI318 specifically covers the 90 vs 135 degree requirements.

Alternate 90 degree is not allowed above a certain concrete strength, load ratio and for Special Moment Frames. Clause 18.7.5.2 in the 2014 version

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

Okay rapt. Thanks!

BA

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

Testing typically shows the 90 degree hooks are less effective because once the cover concrete has spalled in hinge regions that a good deal of the confinement and anchorage is compromised. As you no longer have anchorage of the stirrup or tie leg into the core concrete. Things literally start to unzip from there in a rather predictable manner if you keep cycling things back and forth.

It seems to me to be something ACI should get rid of and retire for the sake of improved detailing and performance. Would imagine the 'we've always done it this way brigade' would have numerous issues with this though. Been to a few talks over the years by visiting American academics where they've mentioned it as something that should really be avoided based on their research findings, especially in walls where the core is less confined anyway and loss of confinement may have a more detrimental effect on wall stability.

In NZ for comparison we can only use 90 degree hooks for transverse reinforcement in beams when there is something confining the hook like a slab (for the top leg of a stirrup with hook returning down vertically). In practice I've never seen anyone actually utilise these provisions though. Otherwise everything is required to be and actually detailed as a 135 degree hook minimum. Even 50 years ago when we started copying/adopting ACI I don't think we ever followed suit in allowing the 90 degree hooks based on never having seen any older plans with 90 degree hooks.

https://engineervsheep.com

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

Agent, do you run in to problems with constructability? I'd think the main reason for allowing 90deg hooks is so you can thread the 90deg part of the tie through the cage and kind of 'snap' it around the opposite bar. Whereas with two 135deg hooks you'd have to assemble the entire 'layer' of ties and thread it down around the long bars? That said, I agree that 135deg hooks are more robust when considering the little 90deg hook of a #3 or #4 bar resisting the buckling of a #8 bar or something, once the cover is gone. 135's hook still stays hopefully embedded in the core.

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

For internal stirrups, which consist of a single bar hooked each end, a bar with good anchorage (large washer or square plate) at each end would provide better confinement and would not be difficult to install, but I suppose contractors would complain about the cost.

BA

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

(OP)

Quote (dold)

Agent, do you run in to problems with constructability? I'd think the main reason for allowing 90deg hooks is so you can thread the 90deg part of the tie through the cage and kind of 'snap' it around the opposite bar. Whereas with two 135deg hooks you'd have to assemble the entire 'layer' of ties and thread it down around the long bars?...

This is why we use the crossties as often as we can instead of a diamond ties or interlocking rectangular ties in precast columns when we have face bars. The cage is built on a frame away from the casting bed . The cage is built by supporting the two corner bars and then putting all of the hoops over then end. These are then distributed along the length and tied off to the top corner bars. Once this is done, all of the rest of the verts are threaded in and tied into place. Last, the crossties are hooked over the verts at each tie. The 90º bend is required to do this process. Any other scheme would be much more fiddly (read labor dollars).

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

Since this is a precast column, a certain symmetry will be lost if the column is cast in the horizontal position. Concrete will settle under the upper 2-#11 bars, leaving a gap below each bar. When the column is erected on site, bars on one side will have poorer bond than those on the opposite side. I don't know whether or not this could result in a significant difference.

BA

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

the reason for consideration of 'top bars' and bond.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

In Manitoba, I'd run with 4 bars, in BC, I'd use 8 with 'diamond' ties if it was my project.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

Quote:

Agent, do you run in to problems with constructability

You get round it by similar methods to which haydynwise eludes to. Prefabricated cages for columns are commonplace, full stirrups can be broken down into a u shaped stirrup with 180 or 135 degree hooks with a closing link with 135 hooks, tie off corner bars after links are in place. In joints often every cross tie and even the outer stirrup is built out of individual links. Smaller diameter hooks 6-12mm can be site bent at a pinch if there is adequate access to do so.

It's the only way we've ever known with 135 degree hooks so we don't really see it as an insurmountable issue.

https://engineervsheep.com

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

Quote:

I don't know whether or not this could result in a significant difference.

You'd always need to consider the orientation at which a member is poured when determining splice lengths. There is an increase in splice lap lengths in most codes to cover this.

https://engineervsheep.com

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

Agent666,

But in the present case, there are no laps. The vertical bars are all of equal length. I wonder if the bars which had been on top during casting, would feel the applied load at a different rate than those which had been on the bottom. There is imperfect symmetry in the column which may tend to make the "pin" less perfect.

BA

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

Quote:

I wonder if the bars which had been on top during casting, would feel the applied load at a different rate than those which had been on the bottom.

I don't believe so, in my opinion it wouldn't be something I'd be too worried about (nor ever worried about in practical design sense).

High bond stresses are only really a consideration for splices where the trapped air bubbles you've alluded to impact development length. Development length in compression may factor in this scenario, but there is obviously enough length over a full storey, the actual bond stress would be the same on the 'bottom' and 'top' bars, and obviously below the capacity in ultimate limit state design. OP can comment on what the bar stress actually is under their compression loads, I bet nowhere near yield in pure compression though where bond is an issue?

I don't personally think it's that great a pin until you fail the slab over at least, as beam rotates under load to act as a simply supported beam it just attempts to rip slab apart in the gap I would have thought. Start cycling it back and forth under seismic and the shear in slab through the gap will obviously cause the slab some distress/damage. The sand is potentially likely to be a durability issue long-term depending on how protected this location is from weather. If a carpark for example, a nice big crack in the slab and lots of water trickling through and wicking into the sand and you've got some great conditions for corrosion of those through bars.

You have to wonder generally if a 'good' pin in one direction, how pin like is it in the other direction (I'm assume 2-3 bars probably here across beam width here to make it less pin like in the orthogonal direction), and whether this results in some concerns regarding the stability of the system as a whole.

https://engineervsheep.com

RE: 24" x 24" precast columns with (4) #11 vert reinforcement

If a 1200WB with 9 bolts is a pin, I'd accept this as a pin.

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