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

[Prestressed Guy]

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.

 

[KootK]

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

 

[jittles]

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.

 

[Prestressed Guy]

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.

 

[Prestressed Guy]

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

 

[Prestressed Guy]

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

 

[jittles]

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.

 

[Prestressed Guy]

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.

 

[jittles]

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.

 

[BAretired]

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.

BA

 

[Prestressed Guy]

@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.

 

[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.

https://engineervsheep.com

 

[Prestressed Guy]

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.

 

[Settingsun]

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.

 

[HTURKAK]

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 ).

 

[BAretired]

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

 

[BAretired]

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

 

[Settingsun]

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?

 

[Prestressed Guy]

- 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.

 

[Prestressed Guy]

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.