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.

 

[BAretired]

Okay rapt. Thanks!

BA

 

[Agent666]

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.

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[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? 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.

 

[BAretired]

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

 

[Prestressed Guy]

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).

 

[BAretired]

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

 

[dik]

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

 

[dik]

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

 

[Agent666]

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.

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[Agent666]

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.

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[BAretired]

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

 

[Agent666]

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.

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[Settingsun]

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