Standard hooks on circular ties.

[jwkilgore]

Both ACI 318-11 and -14 require that the ends of independent circular ties terminate in standard hooks. This is a significant change from the old lap terminations.

I work in the power transmission industry, and standard footing designs are typically circular drilled piers. For large engineered poles, typically there is a standard outer circular reinforcing cage with a long internal anchor bolt "cage" at the top (usually made of threaded #18J bars on the order of 10-ft long). This cage will typically have templates of steel rings at the top middle and bottom instead of ties. For the past several years the clients I've been working with have used spiral ties for circular concrete piers. The design is based on standard tie strength parameters, so the spacing is the wider tie spacing.

Now I'm transitioning to clients who use independent ties, and I'm getting pushback on requiring the hooks at each end. I've seen drawings issued very recently with the old lap splices, using the old "but we've always done it that way" argument.

The problem is that the hooks interfere with the anchor rods. The AR template is usually 3" clear from center of bolt circle to edge of template. This, plus allowing for the hook (even a 180° hook), is pushing the concrete diameters up a size from what they are used to.

My solution is to go to spirals designed as ties (quicker, cheaper, etc.) But again, "we've never done it that way".


Just for clarification, does anyone know of any exception in ACI 381-14 to allow for the old lap splice designs for in-ground circular piers in a non-seismic area?

To further complicate the issue, these structures usually aren't required to meet IBC requirements. They are designed to IEEE C2 (National Electric Safety Code), which literally states "Foundations, settings and guy anchors shall be designed or be determined by experience to withstand the loads in [list of load tables]..." and that's it. But the current "standard of care" is the ACI 318, and I'm really hesitant about not following it.

 

[JAE]

In ACI 318-11 - see 1.1.6 - where it states that 318 does not govern drilled piers.
For piers see ACI 336.
For piles see ACI 543.

Under 336 - they require vertical bars to be spliced per 318 but don't mention ties.



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[BridgeSmith]

For bridge columns and drilled shafts, we rarely use ties (hoops) unless we absolutely cannot use spirals. Their detailing is several decades out of date.

If they insist on using the hoops that are anchored, but end hooks are unacceptable, there's always the option of mechanical or welded splices.

 

[jwkilgore]

ACI 336: My understanding here is that any topics directly addressed by 336 are controlled by 336. Otherwise everything defaults to 318.

Splices: I'm pretty sure I could sell spirals easier than splices.

Out of date: You'd better believe it. Some clients are still building and installing lattice towers originally designed in the 70's. Designed by hand using graphical truss analysis, with full-scales models tested to destruction. BUT, again, this type of construction is specifically excluded from the IBC and seismic never controls. So they haven't been forced to do any major updates.

 

[JLNJ]

Here how I think about it:

The ties are there to prevent the bars from buckling under high axial compression loads. Or maybe flexural compression in a doubly reinforced member.

If your foundation element is reinforced with a nominal number of bars, say for net tension or a small amount of flexure, the ties probably are not needed to prevent a bunch of fully-developed compressions bar from buckling.

If you do in fact have a very, very high load and need the bars to carry significant compression, then design it as a column with properly developed ties. If not, don't sweat it.

 

[jwkilgore]

For engineered poles, especially anchor poles, the moments are very high with relatively low shear and minimal axial load. Foundations in the 5,000-ft-kip and up range are common, with the occasional pole over 10,000-ft-kip. I'm sure bridge piers go higher, with high axial also, but we're up there.

But again, this is a soil-concrete interaction. The initial moment is transferred from the pole base plate through a circular anchor rod cage to an outer reinforcing cage. The pier diameter has to be 1.5'-2.0' larger than the steel pole diameter to meet all clearance and cover requirements. Unless we're in rock or very stiff soil, the soil part of the equation controls everything. The vertical steel is usually determined by deflection requirements. Shear strength does sometimes require additional ties along the length, but it goes from the "concrete carries everything" minimum to "concrete needs a little help from the steel" minimum.

We never have any concrete strength requirements as high as those of a building moment frame or large bridge in a high seismic area. We never have a situation such that we would need to worry about confining the core in a destruction-level event.

Honestly, because our governing code doesn't directly reference ACI 318, and because this industry is generally considered exempt from IBC requirements, I'm having a hard time finding a solid reason to force the clients to change their standard detail. I'm still going to push for spiral ties, and make my objections clear if they don't change. But in the end we're working for people who are engineers themselves and with long corporate histories of satisfactory service with lap-spliced ties.

If anyone else comes up with any new info I'd love to read it. Thanks!

 

[BridgeSmith]

In regions with low seismic potential (Category A or 1) we allow standard lap splices for spirals in both drilled shafts and columns. In the higher seismic regions, we require continuous spirals. This requirement is typically met using coiled reinforcing (a long 'roll' of deformed bar), which apparently is common enough that it is typically used even when we don't specify continuous spirals.

Edit: To be correct, I should have substituted "the AASHTO bridge design spec" for "we" in the paragraph above.

 

[jwkilgore]

Right, here's an example creating #6 spirals from coiled stock:

https://www.youtube.com/watch?v=HaebUjzbMik

 

[BridgeSmith]

Cool video. I figured it was something like that. Thanks!

 

[dauwerda]

To add to what JAE stated, section 1.4.6 of ACI 318-14 specifically states that it does not apply to the design and installation of drilled piers (and concrete piles) unless they are in seismic categories D, E, or F. I also attended an ACI workshop when the 318-14 came out in which they specifically addressed this topic and said their intent was not to require hooks on hoops unless it was a high seismic region.
That said, it does point you to ACI 336.3R which in turn points you back to "may be designed using reinforced concrete in accordance with the provisions in ACI 318 and applicable local building codes." Note the "may", not "shall". I take this all to essentially mean the requirement is not exactly spelled out in the code and engineering judgement should be used for how the hoops are detailed.

 

[slickdeals]

@dauwerda,
Can you elaborate more on the issue about not requiring hoops for low seismic areas from the workshop? Chapter 25 doesn't seem to distinguish between seismic or non-seismic, looks like a standard detail for circular columns. Refer to Section 25.7.4.2.4.1 and Figure R25.7.2.4 (Page 440).

 

[jwkilgore]

This information definitely makes me feel better.

I'm reading here that the actual legally-binding wording in the code does not adequately convey the intent. Hopefully the next edition will address this.

 

[slickdeals]

Don't see any change to Section 25.7.2.4.1 in ACI 318-19 (Public draft).

 

[dauwerda]

@slickdeals,
Note, I stated it does not require hooks on hoops (hoops would still be required unless you were designing plain concrete, which ACI 336.3R does allow). The exception is not made in chapter 25, it is made in chapter 1. Section 1.4.6 states:

"This Code does not apply to the design and installation
of concrete piles, drilled piers, and caissons embedded
in ground, except as provided in (a) or (b):
(a) For portions in air or water, or in soil incapable of
providing adequate lateral restraint to prevent buckling
throughout their length
(b) For structures assigned to Seismic Design Categories
D, E, and F"

The commentary then points you to 18.13.4. for supplemental requirements for piles, piers, and caissons in seismic categories D, E, or F.

I would say this makes it pretty clear that the ACI 318 does not apply to drilled piers unless you are in a high seismic region, and this is what was pointed out in the workshop. (They were also using this as an example of how important it is to read the first chapter of this code (and most others as well).

 

[Bobby46]

CRSI has a good article on it in their Detailing Corner series. It is called RFIs on Circular Ties, Rotating Hooks, Staggered Lap Splices and Closure Strips.

https://www.construccionenacero.com...u11/ci_31_detallado_del_acero_de_refuerzo.pdf

 

[jwkilgore]

@slickdeals could you provide the page and line number of the start of the section defining "complete circular ties"? I'm going to submit a comment, but I don't have an ACI membership so I can't access the draft.

Thanks!


[EDIT: I can't seem to find a way to tag someone or to send a private message. Is this possible? If not, can anyone else get the info?]

 

[slickdeals]

IBC 2015 Section 1810.3.2.1.1 specifically says that for SDC C and higher, we need to have the ends of hoops, spirals and ties to terminate with a seismic hook.
You don't have to be an ACI member to access the public draft.
Link below:

https://www.concrete.org/publications/standards/upcomingstandards.aspx

 

[jwkilgore]

I found it and downloaded the document, but it requires a "license key" to open.

 

[BridgeSmith]

Mine also asked for a license key to open the "318 Public Discussion Draft", but not the other 2 documents listed.

Edit: I just looked at the page again and the required license key for 318 is given right above the list of documents - it's "concrete".

 

[jwkilgore]

Well, duh... "concrete"... missed that one.

25.7.2.4.1 Did NOT change, as stated earlier.

BUT!
1.4.6 (now 1.4.7) changed. It points to 13.4 for deep foundation design, and the commentary now states, "In addition to the provisions of this code, recommendations for ... drilled piers are given in ACI 336.3R ...". So now ACI 318 DOES cover drilled shaft design.

13.4 has been greatly expanded, pulling a lot of information out of the IBC. You now have full design requirements for drilled concrete shafts.

18.13.4 (now 18.13.5) has also been greatly expanded
Specifically 18.13.5.4, "For structures assigned to SCD D, E, or F, hoops, spirals, and ties in deep foundation members shall be terminated with seismic hooks." Seismic hooks are defined as minimum 90° bends for circular ties.

I'm going to submit a comment on 25.7.2.4.1 referencing the exception in 25.7.3.3 and the statement in 18.13.5.4. It will request an exception to the circular tie hooks for deep foundations in seismic zones A, B, and C.

Probably won't go anywhere, but we'll see.