Why is a decrease in tension lap splice length not allowed for excess reinforcement?

[Mike Mike]

A situation that arises on many projects: contractor forgot U bars at column tops or installed them at the wrong elevation, poured columns, and then asks me for a correction. My intent is to transfer some moment between slab and column, and I frequently find my column tensile steel is like 5 or 10 times what's needed. A class B lap on a typical column vertical requires a drill and epoxy solution with a drill bit several feet long, and it appears ACI ties my hands by forcing me to develop the full yield strength of the bar: "Reduction of development length in accordance with 25.4.10.1 is not permitted in calculating lap splice lengths." (ACI318-14 25.5.1.4)

ACI408R-03 says "The extra length required [by ACI318] for Class B splices is not based on strength criteria but rather is used as an incentive for designers to stagger splice locations."

Darwin, Lutz, and Zuo say "Although tests demonstrate that splice strength where (As required) / (As provided) < 1 is conservatively represented by the modification factor in [section] 2.5, this factor is not used to increase the level of reliability of lap splices."

I understand the committees want to incentivize me to detail my splices to be staggered in new construction. But in my situation, if I calculate a one foot embedment satisfies the remainder of code provisions and presumably provides a reliability index of at least 3 or 4 (failure probability of one in several thousand), why do we need to punish the iron workers with having to drill several feet into column tops? Shouldn't the committees be striving to achieve uniform reliability across design? Let me know if I'm missing something here.

 

[bugbus]

I have no doubt that there is some linear relationship between lap length and strength (same as for development length, where you are allowed to reduce it according to how hard the bars are working). But one reason the lap length reduction might not be allowed could be ductility (?), I'm not so sure.

Can you not site weld the bars together in that situation?

 

[Greenalleycat]

Our seismic assessment guidelines could help provide some guidance as to realistic ideas of laps

http://www.eq-assess.org.nz/wp-content/uploads/2018/11/Technical_Proposal_to_Revise_C5_30112018.pdf

See: C5.4.4

I imagine you can't actually rely on this but at least there's some discussion in here that may help inform you

Some food for thought though: if you designed and specified some U-bars extending from your column into your slab to develop moment, then the contractor should provide it
Regardless of your numbers, an epoxy solution is never as good on many levels
If it's a minor connection, then you can be a nice guy, but for someone that sounds relatively critical (a permanent gravity load moment connection) is it right to allow the substitution?
The client is paying their money for what you've detailed, not for the contractor's bodged solution to their fuckup

We're dealing with a similar situation today where the contractor cast their duct in the wrong place to meet our starter bar for a precast concrete wall
We said we would accept a solution of them core drilling 1.2m up the panel and epoxy grouting the starter in place IF they could provide us assurances they wouldn't hit steel, and could give us their QA processes etc
They've called us a few rude things and asked why we can't just drill and epoxy starters "like we normally do"
The answer is simple - they've fucked up, and drilling and epoxying is not equitable in performance (under fire or earthquake) to the neatly cast-in starter that we had ready and waiting for them to drop the wall onto
SO, no doubt going to be an expensive day for them

 

[Mike Mike]

Cat, I've been in the same situation. Always cracks me up when contractors f everything up and then ask us how to fix it. My favorite is when instead of calling me to describe the situation they just email me a photo of the disaster.

You're right, the owner deserves the product he paid for, not a mound of duct tape. But if I can find a less expensive fix that's still building code compliant and will not cause serviceability or any other issues, I'd rather not waste the contractor's time and money, although I realize a lot of engineers disagree with me on this one. Also, I just like to understand what I do. Even if it takes my time away from making money, I enjoy digging into the codes.

What am I reading here? the New Zealand code? Are you in New Zealand? Seems to allow lap splices shorter than full bar yield strength even in high seismic concrete columns where inelastic deformations and yielding are expected, if I'm reading this right.

Gus, yes, ductility is a big one, which is why neither splice lengths nor development lengths may be reduced by excess reinforcement in high seismic zones. Good thing most of my projects are low seismic. Is it possible to weld normal A615 rebar?

 

[Greenalleycat]

That guidance isn't for new builds, we just have detailed info for assessing existing buildings. So if you need to calculate the capacity for an underdeveloped bar it will get you an answer where otherwisewise you'll be stuck saying "0 capacity" for something that has 90% but not 100% of code development

I agree with trying to be reasonable and find ways to help your contractors, but I'm not really happy with solutions that require substituting cast in bars for epoxy pwrsonally. If I would accept epoxy I would have specified it originally, they are very clearly differeny systems in many ways.

In your case I'd be inclined to tell them to hydro demo the concerete and repour the top metre or so

 

[phamENG]

Mike - how big are these columns? I've only been in this particular situation a couple times (I don't do much elevated concrete design), and I've never been able to approve post-installed anchors. Drilling that large a hole that deep requires more precision than any mere mortal contractor can provide. Steel will be cut. If this is a particularly large column with widely spaced bars, maybe you can manage.

As for welding A615 - yes, but there are strict preheating rules. You'll have to look into D1.4 for all the details. A706 is intended to be welded without preheating, which is easier and less expensive in terms of field labor, but A615 can be welded with the right prep in a repair situation.

 

[Brad805]

Would something like the HRC 670 end work in this case? HRC 670

 

[KootK]

For an aseismic situation, I'd be willing to wave the "no proration" business.

1) If your u-bars are smaller diameter than your column bars, as I would expect, I think that you only need to splice for the capacity of the smaller bars at most.

2) In many ways, I view this as part of a "joint" moreso than a conventional lap splice situation. In many, common lap splice situations it can be difficult to say with any certainty which areas of the member will see the most tensile demand (think slab or wall bars). In such cases, reliability concerns are created by the potential for some lap splices to get over loaded before redistribution occurs. In your beam to slab "joint", it seems to me that this issue is much less of a concern.

Why U-bars? I would think that bent, L-shaped dowels would be easier and better from a detailing perspective. Moreover, I thought that the drill and epoxy business required dowel torqueing that made it impractical to orient a shop bent bar in any particular direction.

 

[BridgeSmith]

A little off topic, but for anchors that we use when we need to hold down bridge girder ends because there's uplift on the girders, we now detail U bolts specifically so they can't 'forget' to cast them in, and epoxy them in later. We made the change after one particular project where they did their typically crappy job of installing the anchor bolts with epoxy, instead of casting the anchor bolts in as we specified, and the girders popped up when they let the center sections of the girders off the crane. It was a huge mess - recalculating screed elevations, girder stresses, ratings, etc., as well as reviewing (and rejecting several times) the contractor's plans to get the girders back down on the bearing seats.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Mike Mike]

Koot K., changing the bar size is something we have done in the past, although in some cases I think it incentivizes clumsy work. Most commonly we specify the same large bars as the contractor was supposed to have in there per contract documents, but we allow a shorter lap onto the column verts, specifying instead the splice length of whatever smaller bar size would meet moment demands. Yes, for a drill and epoxy fix we specify hooked bars. Drawings specify U bars.

phamEng, typically columns are a couple feet by a couple feet. And yes drilling in a few feet into the column top is gonna jack something up for sure.

Brad, yes HRC is an awesome solution, I will suggest this next time it happens, thanks.

 

[FAZPATEL]

Cat,
What if the hook for splice of the horizontal (shear) reinforcement for shear wall is not confining/cogging around main vertical bars? Do we need remedy for this if minimum straight lap length is met?

Thanks.