Mechanical Bar Splicers and Drilled Shaft Spiral Cage Clearance

[jbuening]

This may be a contractor issue, but figured I'd get some insight from other designers on here. We did some plans for a bridge that used 6' diameter drilled shafts with #14 vertical bars. The contractor ordered the bars straight with the ends threaded where the mechanical splice occurs just above where the drilled shaft and column meet. With a #14 bar, the diameter and length of these mechanical splicers are quite large. The spiral cage is #6 with a 4" pitch. I think he is going to have issues getting these mechanical splicers in there when the cage is in place, and field bending a #14 bar is near impossible. Is this a scenario where the contractor should have ordered a slight double bend, like _/- that, near the end so that the splicer clears the cage (this would put the centerline of the bars slightly further towards the center of the column), or is common practice to modify the pitch of the spiral in the region of the splicers (increasing pitch to clear splicers)?

Admittedly my construction experience is limited to smaller bars where the vertical bars are just bent slightly in the field so that the splicers clear the cage. This contractor doesn't appear too experienced with large bars in a column scenario and is asking for advice on how to proceed. I'm merely curious if you do any special detailing with large bars using mechanical splicers in columns, and if you have any field experience on how this is handled with large bars. Thanks!

 

[hokie66]

So is the column above the same size as the drilled shaft? And with the same reinforcement, including the spiral? Maybe a sketch would help to explain your problem.

 

[Lomarandil]

I don't know if this would limit the effectiveness of the spirals, but could you have the spirals terminate just above and below the splice location, then use hoops in that region?

 

[SlideRuleEra]

What is the outside diameter of the rebar couplers the Contractor plans to use?

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[jbuening]

Shaft is #14 bars and column is #11, so it is using transitional mechanical splicers. Shaft is 6" larger diameter than column (6' shaft, 5.5' column). The splice occurs 2' and 4' (alternating splicer locations) above the top of the shaft. Attached is partial drawing.

The diameter of the splicers is an inch larger than the #14 bar, so 2.7" or so.

Again, its not the my job as a designer to find a solution but was just curious if other designers on here do details differently when you get into large bar size and splicers so that they don't interfere with the cage....or is this a situation where the contractor needs to identify and plan accordingly.

 

[BUGGAR]

We do big piers and columns for marine work. As designer, we make sure it is possible to make sure that whatever mechanical splices are available will work. It may be the Contractor's responsibility for some of this stuff but if we give him an impossible situation, we've bought it. It is not unusual that rebar requirements set the size of beams and columns on heavy work (without architects). We don't bend big bars.

 

[jbuening]

Buggar, so how do you handle mechanical splicers and spiral cages when dealing with large diameter bars? Do you set spiral pitch to be greater than a typical mechanical splicer length (and typical can vary depending on type of mechanical splicer used), increase spiral cage diameter in region near mechanical splicers, or any other solutions?

I'm always looking out for potential construction issues and never try to push it onto the contractor. All example plans I've found with large bars have them detailed just as we do, so I know this isn't an uncommon situation. I am merely trying to determine a way to eliminate this issue on future projects and to pick other peoples brains on this matter.

 

[SlideRuleEra]

jbuening - I understand you are using this problem as a learning experience, and commend you for doing that. As a former Bridge Contractor, the design diameter of circular columns needs to be coordinated with the availability of suitable circular concrete forms - don't just pick a diameter that (theoretically) works.

Allow as much concrete cover for reinforcing steel as is reasonable, with the expectation that field solutions to problems can safely reduce the cover, if necessary. On this column, I see that the rebar has only 2" cover - no "wiggle room" at all. If the cover were thicker, Lomarandil's suggestion could be an easy, practical solution.

As a retired Owner's Representative, I agree with BUGGAR's statement. IMHO, the Engineer's design showing mechanical splices neatly (and unrealistically) fitting between 4" spirals is misleadingly simple. Also, the Contractor should have thought through the construction aspects and questioned the design before purchasing the rebar accessories. I would equally split financial recovery cost between the Contractor and the Owner. (Note: On a sophisticated project, such as this appears to be, it would not be unusual for the Owner to pay for a problem of this nature, instead of squabbling with his Engineer about whose fault it was). However, behind closed doors the Owner and Engineer should have a frank, constructive discussion about how to prevent this type problem in the future.

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