Lapping of reinforcing bars with different spacings

[gharli]

Hi All,

I understand it is good practice and will make fixing of reinforcing a great deal simpler on site, but is it really required (from any code point of view) to maintain spacing of reinforcing over a change in moment?

i.e. In a wall to wall corner I require 16mm bars horizontally @ 150 spacing (on the outside face). Still on the outside face but moving away from the corner I can comfortably change the spacing to 200 centers. There is a sharp drop in the moment. Is it ok to do this?

Thanks.


_________________
Jones & Wagener

http://www.jaws.co.za

 

[EngineeringEric]

I would drop the diameter of the rebar and maintain the spacing (or multiple of.. 150 on corner and 300 in field) You need the bars to lap for their development length, how would you transfer tension in the bars across that plane?

 

[Klitor]

According to my code... There is allowed clear distance between lapped bars equal to 4*Diameter.
In practice,I never detailed it like that.

 

[KootK]

It could be done one of three ways as far as I know:

1) Remain in compliance with your code's max spacing for non-contact lap splicing.

2) Treat the unconventional lapping as a strut and tie problem and supply suitable vertical rebar to make that work.

3) Use an extended lap length if your code has provisions for that. I believe it is NZ that allows you to space your bars further apart if you add 1.5X the space to the length of the lap.

I detail with #3 and verify with #2 sometimes.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

With your max spacing between bars at 3", it's hard to imagine that this wouldn't work by your code's non-contact lap provisions. You're allowed 6" in the US and Canada and, apparently, 4" in Kiltor's jurisdiction.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Teguci]

I believe the ACI case study that caused this requirement involved the failure of a concrete portal frame at the eave. It's been a while, but I remember the concrete cracked longitudinally along the roof beam splitting between the post developed reinforcement and the roof beam reinforcement.

In my experience, this has been a problem when developing tension from a superstructure element into a drilled pier. The anchor rods are in the center of the pier while the pier reinforcement is around the exterior which could be 12"+ away. To justify the correct embedment length, I would assume a concrete cone failure for the embedded anchors and then make sure I had the drilled pier reinforcement developed by the time it intersected with the plane of failure. Appropriate ties prevent the splitting that happened in the ACI case study.

In your case, Unless you have perpendicular reinforcement to the outside, I'd be concerned with the reinforcement bursting the face of the wall and losing development.

 

[KootK]

In your case, Unless you have perpendicular reinforcement to the outside, I'd be concerned with the reinforcement bursting the face of the wall and losing development.

So long as the reinforcing remains within a single plain, as I believe the case to be here, there should be no cross wall bursting tendency to deal with.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[TehMightyEngineer]

What about bending the bars at slight angles in two places so it offsets into the correct non-contact lap distance. Forgive the ASCII graphic below.

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Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com