Continuous Reinforcement Requirements

[umrce]

This is an incredibly tedious question so bear with me.

I'm trying to terminate some negative moment reinforcement and AASHTO Section 5.10.8.1.2c states that At least one third of the total tension reinforcement provided for negative moment at a support shall have an embedment length beyond the point of inflection not less than the greast of the following....

The bars I'm terminating are #5's @ 6" but instead of taking them out to the inflection point, I was going to terminate them early (per the general requirements of 5.10.5.1.2a) and use the continuous temperature and shrinkage reinforcement (#4's @ 12") to meet the 1/3 requirement.

Unfortunately 1/3 of 0.62 in^2 is 0.207 in^2/ft so a #4 @ 12" violates this ever so slightly. Would any of you be OK with this? I don't feel I can justify basing it on the required steel because the code clearly states that the 1/3 is based on the total steel provided, not required.

 

[BridgeSmith]

I wouldn't put my stamp on it. I don't believe the shrinkage and temperature reinforcement counts as tension reinforcement, anyway. I'm fairly sure the intent of the spec is that you extend 1/3 of the #5 bars that are your tension reinforcement beyond the inflection point the specified distance.

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

 

[umrce]

Thanks Rod

 

[Settingsun]

I wouldn't have a problem provided your #4 bars are laid out so they will be effective as strength reinforcement. I'd also be happy to base on required reinforcement, with anything extra being considered to have been provided for other reasons.

BTW, for your other interpretation (ie contrary to mine above), you would have to add the #4 and #5 bars and provide 1/3 of that. ie if your #4 bars can meet the 1/3 requirement then they have been 'provided' as strength reinforcement.

 

[BridgeSmith]

As I reread your OP, I'm wondering why you have S&T and design tension reinforcement. As far as I know, the reinforcement in the faces of a beam or slab has to meet or exceed the S&T steel requirements, but it doesn't have to be provided in addition to other steel. You should be able to provide the what's required for the negative moment, and terminate some of the bars past the inflection points, leaving enough to meet the S&T requirements.

Honestly, when we do continuous beams, we never terminate any of the negative moment reinforcing. The rebar is so cheap, it's not worth the hassle for us to detail it, the rod busters to place it properly, nor for our field people to make sure they placed it properly. With all of that, there's still the potential for construction errors; it's just not worth it. We terminate some of the positive moment steel at the columns, just to cut down on the congestion between the column steel and the tension steel in the bottom of the beam/cap. Of course, with bridge substructures, the inflection points can move quite a bit with different live load patterns (truck positions).

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