Rebar slices where tension is changing rapidly

[curvbridger]

The aashto and aci codes allow adjustment of the required reinforcement bar splice length based on the ratio of steel area provided to area required.

I am looking at some structures which include marginally spliced bars in cantilevered concrete bending members. Accordingly, the tension requirements in the member drop rapidly and are significantly lower at the end of the splice than at the beginning.

Has anyone seen a treatment of the performance of splices where the tension requirement across the splice is not constant?

Curvbridger

 

[rowe]

For any loaded beam, the moment (or "tension requirement&quot will vary along the length of the splice(except for beams with equal and opposite moments applied at the ends).

For analyzing a tension lap splice, I would use the highest moment (Mu-max) along the splice length (usually at one end of the splice or the other) to determine As(req'd) and then divide by As(provided) (if you are splicing one #5 bar, then As(provided) = 0.31sq.in.)

Design of a tension lap splice requires some judgement and might require one or two iterations when calculating a "minimum code compliant" lap splice length. My advice would be to be choose a location for the beginning of the splice and then analyze it to see if the calculated splice length is adequate.

I hope this helps.

 

[curvbridger]

rowe,

Thanks for your response. The method you describe is conservative, and is appropriate for design.

My problem is that the splices are part of a bridge we are rehabilitating and are in place. My task is to analyze their adequacy. I don't have the luxury of assuming a constant moment.

Maybe I am wrong, and the state of stress through the length of splice is insignificant in terms of the performance of the splice? It seems intuitively, though, that it does matter. Isn't that why splice length and development length are different (that is, required development length assumes no tension in the concrete along the developing bar)?

Thanks again for the interest.

Curvbridger

 

[rowe]

It seems to me that the development length of rebar is primarily based upon performance through use of the factors (AASHTO 8.25.2-3) Likewise, splice length is also based upon performance (8.32).

For one bar, an ideal splice is assumed to exhibit the same effective As along the entire splice (along the splice length, the effective As of one bar goes from 100% to 0% linearly and the eff.As of the other bar goes from 0% to 100%) Therefore, As-prov. is constant. Since the As-prov. is constant, the As-req'd used in the ratio factor should be the highest As-req'd within the splice length.

For many cantilevered members, designers decrease the number of rebar as As-req'd dimenishes. This situation is slightly different than analyzing a "splice". If this is the case, you must satisfy AASHTO 8.24.1.2.1 as well as the development length. I believe 8.24.1.2.1 is to ensure that rebar is sufficient if the moment diagram shifts laterally due to changes in the beam properties (cracking, construction tolerances, etc.). When analyzing bar cutoffs, it can help a great deal to sketch the moment diagram and the capacity of the rebar.