Reinforcement Laps - RC section resistance 3

[kostast88]

I've seen a few different opinions on the matter, so would like to ask the community as well.

In a RC moment joint, (say culvert wall base) what is the reinforcement for Section Resistance (bending)? Can we add the starter bars to the main wall bars (lapped) to determine the resistance?

If so, what is the detailing condition for that (length required for strength + anchorage length or pure lap length)?

 

[HTURKAK]

The starter bars SHALL resist to the moment developing at joint.. In case box culvert, the starters could be combined with exterior reinf. in U shape depending on the height..

Pls look to the following reinforcement detail to get the concept..

https://www.boxculvert.com/manufacturing/reinforcement/

If this question is for a specific case , pls post the details to discuss the probable best reinforcement set up..

 

[kostast88]

The reinforcement arrangement is as shown. No option for alternative (U-bars etc).
We use rigid joint assumption, so the values are taken at an offset from the joint.
Starter bars are fine, its the wall I'm trying to satisfy.

 

[HTURKAK]

It is not clear if the sketch for a box culvert .. Will you pls post the full section?

If the wall for a box culvert, you should need span reinf .( at interior face) .. If the exterior reinf. ( H20@ 125 mm ) is necessary for the full ht, the lap length should be adjusted for the splicing of full reinf.

 

[centondollar]

"In a RC moment joint, (say culvert wall base) what is the reinforcement for Section Resistance (bending)"
That depends on what way the moment is directed. Is the moment opening (angle >90 degrees) or closing (angle <90 degrees) the joint?


"Can we add the starter bars to the main wall bars (lapped) to determine the resistance?"
If you ensure sufficient lap length for development of full bond, yes. This may be done using standards. The easiest way to avoid this issue is to avoid making lap slices in a moment joint.

"If so, what is the detailing condition for that (length required for strength + anchorage length or pure lap length)? "
You design to some code, I presume. Look it up there. The basic idea is to ensure that concrete is bonded to steel (idea: "Force in steel bar = bond stress*perimeter*lap length --> Fs*As=tau*pi*d*L.lap") and that stress transfers from one rebar to the other (this is found in the standard you design to).

 

[ggcdn]

Maybe I misunderstand the issue, but at the location of the arrow, your wall bars are undeveloped. They do not contribute to bending resistance at that location and you need to design the hooked dowels for 100% of the moment.


-JA
try [link calcs.app]Calcs.app[/url] and let me know what you think

 

[KootK]

Can we add the starter bars to the main wall bars (lapped) to determine the resistance?

Yes. We had a pretty good discussion of this issue here including the sketch below by yours truly.

 

[Settingsun]

I interpreted the question the same way as Sonofatkins. At the green section, the capacity comes from just the starter bars (H20 @ 125). You can't double the reinforcement in your calculation to be 2H20@125 because the wall bars aren't developed there.

 

[KootK]

I interpreted the question the same way as Sonofatkins. At the green section, the capacity comes from just the starter bars (H20 @ 125). You can't double the reinforcement in your calculation to be 2H20@125 because the wall bars aren't developed there.

Oh... in that case I change my answer. The rebar under consideration should simply be considered a lap splice.

 

[BridgeSmith]

I don't think you're allowed to count lap spliced bars as more than one bar, unless the overlap is more than the required lap.

Unless the starter bars are extended alot farther up the wall to provide 'positive' moment (tension on the inside face of wall) reinforcement, in the case under consideration, only one bar can be counted. The max 'negative' moments will occur at the corners and a 'positive' moment may occur near at the midheight of the wall (depending on the span/height ratio), but no matter how you assess it, there's only one bar at the critical sections at the corners.

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