Reinforcement around Slab Openings

[miecz]

Article 13.4.2 of ACI 318 (and 350) require reinforcement to be added to the sides of an opening equal to the interupted reinforcing. Our standard detail has always called for this reinforcing to extend to the support. However, in the case of a tank wall, the support for the horizontal reinforcing can be 100 feet away. How far do I need to extend the additional reinforcing?

 

[UcfSE]

It's not really additional. It's just what was displaced by the opening. I'd take it all the way down, or to wherever else the rest of the bars it displaced go. Every other bar has to do the same.

 

[miecz]

Guess I wasn't clear. We don't eliminate the bars that would have passed through the opening. We merely cut them out, at the opening. I believe Article 13.4.2 refers to these bars as "interrupted." So, if we cut six horizontal bars at the opening and add three to the top and the bottom, then, to the left of the opening we have six more bars than we would have had, had there been no opening. Seems like overkill to carry these bars 100 ft to the end wall.

 

[jt12]

I would at least extend bars Ld past the face of the opening in each direction. This is what most of the details I've seen show. If the bars are at a highly stressed region due to tension or out-of-plane moment, I might think of extending the bars splice length or more to help transfer the bar tension between the trim bars and the interupted bars.

 

[UcfSE]

Is this a slab opening or a beam opening?

 

[miecz]

UcfSE-

It's a tank wall. The wall is 25 feet tall and 127 feet long. The openings for pipe penetrations can get up to 36 inches.

jt12-

Yes, the bars can be highly stressed due to out of plane bending. I've been thinking splice length plus hole radius, but 12.14.2.3 of ACI 318 doesn't recognize splices for bars further than 6 inches apart.

 

[pappyirl]

I'm not sure if you can rely on some heuristic rules to determine the reinforcement required for this arrangement. If out of plane bending is significant the quantity of reinforcement should be determined from the analysis of the reduced wall section at the level of the openings.

Generally I would extend the bars a splice length past the opening. I can't understand why you would require the "interrupted" bars to continue to a support. In the case of a tank, the bars directly under the opening must contribute to the section capacity at some stage. Is continuing the bars to the support not doubling up the reinforcement at a lower section?

 

[dik]

I typically use full tension splice development length as the distance to extend the reinforcing beyond the opening.

Dik

 

[jt12]

Since you have out of plane moment you are concerned about: If you are looking for a rational design, you could develop a strut and tie mechanism for the situation (methodology is kind of described in an appendix in the latest 318 codes). You basically need to transfer a tension force from each trim bar to each interupted bar over a distance greater than the 6" limit for an out of plane splice. Essentially, you are going to figure out where each bar is developed for the required tension and make sure there is enough of an overlap to develop a diagonal strut between these two points. Using this methodology, there is some benefit to putting std hooks at the ends of interupted and possibly also trim bars to help cut down trim bar length.

 

[miecz]

Thanks, everyone. I like the strut and tie idea. It's kind of where I was headed with the "splice length plus hole radius." Haven't used strut and tie for anything before, but this seems like the perfect application.

pappyirl-

yes, continuing the discontinued bars and the additional bars to a support provides double the reinforcing away from the hole. That's what we've always called for in the past. I'm trying to eliminate the doubling up, by extending only the discontinued reinforcing to the support.

 

[miecz]

I've finally approached this problem using strut and tie methods, and, well, the result seems excessive to me.

As jt12 said, S&T requires more than twice the development length, i.e., Ld for each bar plus the component of the diagonal strut. For a straight #8 horizontal bar, it works out to 90 inches, or about twice the splice length.