One Way Slab

[slickdeals]

All:

What is the standard detailing practice for top reinforcement perpendicular to the span of one way slabs when it hits a support.

Example: 5" span spanning 20' between beams. Beams are in turn supported on edge girders. What is the top reinforcement required in the slab perpendicular to the girder and how long should it extend?

One of the examples I saw talks about the reinforcement extending 4'-0" past a 45 degree line drawn from the intersection of the beam and girder.

Unfortunately, couldn't find anything in the code regarding this. Usually, this steel is provided per "engineering judgment".

 

[KootK]

Great question.

1) since it's crack control reinforcing, the theoretically correct thing to do would be to figure out where the rebar is required for crack control and run it out that far. Of course this is almost never done as it's a lot of work and not a strength concern.

2) I've seen standard details requiring as little as (5 x t) extension beyond the girder.

3) I've seen the 45 degree business and like it. I didn't know about the extra 4' but sure.

4) The code has corner reinforcing mat requirements for slabs at stiff supports. This is my favorite way to deal with this. If I remember correctly, the bars extend at least 1/5 the span in either direction. That looks about right to me in plan and is often close to the 45 degree value. For a very long, narrow slabs, the bars get too long and I'll switch back to the 45 degree algorithm.

So yeah, judgement all the way.

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.

 

[JAE]

A 5" slab spanning 20 feet is too thin isn't it?
Per ACI thickness tables for continuous slabs you are looking at a min. thickness of L/28 = 8.5 inches at least.
This isn't anything to do with your question but just thought I'd point that out.



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[slickdeals]

JAE: Slab is prestressed. Clear span is actually more like 18'-6".

KootK: What I was referencing is in the document below (Figure 3)

http://www.post-tensioning.org/Uploads/FAQ no. 5.pdf

 

[KootK]

Ah. I had not realized that it was a post-tensioned slab although, in retrospect, the span to depth ratio is a pretty good giveaway. In the past, I have used the 45 degree + 2' method as described for the pre-stressed "shadow" area in the referenced PTI document. Frankly, I wasn't aware that PTI considered that to be a non-compliant system for T&S.

Your beams are post-tensioned but the slab perpendicular to the span is not, right?

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.

 

[TXStructural]

As a general rule, reinforcement is required to extend a full development length beyond the point at which is it required. For flexure, this may be reduced if there is more steel than required for flexure. Since this application does not have vertical stirrups, the steel will have a role is resisting shear, which means full development beyond face of support.
If this a continuous slab or is cast integrally with the supporting member, or the beam is a steel composite beam, you also have moment at the support and/or need to restrain the slab across the support.

 

[slickdeals]

Ok, I am sensing we are going off-topic. Read OP

 

[TXStructural]

Your OP doesn't specify what kind of girders. You do mention extending bars four feet beyond the supports, which wouldn't work if the slab stops as the support, but you mention "edge girders." If you provide more details, we can provide a more complete answer.

 

[slickdeals]

Hope the attached helps. I am not sure what the kind of girders has to do with the question I am asking, hopefully I will learn why shortly.

 

[KootK]

If you're placing those bars to deal with negative flexure, then my first comment would apply.

If you're placing those bars so that you can omit N-S T&S mild reinforcing and, instead, use the PT in the N-S beams for that purpose, then my second comment would apply.

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.