Concrete Slab Design - Reinforcement

[Natedj]

I'm a draftsman for a structural engineering company and I've worked with a number of engineers in my career.
I thought I understood the basics of slab reinforcement pretty well, but the engineer I'm currently working with (a very seasoned one at that) is telling me something different than what I've picked up from other engineers in the past.

This is a two part question about longitudinal and transverse bars as it pertains to an elevated concrete slab (lets say its 14" thick) and its supported by concrete columns.

I've always shown the longitudinal bars in the slab as the bars that are parallel to the the longest side of the entire building slab and alternatively, the transverse bars run the shorter length of the building slab.
The engineer I'm currently working with says that's not entirely true. He says the longitudinal bars should be determine by the column spacing and not the overall size of the building slab. So if we have a building that's 120' wide x 360' long and is being supported by columns on a 20'x25' grid, the transverse bars would be dictated by the 25' dimension span between the columns and not the 360' length of the entire slab. This is only regarding the slab reinforcement and not any additional reinforcement. Any thoughts on this?

And secondly, when placing the bars in the slab I've always shown the transverse bars on the outer layer and the longitudinal bars on the inner layer. In other words, my top mat of reinforcement would always show the transverse bars above the longitudinal bars and the bottom mat would show the transverse bars below the longitudinal bars. Now I'm being told that its supposed to be the other way around. Can someone confirm this?

 

[phamENG]

In both cases it will be up to the engineer as it impact the analysis. It sounds like you're specifically talking about one way slab designs, though a 20x25 is a pretty good candidate for 2-way analysis. The longitudinal bars are typically the bars parallel to the span (i.e. running from one support to the other). I would typically design that as the short direction, so my longitudinal bars would be running in the 20' direction of your 20x25 bay. However, there are different design methodologies out there and the engineer for whom you are working may have a perfectly good reason for detailing it as he is.

As for the ordering of the mats, I would place my longitudinal bars on the outside as this generally increases the strength of the slab. Again, I say generally - the engineer needs to make a decision based on the demands on the structure. This can be highly variable.

Have you asked this engineer why he does things differently? A draftsman/CAD tech who wants to understand what they're drawing can be a valuable commodity. Hopefully your employer realizes that.

 

[Tomfh]

The engineer is correct in that the support spacing is ultimately more important than the overall footprint in determining bar order.

 

[rapt]

There is no logic to "longitudinal" and "transverse" bars controlling anything, or length of building either.

Longest span bars would normally be at minimum cover, top and bottom. But it depends on what the designer assumed in the calculations.

 

[BridgeSmith]

In my line of work (bridge design), longitudinal is the term typically used for the direction parallel to the primary direction the member spans. Usually for a slab, that would mean the longitudinal bars would go on the outside, to get the greatest bending capacity possible for the span in that direction.

The short answer for bar placement is put the bars where the engineer tells you to.

As far as labeling them, if the typical convention for your construction sector is different than the engineer's convention, you should discuss it with him or her, to be sure the design and details will be understood correctly by others who will read or review the work.

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

 

[r13]

I suggest you ask your engineer which way is the major direction (used in design), and place the bars at the outermost layers.

 

[JoelTXCive]

Here's two videos describing the difference between a 1-way and 2-way slab.

https://www.youtube.com/watch?v=SVC2BeqRKGc

https://www.youtube.com/watch?v=VEFk30zcnOs

 

[BridgeSmith]

I must disagree slightly with your suggestion, retired13. I suggest it's better to ask the engineer which layer of steel goes on the outside, rather than asking about the major direction and inferring the order of the layers from the answer.

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

 

[r13]

BridgeSmith,

Agree. Actually I used to hand to the drafts person/technician a simple sketch, so there is no confusion.

 

[Natedj]

Thanks for the responses guys. I did ask the engineer his intent for the reinforcement and his response was mentioned in the initial post. The reason I brought up the question was not necessarily directed to a particular project, I just wanted to clarify the rule of thumb among the design professionals since I know opinions can vary. Like I said, I was lead to believe differently in the past. I guess the "rule of thumb" is limited to the particular engineer who made the rule
Thanks again guys.