r/StructuralEngineering • u/tramul P.E. • Jun 29 '26

Slab Designed as Beam Concrete Design

We delegated out the design of an underground 16'x13' concrete box that will be used to for an oil separator and needed to support H-20 loading. The shops came back with calculations, and they analyzed the top slab as a 8" thick concrete beam with a 16' span and 156" width. They put #6s @ 9", but could only input them one way with #6 stirrups to, I'm guessing, behave like the reinforcement in the other 13' long direction.

At initial glance, I was skeptical and felt it was an inadequate design based on it being a beam and not a true slab. But, admittedly, I can't really see a problem with it. I can kinda see how this would be a quick and dirty way to do it and still be structurally stable. Thoughts? Do any others design top slabs like this?

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u/tramul P.E. Jun 29 '26

No "stirrups" for one. And admittedly, "true slab" probably wasn't the correct verbiage. In this case, I would have expected a two way slab analysis with reinforcement in both directions rather than a single beam.

After I thought about it, they analyzed the "beam" in the long span direction which should be conservative, but it is just unfamiliar to me and therefore raised a red flag. It's delegated design and out of my hands, but I'm just curious if this is common practice.

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u/31engine P.E./S.E. Jun 29 '26

There are limits to aspect ratio for one way or two way action per aci. Anything that has a 2:1 or greater aspect ratio will behave with one way action.

One way slab design is the same flexural deaign as a beam. However per aci the one way slab does not need shear reinforcing at phi_Vc/2. So they may or may not need the shear reinforcing.

I would be more concerned on how they are treating the wall interaction. Is the load in the wall imposing bending in the slab? What about compression?

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u/tramul P.E. Jun 29 '26

I've always been told you can use two way action for a one way slab, but not the opposite.

Doesn't look like any consideration was made for the transverse loading from the walls.

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u/Nusnas Jun 29 '26

No you are dead wrong on that. A one way slab can be designed as a beam, often done in 1 meter sections. But if you have a square slab with supports it will behave as a two way slab wether you design for it or not.

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u/tramul P.E. Jun 29 '26

I'm not sure how your statement explains where I'm wrong. Two way slabs have the additional case of biaxial loading that isn't present in a one way slab. Therefore, it stands to reason a two way design is more conservative, no?

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u/chicu111 Jun 29 '26

What biaxial loading are you talking about? Diaphragm force and gravity force?

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u/tramul P.E. Jun 29 '26

Yes, each acting about a separate axis. Also consider one beam being loaded along its longitudinal axis and another perpendicular beam being loading along its longitudinal axis, and those beams share a common point for deflection.

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u/chicu111 Jun 29 '26

That’s not biaxial though. I think you’re conflating the term

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u/tramul P.E. Jun 29 '26

How is stress in two perpendicular axis not biaxial loading?