RC Beam - RC Slab Connection Detail

[Grant M]

Hello all,

I am currently designing a large underground cast-in-place stormwater collection structure. The roof of the structure consists of a 27" concrete slab which spans to concrete walls on two sides and to two concrete beams (which connect to the walls on each end) on the other sides. Initially, I had the concrete slab connecting to the top of the concrete beam; however, due to some constraints (the bottom of the beam interfering with the swing radius of some flap gates) I was asked if I could raise the beam up, so the slab connects into the bottom of the beam. I have attached a PDF file showing the before and after of what I have just described.

After discussing with a few fellow structural engineers in my company, we did not see any issue with bringing the slab into the bottom of the beam, as we believe it would still load the beam in the same way. But I thought I would reach out to see if anyone had any comments/concerns with this proposed beam/slab connection. Any help you can provide would be greatly appreciated!

Thank you,
Grant

 

[r13]

Can be done, but

 

[EZBuilding]

Two initial things come to mind.

The slab is no longer part of your compression block in the design of your reinforced concrete beam.
The design of your reinforced concrete beam for shear should have it's design section at the face of it's support and not a distance "d" away.

 

[Agent666]

Think of the only difference for vertical loading being that instead of providing a vertical compression force to the top of the beam that can strut to the bottom of the beam, you're instead delivering a vertical tension to the stirrups which needs to drag up to the top of the beam. Lookup your code regarding any hanger steel required in this situation to drag the reactions at the bottom of the beam to the top of the beam s if the loads are significant. Usually its phrased in terms of beams supporting beams, but can equally apply to the design of heavily loaded slabs.

If you do a strut/tie of the scenario with load on top and bottom you'll see the shift in the vertical tie force. As EZBuilding noted, this basically means you're designing for your max shear slightly differently than normal due to this effect.

 

[Blackstar123]

I don't know about the design practice of other engineers here, but when designing an upstand beam, I usually ignore the shear strength of concrete on the basis that compression struts of beam will be in tension when load is transferred from the bottom.

Is your sketch to the scale? For a slab as thick as yours, I don't think provided beam size will provide any meaningful support.

 

[Blackstar123]

By this

The design of your reinforced concrete beam for shear should not have it's design section at the face of it's support and not a distance "d" away

Do you mean this?
The design of your reinforced concrete beam Slab for shear should not have it's design section at the face of it's support and not a distance "d" away.

 

[EZBuilding]

Blackstar - thank you for the correction I will edit my post. I also agree with your concerns regarding beam size and slab depth...

 

[KootK]

I'd like to see closed stirrups designed for at least compatibility torsion. The beam's going to roll with the end slope of the slab and I'm guessing that it will have meaningful torsional restraint at the end connections to the supporting walls.