Elevated Concrete Slab Design 1

[RM87]

Hello folks,

My apologies for the relatively banal question, but I'm drawing a blank. It's been a while since I've designed an elevated concrete slab and I was wondering if anyone could give me any design tips on that re-entrant corner? I'm planning on designing this to ACI 318-11 standards, and as a two-way slab.

The current draft attached was for the client as a pricing set - using a one-way slab design but it should largely illustrate the situation. It's an elevated slab over an existing garage. Any tips folks? Anything I should watch out for? For the more experienced sages in these forums, any words of caution I should carve into my memory?

Thanks,
Rod

 

[hokie66]

The spans are not large, but to me the support conditions are unclear. There seem to be walls left, bottom, and at the stair, but what do the other hidden lines represent?

I hate CAD, CAD drafters never seem to know what they are drawing. Pretty pictures, signifying nothing.

 

[BAretired]

I agree with hokie. The drawing is not clear, but I would be concerned about the unsupported corner of the slab.

BA

 

[TehMightyEngineer]

This is why we only do things in 3D models in CAD. Isometric views for everyone.

Maine EIT, Civil/Structural.

 

[JAE]

For the re-entrant corner, usually I will use heavier amounts of reinforcing both ways (east-west and north-south) at the corner to help transfer the loads.
Sort of like establishing an "in-slab" pair of orthogonal beams to carry the load and stiffen those edges near the corner.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[TehMightyEngineer]

What JAE said, design them like discrete, orthogonal beams to transfer the load around the corner. Simple and conservative. Add some diagonal bars to stop the crack that will form at the sharp corner.

Maine EIT, Civil/Structural.

 

[BAretired]

Mighty,
Do diagonal bars do anything more than bars in each orthogonal direction? He's already got four layers of steel.

BA

 

[TehMightyEngineer]

As best I can tell it's not always required but does improve the crack resistance of the corner. My boss and I have always specified at least a 24" long #4 diagonal at re-entrant corners as it's cheap insurance against cracking and will cross the crack orthogonally vs 45 degrees.

In addition ACI recommends it and in some cases requires it. For example ACI 332 for residential construction specifies in section 7.2.10 that at re-entrant corners in walls a 24" diagonal bar shall be provided at the corners. ACI 551 for tilt-up construction also requires this as well if I recall, even if you have orthogonal bars parallel to each sides of the corner.

Some further information:

http://www.concreteconstruction.net...a-reentrant-corner-crack-ruin-your-image.aspx

Maine EIT, Civil/Structural.

 

[RM87]

Ya, the drafting is poor. The condition below the proposed slab are retaining walls on the N, E, W directions, and a concrete beam across the S (front) side of the garage.

The hidden line down the center signifies the "peak" of the slab for drainage sloping purposes, and the other dashed line at the N retaining wall is an existing, large monolithic concrete piece of an older retaining wall that is expected to remain. Basically, the slab is being supported on three sides with retaining walls and on the south side with a concrete beam with its reactions on the retaining walls.

@JAE and TehMightyEngineer - Thanks for the recommendations. Sounds good, #4/24-36" should work out nicely.
@BAretired and hokie66 - I appreciate the input.

If anything else comes to mind, I'm all ears. I'm thinking about the temporary shoring of this structure too... so I'm open to ideas outside of the standard steel soldier piles and lagging.