Precast Slab Design - Transverse Reinforcement (HS20-44) 1

[astructurale]

I am designing a 12" deep precast slabs that span over a creek at a residence to resist HS20-44. (2-10ft wide, 20ft span, 2" composite topping) I have designed it for flexural capacity and shear, and it meets the deflection and crack criteria. How do I specify the transverse steel? Is there a minimum required by ACI 318-99? The only examples I can find are for transverse slab design when you have stems supporting the slab, and this is not a stemmed member. Or is stripping and handling the only case I need to design for in the transverse direction?

 

[jike]

The design of the transverse steel is extremely important! It carries the bending moment caused by your wheel loads trying to spread out laterally to be resisted by an effective width of slab. If the transverse steel is undersized, then the main steel could be overstressed, since the effective width carrying the concentrated load would not be as great as one would expect.

There is nothing in ACI 318 that addresses this. I do not have the current AASHTO spec but I believe there is a requirement in terms of the % of the main reinforcement. Westergaard also has charts that can be used to determine the required transverse reinforcement.

I suppose another way to approximate the distribution steel might be to estimate your effective width and then figure it like a footing with a concentrated load spreading the load out to the soil over the footing width (equal to the effective width of the slab). Note that the effective width becomes narrower when the wheel load spacing is less than the effective width.


Good Luck!

 

[jike]

Don't forget the effective width for concentrated loads on precast slabs is less than for poured monolithic slabs. I assume this is because of the keyed joint. For concentrated loads near an unsupported edge of the precast slabs, the effective width has an additional reduction.

Check with Spancrete or other precasters on the what they recommend for effective widths.