center rebar impact on reinforcement calculations for a suspended slab 2

[jpyeron]

per DCID 1/21 (

http://www.fas.org/irp/offdocs/dcid1-21.pdf)

4.1.1 Reinforced Concrete Construction: Walls, floor, and ceiling will be a minimum thickness of eight inches of reinforced concrete. The concrete mixture will have a comprehensive strength rating of at least 2,500 psi. Reinforcing will be accomplished with steel reinforcing rods, a minimum of 5/8 inches in diameter, positioned centralized in the concrete pour and spaced horizontally and vertically six inches on center; rods will be tied or welded at the intersections. The reinforcing is to be anchored into the ceiling and floor to a minimum depth of one-half the thickness of the adjoining member.

Looking at ACI 318-08 chapters 7, 9, 13 do not seem to address the #16 rebar centered in a 203.2mm slab. It does indicate that the rebar used should be no closer than 20mm (7.7.1) and my understanding is that it should not be more than 40-50 mm from the surface.

Would the rebar in the center influence the calculations for the top and bottom rebar?

p.s. I am using the calculators at

http://www.civl.port.ac.uk/rcc2000/

 

[TXStructural]

It is really not practical to weld bars in the field if they are to develop strength, but is sounds like these are solely to prevent access through the wall or slab. Tied reinforcing does not provide any greater strength than loose bars, and mechanical couplers would be ideal, but are not specifically allowed.

There is no maximum on cover and it is acceptable to have only centrally-located reinforcement. It is relatively common to have only one layer at the center of walls. In a flexural member, the crack width can be markedly wider on the tension side with deeper cover.

It sounds like the only non-prescriptive requirement would be for self-weight of the top of the vault. If the strength is adequate to support the span, you can simply place the rebar in the center of the 8" slab and design for d=4" It is not structurally efficient, so you might choose to reinforce with additional reinforcement.

 

[hokie66]

Agree generally with TX. I would specify the central bars in all the slabs and walls where this applies, but would design flexural slabs with appropriate reinforcement in the tension face. So slabs on ground and walls reinforced with the central bars, and suspended slabs with both central and tension surface bars.

 

[jpyeron]

Thanks for the clarity TXStructural & hokie66.

For the purpose of calculation on the suspended slab, should the central rebar be ignored as part of the calculations?

 

[TXStructural]

Depending on how large the vault will be. It would be acceptable to ignore the center bars, but the slab would need to be designed for brittle failure. The central bars would not compromise strength, but would result in an over-reinforced section, where the compression concrete would fail before the reinforcement yields. This is probably irrelevant if there is no live load applied to the slab.

 

[jpyeron]

Thanks I will go back and read up on the brittle failure issues for the two slabs (12'x40' and 12'x20') slab.