Structural Slab - Thickness for Constructability

[ChrisKn]

I've run across a few people at my company who will use a minimum of a 7" thick slab for structural slabs requiring T&B steel. Its not so much a design issue as it is a constructability.. that is i have been told increasing a structual slab from 6 to 7 inches helps with the placement of T&B bars. Has anyone heard this school of thought on this issue?

Just wondering cause I'm designing a building on horrible soil conditions so its gonna be a structural slab with grade beams and piles. I'm gonna break down teh interior of the building into a 1 way cont slab with 5 spans of approx 12 feet on grade beams. By design espicially with the use of continuous slabs (1/10 moement instead of 1/8) the 100psf LL will yield a 6 inch slab with relatively small T&B steel. But most of the people at my company seem opposed to using 6 inch slabs with T&B steel and some say if you do use a 6" slab that the reinforcing should be called out as staggered?

Well anyway..What are your thoughts on this??

Chris

 

[emart0429]

Chris,

I would tend to agree w/ the others in your company. For a slab on grade I'd assume you're maintaining 3" clear to the bottom mat of steel. Then if you've got 4 layers of #4 bars (0.5" nominal, 0.56" outside width), the top mat of steel will be at best 1" below the top of the slab. In our bridge designs, we have to maintain a minimum 1" gap between the top mat and the bottom mat of deck reinforcing to allow the concrete to "flow" through and around the reinforcing. Without the gap you may not get proper consolidation of the concrete around each bar.

My 2 cents,
Erik

 

[ChrisKn]

Erik.. Thank you for your reply. The more i think about the issue I can see everyones point. Just its one of those things i never contemplated before.

On a side note though our general notes say to provide a 2" clear for bottom reinforcing in a structural slab (1 1/2" for top). So i guess with 2+1.5+.625+.625 (.625=dimaeter #5 bars) would still leave 1 1/4" gap between bottom & top mat. Even thoguht this would work, the placement of reinforcing and concrete work is far from an exact science and it seems that an extra inch would lend itself to proper consolidation of the concrete.

Chris

 

[civilperson]

The outside diameter of nominal size bars is greater than the equivalent diameter for area due to the deformations. Thus, a #5 bar crossing a #5 bar at right angles is 1.375" from out to out.

 

[ChrisKn]

Thanks civil person, never knew that, but it makes sense since the reiforcing is deformed and not smooth. Besides that my previous post had an error becuase the actual measurements were off because i neglected to include the temperature steel in my calc. So lets say #4 temperature steel perpendicular to the #5 main reinforcing steel. Not sure what the measurement of a #5 crosing a #4 is...but even using nominal dimensions it defintely wont work..LOL..

Thanks you for all your help... i should know these things but somehow i managed to do very little real structural engineering work in the first 10 years of my career. Now the tides have turned and I'm into it full go, so i gotta refresh myself. But on the flip side i know way too much mehcanical/HVAC and plumbing from my past endevours. I guess knowledge in any area doesnt hurt.

Thanks again,
Chris

 

[apsix]

Adding 10% to the design diameter is a pretty good estimation of the actual maximum diameter across the ribs.