90' in Elevated Exterior Slab w/ no CJ's

[PEnSE]

I have a small concrete grandstands project where the arch. would like to use the space below. In order to get enough clear height he is wanting to lower the floor level below grade and have a small structural section height. Instead of using the typical raker beam with the stairs used as structural members spanning perpendicular to the raker beams, he'd like the slab to span in the direction that there would typically be a raker beam.

Without raker/concrete frame lines, I'm thinking of using the stands as a sloped diaphragm spanning to shear walls. He is also talking about adding a canopy so lateral loading from the canopy and the stands would have to flow to the shear walls.

My concerns are:
Could I span about 90' without adding any CJ's (sawed joints) without getting objectionable cracking by the owner? Any admixtures I could add that could handle this? This is an exterior elevated stand in the SE part of the US?

Could I cut CJ's about every 20'...say 1/4" wide by 1" deep..and provide 2" of cover for the top bars? What should the sawed joint be caulked with to perform the best long term?

How can I prevent moisture problems from water getting through the concrete to the space below?

Do you see any construction issues? Such as cold joints in the contractor's pours or cutting CJ in the stairs?

Please see attached sketches and I appreciate all input

Thank you!

 

[PEnSE]

See Attached

 

[hokie66]

A few things:

1) You are not spanning 90'. The span is the distance between bearing walls.

2) Sawn joints are not a reliable means of controlling cracking in suspended slabs. The cracks are just as likely to form away from the sawcuts. In your case, cracking will be due to restrained shrinkage. The longitudinal walls will restrain the slab, and cracking is inevitable. Crack control (not prevention) reinforcement of about 0.6% Ag would be required.

3) Watch bending bars around re-entrant corners. As the bars are stressed, they tend to pop out the corners.

 

[rowingengineer]

Agree with everything Hokie said.

a crystal type water proofing additive to concrete might help with water ingress.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[PEnSE]

Hokie,
If I reinforce the steps in the slab with 0.6% Ag, what kinds of cracks are we talking about? Would the cracking be acceptable to the typical owner? I would specify a maximum water/cementitious material ratio of 0.5(by weight) or below for the concrete. Are there any admixtures in addition to the 0.6% steel that would help spread the cracks out?

Do you think this framing system is a bad idea and I should stick with a typical framing system with rakers spaced every 20' or so with the slab spanning between the rakers? I could use precast such as hollow core, or precast risers with a joint every 20' @ the raker beams. This is such a small job though that with the cost of mobilization, precast might not be the most economical solution.

With the layout being roughly 140' x 140' do you see the need for any expansion/movement joints...i.e. maybe adding 1 double column line..directly behind wear home plate would be (2 separate structures)

Thanks for your help!

 

[JAE]

I think I would tend to separate the three wings of your plan layout with expansion joints.

Not sure I'm comfortable with that large canopy - you have three cantilevers extending off each other.
With high gust wind forces that thing would gallop.





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