Joints in elevated concrete slab on form deck 5

[vincentpa]

We have an elevated concrete slab on form deck in a large industrial building. How often do you space the control joints? How often do you space the expansion joints? Where do you locate the control and expansion joints? I imagine that the location of the joints will differ with the orientation of the infill beams. Will the spacing and location of the control joints be the same as a composite slab (beams, concrete with studs)?

 

[vandede427]

I would not put any joints in an elevated floor even if it's on form deck.

 

[JAE]

no joints

 

[jike]

I would not place any control joints in the slab. That being said, make sure you have proper and sufficient rebar in the slab.

I would place expansion joints in the 200 to 250 foot range. Closer, if in an unheated or in a non-(air)conditioned building.

 

[csd72]

If you are using this as a floor diaphragm you definately do not want to put any weaknesses in it.

 

[tolchijb]

I would make sure I have top reinforcing bars over the girders or else the slab will crack over the girders and I will have "control joints" in the slab anyway.

 

[vincentpa]

If you don't use control joints, for a slab on form deck, would you use ACI minimum temperature and shrinkage (T&S) steel requiremente (rho=0.0018) for reinforcement perpendicular to the main flexural reinforcement? Or would you place welded wire fabric (WWF)? Our slab is total 8" thick with 1.5" form deck. That leaves 6.5" over the flutes. The slab is an overdesign requirement. I am worried about cracking.

I am trying to get the customer to use composite steel deck because the deck will not be subject to corrosion. This will eliminate the bottom flexural steel. Steel composite deck manufacturers put recommendations for T&S steel, usually very lighter gage WWF. I was always concerned about the negative flexural cracking over beams and girders. I don't believe the lighter gage WWF is adequate to prevent wide flexural cracks. What has your experience been using composite deck? What do you do when using composite deck? Do you place rebar over the beams and girders to keep the negative flexural cracks tight or do you use a heavier gage WWF?

Does anyone know of a good design guide or rule of thumb for designing T&S steel on elevated metal deck? I have a ton of literature for slab-on-grade but nothing for metal deck.

 

[demayeng]

Flexural cracks: normally extra bars are required (in addition to the T&S steel) over the negative reinforcement zones, you can either go off manufacturers tables for the metal deck based on the spans you have or you can work it out using the normal suspended slabs principles. Sometimes for short spans the fabric is enough.

T&S steel: Here in aus 0.6% of the cross-sectional area (using the cover concrete thickness 6.5") is considered 'strong' crack control.