Control joints in elevated composite slab 2

[WWTEng]

4.5" concrete slab over 2" composite metal deck (6.5" total depth). Normal weight, 4000 psi concrete. Beams below spaced @ 8' oc in a 30'x24' bay. Studs over WF beams are 5.5" long.

Where would I find the requirements for locating control joints?

 

[hokie66]

You don't use control joints in an elevated slab.

 

[WWTEng]

Hokie, Thanks. I have heard that but how would you control cracking in elevated slab then?

 

[haynewp]

The cracking that will happen comes from deflection differences over stiffer elements. I add in bars over the top of all girders and moment frames on the floor (#4@18" o.c. or similar close to the top of the slab) to keep the cracks tighter and I don't depend on the WWR to control it. I have never put in control joints in the case you are describing either.

 

[hokie66]

You control cracking (not prevent cracking) by crack control reinforcement. The amount of reinforcement required depends on the useage of the slab and how important it is to control the cracks to a given width. In a slab cast on metal deck, restraint shrinkage cracking perpendicular to the ribs would in most cases be controlled adequately by the top flexural reinforcement. Restraint cracking parallel to the ribs is controlled by what is commonly called "temperature and shrinkage" reinforcement. Whether or not the minimum T&S reinforcement is enough depends on the project.

 

[haynewp]

Right, I should have said the cracking that you will notice will be over stiff elements I described as long as WWR is installed throughout to help control shrinkage crack widths.

 

[WWTEng]

hokie, haynewp.

Thanks for the input. Now what about location of construction joints. Whats the recommended place for these joints.

 

[haynewp]

Out of the beam effective slab flange width for joints running parallel to the beam or girder.

 

[WillisV]

http://www.ascconline.org/PDF/PositionStmt_23_Web.pdf

 

[hokie66]

Construction joints in suspended slabs are almost always best located at centre span, the point of minimum shear. Where beams are not composite, you could also centre them on the beams, but not in your case, as the studs can't be in a joint. In the other direction, parallel to the deck ribs, I would avoid construction joints near the centre span of the beams, just because you want the concrete flange to be in intimate contact. A joint right over a girder is not a bad place, as that will tend to act as a "control joint" for the type of deflection cracking which haynewp described over the girders.

 

[slickdeals]

Another reference is here

http://www.concreteconstruction.net/images/Let It Crack_tcm45-343570.pdf

 

[haynewp]

See ACI 302.1 R-04, section 3.3.8.2. It the gives recommendations on construction joint location relative to primary beam effective concrete flange widths, mid-span between secondary beams, etc..