Composite steel/concrete decks - crack avoidance 1

[Wopshistos]

Does anyone have proper advice as to the extent pours can be placed without control joints in this type of construction. My problem relates to a 30m x 90m upper floor where the deck acts compositely with the support steel.

Surface is to be power floated to a good standard so cracking - thru shrinkage or flexure needs to be minimised. Obviously the ribbed deck steel construction provides reinforcement as well as friction restraint on the underside. How much reinforcement in the top then?????????

If joints are required, how should they be constructed?

 

[pba]

Certainly consider the use of fibre reinforcement as this better controls cracking. It also aids program.

Your biggest problem will be the deflection of the steel frame. If possible design the beams as unpropped so that the dead weight deflections occur while the concrete is still wet.

We recently cast a slab 72m by 36m as one pour with A142 mesh (6mm bars at 200mm spacing in both directions)as the anti-crack reinforcing. This has not cracked in any noticable way but much is dependant on the concrete mix and quality of the workers.

 

[jike]

I have always had fairly good luck with not having any control joints in a composite slab.

I use draped WWF over the beams. I require #3 support bars wired to the studs to hold the wire up at the beams and let it drape between beams. I size the wire for the slab negative moment over the beams.

I add short bars over the girders. Please note that he effective slab depth over the girders is much, much less than over the beams since the deck ribs are parallel to the girder so the slab is really the total slab thickness minus the deck thickness.

I require the construction joints to be outside of the effective width of the composite beams. If joints need to be placed within the effective width, I would require dowels across those joints.

I have also seen some engineers design non-composite beams so that construction joints can be placed directly over them.

I hope this helps!

 

[14159]

Are you concerned with large cracks or small ones? My experience is that the larger ones can be avoided by rebars places in strategic locations and reasonable pour sizes. The small ones are very much more difficult but they usually don't matter anyway unless you have sensitive flooring such as sheet vinyl.

I don't think you can completely eliminate small cracks over beams on column lines regardless of pour size or reinforcement.

For shored construction, the slab is much thinner at the beams and girders along column lines and thicker in the middle of the bays. It's also restrained in-plane by stud connections to the beams. We placed significant top rebar transverse to the beams along column lines but the small cracks often remained and caused problems with sensitive flooring.

For unshored construction, the slab has a fairly uniform thickness, but will have negative moments at these same locations when the shores are removed. Either way, I think the small cracks can't be reliably eliminated regardless of pour size.

The huge question is: What type of flooring will you have?

DBD

 

[pba]

DBD,
Don't you have shored and unshored the wrong way round?

 

[14159]

"Don't you have shored and unshored the wrong way round?"

Oops! That's correct. Thank you for locating my error.

DBD