construction joints in composite slabs

[a2mfk]

I did some searching in the past Tips on the subject, and still have a couple of questions. This was the best one I found:

http://www.eng-tips.com/viewthread.cfm?qid=290066

Typical office building floor, steel wide-flange composite beams with composite steel deck and concrete slab system. WWF is provided for shrinkage crack control only, slab spans are designed as simple.

We need to detail where and how to place construction joints for logistical reasons in the field. I understand to place construction joints outside the effective flange width of the composite beams, so I suppose as close to this as possible is ideal. Any other thoughts on where to place them would be great.

Since the slab is in compression, the only issue would be shear that I can think of- but you have WWF and the deck also. Is it standard to provide perpendicular dowels across the joint for shear transfer or are these just for "warm fuzzy" feelings...? #4 @ 18" o.c. x 24"?

Thanks in advance for your input.

 

[JAE]

I'm not sure of the efficiency (efficacy?) of using WWF for shear friction at a construction joint. I'd go with the WWF typically and then require #4 @ 12 to 18" o.c. across the joints - fully developed each side.

 

[hokie66]

For joints parallel to the composite beams (perpendicular to the deck spans), I would place the joints half way between beams. The steel deck will transfer whatever incidental shear which occurs at that location. For joints in the other direction, locations don't matter, as long as they are far enough away from any composite girders.

I don't like the practice of designing for simple spans, thus intentionally allowing the slab to crack over the supports.

 

[csd72]

a2mfk,

How big is this slab that you cannot pour it in a day?

For anything more than about 50m (160 foot) you should provide a proper expansion/contraction joint unless you can justify it by calculation.

If you do need to push the boundaries then you should consider a pour sequence that skips alternat bands to allow the orifginal bands to shrink prior to pouring the infill. This works better with insitu slabs though it should make a difference on composite.

hokie,

It depends on the fabric and cover used, often the fabric is sufficient to provide for serviceability moments and therefore will be sufficient to limit cracking.

 

[rowingengineer]

would agree with Hokie in regards to not designing as simple span.

however I do have a question for Hokie. would your advice about shear transfer still apply if the building was in a seismic zone thus the slabs could act independently in the along joint direction if the other structural components didn't have enough strength?

How could you do anything so vicious? It was easy my dear, don't forget I spent two years as a building contractor. - Priscilla Presley & Ricardo Montalban

 

[a2mfk]

Thanks guys. Building is under construction, the request was from the contractor to have a construction joint(s). The floor is good sized, not sure of the exact square footage, but nothing extreme. Maybe limited amounts of finishers available or some other sequencing reason. Anyway, I did forget to say that we are not using the floor for a diaphragm, but we went ahead and provided a detail with bars across the joints because it makes us feel good even if there is no great engineering reason The joints will be placed at mid-span so the out of plane shear in theory will be zero. Thanks again.

 

[hokie66]

RE,
Not sure I understand your question, but I can't see the slabs on either side of the construction joint acting independently due to seismic load. The deck continues across the joint, the shrinkage reinforcement continues across the joint, and the construction joint is just a joint in concrete, not a slip joint.

 

[rowingengineer]

Hokie,
on seismic projects I have always spent time reinforcing all construction joints in slabs, due to the large potential load that could occur along and across the joint. I guess I just don't like relying on mesh, deck and a little bit of stone friction.

How could you do anything so vicious? It was easy my dear, don't forget I spent two years as a building contractor. - Priscilla Presley & Ricardo Montalban

 

[a2mfk]

Rowing- I would be concerned with shear if I was transferring in-plane diaphragm shear (not in my case). It would be on a case by case basis depending on the magnitude of the shear and location of the joint. In many cases, as Hokie explained, I think the deck would suffice for diaphragm shear transfer.