Location of construction joints on flat slab! 2

[engr567]

What is the ideal location of construction joints on a flat slab? Does ACI have any reference on this? In my case, the contractor wants to locate the construction joints at the mid-spans between the columns,; however, I am not comfortable with that because there might be a maximum positive moments depending on the span lengths. What is the industry practice?

 

[JAE]

CJ's should be in the center 1/3 of any span. Avoid placing CJ's near columns.

 

[cancmm]

Shear is generally your primary concern with construction joints. That is why they are, as JAE mentioned, typically placed in the center 1/3 of a span and away from supports.

 

[engr567]

Thanks JAE and cancmm.

 

[archeng59]

Might be a dumb question, but are you asking about CJ's in an elevated flat slab or slab-on-grade? My first thought was elevated structural flat slab, but second guessed myself.

 

[cancmm]

The concern towards moment capacity tipped me off that this is an elevated slab.

 

[Doublest]

To me sounds seismicpe is talking about slab-on-grade. I can't imagine how elevated flat slab with construction joints in the center of the span can work, how do bending moment or shear go through these CJ in elevated flat slab? If I am wrong, please help me to understand.

Thanks.

 

[cancmm]

First, a slab on grade wouldn't span any distance since it is continually supported.

The center third of a span will generally have the highest moment and lowest shear. See ACI 6.4.4 for the "middle third" note and commentary

Moment is taken up by the compressive strength of the concrete and tensile strength of the steel. Remember that concrete is assumed to have zero tensile capacity.

What little shear is at this point is taken by friction at the joint and steel carried through it

 

[Doublest]

cancmm,

Thank you very much for your answer.

I am not quite a real world concrete guy, however, I think there is no need to discuss the concrete theory. We all know it very well. What I would like to know is how in real world can that Construction Joints achieve flexural ability (I can accept that shear ability can be achieved through shear friction reinforcement). Please see attached sketch, if the CJ is like what is shown in the sketch, I still can't understand how the flexural ability is achieved.

 

[hokie66]

Doublest,

The OP's contractor, JAE, and cancmm are correct. What you depicted is some sort of movement joint, but what is being discussed here is simply the location of construction joints. In a typical flat slab, these joints are just vertical breaks between pours. On a large job, you can't cast the whole slab at once.

 

[JAE]

OK - we are talking about CONSTRUCTION joints - not expansion joints. In an elevated slab there are times when the volume of concrete that can be placed at one time is limited. Thus, there is a need for construction joints -

In a CJ, all the reinforcement that is typically in place without a joint is exactly the same when there is a joint. The joint is a keyed, or preferably a rough bulkhead.

The flexure is no problem. In concrete the tension is taken by the reinforcement and the compression block doesn't get affected by a joint perpendicular to the compressive stress.

This is done all the time - check out ACI's detailing manual and other concrete books as well.

 

[apsix]

And an apology to cancmm wouldn't go astray.

It could not be assumed from Doublest's question that "..concrete theory. We all know it very well".

 

[ron9876]

Yes unless it is a PT slab then the joint should be placed where the cable profile is at the center of the slab.

 

[Doublest]

OK, I am wrong.

I have always been believed that elevated slab has to be a monolithic slab. When I saw the phrase “construction joint”, I was thinking it means an expansion joints. That was where the mistake came from.

Thanks guys.