CONSTRUCTION JOINT @ COLUMN FACE 2

[lutein]

I am working on a conventionally reinforced slab that is under construction.
Contractor has requested to locate the construction joint at the face of column, which is not desirable structurally.

I was wondering if you have any experience locating the construction joint at the face of columns?

There are a list of items I could think off that might impact the design:
1. punching shear at the column. Since the shear face is now at the face of column, so the columns are no longer an interior columns - Is this correct?
2. Maximum negative moment at column could be overcome by adding more temperature steel.

Did I miss anything?

I would appreciate your valuable input based on your experience on this.

Thank you very much in advance.

 

[JAE]

Usually, as you suggest, you want to avoid joints in areas of high shear.

one detail that we have done in the past is to inset the slab into the column face by 2" (or 1 1/2" to avoid rebar conflict. You could, I guess, go even deeper with the joint. This creates a keyed type joint where the shear on a plane converts to bearing on the concrete key.

The key would/could be the full depth of the slab.

But I would still be reluctant to do this unless there...was...just...no...other...way.

 

[jike]

Is this a 2 way flat slab, flat plate or a oneway slab on beams?

Generally, I do not allow a construction joint at the face of a column. You want the construction joint in a zone of low shear.

 

[WillisV]

You could also add additional shear friction reinforcing near midpoint of the slab hooked into the column.

 

[mitchelon]

Regardless of where you place the construction joint, it will need to be engineered. You can place it anywhere you want as long as the joint has the necessary shear and flexural strength required by your structural analysis. A good location is typically where the moment in the slab is zero. This way you only have to provide reinforcement (smooth rebar preferably) for shear strength. Are there any other joints specified in the design?

 

[JAE]

For construction joints in concrete systems, you usually don't put the joint in places of zero moment....but low shear.

 

[mitchelon]

I apologize. JAE is right. ACI recommends they be located within the middle third of the span, or as JAE said, where shear stresses are low.

 

[lutein]

Thank you for the replies.
We would definitely engineer the joint with sufficient reinforcing.
The thing that gets me is ACI 318 Section 6.4 that says "Contruction joint shall be located at 1/3..." and does not allow any engineering analysis to overwrite this.

 

[MotorCity]

From the previous responses, it sounds like this is not a slab on grade. For elevated slabs I would agree that locating a construction joint near a column face is not desirable. However, for slabs on grade it is quite common for joints to occur at a column face in a "pinwheel" arrangement. This detail has all joints at a column tangent to a different face of the column. The resulting pattern resembles a pinwheel in plan.

 

[jen4950]

We always specify the middle third of the span- and I just wouldn't accept a construction joint at the face of the column- period. You are the one sealing the final design- not the contractor.

If you are backed into a corner (situation to avoid in the future, especially if coming from a contractor) be sure to follow thru to the tee, and don't skimp. Pure shear-friction steel for the interface (times 3?) and roughen all surfaces immediately comes to mind.

 

[KCRatnayake]

JAE, We normally place the construction joint at the middle third of the span. I.e. generally at the point of contraflexture (Zoro moment) of a continuous beam. What is this low shear story?

 

[JAE]

clefcon - well, the concept behind ACI 318's requirment for the middle third of the span (per lutein above - section 6.4) is that a vertical construction joint in a flexural element (be it slab, joist or beam) significantly affects the shear behavior more than the flexural.

With flexural behavior, you essentially have your primary stresses perpendicular to the vertical face; thus, the joint doesn't really affect the transfer of orthogonal stresses through the joint. On the tension side, the concrete isn't doing much of anything and the rebar simply continues on through. On the compression side its simply concrete on concrete in direct bearing so again there's no significant affect.

For shear, however, the vertical joint aligns more with the vertical component of the shear and you must depend upon the roughness of the interface and any rebar that crosses the joint to resist the ability of one side slipping with respect to the other.

Placing the joint at the point of inflection (zero moment) doesn't provide you with any significant advantages. Keeping it in the middle third maintains a low shear magnitude at the joint.

 

[jike]

I would like to add a little bit to what JAE mentioned.

Although in theory the shear at mid span on a uniformly loaded beam is zero, in reality this is often not the case. A simple beam with uniform load on half the span will generate a shear at the mid-span equal to 1/4 of the end shear on a fully loaded beam. We, therefore, think it is always prudent to require a key of 1/3 the depth of the beam at the construction joint.

 

[lutein]

Thank you all for your valuable input.
Just as an update - We have decided to engineer the joint at the face of column, because it will help expedite the construction and save the owner at least 1 month of construction time by doing this, due to formwork-shoring process.

What we have done is to check on several items:
1. Shear friction transfer along the slab. By obtaining the shear at the joint location from FEM program, we designed additional shear friction bars at the mid depth of slab.
2. Punching shear at column - Recheck them and add whatever required shear over column head.
3. Apply bonding agent with continuous key along the joint.

 

[KCRatnayake]

Thanks JAE. Noted.