huangyhg

construction joint @ column face
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.
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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.
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.
you could also add additional shear friction reinforcing near midpoint of the slab hooked into the column.
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?
for construction joints in concrete systems, you usually don't put the joint in places of zero moment....but low shear.
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.
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.
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.
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.
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?

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