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Column/beam/slab junction - high strength concrete

Column/beam/slab junction - high strength concrete

Column/beam/slab junction - high strength concrete

(OP)
In a slab-beam-column type construction in multistory building the practice is to many times use higher strength concrete in columns as compared to beams and slabs.  The concrete is placed in the following stages:

1. Column is cast up to bottom of beam using high stength concrete.
2. Beams and slabs (Including junction) is cast using lower strength concrete.
3. Column above slab is again cast with high strength.

This sequence results in lower strength concrete in the beam-column junction portion of the column compared to the rest of the column.

Is it ok to use this method? If not, is it typical to puddle high strength concrete in the areas around column junctions? Is this addressed in ACI anywhere?

My thoughts:  

Beam is designed for forces at face of column. - Therefore, beam-column junction is a part of the column and not the beam.  Therefore it should be high strength.

Beam-column junction is the weakest zone (especially in seismic loading) so this portion should always be detailed with caution.

ACI-318 stipulates strengthening the joint when stength of concrete in beam is lower than concrete in column by 40%.  However, ACI has no special provisions when the difference in concrete strengths is less than 40%.  

Please let me know your thoughts.  

RE: Column/beam/slab junction - high strength concrete

ACI 318, Section 10.15 addresses this topic a bit.

What I've done in the past is keep the column strength within the tolerance given in 10.15 (f'c of column <= 1.4(f'c of floor)) and proceed with design using the f'c values for each - no other design precautions need to be taken.  (see the commentary for 10.15)

 

RE: Column/beam/slab junction - high strength concrete

Badger,

The reason you can use the lower strength concrete to transfer the column force through the floor is that the concrete in that area is confined laterally by the floor.  If the code specified ration of strengths is exceeded, it is relatively common, but tedious, to puddle the joint area.

Incidentally, the Australian Standard allows for transmission of the column force through the slab or beam if the column/floor strength ratio does not exceed 2.0, provided that specified restraint is provided.

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