Concreting of RC Column

passingtongreen is very correct with his comments, and I agree with them wholeheartedly. GA for my Bud!

Design Shear is not the only factor that must be considered when designing a reinforced concrete column. If the concrete frame of a building is designed to withstand earthquake and wind loadings, then there is a high probability (depending on the magnitude of the quake of wind loads) that the applied Design Bending Moments (Mu) due to those loads will be present throughout, and they will occur not only at the top and bottom of the column @ each story but in the middle of the column. Where, the "Inflection Points" of the Moment Diagram, where Mu = zero, will occur above and below the column mid-height. There will be Negative Mu at the top and bottom of the columns as well as the Positive Mu present at the column mid-height.....where we find the shear forces at the mid-height will be close to zero, whereas they will be maximum at the floor slabs.....because normally the column will be designed for "FIXITY" at it's base as well as it's crown. Placing a construction/cold joint at mid-height of a column is a very bad idea because there would be no continuity of the "transformed section", and the column would only be relying on the steel rebar and not the interaction between the concrete and the rebar to withstand the applied Bending Moment (Mu).

According to the ACI 318 Standard here in the USA, all columns are subjected to at least a minimum prescribed eccentricity (in each column axis) which would induce Bending Moments in the column. In many cases, there will be Bi-Axial Design Bending Moments, Bi-Axial Design Eccentricities, and Bi-Axiual Design P-Delta loads present, not just Design Shear Forces.

[Additionally, don't forget P-Delta Effects, as mentioned directly above, that are induced into the building frame. These too induce Bending Moments......no column is ever perfectly plumb, because there are inducted eccentricities due to the construction tolerances. I don't care how good a concrete contractor is, no one can build a concrete building to absolute zero deviation of plumb].

The reason that construction joints occur at the column-slab interface is that the lateral shear forces are easier to withstand because of the abundance of steel reinforcement located there, if designed correctly and in accordance with the concrete design codes (no matter where you are located in the world).....and in the case of a cold joint, the steel (if properly designed) is fully capable withstanding the shear forces alone, in necessary. Obviously, it is never a good idea to ever produce a cold joint whenever possible, because we'd like to see a homogeneous concrete placement throughout, and thus a resulting high degree of continuity and structural integrity.