Axial transmission of column load through floors

[blihpandgeorge]

AS3600 Cl 10.8 gives guidance on determining the effective concrete stress (f'ce)through a column / slab joint.

My question is, assuming you need the full strength of f'c in the column, does f'ce need to meet f'c or 0.75f'c as per 10.8a? My first instinct is the full f'c (or whatever is required for the column design), however i have had others remark that the reduction afforded by Cl 10.8a can be extended to this.

 

[Agent666]

While I can't speak directly to the AS provisions (don't have a copy of latest AS3600:2018), in NZ the requirement would equate to roughly a 0.71 f'c requirement. You are allowed a lower strength in the floor concrete carrying through the column, its because the floor plate usually confines or buttresses this concrete, such that its more or less equivalent in a way to the higher strength concrete of the column.

If your floor concrete is lower strength, then either you typically need to up the concrete strength until it satisfies the 0.75f'c requirement, or often there are provisions allowing you to pour higher strength concrete locally and extending say 600mm all round the column (so you don't need to increase the strength over the full floor plate, but consider practicality of this on site), or alternatively codes can allow the addition of further reinforcement to maintain column strength across the floor pour with the lower strength concrete. Sometimes if there are beams framing into the columns all round then a lower strength requirement in the joint region can be used as well without any penalty, again due to the confinement afforded by this arrangement.