High strength columns and normal strength slabs
The economic design of a high-rise frame building may call for high strength concrete in the columns and a lower strength in the floor slabs. Concern is often expressed about the capacity for the column loads to be transferred through the relatively weaker slab. Some guidance is given in the American Concrete Institute ACI 318 Building Code requirements for structural concrete and Commentary. The Commentary to Clause 10.15 states that “When the column concrete strength does not exceed the floor concrete strength by more than 40% no special precautions need be taken”.
Clause 10.15 continues by giving guidance for when the concrete in the column is greater than 1.4 times that in the floor system, generally requiring that “Concrete of strength specified for the column should be placed in the floor slab at the column location …and the top surface should extend 600 mm into the slab from the face of the column”. Alternatively, for columns supported on four sides by beams or by slabs (i.e. for internal columns) “the strength of the column shall … be based on an assumed concrete strength in the column joint equal to 75% of the column concrete strength plus 35% of floor concrete strength”. No guidance is given for edge and corner columns.
An additional requirement is that the design strength of the concrete in the column should not be more than 2.5 times the strength of the concrete in the slab. This is because it has been found experimentally that low strength slabs do not provide adequate restraint to the concrete under the column.
An alternative approach given in the Australian Code (AS3600), for the situation in which the strength of the concrete in the column lies between 1.4 times and 2.0 times that of the concrete in the slab, is to provide additional vertical reinforcement through the slab and design the area as a column using the lower concrete strength. Restraint from slabs or beams on all four sides is still required, i.e. the guidance is for internal columns. Some guidance is given for limited edge column situations.
The Concrete Society Technical Report 64 Guide to the design and construction of reinforced concrete flat slabs states that “Where the concrete strength in the slab is less that 70% of that in the column, bursting effects within the slab depth should be considered. These bursting effects can be resisted by the use of special confinement reinforcement within the slab.” A method is provided for internal columns, which can be applied to edges where the columns are set back. Hoop reinforcement is provided to contain the concrete in the slab and hence enhance its effective strength. The required area of the hoop reinforcement, As , is given by:
Asfyd/Rh > 0.4fck,col – 0.45fck,slab
where R is the radius of the hoop.