3/6/13 High strength columns and normal strength slabs

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Fingertip High strength columns and normal strength slabsDescription

The economic design of a high-rise frame building may call for high strength concrete in thecolumns and a lower strength in the floor slabs. Concern is often expressed about thecapacity for the column loads to be transferred through the relatively weaker slab. Someguidance is given in the American Concrete Institute ACI 318 Building Code requirements forstructural concrete and Commentary. The Commentary to Clause 10.15 states that “Whenthe 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 greaterthan 1.4 times that in the floor system, generally requiring that “Concrete of strengthspecified for the column should be placed in the floor slab at the column location …and thetop 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) “thestrength of the column shall … be based on an assumed concrete strength in the column jointequal to 75% of the column concrete strength plus 35% of floor concrete strength”. Noguidance is given for edge and corner columns.

An additional requirement is that the design strength of the concrete in the column should notbe more than 2.5 times the strength of the concrete in the slab. This is because it has beenfouind experimentally that low strength slabs do not provide adequate restraint to theconcrete under the column.

An alternative approach given in the Australian Code (AS3600), for the situation in which thestrength of the concrete in the column lies between 1.4 times and 2.0 times that of theconcrete in the slab, is to provide additional vertical reinforcement through the slab anddesign the area as a column using the lower concrete strength. Restraint from slabs or beamson all four sides is still required, i.e. the guidance is for internal columns. Some guidance isgiven for limited edge column situations.

The draft Concrete Society Technical Report Guide to the design and construction ofreinforced concrete flat slabs states that “Where the concrete strength in the slab is lessthat 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 withinthe slab.” A method is provided for internal columns, which can be applied to edges where thecolumns 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/R > 0.4fck,col – 0.45fck,slab

where R is the radius of the hoop.

Acknowledgement The Concrete Society

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