CROSS Safety Report
Compatibility at RC column to slab joint - strong column/weak slab
This report is over 2 years old
A reporter who was reviewing the structural design of a commercial building raises concerns over the compatibility at column/slab joints when different grades of concrete are used.
Key Learning Outcomes
For civil and structural design engineers:
With the growing use of high strength concrete in columns, a common problem in design is to know how much the effective strength of the column is reduced by a weaker intersecting slab
The Concrete Society have an advice sheet on the subject. This is available free to Society members, also through IHS or as a download High strength concrete columns and normal strength slabs.
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A reporter worked as an independent engineer to review the structural design of a project. It is a commercial building of about 30 storeys with a central reinforced concrete (RC) core, RC internal and external columns, and flat slab floors. Construction is in-situ and columns have at least 4% of reinforcement. Their concern is that the strength of concrete in the columns, grades C70/85, is greater than the strength of the slabs, C30/37, so there is a layer of weaker concrete at each floor level.
Under vertical loads, continues the reporter, there will be negative moments over the columns and to the tensile bending stress in the top of the slabs will be additional axial tensile stress from diaphragm loads in the slabs. As the concrete will be cracked in the tension zone they wonder if it will properly constrain the compressive loads from the columns. They believe that compatibility at the column/slab joint is not satisfied.
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With the growing use of high strength concrete in columns, a common problem in design is to know how much the effective strength of the column is reduced by a weaker intersecting slab. Although this can theoretically be avoided by casting a ‘pool’ of high strength concrete in the slab at each column location, in practice this requires very careful site management and supervision, and a design solution accepting some limitation in the column strength is usually preferred. BS 8110 does not cover the situation.
Neither is the subject dealt with directly in Eurocodes although there is mention of confinement. Some guidance is given in Concrete Society Technical Report 49 Design guidance for high strength concrete (cl 6.3). Relevant research has been published by various authors in Germany, Canada, and Australia.
Concrete Society Technical Report TR64 suggests the following options:
Increase the strength of the concrete in the slab
Cast the slab concrete over the column perimeter and within a prescribed area outside the column perimeter with the higher grade concrete
Cast the lower column through the slab
Confine the concrete by using large diameter links in the slab
Whilst there is still debate over the best model a suitable approach would be to consider possible effects and make design decisions accordingly.
The Concrete Society has pointed out that they have advice sheet on the subject. This is available free to Society members, also through IHS or as a download High strength concrete columns and normal strength slabs.
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