CROSS Safety Report
Compatibility at RC column to slab joint - strong column/weak slab
This report is over 2 years old
Overview
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:
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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
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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.
Full Report
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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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Feedback
I read this report with interest.
1. This deals with the compatibility of the concrete at column/slab junctions where the column concrete is of a characteristic strength much higher than that of the slab and where the slab is cast over the column head.
2. This is a common occurrence in tall buildings design.
3. The ACI Code 318 gives empirical guidance, stating that if the column concrete characteristic strength is limited to not more than 1.4 times the slab characteristic strength then the full column strength can be used. This removes the problem of having to cast a small area of the 'column strength' concrete along with the much larger area of the 'slab strength' concrete.
4. For example, slab concrete C37 can be used with column concrete C37 x 1.4 = C52
5. The example quoted, C85 column concrete and C37 slab concrete would seem extreme. It would benefit from having larger columns (with lower strength concrete) and higher strength concrete slabs at lower levels which are presumably the critical levels. C55 column concrete, C40 slab concrete would be satisfactory. For a given load the column area would be only about 35% greater with C55 concrete.
6. Although it appears that this ACI 318 concession will apply well to interior column/slab junctions, where the concrete at the column head is confined all around, edge and corner columns have less confinement if the slab is flush with a column face. It would be logical to provide additional "containment" reinforcement, extending to the column face, within the slab thickness. Where the slab extends beyond the column face by a full anchorage length then the edge or face column can be treated as internal.
7. The condition about tensile stresses in the top of the slab will occur for any concrete strength. If there is concern about this, simply continue the column links through the depth of the slab. This should provide sufficient confinement to the weaker slab concrete where it passes over the column head. In geographical areas of even medium seismicity it is common practice to provide links at close centres (eg 100mm c/c) for a short distance below and above the column/slab intersection.
8. This situation is referred to in the Concrete Society Publication 'Guide to the Design of Concrete Structures in the Arabian Peninsula', 2008, Section 7.8.4.
I would like to draw attention to the fact that the robustness of a column-slab structure increases very significantly if the columns are continuous and the slaps rests on plinths. This could ensure that failure of a slab does not drag the column with it.
There is a Concrete Society Advice sheet entitled High strength concrete columns and normal strength slabs available from:
http://www.concrete.org.uk/fingertips-nuggets.asp?cmd=display&id=680
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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.