Axial shortening of columns
Reinforced concrete columns and walls shorten owing to elastic compression, creep and shrinkage, although the effect is not significant for buildings less than about 10–15 storeys. As explained in Concrete Society Technical Report 67, Movement, restraint and cracking in concrete structures, columns and walls in multi-storey buildings shorten by different amounts and at different times.
Analysis of vertical shortening has to take account of the following:
• Axial strain. Each increment of load causes an initial elastic strain which increases over time by a process known as creep.
• Shrinkage. Shrinkage starts immediately the early thermal contraction cycle has passed, and then continues at a decreasing rate.
• Construction sequence. Each new floor is cast at a level which overrides all the shortening which has taken place beneath it.
• Loading sequence. After a floor is constructed, the remaining load is added incrementally, usually in the sequence: screed or raised floor; walls and partitions; ceilings with lighting and other services; furniture and occupants.
• Time-dependent effects. The overriding problem is that creep and shrinkage are both very much dependent on the age of the concrete, and with each storey cast at a different time the total shortening at any one time is the sum of movements which all started at different times and have progressed to different stages.
• Differential shortening. Generally it is differential shortening which is important, particularly between columns, which are generally heavily loaded, and core walls, which are usually more lightly loaded.
• Shortening in a single storey height is important for added elements which are not flexible, such as cladding and partitions.
Technical Report 67 considers the shortening of a range of columns (different concrete strengths and reinforcement percentages) and concludes that a maximum shortening of 1.4mm/m is possible, i.e. 4–5mm in a typical storey height. The Report suggests that it is difficult to reduce the shortening significantly. A better strategy is to limit the differential shortening by designing all columns to the same criteria, and by keeping long clear spans between different structural types, i.e. between interior columns and cores and shear walls on the one hand and perimeter columns on the other.