Design for movements in buildings
The greater parts of codes of practice and textbooks on reinforced concrete focus on design to resist externally applied loads, deriving the reinforcement needed to resist axial loads, bending moments and shear forces. However, many concrete elements are lightly loaded or are affected principally by other actions, such as early-age contractions, temperature and humidity effects, creep, and long-term drying shrinkage. These all generate movements, and although they rarely determine the ultimate capacity they often affect serviceability, particularly cracking.
The first step in understanding movement is to distinguish clearly between types of movement (e.g. deflection, shortening, linear expansion and contraction) and sources of movement (e.g. dead and imposed loads, early-age contractions, drying shrinkage, temperature variations). These all take place over different timescales, and time is an important parameter in understanding movement. It is also important to recognise that the sources act cumulatively, so that any cracking or deformation is usually at least the result of shrinkage and temperature added to early-age effects, and often with contributions from other sources.
The importance of movement is highly dependent on whether it is restrained or not. Restraint occurs in many ways: internally (from reinforcement or temperature differentials), at an interface (fresh concrete is placed on an older substrate), at an edge (when a wall is poured onto a footing or a slab against a previous pour) and between ends (between stiff points, such as cores). In practice, all restraint is partial as restraints will usually ‘give’ under the huge forces that can be generated.
Creep is beneficial in reducing the stresses induced by restraint, especially at early ages when a 50% reduction can be achieved in a few weeks. Additionally, while the forces are potentially very large they are strain-induced, so that if the restraint is removed they reduce or even disappear. A final point is to understand cracking. The likelihood cracking occurring is very difficult to predict, and the preferred strategy is to assume that cracks will occur and to provide enough reinforcement to control them.
Guidance on these various aspects is given in Concrete Society Technical Report 67, Movement, restraint and cracking in concrete structures.