Abstract
Modern seismic design codes, which are based on capacity design concepts, allow formation of plastic hinges in specified locations of a structure. This requires reliable estimation of strength of different components so that the desired hierarchy of strength of the structural components can be ensured to guarantee the formation of plastic hinges in the ductile elements. As strength of longitudinal reinforcing bars governs the strength of reinforced concrete members, strain-ageing, which has significant effect on the strength of reinforcing bars, should be given due consideration in capacity design. Strain-ageing can increase the yield strength of reinforcing steel bars and hence the strength of previously formed plastic hinges, thereby likely to force an unfavourable mechanism (such as strong beam-weak column leading to column hinging) to take place in subsequent earthquakes. In this paper, the strain-ageing effect of commonly used New Zealand reinforcing steel bars is experimentally investigated. Common New Zealand steel reinforcing bars are tested for different levels of pre-strain and different time intervals up to 50 days, and the results are discussed focussing on the extent of strain-ageing and its possible implications on seismic design provisions. The results indicate that designers need to use a higher flexural strength (in addition to overstrength) for the weaker member in checking the strength hierarchy in capacity design of reinforced concrete frames. Similarly, in designing retrofit measures to restore a damaged reinforced concrete member engineers need to take into account an increase of yield strength of the reinforcing steel bars employed in the member due to the strain-ageing phenomenon and the extent of increase in the yield strength depends on the level of damage.
Highlights
Uni-axial stress-strain behaviour of steel has been investigated over the past century
This work shows that the strain-ageing phenomenon does exist for the investigated reinforcing steel bars which are commonly used in New Zealand
The effect is most pronounced when steel bars are subjected to a high residual strain and again exposed to a plastic loading after a long time
Summary
Uni-axial stress-strain behaviour of steel has been investigated over the past century. Limited research has been carried out on the effects of strain-ageing on the mechanical behaviour of reinforcing steel under monotonic and cyclic conditions. Bauschinger (1887) observed an increase in the maximum load-carrying capacity of a piece of steel after it was tested in the inelastic range, left for some time and retested. Physical basis of this phenomenon can be interpreted by time dependent diffusion of small elements such as carbon and nitrogen to crystal flaws (e.g. dislocations). Plastic deformation (yielding) is triggered by movement of dislocations through the crystal, whereas the presence of small atoms (e.g. carbon and nitrogen) can markedly increase the sliding resistance between the atoms and increase the yield strength
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