What is meant by reinforcing steel?

In reinforced concrete structure, steel which is equally strong in compression and tension, is used to combine with concrete to improve the resistance of concrete to tensile force!, The steel used to provide reinforcement in concrete structures is termed reinforcing steel. In earthquake resistance reinforced concrete structures, reinforcing steel plays an extremely important role which is significantly more demanding that its basic function. This is due to the philosophy of capacity design of reinforced concrete structures to utilize both strength and energy dissipation characteristics of the system • The energy dissipation characteristics are utilized to absorb and dissipate the dynamic seismic loads to avoid brittle failures. This energy dissipation mechanism relies on the ductility of the structure in the post-elastic range. In the strong column-weak beam design concept, the ductility of the structure is ensured by the development of plastic hinges in beams adjacent to column-beam joints in preference to hinges forming in the columns. The absorption and dissipation of energy by post-elastic deformation in plastic hinges depends almost entirely on the ductility of the reinforcing steel • Steels used for reinforcement in this structure should be capable of accommodating significant amounts of strain without failure. Therefore, the ductility of reinforcing steel becomes an important requirement in the design of earthquake resistant reinforced concrete. The plastic hinge behavior of reinforced concrete members is also very dependent on the stress-strain characteristic of the reinforcing steel • During an earthquake, strains in the steel of plastic hinge regions may increase beyond the Luder strain, consequently strain-hardening occurs. This strain increase may lead to large strength increases, particularly if the strain hardening rate of the steel is high and if the steel has a short Luder strain which results in strain hardening occurring soon after yielding. Ideally, the Luder strain should be as large as possible so that the plastic strain is accommodated with a minimum of strain hardening. As a result of this flexural over strength, during subsequent earthquakes, plastic hinges may be formed in regions which have not been designed as such. Thus, relocation of the plastic hinge within the structure could give rise to an undesirable failure mode. In the design of seismic resisting concrete structures, an over strength factor which is greater than unity is included in the calculation of the steel stress at the beam plastic hinges to take into account the possibly large increase in flexural strength. Strain ageing of reinforcing steel also has a significant effect on the properties of seismic reinforced concrete structures • Strain ageing of the longitudinal reinforcing steel at plastic hinges subsequent to the first formidable seismic loading can increase the flexural strength at the plastic hinges as a result of the increase in yield strength of the steel during the ageing process. The flexural over strength brings the same effect as when strain hardening of steel occurs, i.e. causing the plastic hinges to form at alternative and undesirable regions in the structure during subsequent earthquakes. Cold bent reinforcing bars in the form of standard bends, returns or hooks contained in most regions in reinforced concrete structure will strain age during service at ambient temperature • As a result of strain age embrittlement, these strain aged regions will be susceptible to brittle failure, which may cause catastrophic fracture of the structure. It is therefore, very important to understand the effect of strain ageing on the mechanical properties of reinforcing steel used in earthquake resistant reinforced concrete structures. Unfortunately, information regarding strain hardening and strain ageing of reinforcing steels are not specified in appropriate standards, nor is the data on Luder strain.