Civil Engineering Association

Reinforced Concrete under Large Seismic Action

Seismic design procedures were first incorporated in building design codes in the 1920s and 1930s when inertial loadings began to be appreciated. In the absence of reliable ground measurements during an earthquake as well as detailed knowledge of the dynamical response of structures, the seismic action was taken into account for design purposes as a statical horizontal force corresponding to about 10% of the weight of the structure.
By the 1960s ground measurements during an earthquake in the form of accelerograms were becoming more generally available. At the same time the development of strength design philosophies and of computer-based analytical procedures such as the spectral-modal analysis and the time-history analysis, facilitated the examination of the dynamical response of multi-degree-of-freedom structures (MDOF). According to these procedures, the calculations were carried out in a deterministic fashion. The response of the structure was assumed to be in the elastic range and the earthquake loading was taken into account considering the typical seismic intensity and soil nature of the site of the structure.
As the records of strong ground motion were increasing, it became apparent that the code provisions were inadequate in providing the required structural strength of the building to withstand an intense earthquake. This was recognized analysing the damage in structures that had been close to resonance. In fact the vibrating masses of structures in such a situation had often been exposed to accelerations from two to six times the maximum base acceleration that, of course, would induce forces on the structural elements much larger than expected in the design phase. However the lack of strength did not always result in failure and sometimes not even severe damage. On the other hand in specified regions of the structure (especially the ones with shear dominated behaviour) a rapid reduction in strength (brittle failure) was observed leading to local failure that often resulted in the formation of mechanisms and consequently collapse of the structure.
This type of observations called the attention of structural engineers to the property of the materials or of the structures to offer resistance in the inelastic domain of response. This property is generally known as ductility and includes the ability to sustain deformations in the inelastic range without significant loss of strength and a capacity to absorb energy by hysteretic behaviour