Assessment of the behaviour factor for the seismic design of reinforced concrete structural walls according to SANS 10160 : part 4
Thesis (MScEng (Civil Engineering))--Stellenbosch University, 2008.
The South African code for the design loading of building structures, namely SABS 0160 (1989), was revised with the requirements for seismic design prescribed in SANS 10160: Part 4: Seismic actions and general requirements for buildings. SANS 10160: Part 4 incorporates the seismic design provisions of several seismic codes of practice, however, the influence of the value prescribed for the behaviour factor has not been established with regard to South African conditions. The behaviour factor is used by most seismic design codes to account for the energy dissipating effects of plastification in structural systems when subjected to earthquake ground motion, to reduce the elastically determined forces to be designed for. However, a considerable difference is observed in the values of the behaviour factor prescribed for the design of reinforced concrete walls between the leading international seismic codes. The aim of this study is to assess the value of the behaviour factor prescribed in SANS 10160: Part 4 for reinforced concrete structural walls under the influence of South African seismic conditions and code requirements. A method of quantifying the value of the behaviour factor was developed and implemented in the study by Ceccotti (2008). This method entails estimation of the maximum analytical behaviour factor as the ratio of seismic intensity at failure of the structure to the seismic intensity prescribed by the design code. Such a method is adopted for this study where the lateral force resisting systems of six-, eight- and tenstorey buildings are investigated with nonlinear static analysis to quantify the maximum computationally-determined value of the behaviour factor. Firstly, it is observed that it is possible to quantify the value of the behaviour factor through the use of a computational study. The nonlinear static method of analysis is shown to provide reliable results in the estimation of the behaviour factor for a sixstorey building, however, does not perform well for taller buildings. Further investigation with the use of dynamic time-history analysis is proposed to evaluate the influence of the factors identified in this study. The behaviour of structural walls, designed for reduced forces with the prescribed behaviour factor of 5.0, exhibits high yield strengths and resists the design seismic action entirely elastically. This high strength is found to be due to the reliability/redundancy factor prescribed by SANS 10160: Part 4 and because of the high values of structural overstrength. Similar studies observed high values of structural overstrength for buildings designed for low seismic intensity, which were shown to result from the fact that the resistance required to gravity loading became more critical than the seismic loads in the design of the structural system. This study identifies several factors that influence the value of the behaviour factor, such as the number of walls in the lateral force resisting system; the number of storeys of the buildings; available displacement ductility of the structural system; and the ground type designed for.