The results of several thousands of inelastic time history analyses, which have been made on single degree of freedom structures to assess P-delta effects induced in earthquakes, are reviewed. The principal factors influencing P-delta actions are shown to be the ductility, the duration of the severe ground motion, the level of damping and the period of the structure. A method of designing for P-delta effects for single degree of freedom structures is presented. A limited number of analyses of multi-storey frames and walls indicate that the approach may be used for multi-storey structures. This paper gives background information on the P-delta method of analysis given in an appendix to the commentary of the proposed loading code.

A study group of the New Zealand National Society for Earthquake Engineering has recently completed recommendations for the Seismic Design of Storage Tanks, in a form suitable to be used as a code by the design profession. The recommendations cover design criteria, loading, actions and details and are based on a consistent philosophy of serviceability under the design level earthquake. This paper provides a review of the study group's recommendations.

An investigation considering the influence of loading histories on the performance of a reinforced concrete beam plastic hinge is described. Twelve loading histories were considered, including conventional procedures employed in the United States, Japan and New Zealand, and artificially generated histories derived from recorded earthquake ground motions. Details of the prototype structure and the test beam are described, followed by comprehensive reporting o f experimental data. Performance descriptors and further treatment of the experimental data are presented in a companion paper.

In a major earthquake the beams in moment resisting frames may develop either reversing or unidirectional plastic hinges. The form of plastic hinge depends upon the ratio of the moments induced by the gravity loading to those induced by the seismic actions. Where this ratio is low the plastic hinges form at the ends of the beams and the sign of the inelastic rotation changes with the direction of sway. These are reversing plastic hinges, and the magnitude of the rotation that they sustained is closely related to the inter-storey displacement. However, when the moment ratio exceeds a certain critical value, unidirectional plastic hinges may form. In this case negative moment plastic hinges develop at the column faces and the positive moment plastic hinges form in the beam spans. As the earthquake progresses the positive and negative inelastic rotations accumulate in their respective zones so that peak values are always sustained at the end of the earthquake. With this type of plastic hinge no simple relationship exists between inter-storey drift and inelastic rotation. Several series of time history analyses have been made to assess the relative magnitudes of inelastic rotation that are imposed on the two forms of plastic hinge. It is found that with design level earthquakes typically the unidirectional plastic hinge is required to sustain 21/ 2 to 4 times the rotation imposed on reversing plastic hinges, with the curvature ductilities ranging up to 140. These values are appreciably in excess of the values measured in tests using standard details. This indicates that in structures where unidirectional plastic hinges may form, the design displacement ductility and or the allowable inter-storey drift should be reduced below the maximum values currently permitted in the New Zealand codes. The problems associated with the formation of unidirectional plastic hinges can be avoided by adding positive moment flexural reinforcement in the mid regions of the beams. By this means the potential positive moment plastic hinges can be restricted to the beam ends.

A series of ductile moment resisting reinforced concrete frames are sized to meet the minimum seismic provisions of the New Zealand Loadings Standard, NZS 4203-1992, the Draft NZ/Australian Loadings Standard, the Uniform Building Code, UBC-1997, the International Building code, IBC 2000 (1998 draft) and Eurocode 8 (1998 draft). The results of the analyses allow valid comparisons to be made between the different codes. It is shown that comparisons of individual clauses can be misleading due to the many interactions that occur between clauses. Comparative analyses were made for the buildings described above located in both high and low seismic regions. It is shown that the strength and stiffness requirements for both the New Zealand Loadings Standard and the Draft Standard are low compared with the other codes of practice in the high seismic zone. It is recommended that the required design strengths in the Draft NZ/Australian Standard be increased.