Acceptable interstorey drifts at ultimate limit state

Technical Abstract

Design engineers often use simple procedures to analyse buildings and to calculate the displacements between adjacent storeys which are referred as “interstorey drifts”. These calculated interstorey drifts need to be scaled up to account for the building’s ability to deform beyond its elastic range during a design level earthquake. The increased drifts are required to be within acceptable limits prescribed in the earthquake loadings standard NZS1170.5 (Standards New Zealand 2004). The aim of the current research is to confirm the published interstorey drift scaling method and, if necessary, to develop rational alternative scaling procedures to justify compliance with the drift limits stated within that standard.

The methods of analyses that are recommended in the standard are: (a) “elastic design” procedure, which is simple and reasonably accurate within the elastic range but requires the above mentioned scaling, and (b) “inelastic time history analysis” (ITHA) procedures which is considered to be “time consuming” but to provide “accurate” results. It is important that there is consistency between the drifts calculated using ITHA (seldom used for design in NZ) and those derived using elastic analysis techniques.

The study has involved the development of fifteen buildings with different structural forms (frames, walls and braces) and construction materials that were compliant with the interstorey drift criteria stipulated in the loading standard and their respective materials standards. The selection of the building form used involved two seismic settings (High – Wellington and Low – Auckland), two site classes (shallow soil and deep soil, each of which was assigned a suite of 7 compliant ground motion records), three structural forms (Reinforced Concrete Frame, RC Wall and Eccentrically Braced Steel Frame) and three storey heights (3, 10 and 20 storey plus light roof above each upper floor). Each structure was then re-analysed using ITHA techniques with selected earthquake records that complied with the provisions of 1170.5.

Three key findings evolved from this study:

• The post-elastic displaced shape of the wall and braced buildings did not align with the elastic displacement shape. Therefore scaling of the elastic profile for these buildings remains a dubious approach. However, for reinforced concrete frame buildings, the post-elastic displaced shapes were similar to the elastic profile. 
• The inelastic responses were more localised in the bottom storeys of the buildings and the extent of inelastic behaviour was dependent on the structural forms and the seismicity assumed for the buildings.
• Consequently, a single value of drift modification factor need not be applied up the height of the building and for all types of structural form as recommended by current provisions rather the factor can be appropriately varied as suggested in this study.