Performance of ductile reinforced concrete moment resisting frames subject to earthquake actions

Abstract

With the introduction of the Loadings Standard NZS 4203:1992 (Standards New Zealand 1992), there was a marked reduction in the minimum strength required in multi-storey buildings. It has been shown that the strengths specified in this code to resist seismic actions are low when compared with major international design codes (Fenwick and Davidson 1994; Fenwick et al. 2002). Few modifications to these low strengths, other than an increase in the minimum permissible base shear, have been made in the draft revision of the Standard, NZS 1170.5. The first objective of this work is to investigate if these strengths are adequate to meet the stated objectives of the new Loading Standard for seismic actions, NZS 1170.5.

To design a ductile multi-storey moment resisting frame structure, it is essential to provide the columns with sufficient strength to prevent the premature formation of a column sway mechanism. In the 1970s a method was developed and adopted in the structural concrete Standard NZS 3101:1985 and 3101:1995 (Standards New Zealand 1995). This approach introduced the concept of a dynamic amplification factor, which was applied to the column moments to allow for "higher mode effects", which cause the distribution of column actions to diverge from those found in a static elastic analysis.

However, due to technology restrictions at that time, the dynamic amplification factor was estimated through a limited number of non-linear time-history studies using a bi-linear hysteretic model and without considering P-delta effects (Park 1995). The second objective is to examine how well this method of determining column actions works when using the lateral loading specified in NZS 4203:1992 (Standards New Zealand 1992) and the draft provisions of the proposed Loadings Standard NZS 1170.5 together with a more realistic hysteretic model and the inclusion of P-delta actions in the analysis.

When the proposed new Loading Standard NZS 1170.5 was being developed, an anomaly was apparent in the current requirements for columns. In ductile structural steel moment resisting buildings much lower strengths were required than the corresponding levels in reinforced concrete frame buildings. In an attempt to remove this anomaly, a second method of defining the design actions in columns was introduced. The intention was that this could be used for both structural steel and reinforced concrete multi-storey moment resisting frame buildings. With this approach, in a major earthquake plastic hinges could be expected to form at several levels over the height of the columns, while with the approach given in NZS 3101:1995 (Standards New Zealand 1995) plastic hinges would only be expected at the base of the columns and in the top storeys. In the revised edition for the NZS 3101:2006 it is intended to give the designer the option to select either method to determine the required column strength. However, when plastic hinges are allowed, it should not be overlooked that potential plastic hinge regions need to be detailed accordingly so that proper confinement, antibuckling reinforcement and adequate shear strength is provided by the transverse reinforcement and lap splices are located away from the potential plastic hinge zones.

The third objective is to compare the performance of multi-storey moment resisting frame
buildings where the columns are modelled as:

1. Elastic responding columns except at the base,
2. Columns designed to meet the minimum requirements as given in NZS 3101- 1995,
3. Columns designed to meet the minimum strength requirements as defined in the 2004 draft of NZS 1170.5 where limited protection to plastic hinge formation is given.