Modern multi-storey buildings and moderate earthquakes

Technical Abstract

Recent moderate earthquakes overseas have highlighted that the combined cost of direct damage and indirect costs from such events can be significant. In the case of multi-storey buildings the non-structural damage is likely to be considerable, such that the repair and reinstatement process will take a long period of time and involve extensive occupancy disruptions.

This project integrates recent work from a number of different sources to enable a realistic moderate earthquake scenario to be portrayed, and foreshadows the likely impacts and process issues.

Key Issues Examined

1. Characterising moderate earthquakes in terms of parameters used in the engineering design of new buildings.
2. Quantifying the level of lateral drifts anticipated for modern multi-storey buildings.
3. Identifying the non-structural damage that could potentially be caused by such movement.
4. Highlighting the major issues and likely durations associated with the damage assessment and repair process.

Principal Findings

• A moderate earthquake can be characterised as one that generates MM8 intensity on intermediate soils. In Wellington, such an event is considered to be represented by a magnitude 6.0 to 6.5 earthquake which has a return period of approximately 140 years (probability of occurrence 30% in 50 years).
• The maximum likely interstorey drift of a 13 storey moment resisting frame building designed to NZS 4203:1984 in such an event was found to be 10 to 15mm, and the corresponding local displacement ductility demand between 1.5 and 2. This indicates that a moderate earthquake is likely to cause structural damage to a number of buildings of this type, in addition to extensive non-structural damage.
• The likelihood of structural damage occurring under MM8 shaking contrasts with the qualitative damage descriptions for the lower values of the MM scale, whereby damage is not implied to modern construction until higher levels of shaking.
• Damage ratios that are commonly applied to modern multi-storey buildings for MM8 intensity shaking are around the 6 to 8 % level. First principles assessments based on the above drift levels are used to suggest that ratios of 7 to 30% may be more appropriate. This indicates that typical damage ratios for moderate earthquake intensities represent lower bound damage estimates only.
• Consideration of the insurance and engineering processes for damage assessment and repairs following earthquakes has highlighted that even for buildings sustaining non-structural damage, there are a number of interactive steps involved. These steps can involve owners and tenants with separate but inter-related building, contents and business interruption policies. Analysis of the impact of the likely interaction plus the disruptive effect of repairing non-structural damage has indicated that some multi-storey buildings could take between 5 and 7 weeks before full re-occupancy can be assured.
• An empirical approach for assessing indirect losses for businesses operating in modern multi-storey buildings has been developed. While further development of this methodology is required, it is thought to be of value to tenants in establishing the level of business interruption cover they should have in place.
• An indicative descriptive scenario which emphasises the human reaction to and impact of a moderate earthquake is also included.

Recommendations

General

• There needs to be a better awareness of the level of damage and disruptive impact of earthquakes that are smaller than "design" events but correspondingly more likely to occur.
• The limited protection to non-structural and contents elements in moderate earthquake events afforded by modern ductile frame structures needs to be more clearly conveyed by designers to owners and prospective tenants. This category of construction is essentially untested in terms of actual earthquake exposure in New Zealand.

Specific

• A working party comprising insurance adjusters and earthquake engineers be established to map the claims process for commercial premises following earthquakes, and to identify prior training needs and post-event guidance requirements for the engineering profession and the insurance industry.
• Given the extensive use of the MM scale in the analysis and description of historical earthquakes, an improved set of damage descriptors for modern buildings under MM8 and MM9 shaking in terms of both non-structural and structural performance should be developed.
• The findings from this study in terms of structural performance parameters should be further examined with regard to the frequency and duration of shaking from the scenario spectra. This aspect could involve the application of appropriately scaled time history earthquake traces to sample buildings.