56 — Build 141 — April/May 2014

Resilient buildingsFEATURESECTION

BUILDINGS ARE CENTRAL to people’s lives, and their continuous function after a destructive event is a major challenge to the resil-ience of a city or even a country.

Buildings saved lives but at a costIt was not until the Canterbury earthquakes in 2010 and 2011 that people understood the importance of buildings continuing to function after a catastrophic seismic event.

One lesson from the earthquakes was that there is a significant gap between the performance criteria in the New Zealand Building Code and general societal expectations for building performance in earthquakes.

Most modern designed buildings in the Christchurch CBD achieved the Code-specified performance criteria at ultimate limit state – the

Is it worth it?

buildings did not collapse, presenting a life safety issue. They were extensively damaged, however, and in many cases needed to be demolished and rebuilt, causing significant and lengthy disruption to business activities and normal daily living.

Damage was well beyond people’s expectations and resulted in a considerable cost to the country.

Looking for the financial thresholdThis research project’s premise is that there will be a financial threshold at which the increase in the initial construction cost for a building will be balanced by the economic benefits over the entire building life cycle.

The current Building Code is performance-based with the aim of ensuring life safety. From an economic standpoint, information on the investment versus financial benefit relationship is urgently needed by all stakeholders to assist decision-making for targeted building performance. It will also be essential for consideration during future changes to the Building Act and Building Code.

Different building types and lateral seismic resistanceThis project considers the relationship between cost and benefit for typical building types.

Additionally, it will also look at the variations in the inherent resilience of different lateral seismic resisting systems. Potentially, different lateral seismic resisting systems could have very different earthquake resilience levels, and the determination of the seismic design level for a certain type of building is the result of a risk versus economic consideration.

Resilience of buildings to earthquake events is being studied separately for different lateral load resisting systems. A small sample

A current BRANZ research project is studying the economic benefit of designing buildings for increased resilience under seismic loading

throughout their entire life cycle.

BY ANGELA LIU, BRANZ SENIOR STRUCTURAL ENGINEER, GRAEME BEATTIE, BRANZ PRINCIPAL STRUCTURAL ENGINEER, AND IAN PAGE, BRANZ MANAGER ECONOMICS

Build 141 — April/May 2014 — 57

of model buildings is being designed for each system and will be subjected to different earthquake intensities. A wide-ranging cost-benefit study will be carried out for the buildings’ entire life cycle to investigate whether higher performance is likely to be economically justified.

Indirect and direct costs studiedThe economic study includes both the direct and indirect costs from earthquake events, including aspects such as business interruption.

A comparison of the inherited resilience of different structural systems to earthquakes is being carried out in order to better inform stakeholders’ decision-making on the structural design.

First up – reinforced concrete framesThe first stage of the project is well under way and has focused on reinforced concrete-frame structures. The subsequent stages of the research will focus on other structural types and a comparative study of the seismic performance of different structural types.

Two identical reinforced concrete-frame buildings were designed and analysed, with different lateral seismic load resisting systems. They were designed to the same current seismic design standards but using different ductility assumptions. As a consequence, the lateral load resisting systems of the two were different.

Findings raise issue with regulationsStructural analyses carried out so far have concluded that reinforced concrete-frame buildings designed to the same current seismic design standards would be expected to sustain different damage levels, and therefore different economic consequences, in the same design earthquake event if different ductility assumptions were made.

The study suggests that building structures of different structural systems would potentially be expected to achieve very different performance levels for the same design earthquake. This implies a very different economic scenario, especially over the life cycle of a building.

This is contrary to the principles used in developing the current building regulations. In these, buildings designed according to the current design standards are meant to achieve the same minimum building performance levels, regardless of the structural types and design assumptions.

Study outcome to better inform allThis project runs until late 2015 and expects to better inform owners, engineering practitioners, banks and insurance companies and the building regulation policy-makers about the balance that needs to be struck between acceptable levels of risk and the costs of mitiga-tion and resilience.