Monday, April 23, 2012

The Case for Building Instrumentation


Should more buildings in New Zealand be equipped with earthquake recording instruments to measure their response to shaking? GeoNet data have proven to be very important for understanding the extensive damage caused by the Canterbury earthquakes during the last 18 months. We can thank the vision of John Berrill, formerly of the Engineering School at the University of Canterbury, for the high level of GeoNet instrumentation in the Canterbury region (although the major target was recording  an Alpine Fault rupture; see "CanNet: the little network that could!” in GeoNet News October 2010). The many GeoNet strong motion stations provided a very good indication of the extreme levels of ground shaking caused by the major earthquakes (see Figure 1),  but only a single building in Christchurch had been instrumented in the GeoNet building instrumentation programme.  What would more instrumented buildings in the region have told us? Could they have identified buildings damaged in the Darfield (September 2010) earthquake and helped with post-event building evaluations? Could they be helping us make decisions about the rebuild process? Should future buildings over a certain size be instrumented to a specified level as is required in California? Can the one instrumented building in Christchurch provide an insight into the answer to these questions? I will give a little background and then come back to these questions.

Figure 1: The levels of shaking for the top six high impact Canterbury earthquakes. The length of the bars show the vertical and horizontal shaking levels at the indicated sites around the Christchurch area. The vertical shaking in the Christchurch (22 February 2011) Earthquake  exceeded 2 times the force of gravity, and a similar level of horizontal shaking occurred during the June 13 2011 earthquake.

I recently attended the annual conference of the New Zealand Society for Earthquake Engineering (NZSEE) at the University of Canterbury in Christchurch. The theme of the conference was "Implementing lessons learnt” from the Canterbury earthquakes.  The major Canterbury earthquakes were high impact events which inflicted higher than expected levels of damage. There are many reasons for this, the most important are:

  • the closeness of the earthquakes ruptures to Christchurch city;
  • the very high shaking levels;
  • the extensive liquefaction.
Many of the papers presented at the NZSEE conference used GeoNet data as the basis for their analysis (although with limited acknowledgement of GeoNet and its sponsors – the Earthquake Commission (EQC), Land Information NewZealand (LINZ) and GNS Science). What is clear is that much of the damage would have been very difficult to understand and explain without the availability of the GeoNet data showing the actual level of ground shaking. Without data it is very difficult to match expected and actual levels of damage.

It is a common misconception that the aim of our current building codes is to ensure buildings are not damaged by major earthquakes. It is not - the aim is to ensure life safety. Buildings that perform well and save lives may still need to be demolished and replaced following a major earthquake. Critical buildings such as hospitals are built to higher standards, and one way this is done is by using base isolation so the building does not respond as violently to the ground shaking.  Base isolation and other means of shaking energy absorption appear to be very effective at reducing the level of damage, although very few (around a dozen) buildings have base isolation in New Zealand.  Papers presented at the conference suggested the additional cost of base isolation is usually less than 10%.

The GeoNetBuilding Instrumentation Programme (see Figure 2) aims to install multiple seismic instruments in about 30 representative buildings (commercial and residential) and bridges throughout New Zealand to gain insights into the earthquake engineering performance of those structures. To date 10 installations have been completed and several others are in progress. A brochure on the programme can be found here. The list of buildings are chosen to cover the range of building types and were identified largely on the likelihood of capturing useable data, so most are in Wellington or along the east coast of the North Island. There were some planned for the South Island but originally very few for Christchurch.

Figure 2: A typical schematic representation of the components of the seismic instrumentation deployed within a building. The sensors are distributed at various levels of the building and connected through computer network cables to the central recording unit. The GPS receiver provides accurate timing (to less than 1 ms). Wherever possible, one of the sensors is mounted in an enclosure a short distance from the building so as to record shaking levels away from the building. Diagram courtesy of Canterbury Seismic Instruments Ltd.
What is clear is that without instruments in buildings it will always be impossible to know if the damage caused by past large earthquake could have been identified using such instruments. For example, we will never know if some damage could have been detected instrumentally after the Darfield earthquake and before the Christchurch earthquake. The two major changes which could be identified are inter-story drift (floors moving horizontally, relative to each other) and the frequency of the modes of oscillation of a building. Research is needed to identify how much use building instrumentation would be and how the data from instrumented buildings can best be used to detect what has come to be known as “building health”. But in my opinion we should be instrumenting as many buildings as possible, and perhaps there should be a minimum instrumentation standard for buildings of a given size or complexity. It seems like an oversight that few base isolated buildings are currently instrumented. Based on the usefulness of the ground-based GeoNet data for the understanding of the Canterbury earthquakes, how much more could have been added if a selection of the most damaged buildings in central Christchurch had also been instrumented before the earthquakes occurred?

Links:
GeoNet Website
GeoNet Rapid (Beta)
GNS Science
Earthquake Commission
Land Information New Zealand
GeoNet News, Special Darfield Earthquake issue, October 2010
New Zealand Society for Earthquake Engineering
NZSEE 2012 Conference
GeoNet Building Instrumentation Programme
GeoNet Building Instrumentation Programme Brochure

No comments:

Post a Comment