Christchurch NZ
AEES Earthquake Reconnaissance Mission

Dr Helen Goldsworthy and Prof. Hong Hao
Wednesday 9th to Monday 14th March 2011

Dr. Helen Goldsworthy (University of Melbourne, AEES committee member)
Professor Hong Hao (University of Western Australia, President of the AEES)

(Follow-up report here)

Introduction

Helen arrived close to midnight on Wednesday, 9th March. In the taxi on the way to her motel some collapsed masonry facades and fences, and crack and bumps on the road, were reminders as to why she was in Christchurch. An aftershock during the night and the sound of helicopters buzzing around in the morning were further reminders. She joined Hong Hao, Associate Prof. Nawawi Chouw from the University of Auckland and his student Sushil on the Thursday to be shown around by Greg Cole, a Ph.D. student from the Department of Civil and Natural Resources Engineering at the University of Canterbury and a excellent guide. The group observed first hand the dramatic effects of liquefaction, ground vibrations and landslides on buildings, roads and bridges in the area surrounding the city centre. On the Friday, Associate Professor Rajesh Dhakal, also from the Department of Civil and Natural Resources Engineering at the University of Canterbury, generously spent the day driving the group around to study the damage to bridges in particular, and also building damage close to the fault rupture at Lyttleton.

The cordoned-off city centre was only accessible to people who'd been through an induction process and who needed to be involved in some legitimate endeavour in that area. Nawawi and Sushil had already obtained a "red pass" from an induction they'd completed after the Darfield earthquake. Helen and Hong obtained the pass after completing an induction together with other teams from around the world, as well as volunteer structural engineers from around New Zealand. Bruce Deam, a lecturer at the University of Canterbury, was in charge of the organisation of the overseas groups; over 80 people in total we were told. Professor Roberto Leon from the Georgia Institute in the U.S. was one of those people; Helen had met him previously at the ICSCS conference in Sydney in 2010. The inducted groups volunteered to help the council with building assessments in the cordoned-off city centre. On the Saturday Hong, Nawawi and Sushil joined one team and Helen joined another smaller group. Hong's group managed to complete their assessment tasks very efficiently leaving some time for an informal tour given by their group leader who pointed out some of the more interesting failures. Helen's team spent quite some time waiting for the locksmith to arrive to open a couple of her team's buildings, and so all of her day was spent on the building assessments. Hong, Nawawi and Sushil left on the Sunday. Helen remained to continue the building assessment tasks with her CPEng colleague Steve Lough, director of LoughDowney in Auckland. This time there were some interesting buildings to assess and a chance to look around at other failures, once again while waiting for the locksmith to arrive. She met another AEES member, Peter McBean (Director of Wallbridge and Gilbert), by chance at lunchtime. He'd taken 2 weeks off a busy work schedule in Adelaide to volunteer for USAR activities. His group had been inside the Grand Chancellor building (access was severely restricted because this 27 storey hotel had been severely crippled; at one stage there were fears that it would topple) and he showed her various photos that illustrated the failure mode and the collateral damage that followed. Peter mentioned that another AEES member, former president, John Wilson, had only recently left Christchurch; he'd also been acting in a USAR capacity.

General comments

In the September 2010 Darfield earthquake the presence of a layer of gravel hundreds of metres deep under Christchurch was said (in a report by Tonkin and Taylor) to be responsible for the amplification of the acceleration response in the higher period range. In the Darfield earthquake a hump in the spectra in the higher period range was centred at a period of about 2.5 seconds. In the February 22nd 2011 Lyttleton earthquake an even higher acceleration response was recorded in the city centre and it remained high up to a period of about 2 seconds.

Some low-rise rigid structures in the city centre with lateral force-resisting systems provided by shear walls or stiff frames behaved very well. They had sufficient strength to withstand the high accelerations and were not subject to a large displacement response. However, the presence of heavy masonry elements that were poorly attached to the structure, or of plan irregularities that induced a strong torsional response, resulted in considerable damage in some of these low-rise buildings.

It is of interest to note that design or retrofit of buildings using base isolation is likely to be ineffective for many buildings in the Christchurch city centre because of the nature of the response spectra. It could even aggravate the problem if, for example, a stiff (low period) structure were moved into the higher period range (with potentially higher acceleration response if the period were still less than 2.5 seconds).

Liquefaction

• The approach to bridge walkways were often severely affected by lateral spreading and settlement
  
• Path and roads running parallel to the river were sometimes badly damaged by severe fissures and settlement.
  
• Bridge abutment piers have not always been driven at an angle. Even if they have been, liquefaction causes the banks to move inwards and the abutments tend to tilt backwards as a result. The bridge deck is put into compression and this sometimes causes localized damage at the ends. It may be possible to allow for this inward movement of the abutments in future designs, while also giving consideration to providing a rubber barrier to absorb the energy from potential pounding.
  
• Evidence of rigid body torsional motion of bridge decks, possibly due to relative lateral movement of the abutments
• Slabs on ground and building structures sometimes highly distorted by the effects of liquefaction, eg. heaving, fissures, uneven settlement, partial loss of foundation etc
  

Masonry Construction

In many cases masonry cavity walls fell out of plane in their entirety or partially. Sometimes both the outer and inner skins fell and in other cases just the outer skin. It was often observed that there were very few ties, and/or that the ties that were there were poorly designed, and/or the mortar was very weak.

Out-of-plane gable failures were prevalent. The flexible nature of the roof and ceiling diaphragms in some of the older buildings and churches may have been a contributing factor. Sometimes anchors had been installed in a prior retrofit, however the triangular tops of the gables were still vulnerable.

In some cases parapets were supported by running a beam behind them, but there was still evidence of loose capping and masonry that would present a falling hazard at the top and front of the parapet. Unless it were possible to reliably secure these heavy falling hazards, it would be prudent to remove the parapet in its entirety.

The band beam at the roof level of some of the masonry structures was often not tied in well to the rest of the structure. In some cases the beam itself became a heavy dangerous falling hazard.

Rods were sometime observed at intervals along the top timber beam in shops. They'd been attached to the masonry facade by drilling holes into the masonry and filling this with epoxy. This anchorage was inadequate and the masonry had fallen.

Glass panels

Edge supports for glass panels often not detailed sufficiently to allow movement, even in some of the modern buildings.

Pounding

Minor effects of pounding were observed when buildings were close together.

Pounding due to high vertical accelerations was an issue in bridges. This caused crushing of the cover concrete in bridge piers in one case. In another the edge of a long wall support was knocked off at the top due to intense localized vertical forces from the bridge bearings (as the wall pounded on the bridge deck from below).

Torsional effects

This was undoubtedly a problem in many buildings, eg. buildings on corners with stiff walls along two adjacent edges and flexible frames along the other two.