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Christchurch earthquake reconnaissance part 2: Day 4

Earthquake performance of a seismically retrofitted building.  This building had steel moment frames around the perimeter of the first level with plywood diaphragm strengthening under the second floor framing.  Collapsed URM second level exterior walls and a collapse prevention performance level for level 1.  Christchurch CBD, New Zealand.

Day 4 – Monday 3/14/11

With a large team consisting of engineers from the EERI Reconnaissance Team and SEAOC (Structural Engineers Association of California), I start the day investigating buildings surrounding the locations of 18 accelerographs, or strong ground motion sensors.  These strong ground motion sensors are devices that measure the accelerations caused by earthquake shaking.  They measure the accelerations in 3 principal directions, X, Y, and Z (up and down).  These accelerations impart forces on buildings, bridges and other structures.

The goals of our investigation for today is to determine the spatial distribution of earthquake damage relative to building type and gather data from the accelerographs. The engineers from SEAOC are using the data gathered to enhance and validate PBEE (Performance Based Earthquake Engineering) design guidelines and standards.  The buildings we investigate consist of low, mid, and high-rise commercial, some residential and several large historic churches.  The types of engineered structures are mostly cast-in-place or precast concrete frame and shear wall with some steel frame buildings.  Buildings that were not engineered are mostly residential and light mixed use commercial buildings consisting of wood frame and masonry construction.  Unfortunately, but as expected, damage patterns were as follows:

1. Significant collapse in weak and heavy URM (unreinforced masonry) and non-ductile concrete buildings.

Earthquake performance of a seismically retrofitted building. This building had steel moment frames around the perimeter of the first level with plywood diaphragm strengthening under the second floor framing. Collapsed URM second level exterior walls and a collapse prevention performance level for level 1. Christchurch CBD, New Zealand.
Earthquake performance of a seismically retrofitted building. This building had steel moment frames around the perimeter of the first level with plywood diaphragm strengthening under the second floor framing. Collapsed URM second level exterior walls and a collapse prevention performance level for level 1. Christchurch CBD, New Zealand.

2. Less damage occurred in light wood frame buildings compared to heavier buildings.
3. Building corners and overall building performance was significantly affected by discontinuous structural elements and irregularities in construction.
4. Collapse of a number of street facing URM facades was exacerbated by falling parapets and façade supported awnings.

Another seismically retrofitted building. Out of plane wall failure of the level 2 URM wall onto the level 1 entry awning below. Note the vertical steel channel wall “pilaster” at the left side URM wall corner. Christchurch CBD, New Zealand.

5. Wall anchors with rosettes worked well in supporting URM walls with the exception of gables.
6. Many historic churches constructed of unreinforced masonry and stone with wood arch roofs did not fare well.

This church had moderate damage from the September 4, 2010 Darfield Earthquake and was under repairs at the time of the February 22, 2011 Earthquake. Damage consisted of out-of-plane failures to URM gable walls and complete collapse of the URM bearing wall supporting roof structure of the main sanctuary (on right side in the photo). The heavy timber roof structure rode down the crumbling bearing walls and was resting relatively intact in a grassy yard partially on top of some trees in a adjacent property to the right (not shown in photo). Christchurch CBD, New Zealand.

As in the previous days’ investigations, we see that even though some of these buildings had stabilization measures and seismic retrofits installed following the September 4th earthquake, they still collapsed.  As expected, some of the collapses were due to interruptions in the seismic system load path or inadequate ductility and toughness in building columns, walls, and connections.

Earthquake damage to a bike shop in Christchurch CBD. Note the small section of tall parapet wall still standing due to lateral support by the parapet wall brace. Christchurch CBD, New Zealand.

We finish our morning investigation and data collection and later in the day we travel to the small towns of Heathcote, Sumner, and Redcliffs, to observe buildings in these more rural and seaside residential communities.  A popular summer beach destination, the town of Sumner is nestled in a small seaside valley with only 2 ways in or out of this tranquil seaside town.  You either use a single 2-lane steep mountain road or a single multi-lane highway along the water.  Unfortunately, several residents that I speak with note that the earthquake has significantly impacted recreation and tourism at this and many other seaside towns.  We find the beach sparsely being used on this warm late summer day (it is summer, soon to be fall, in the southern hemisphere). There was significant loss of life due to rock falls onto roadways and buildings adjacent to steep slopes and cliffs in this region.

Roadway damage from slope instability along a steep road near Sumner. This type of road damage was prevalent along steep mountainous roads. A particular problem for narrow roads on curves as roadway damage causes the narrow 2 lane roads to become narrow one lane roads until slope stabilizations and repairs can be made. WInter rains and weather will make these damaged roads even less stable if these are not repaired quickly. Sumner, Christchurch Suburb, New Zealand.

Back in Christchurch for the evening, we attend a meeting and briefing hosted by the University of Canterbury Civil Engineering Department where university researchers and engineering research, reconnaissance, assistance teams share their experiences.  We, as well as teams from Japan, Australia, England, and Taiwan, are asked to make a brief presentation on our initial observations.  Several public officials provide briefings on current issues and their current needs.  They ask the structural engineering community in attendance for any assistance and advice we can provide.  Specifically, they are seeking advice and ideas on the following:

1. Stabilization of partially collapsed buildings.
2. Reopening of roads and public ways adjacent to these structures.
3. Insight on how other communities instituted public processes for building deconstruction when these buildings pose a public safety threat.
4. Installing emergency stabilization measures for cracked rock faces and landslide prone areas above homes and roadways before the coming winter (it is currently fall).

With those in mind, I am following up with our New Zealand counterparts with some post-earthquake recovery information that we have used in the United States.  Items such as our sample disaster recovery ordinances, building demolition process ordinances, building code excerpts, and sample technical standards and guidelines for the repair of earthquake damaged concrete and masonry buildings (FEMA 306, 307, 308).  I am also providing details for the reconstruction of masonry chimneys and related building repair standards that were developed by the Cities of Seattle, San Francisco, and Los Angeles after our damaging west coast earthquakes in 1989, 1994, and 2001 and forwarding rockfall slope stabilization standards from WSDOT, ODOT and CALTRANS.  This information will be placed on the University of Canterbury Earthquake Clearing House website.

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