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Preliminary Report on Bridge Damage from the Darfield (New Zealand) M7.1 Earthquake of September 4, 2010



Figure 1: Location of bridges inspected, strong motion sites and earthquake epicenter

MCEER investigators Michel Bruneau (Professor, Dept. of Civil, Structural and Environmental Engineering, University at Buffalo) and Myrto Anagnostopoulou (SEESL Structural Engineer, Dept. of Civil, Structural and Environmental Engineering, University at Buffalo) conducted post-earthquake investigations in New Zealand on behalf of MCEER and EERI's Learning from Earthquakes Program (funded by the National Science Foundation). The following preliminary report was submitted by Bruneau, Anagnostopoulou and Alessandro Palermo (Senior Lecturer in Structural Engineering, Dept. of Civil and Natural Resources Engineering, University of Canterbury, Christchurch, New Zealand) on September 23, 2010.

Overall, bridges in the Canterbury area have suffered little damage, and this damage was mostly limited to the Christchurch and Kaiapoi areas. A number of factors contributed to this behavior.

  1. Most bridges in the Canterbury area are small to moderate spans; such spans are recognized to generally exhibit a more sturdy seismic response, due largely to their symmetry and limited reactive mass.
  2. Like many buildings and other infrastructure in the areas subjected to the earthquake excitations, bridges were generally designed to resist forces substantially larger than the demands imparted by this particular earthquake.
  3. Bridges shared a number of common design features that gave them high seismic resistance.

The bridges that were inspected after the earthquake of September 4, 2010 in the Canterbury area and their location are illustrated in Figure 1 and summarized in Table 1. In addition, Figure 1 identifies the location of the epicenter of the earthquake as well as, the location of six strong motion sites in nearby locations to the inspected bridges.


Table 1: Identification and location of the bridges inspected

ID

Name

Coordinates

A.

Bridge on Gayhurst Rd, Christchurch

43°31’17”S - 172°40’21”E

B.

Bridge on Bridge St, Christchurch

43°31’30”S - 172°43’26”E

C.

Pedestrian bridge on River Rd and Avonside Dr, Christchurch

43°30’55”S - 172°39’56”E

D.

Pedestrian bridge, Christchurch

43°31’04”S - 172°40’37”E

E.

Pedestrian bridge, Kaiapoi

43°23’43”S - 172°39’36”E

F.

Mandeville pedestrian bridge, Kaiapoi

43°22’52”S - 172°39’20”E

G.

Chaney’s Overpass on HW1

43°25’47”S - 172°38’46”E

H.

River bridge, Lincoln

43°28’47”S - 172°30’52”E

I.

Highway 1 bridge over Selwyn River

43°38’55”S - 172°13’40”E

J.

Avondale Rd bridge, Brooklands

43°28’47”S - 172°41’16”E

K.

Pages Rd bridge, Brooklands

43°28’47”S - 172°41’26”E

 

Ground Motions and Comparison of Earthquake Intensity with Design Intensity

Six strong motion sites that are located nearby the inspected bridges are selected and the recorded acceleration time histories are presented in order to give an insight to the damage observed. Table 2 summarizes the identified strong motion sites and specifies their geographical coordinates (http://www.geonet.org.nz/resources/network/index) as well as, the site subsoil class according to NZS 1170.5 (2004).


Table 2: Location of strong motion sites

Site ID

Name

Coordinates

Site Class

CHHC

Christchurch Hospital

43°32’08”S – 172°37’34”E

D

HPSC

Hulverstone Drive Pumping Station

43°30’12”S – 172°42’07”E

U

KPOC

Kaiapoi North School

43°22’41”S - 172°39’49”E

E

PPHS

Christchurch Papanui High School

43°29’40”S – 172°36’24”E

D

PRPC

Pages Road Pumping Station

43°31’39”S - 172°40’58”E

E

SHLC

Shirley Library

43°30’25”S - 172°39’48”E

U

D: Deep or soft soil, E: Very soft soil, U: Unknown


The acceleration time histories for the three orthogonal components are plotted for each of the above strong motion sites (Figure 2a-c, Figure 3a-c, Figure 4a-c, Figure 5a-c, Figure 6a-c, Figure 7a-c) using the ‘Vol2’ recorded acceleration time histories data as provided by http://www.geonet.org.nz (full processing in the frequency domain, correction for the dynamic instrument response, high- and low-pass filtering). Moreover, the acceleration response spectrum for each site and each of the three orthogonal components are plotted assuming 5% of critical damping (Figure 2d, Figure 3d, Figure 4d, Figure 5d, Figure 6d, Figure 7d). Table 3 summarizes the peak ground acceleration for each considered record and component.

Table 3: Peak ground accelerations for considered sites

Site ID

Components

PGA (g)

CHHC

N01W - S89W - Vert.

0.20

0.15

0.16

HPSC

N04W - S86W - Vert.

0.16

0.12

0.13

KPOC

S75E - N15E - Vert.

0.30

0.36

0.08

PPHS

S33W - S57E - Vert.

0.22

0.17

0.28

PRPC

W – S - Vert.

0.20

0.24

0.32

SHLC

S40W - S50E - Vert.

0.18

0.18

0.14


The horizontal acceleration response spectra of each considered strong motion site are compared to the Horizontal Design Response Spectrum as defined in NZS 1170.5 (2004) accounting for the site’s soil class (Figure 2e, Figure 3e, Figure 4e, Figure 5e, Figure 6e, Figure 7e). In the case of strong motion sites that the site class is unknown (identified by U in Table 2), the Design Response Spectra for Soil Class E has been used which is defined as very soft soil sites (NZS 1170.5, 2004).

As shown in Table 3, the maximum horizontal peak ground acceleration (PGA) of the considered strong motion sites is observed at the KPOC station at Kaiapoi (refer to Table 2) and equals 0.36g. In addition, Figure 8 and Figure 9 compare the horizontal and vertical acceleration response spectra for all considered sites with the corresponding horizontal and vertical acceleration design spectra for soil classes D and E, as defined in NZS 1170.5 (2004).


Figure 8

Figure 8: Horizontal Acceleration Response Spectra (ARS) for the six considered sites with Design Response Spectra (DRS) for Soil Classes D and E

Figure 8 shows that there is a significant variation in the intensity of the strong ground motions recorded throughout the Canterbury region. The majority of the response spectra are below the design spectrum for a wide range of periods. However, the response spectra from KPOC and PPHS stations are higher than the design spectra for periods under 1.0 seconds which can be attributed to the existence of very soft susceptible to liquefaction underlying soils.

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Gayhurst Road Bridge, Christchurch

Figure 10a-c shows the Gayhurst Road bridge over the Avon River in Christchurch, in a neighborhood where extensive soil liquefaction took place. River banks had multiple sand boils, the ground surface exhibited many large open cracks due to lateral spreading of the soil, and many residential homes suffered extensive damage due to differential settlements. At that location, in spite of this extensive liquefaction and soil movements, that bridge did not suffer any significant structural damage, except for abutments where vertical cracks in the wall at the edge of bridge deck width developed under the applied soil pressures. This can be explained by the sturdy monolithic structure of this bridge, in both its longitudinal and transverse directions; wide wall piers provided a stiffness and strength largely in excess of the values needed to resist severe seismic excitations transversely to the axis of the span, and continuity of the superstructure from abutment to abutment provided a rigid behavior in the longitudinal direction. Note that while the bridge's wall piers were likely supported on piles, damage to such piles would be difficult to identify if at all present (this would incidentally be the case for all the bridges inspected as part of this earthquake reconnaissance visit).

Gayhurst Road Bridge, Christchurch - Figure 1a
Photo by Michel Bruneau
 Photo by Michel Bruneau
Photo by M. Anagnostopoulou

Figure 10a-c. (43° 31’ 17” S - 172° 40’ 21” E) In spite of extensive liquefaction and soil movements, the Gayhurst Road bridge did not suffer any significant structural damage

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Bridge Street Bridge, Christchurch

The longer Bridge Street bridge connecting South Brighton to Christchurch behaved similarly, with the difference that, in this case, the lateral forces due to lateral spreading at the abutments rotated those abutments about a contact point at the slab level, resulting in a residual rotation of the abutments as well as distortion and sliding of the neoprene bearings (as seen in Figure 11a-c) – the original location of the bearings could be clearly seen on top of the abutments at the point of girder supports (Figure 11-e). As a result of soil lateral spreading and settlement, both abutments moved closer to each other, the West Abutment (from Bexeley Road) moving in contact with the deck.

Photos by M. Anagnostopoulou

Figure 11a-c. (43° 31’ 30” S - 172° 43’ 26” E) The longer Bridge Street bridge connecting South Brighton to Christchurch behaved similarly to the Gayhurst Road bridge.

Photo by Michel Bruneau
Photo by M. Anagnostopoulou

Figure 11d-e. The original location of the bearings could be clearly seen on top of the abutments at the point of girder supports.

 

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Pedestrian Bridges, River Road – Avonside Drive, Christchurch

Some bridges not endowed with similar strengths and stiffnesses to resist the demands applied to them as a consequence of the liquefied soils did not fare as well.

For example, a pedestrian steel truss bridge over the Avon River in Christchurch buckled axially under the axial thrust imposed by the movements of its abutments due to the lateral spreading action of soils on top of liquefied layers (Figure 12a-b). This buckling introduced compressive forces in the bridge truss chord. These forces, unanticipated in the original design, consequently led to local buckling of some chord members, twisting of the superstructure, and uplifting at a support location above one of the braced piers in the river.

 Photos by M. Anagnostopoulou

Figure 12a-b. (43° 30’ 55” S - 172° 39’ 56” E) A pedestrian steel truss bridge over the Avon River in Christchurch buckled axially under the axial thrust imposed by the movements of its abutments due to the lateral spreading action of soils on top of liquefied layers.

 

A pedestrian arch bridge also over the Avon River twisted slightly at its southern abutment; slight spalling was observed at the center span of one of the arches, and a sign posted the bridge as closed due to hazards created by utilities (power pipes) carried by the bridge and exposed at the abutment by the liquefied soils (Figure 13a-b). Horizontal cracks parallel to the axis of the bridge were also observed in the soil at the abutment and along the road, as evidence of local resistance of the bridge at that location to the lateral spreading of the soil, as well as horizontal cracks in the abutments in response to the pressures excerted by lateral spreading of the soil.

Photo by M. Anagnostopoulou
Photo by Michel Bruneau

Figure 13a-b. (43° 31’ 04” S - 172° 40’ 37” E) A pedestrian arch bridge also over the Avon River twisted slightly at its southern abutment.

 

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Pedestrian Bridge, Kaiapoi

A disused railway bridge turned into pedestrian use in (north of Christchurch), consisting of steel girder beam supported on timber pile bents, lost supports over part of its length (Figure 14a-c). Many of those piles split along the vertical plane in which lied the bolts used to connect the spans to the pile bent. Evidence of the lateral pressure on the piles was provided by the large gaps between the soil and piles on one side of the piles.

Photos by M. Anagnostopoulou

Figure 14a-c. (43° 23’ 43” S - 172° 39’ 36” E) A pedestrian bridge in Kaiapoi lost supports over part of its length.

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Mandeville Pedestrian Suspension Bridge, Kaiapoi

The Mandeville pedestrian suspension bridge in Kaiapoi, built in 18744 over the Waimakarini River, also suffered substantial damage when one of its towers rocked longitudinally beyond the point of stability above its support, resulting in a downward span failure in the direction of the falling tower, and concurrent beam fractures in the opposite upward moving span (Figure 15a-b). Cursory inspection of the steel connectors between the timber towers and its supporting timber pile bents did not lead to a clear understanding of the mechanism providing stability to the structure under normal unbalanced load conditions. Lateral spreading may have also partially contributed to this failure, albeit in a secondary way.

Photos by M. Anagnostopoulou

Figure 15a-b. (43° 22’ 52” S - 172° 39’ 20” E) The Mandeville pedestrian suspension bridge suffered substantial damage when one of its towers rocked longitudinally beyond the point of stability above its support.

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Chaney's Overpass on Highway 1, North of Christchurch

In some instances, while the bridges remain essentially intact, the approach spans partially or totally failed, making access to the bridge more difficult or impossible. For example, the twin continuous bridges at the Chaney overpass on Highway 1 north of Christchurch were found to be structurally sound, and tied to their abutment walls to prevent unseating there. However, due to liquefaction at the site surrounding the bridge, the approach to the southbound lanes of that Highway 1 settled by a few inches (Figure 16a-b). After a brief closure for inspection, this busy route was reopened with signage reducing the speed to 30 km/h (down from 100km/h) for the safety of motorists driving across that bridge.

Photos by M. Anagnostopoulou

Figure 16a-b. (43° 31’ 17” S - 172° 40’ 21” E) Due to liquefaction at the site surrounding the bridge, the approach to the southbound lanes of that Highway 1 settled by a few inches.

 

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River Bridge, Lincoln

The approach to the River Bridge, in Lincoln (approximately 8 miles from the fault), exhibited a dramatic evidence of settlement from its original perfectly horizontal alignment (Figure 17a). Again, cracks in the approaches parallel to the axis of the bridge provide evidence of the resistance against lateral spreading provided by this short monolithic span (Figure 17b).

Photos by Michel Bruneau

Figure 17a-b. (-43.670368 S, 172.514425 E) The approach to the River Bridge, in Lincoln exhibited a dramatic evident of settlement from its original perfectly horizontal alignment

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Highway 1 Bridge over Selwyn River

Many bridges at similar distance (or closer to the fault), where soil liquefaction did not occur, did not suffer damage. For example, the Highway 1 bridge across the Selwyn river (Figure 18a), less than 3 miles from the fault, and the railroad bridge adjacent to it, were operational. As the main North-South route from Christchurch to south of the island, this bridge consisted of multiple simply supported spans tied together in pairs using reinforced steel wedges over wall piers supported on piles (Figure 18b).

Photos by Michel Bruneau

Figure 18a-c. (-43.648672 S,172.227802 E) Many bridges at similar distance (or closer to the fault), where soil liquefaction did not occur, did not suffer damage and were operational.

Avondale Road Bridge

Note that similar ties were observed in many other bridges on major thoroughfares, such as the Avondale Rd. Bridge in Christchurch shown in Figure 19a-c for example.

Photos by Michel Bruneau

Figure 19a-c. (-43.500468 S,172.687865 E) Similar ties were observed in many other bridges on major thoroughfares, such as the Avondale Rd. Bridge in Christchurch.

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Pages Road Bridge, Brooklands

Finally, note that soil settlement and lateral spreading at bridge abutments often detrimentally affected the utilities carried by those bridges. For example, fracture of a sewer pipe across the Pages Rd. Bridge near Brooklands north of Christchurch, contaminated the river (Figure 20a-b). Incidentally, flexural cracks were observed on the abutment piles exposed as a consequence of soil lateral spreading (Figure 20c).

Photos by Michel Bruneau

Figure 20a-c. (-43.399603 S, 172.69073 E) A fracture of a sewer pipe, across the Pages Rd. Bridge near Brooklands north of Christchurch, contaminated the river.