Performance of Bridges with Seismic Isolation Bearings During the Maule Earthquake, Chile

Soil Dynamics and Earthquake Engineering 47 (2013) 117–131 Contents lists available at SciVerse ScienceDirect Soil Dynamics and Earthquake Engineering journal homepage: www.elsevier.com/locate/soildyn Performance of bridges with seismic isolation bearings during the Maule earthquake, Chile Mauricio Sarrazin n, Ofelia Moroni, Carlos Neira, Braian Venegas University of Chile, Civil Engineering, Blanco Encalada 2002, Santiago, Chile article info abstract Article history: On February 27, 2010 an earthquake of magnitude Mw¼8.8, with epicenter in Cobquecura, Maule Received 14 November 2011 region, hit the central part of Chile. After the earthquake, a tsunami occurred that caused heavy Received in revised form casualties and damage to buildings and infrastructure. In particular, 4.5% of the overpasses located in 5 April 2012 the affected region suffered some type of damage and 25 bridges and several pedestrian bridges Accepted 23 June 2012 collapsed. At that time, there were about a dozen bridges with seismic isolation bearings in Chile, two Available online 21 July 2012 of which were instrumented with accelerometer networks: the Marga Marga Bridge, located in Vinã del Mar, and an elevated section of the Metro Line 5 in Santiago, at approximately 300 km and 400 km from the epicenter, respectively. This paper analyzes the acceleration records obtained at these instrumented structures and studies the effect of the seismic isolation on their dynamic response. The beneficial effect of the isolation system, especially in the longitudinal direction, is apparent. In addition, some flaws in the collapsed bridges are described. & 2012 Elsevier Ltd. All rights reserved. 1. Introduction the worldwide trend [3], most of the important bridges have some seismic protection system consisting of natural rubber or neo- An 8.8 magnitude earthquake struck central Chile at 3:34 AM prene isolators, friction bearings and/or energy dissipation on February 27th, 2010, followed by a tsunami that swept the devices. In the disaster zone there are two bridges with seismic coastline between Llolleo and Lebu, nearly 700 km apart. The isolation support that are instrumented with accelerometer epicenter was located 35 km deep, off the coast of Cobquecura, networks: Marga Marga Bridge, located in Vinã del Mar, at Maule Region. According to seismic data the rupture zone was approximately 300 km from the epicenter and, a viaduct section 500 km long and 150 wide [1]. This area is one of the most of the Metro Line 5 in Santiago, some 400 km away from the densely populated in Chile and concentrates the majority of the epicenter. The acceleration records obtained at both structures are industrial facilities in the country, excluding mining. The damage analyzed in this paper. The effect that the seismic isolation had on to buildings, industries and road works were substantial. the dynamic response of the structures is also assessed. This type The area is instrumented with a network of broadband of analysis has been performed for bridges in other countries [4,5]; seismometers and strong motion accelerometers, thus a large however the high magnitude of the earthquake and the local soil number of records at the epicentral area was obtained [1,2]. conditions are unprecedented and make this research of interest. The earthquake lasted nearly 140 s with a strong motion part of Failures of highway bridges and crosswalks at various locations in 40–50 s. The largest horizontal acceleration was recorded by the Chile are also described. Melipilla station (0.78 g), about 50 km from the coast and more than 200 km away from the epicenter. This station also recorded high accelerations during the 1985 earthquake. However the 2. Description of damaged bridges damage in Melipilla city was quite modest as compared with places closer to the epicenter, where lower accelerations were Most bridges in Chile are composed of simply supported steel recorded. or reinforced concrete beams, with transverse diaphragms that In the last 15 years, many concession roads have been built or connect locally the beams over the piers and/or at the abutments. renovated in Chile, which include several major bridges. Following In addition, vertical anchorage rods called ‘‘seismic bars’’ are provided at the supports (see Fig. 1). This practice changed n Corresponding author. Tel.: þ56 2231 8406; fax: þ56 2334 7194. around the year 2000, when the use of precast prestressed E-mail address: [email protected] (M. Sarrazin). concrete beams was massified, and the transverse diaphragms 0267-7261/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.soildyn.2012.06.019 118 M. Sarrazin et al. / Soil Dynamics and Earthquake Engineering 47 (2013) 117–131 Fig. 1. ‘‘Seismic bars’’ and transverse diaphragms in typical simply supported Fig. 2. (a) Vespucio Norte Overpass. Insufficient steel stoppers.(b) Overpass near bridge. Rancagua. were eliminated. Instead of them, steel or reinforced concrete Vespucio Norte where the steel side stoppers were insufficient to stoppers were considered at the bottom flange of the beams in the hold the beam flanges. Other factors that contributed to the transverse direction. damage were the lack of vertical restraint bars at the supports Most bridges with large spans and tall piers are equipped with and the small size of the supporting area. Fig. 2b shows a seismic isolation and/or dissipation devices. Using ambient vibra- damaged overpass with a significant lateral displacement of the tion records and FFT analysis, Moroni et al. [6] determined in superstructure that broke the reinforced concrete stoppers. In some of them the fundamental frequencies and equivalent damp- addition, local amplification of seismic waves and poor soil ing. In the horizontal direction, frequencies varied between quality was reported in all these sites. 0.68 Hz and 1.46 Hz and the equivalent damping between 1% Three bridges over the Bio Bio River, connecting the city of and 4.4% Concepcio´ n to the south, partially collapsed (see Fig. 3a). Lique- About 4.5% of the overpasses failed due to the earthquake and faction and lateral spreading of the soil could explain such 25 bridges collapsed, 10 of them on public roads and 15 in damage. Between Curico and Talca, over the Claro River, an old privately managed toll roads [7]. A detailed description of the bridge built in 1890 collapsed. It was a 117 m long bridge, formed failures of 32 bridges, several of them of the skew type, was by 7 masonry arches of about 25 m height. This bridge (Fig. 3b) reported by Yen et al. [8]. It is believed that the skewed shape and had been declared a National Monument a few years earlier. the lack of transverse diaphragm over the abutments were the All seismically isolated bridges, with the exception of Cardinal main cause of the collapse of the overpasses. After the earthquake Silva Henriquez Bridge in Constitucio´ n, had good performance. experience, the Ministry of Public Works decided that the use of Some of them can be seen in Fig. 4. Detailed damage in Cardinal transverse diaphragms should be compulsory. The damage to Silva Henriquez Bridge is described in [8]. The main damage was a highways around Santiago, as Vespucio Norte and South East lateral displacement of the supporting plates that produced fail- Highway, both operated by private agencies, had a big economic ure of the stoppers. The welded connection of the beams to one of impact. Other badly affected areas were the village of Hospital the abutments was also fractured. Despite the damage, light and the Rancagua By-Pass. Fig. 2a shows a collapsed overpass in vehicles were allowed to cross the bridge while it was repaired. M. Sarrazin et al. / Soil Dynamics and Earthquake Engineering 47 (2013) 117–131 119 base and top of pier C4, north and south abutments and several positions on the deck, as shown in Fig. 6. The top of Fig. 6 shows the deck with the sensors in the transverse and longitudinal directions and the bottom shows pier C4 with sensors 2, 5, 8 and 14 that measure in the vertical direction. Using this network for ambient vibrations, frequencies in longitudinal (L) and transverse (T) direction, as well as damping of the superstructure were determined (see Table 1). This table also shows the corresponding frequencies and equivalent damping obtained from the July 24, 2001 earthquake records. At the deck, the predominant frequencies in both horizontal directions were reduced considerably during the strong motion interval of the earthquake, while the equivalent damping in longitudinal direction increased, reaching up to 15%. Ambient vibrations were measured using seismometers at the ground, at the bottom of pier C4 and at the valley, 20 m apart to the west of pier C4. Predominant frequencies of 1.3 Hz in both horizontal directions and 1.8 Hz in the vertical direction were identified. Subsequently, frequencies in horizontal direction of 1.2 Hz at the valley and 4.4 Hz on the rock at the south abutment were obtained from moderate seismic records using Nakamura’s method. [10]. 3.2. Maule earthquake records Figs. 7, 8 and 9 show the acceleration records, in fractions of g, obtained during the February 27th, 2010 earthquake in the longitudinal, transverse and vertical directions, respectively. Peak accelerations are listed at the right end of each diagram. There is a significant reduction of accelerations in the longitudinal direction between the pier and the deck and also between the abutments and the deck. Accelerations at the south abutment are much larger than at the north one, although both sites may be classified as hard soils. The differences in topographic conditions between one abutment and the other could explain the differences in Fig. 3. (a) Bridge in Concepcio´ n.(b) Collapsed Claro river bridge.

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