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Post-Earthquake Reconnaissance Report on Transportation Infrastructure: Impact of the February 27, 2010, Offshore Maule Earthquake in Chile

CHAPTER 3. OVERVIEW OF BRIDGE PERFORMANCE AND SEISMIC DESIGN REQUIREMENTS IN CHILE

3.1 OVERVIEW OF BRIDGE PERFORMANCE

3.1.1 Damage Statistics

Of the nearly 12,000 bridges in Chile, about 200 were damaged in the offshore Maule earthquake, including 20 structures with one or more collapsed spans. These 12,000 bridges include about 4,750 culverts and pedestrian overcrossings and 7,250 highway bridges. Of the highway bridges, 6,800 are publicly owned by MOP and 450 are owned by private companies called concessions, which have designed and constructed several major toll roads in Chile. Table 3 shows some damage statistics.

Table 3. Number of damaged bridges.

3.1.2 Bridges Visited By Reconnaissance Team

As noted in chapter 1, TIRT visited 41 damaged bridges at 32 sites in Chile from Santiago to Arauco over a 9-day period from April 4 to 13, 2010. Table 4 provides a summary of the bridges visited along with brief notes about the observed damage. The locations of these sites are shown in figure 1 and figure 3.

Table 4. Bridges visited by reconnaissance team.

3.1.3 Damage Summary

The following principal types of damage were observed:

• Span unseating due to lack of transverse diaphragms and shear keys in precast pretressed concrete superstructures.
• Span unseating due to skew and insufficient support length.
• Span unseating due to ground movement caused by liquefaction-induced spreading.
• Girder distortion, column damage, and heavy scour due to tsunami wave forces.
• Column failure due to ground movement caused by liquefaction-induced spreading.

Detailed descriptions of the damage types are given in chapter 4 and chapter 5.

Many of the bridges built by concessions used precast prestressed concrete girder superstructures without diaphragms or shear keys for transverse restraint. Vertical rods called seismic bars and hold-down ties were used to prevent uplift after high vertical ground accelerations were recorded during the 1985 earthquake. These rods and ties were largely ineffective in the transverse direction, and many spans slid sideways on their cap beams. This lack of restraint also allowed a number of two-span bridges to rotate about a vertical axis through the pier and slide off their abutment seats.

In addition, several skewed spans with diaphragms and shear keys rotated about a vertical axis and were unseated in their acute corners due to insufficient support length. Straight bridges built before the concession era and those with cast-in-place (CIP) diaphragms and concrete shear keys performed well.

Despite higher than anticipated spectral accelerations, column damage was slight, perhaps because the lack of transverse restraint and insufficient support length allowed many superstructures to separate from their substructures, limiting the demand on the columns. When the superstructure did not separate, column damage was more likely to occur, such as with the shear failures due to imposed displacements from liquefaction-induced lateral spreading in several columns under the approach spans to the Juan Pablo II bridge across the Biobío River in Concepción.

In addition to this bridge, liquefaction-induced lateral spreading or settlement is believed to be responsible for the collapse or serious damage of many other structures along the coast, including the Llacolen, Chepe, Ramadillas, and Tubul bridges.

Bridges on coastal highways also sustained tsunami damage, such as the lateral distortion of the superstructure of the Pichibudis bridge just north of Iloca, the undermining of several piers due to scour, and the puncture of steel pile bents by floating debris in the Cardenal Raúl Silva Henríquez bridge across the Maule River at Constitución.

3.2. SEISMIC DESIGN REQUIREMENTS FOR BRIDGES

In Chile, as in other highly seismic countries, earthquake engineering has evolved over time, and advancements can be linked to the occurrence of large earthquakes.

According to Rodrigo Flores, the first step toward modern seismic design in Chile occurred after the 1906 Valparaiso earthquake, when the government created the Chilean Seismological Institute.⁽⁹⁾

After the 1928 Talca earthquake, another important step was taken with the passage of the 1931 Act of Construction and Urbanization, which established basic requirements for the seismic design of buildings. This document evolved over time until the 1972 creation of the Chilean Seismic Code for Buildings, which was based on U.S. and Japanese seismic codes. After the 1985 earthquake, studies were conducted to develop uniform risk maps for the country and three seismic zones were established, with the highest risk occurring along the Pacific Coast. This zoning was reflected in a 1996 update of the seismic building code.

Seismic design methodologies for bridges have also been based on U.S. and Japanese experience. According to Alex Unión and Rodolfo Saragoni, the design of concrete bridges in Chile before 1950 was based on the handbook published by Alberto Claro Velasco, Normas para el Cálculo y Proyecto de Puentes Carreteros de Hormigón Armado (Standards for the Design and Protection of Reinforced Concrete Road Bridges). After the mid-1950s, most designs were based on the AASHTO Standard Specifications for Highway Bridge Design.⁽¹⁰⁾

Before the mid-1980s, the seismic design coefficient for bridges was 0.12. This coefficient was increased to 0.15 following the 1985 earthquake, and a modified version of Division I-A of the AASHTO Standard Specifications was adopted in 1998. The design coefficient was not changed until 2001 when three seismic zones were introduced with peak ground accelerations (PGAs) of 0.2, 0.3, and 0.4 g (see figure 22). In addition, the soil factors were modified along with the response modification factors, and an allowance for the effect of scour was introduced. Column design was required to be in accordance with the AASHTO requirements for Seismic Performance Categories C and D of Division I-A.⁽¹⁰⁾ These provisions can be found in section 3.1004 of the MOP Manual de Carreteras (Highway Handbook) and are summarized in appendix B.⁽¹¹⁾

Figure 22. Map. Seismic zone map for central Chile.⁽¹¹⁾