SEISMIC RETROFITTING STRATEGIES FOR BRIDGES IN MODERATE EARTHQUAKE REGIONS ABSTRACT Ayaz H. Malik, P.E. Project Engineer New York State Department of Transportation Certain parts of the United States have traditionally ignored the potential for seismic damage since past earthquakes were rare and the majority of these were of moderate magnitude. In the last decade, the emerging evidence indicated that these moderate earthquake regions should provide sufficient ductility to avoid catastrophic failure. This paper presents some of the strategies being used for New York, considered as a moderate earthquake region, with certain areas carrying critical structures, requiring performance based approach. INTRODUCTION Although the Northeastern part of the United States experienced several major earthquakes, including the biggest earthquake ever occurred in the United States of America (Table 1). The mostly felt series of earthquakes occurred in 1811-12 near New Madrid, Missouri. The largest of these quakes was felt over an area of two million square miles - from Canada to the Gulf of Mexico and from the Rocky Mountains to the Atlantic Ocean. Still many parts of the Northeast, including New York, are considered a moderate earthquake region (Table 2). Like New York, there are other parts where the potential for such a big event was not considered since the probability and frequency of such a major earthquake is very low. EXISTING HIGHWAY BRIDGES Most of the existing bridges in the moderate earthquake regions are more than fifty years old. The majority of these structures were designed without any consideration for earthquake forces. Any moderate to major seismic event can cause severe damage to the structures, endangering public safety and interrupting vital lifelines. There are currently more than 20,000 bridges in New York State under the jurisdiction of State, Bridge Authorities, and local bridge agencies. These bridges vary in structural types and materials. The majority of these structures built prior to 1990, did not consider the seismic design forces or was not significant to control the design. DESIGN AND RETROFITTING STRATEGIES With the occurrences of earthquakes in the late 1980's (Mexico, September 1985; Armenia-Spitak, December 1988; and Loma Prieta, October 1989) New York established a 1
comprehensive program to consider significant seismic design forces and detailing for new structures, and to retrofit the vulnerability of the existing structures. With the vast population of existing structures, candidates for vulnerability assessment and retrofitting, the department established the following policy in 1991: It shall be the policy of the Department to evaluate the seismic failure vulnerability of bridges programmed for rehabilitation, to assess option and costs of seismic retrofit measures, and to incorporate into the rehabilitation plans those retrofit measures deemed warranted to eliminate or mitigate such failure vulnerability. The retrofitting program mainly covered the following actions against vulnerability failure, for conventional bridges: 1. Replacement of High steel Rocker and Low Steel Sliding bearings: As observed in the previous earthquakes, as well as in the recent Kobe (Hanshin- Awaji, Japan) earthquake, steel bearings performed very poorly and bearing failure was the cause of the many structural failures. During an earthquake, bearings are subjected to displacements, rotation and lateral forces in various directions, resulting in brittle failure of the unidirectional steel bearings (Figure 2). Replacing these bearings with ductile, multi-rotational and multi-directional bearings provide safety against potential unseating of the superstructure (Figures 3 and 4). 2. Retrofitting for Continuity to multiple Simple spans: Simply supported spans are made continuous, when feasible to provide redundancy. Continuity enhances the seismic response by distributing the in-plane forces to the piers and abutments and prevents loss of end support at piers due to longitudinal movement. When connecting the unrestrained girder ends of the adjacent spans, it is important to provide a complete splice between the flanges and the webs. Bolted splices are used since they provide ductility to the connection (Figure 5). Where continuity is not feasible, restrainers and/or shear blocks are used to prevent unseating of the superstructure girders (Figure 6). 3. Retrofitting of concrete columns: The current practice for earthquake resistance design of columns for bridge piers is to provide sufficient confinement at the potential plastic hinge locations by ties or spirals. Majority of the existing pier columns is not provided with sufficient confinement necessary to improve the compressive strain and to provide proper lateral support to the primary reinforcement. Circular columns with insufficient confinement have been retrofitted with steel jacketing to provide passive confinement. To avoid excessive moment demand on the adjacent cap beam/footing rubber (elastomer) sheet is placed between the in-fill grout and the steel casing around the column top/bottom (Figure 7). 2
Wall type (solid) piers are retrofitted for proper reinforcement by 300 mm (12 inch) thick reinforced concrete jacket all around. Grid pattern drilling and grouting is used to dowel the concrete jacket to the existing pier. In Metropolitan areas with many critical bridges, it is important to adopt strategies that assure minimal interruptions of essential services, after a major event. There are nearly 2,100 bridges in the New York Metropolitan area, under the jurisdiction of various agencies. An expert panel of seismologists was appointed to recommend the seismic hazard applicable for all bridge projects in the New York City area. Dr. McGuire, of Risk Engineering Inc., Boulder, Colorado, panel chair, recommended uniform hazard horizontal acceleration spectra on hard rock for return periods of: 500, 1000, 2500, and 5000 years. Dr. Dobry, Geotechnical and Soil Dynamics expert, extended it for other site conditions ranging from soft rock to soft soil. Additionally, a two level and one level seismic hazard was specified based on the importance of the structure and the related performance criteria (Table3). Based on the importance of access routes to critical and emergency facilities such as: hospitals, police, fire stations, and communication centers, bridges must continue to function and are classified as critical, essential, and other. In all cases, collapse shall be prevented. Critical Bridge: Bridge that must continue to function as part of the lifeline, social/survival and serve as important link for civil defense, police, fire department or/and public health agencies to respond to a disaster situation after the event, providing a continuous route. Any bridge that crosses a critical route should be evaluated on critical hazard levels with the performance criteria of no collapse and the bridge shall not restrict the operation of the critical highway passing below. Essential Bridge: Bridge that must provide limited access after the event and serve as important link for civil defense, police, fire department or/and public health agencies to respond to a disaster situation after the event, providing a continuous route. Any bridge that crosses an essential route should be evaluated on the essential hazard level with the performance criteria of no collapse and the bridge shall not restrict the operation of the essential highway passing below. Other Bridges: Bridges not qualifying as critical or essential. The detailed seismic assessment and retrofitting process are performed in three phases. The first phase may need site specific study for ground motion, depending on the subsurface geotechnical conditions and soil profiles at the bridge. On the other hand, AASHTO rock acceleration spectra or an expert panel recommended spectra may be appropriate to compute the seismic demand. 3
The second phase is a quantitative evaluation of individual bridge elements using the global analysis procedures outlined in the current AASHTO specifications for seismic design of bridges (Division I-A). The resulting forces and displacements (referred to as demands) are compared with the ultimate force and displacement capacities of respective elements. The third phase of evaluation is an assessment of the influence of failure in each element with insufficient capacity to resist the design earthquake. Based on the importance of the structure, as part of the lifeline network system, and the related performance criteria, the bridge elements should be retrofitted. In no case, will collapse be acceptable irrespective of its importance. CONCLUSIONS Classifying the bridge importance and scheduling with the rehabilitation program accomplish the strategies used to assess and retrofit the existing structures, originally designed with no consideration of seismic forces. Improved seismic details for reinforcement, connections, and column confinement are provided as standard details to eliminate seismic failure vulnerability of bridges. Better understanding of innovative technologies and modeling techniques proved helpful in enhancing the performances of highway structures to resist the seismic loads. REFERENCES 1. AASHTO Standard Specifications for Highway Bridges Division 1A with NYSDOT Blue Pages 1999 2. New York State Department of Transportation s Engineering Instruction 92-046. 3. Shirole, A. Malik, A. Seismic Vulnerability Evaluation of New York State Bridges Proceedings of the American Society of Civil Engineers (ASCE), Structures Congress XIII, Boston, MA - April 2-5, 1995. 4. Malik, Ayaz H. Seismic Retrofitting of Bridges in New York, Proceedings of the 13 th U.S.-Japan Workshop, Tsukuba City, Japan - September 27- October 4, 1997, 4
Table 1 Damaging Earthquakes in Eastern North America DATE LOCALITY I o M Feb. 5, 1663 St. Lawrence River, Quebec X >7.5* Nov. 19, 1755 East of Cape Ann, Mass VIII 6.0* Dec.-Feb. 1811-12 New Madrid, Missouri VII >8.0* Jan. 4, 1843 Western Tennessee VIII Oct. 20, 1870 St. Lawrence River, Quebec IX Aug. 10, 1884 New York City VI 5.3* Aug. 31, 1886 Charleston, S. Carolina X 7.0* Oct. 31, 1895 Charleston, Missouri VIII May 31, 1897 Giles County, Virginia VII Feb 28, 1925 St. Lawrence River, Quebec VIII 6.5 Aug. 12, 1929 Attica, New York VII 5.2 Nov. 18. 1929 Grand Banks off Newfoundland X 7.2 Nov. 1, 1935 Timiskaming, Ontario VIII 6.2 March 8, 1937 Western Ohio VIII 4.9 Sept. 5, 1944 Massena, New York VIII 6.0 July 27, 1980 Sharpsburg, Kentucky VII 5.2 Oct. 7, 1983 Newcomb, New York VI 5.1 Nov. 25, 1988 Saguenay, Quebec VIII 6.3 April 20, 2002 Plattsburg, New York VI 5.1 I o - Maximum Modified Mercalli Intensity M- General Magnitude (Richter Scale) *- Estimated magnitude Source: U.S.G.S. 5
Table 2 Significant Earthquakes in New York State DATE LOCALITY I o M Dec. 18, 1737 New York City VI 5.0* Mar. 12, 1853 Lowville VI 4.8* Oct. 23, 1857 Buffalo V 4.6* Dec. 18, 1867 Canton VI 4.8* Dec. 11, 1874 Tarrytown VI 4.8* Aug. 10, 1884 Rockaway Beach (NYC) VI 5.3* May 28, 1897 Plattsburgh VI Mar. 18, 1928 Saranac Lake VI 4.5* Aug. 12, 1929 Attica VII 5.2 Apr. 20, 1931 Warrensburg VII 4.5 Apr. 15, 1934 Dannemora VI 4.5 Sept. 5, 1944 Massena VIII 6.0 Sept. 5, 1944 Massena V 4.5 Jan. 1, 1966 Attica VI 4.6 June 13, 1967 Attica V 4.4 Oct. 7, 1983 Newcomb VI 5.1 Oct. 19, 1985 White Plains V 4.0 April 20, 2002 Plattsburg, New York VI 5.1 I o - Maximum Modified Mercalli Intensity M- General Magnitude (Richter Scale) *- Estimated magnitude Source: U.S.G.S. 6
TABLE 3 Performance Criteria and Seismic Hazard Level -New York City and Surrounding Areas Importance Categories Critical Bridges Essential Bridges Other Bridges Return Period Probability of Exceedance Performance Criteria 2500 Yrs 2% in 50 Yrs No collapse, limited access for emergency traffic in 48 hrs., full service within month(s) 500 Yrs 10% in 50 Yrs No collapse, no damage to primary structural elements, minimal repairable damage, full access to normal traffic available immediately (allow few hours for inspection) 2/3 (2% in 50 Yrs) 2/3 (2% in 50 Yrs) No collapse, repairable damage, one or two lanes available within 3 days, full service within month(s). No collapse, significant but repairable damage in visible areas. Traffic interruption acceptable 7
Figure 1 - New York City and Surrounding Areas 8
Figure 2 Steel Rocker Bearing Figure 3 Elastomeric Bearing with Laminated Load Plates 9
Figure 4 Expansion and Fixed Multi-Rotational Bearings 10
Figure 5 Typical Elevation at Pier Figure 6 Shear Restrainers 11
Figure 7 Steel Jacket Retrofitting Pier Columns 12