SEISMIC RETROFITTING STRATEGIES FOR BRIDGES IN
MODERATE EARTHQUAKE REGIONS
Ayaz H. Malik, P.E.
Project Engineer
New York State Department of Transportation
ABSTRACT
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
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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:
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“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.”
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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 (HanshinAwaji, 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).
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,
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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.
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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.
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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.
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 13th
U.S.-Japan Workshop, Tsukuba City, Japan - September 27- October 4, 1997,
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Table 1 Damaging Earthquakes in Eastern North America
DATE
LOCALITY
Io
M
X
>7.5*
Feb. 5, 1663
St. Lawrence River, Quebec
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
Io- Maximum Modified Mercalli Intensity
M- General Magnitude (Richter Scale)
*- Estimated magnitude
Source: U.S.G.S.
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Table 2 Significant Earthquakes in New York State
DATE
LOCALITY
Io
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
Io- Maximum Modified Mercalli Intensity
M- General Magnitude (Richter Scale)
*- Estimated magnitude
Source: U.S.G.S.
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TABLE 3
Performance Criteria and Seismic Hazard Level -New York City and Surrounding Areas
Importance
Categories
Return
Period
Probability of
Exceedance
Performance Criteria
Critical
Bridges
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)
Essential
Bridges
2/3 (2% in 50
Yrs)
No collapse, repairable damage, one or two
lanes available within 3 days, full service
within month(s).
Other
Bridges
2/3 (2% in 50
Yrs)
No collapse, significant but repairable damage
in visible areas. Traffic interruption acceptable
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Figure 1 - New York City and Surrounding Areas
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Figure 2 Steel Rocker Bearing
Figure 3 Elastomeric Bearing with Laminated Load Plates
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Figure 4 Expansion and Fixed Multi-Rotational Bearings
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Figure 5 Typical Elevation at Pier
Figure 6 Shear Restrainers
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Figure 7 Steel Jacket Retrofitting – Pier Columns
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