Seismic Performance of Highway Bridges
by Rania Bedair, Ph.D. P.Eng.
Bridges are considered one of the most critical components of highway transportation networks, as the closure of a bridge due to partial damage or collapse can disrupt the entire transportation system. A bridge with slight or moderate seismic damage could decrease its serviceability and ability to transport vehicles. Structural integrity and accessibility of bridges are crucial even after a seismic event.
The San Fernando earthquake of 1971 caused substantial damage to highway bridges. A significant amount of the damage was caused by vibration effects since large ground motions and high earthquake-force levels exceeded those that estimated in the design phase, as outlined by Gates, 1979. Most of the damage was caused by excessive forces at the supports and by the weakness of the substructure. Insufficient length of bearings at supports, inappropriate hinge design, and inadequate restraining devices were the causes of spans falling off their supports. Many highway bridges also have suffered heavy damage in recent earthquakes (Loma Prieta, 1989, Northridge, 1994, and Taiwan, 1999). The design procedures that were in use proved inadequate and design codes are being constantly revised to improve the seismic performance of bridges.
"On October 17, 1989, the third game of the 1989 World Series was set to begin at 5:30 pm at Candlestick Park in San Francisco. Many passengers were listening to the game as they sat in traffic. At 5:04 pm, a 7.1-magnitude earthquake struck the Bay Area. The upper deck at pier E9 on the Bay Bridge failed and crashed into the lower deck, also causing the lower deck to fail. There was one fatality. The bridge was then closed for a month-long repair."
A bridge resists an earthquake by a combination of strength, deformability and energy absorbing capacities. The relative effectiveness of these factors depends on the characteristics of the ground motions. The conventional approach to earthquake resistant design is to allow the structural components to deform plastically. Plastic deformation has two effects on the response of a bridge. Firstly, the natural frequency of the bridge is reduced as a result of the stiffness reduction and secondly, the amount of damping is increased because of the hysteretic properties of plastic hinges. The consequence of both effects is done to generally reduce the response of the bridge.
The majority of bridge failures are due to pounding effects, as the gap between adjacent girders is usually not large enough to accommodate the relative displacement between the adjacent components. Pounding is a result of relative movement of adjacent bridge superstructures. Sometimes, the pounding force becomes larger than the designed force, which may lead to major local failure in the bridge. Pounding is a very complicated phenomenon and involves plastic deformations in the materials at the location of pounding, energy dissipation during contact etc. In general, bridges lack structural redundancy and hence suffer severe damage that may lead to failure during an earthquake.
A large number of older bridges have been designed and constructed without considering seismic forces. Therefore, it is very important to evaluate the seismic capacity of existing bridges and to identify the required repairs or retrofitting required in order to optimize the rehabilitation efforts. Briefly, bridges are subjected to two types of loads: static and dynamic. In the present scenario, where earthquakes occur frequently, dynamic force cannot be neglected in the design. In fact, for both newly constructed bridges and older existing bridges, it is desirable to measure the dynamic properties of the structural system (resonant frequencies, mode shapes, and modal damping) to better understand their dynamic behaviour under normal traffic loads as well as extreme loads such as those caused by seismic events or storms.
The Importance of Structural Health Monitoring
Structural health monitoring is a cost effective and reliable innovative technique to evaluate the seismic capacity of existing bridges. Recently, techniques based on ambient vibration recordings have become a popular tool for deriving the dynamic properties and characterizing the seismic response and the state-of-health of civil infrastructure. Accordingly, a smart assessment based on real data from sensors deployed on a structure’s surfaces, rather than relying on visual inspection and simulation techniques used in traditional engineering practices, is achieved. In areas less exposed to seismic hazard, vibration-based techniques also represent important tools for civil infrastructure. A continuous monitoring of such structures allows for the identification of their fundamental response characteristics and the changes of these over time, which represent indicators for potential structural degradation.
The physically tangible relation between stiffness/mass changes and natural frequency/mode shapes changes suggested the use of modal analysis for damage identification. It should be noted that modal information is a reflection of the global system properties while damage is a local phenomenon. For example, the reduced natural frequencies are often marginally influenced by a local damage.
The Seismic Isolation Technique
Seismic isolation is a response modification technique that reduces earthquake effects on bridges and other structures. The seismic isolation technique is commonly used to isolate the bridge deck, which is responsible for the majority of the pier base shear during a seismic event, from the supporting piers. Thus, when an isolated bridge subjected to an earthquake, the deformation occurs in the isolator rather than the substructure elements. Isolation is achieved by interposing mechanical devices with very low horizontal stiffness between the superstructure and the substructure. This greatly reduces the seismic forces and displacements transmitted from the superstructure to the substructure. Improved performance is therefore possible at low cost.
Conventional bridge where deformation occurs in the substructure
Seismically isolated bridge where deformation occurs in the isolator
The main concept in seismic isolation is to shift the fundamental frequency of the bridge system away from the dominant frequency of the earthquake (i.e. towards a longer period). The second purpose of an isolation system is to provide a mean of energy dissipation by reducing the transmitted acceleration into the superstructure.
Vinay Kumar & Shivanand, 2016. “Seismic Performance Evaluation of Existing Bridge”
Gianmario Benzoni & others, 2013. “Structural Health Monitoring of Bridges with Seismic Response Modification Devices”
M. C. Kunde & R.S. Jangid, 2003. “Seismic behavior of isolated bridges: A-state-of-the-art review”
Ian Buckle & others, 2002. “Seismic isolation of highway bridges”
P. J. Moss& others, 1986. “The influence of bridge geometry on the behaviour of bridges on isolating bearings”
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