207
Chapter 19
Seismic retrofitting of curved highway viaducts
G. Moor & C. Mendez
Mageba USA, New York, USA
T. Spuler
Mageba SA, Bulach, Switzerland
ABSTRACT: In recent years, curved highway bridges, which are more prone to seismic
damage than straight ones, have become an important component in modern highway sys-
tems, as the most viable option at complicated interchanges or river crossings where geomet-
ric restrictions apply. Curved structures may sustain severe seismic damage owing to rotation
of the superstructure or displacement toward the outside of the curve line. A commonly
adopted earthquake protection strategy consists of replacing the vulnerable conventional
bearings with seismic isolation devices. This paper describes, with reference to a current
project, how seismic isolation, in the form of Lead Rubber Bearings, can be retrofitted to
curved highway viaducts. As a result of such works, the constant serviceability of the struc-
ture is ensured even after the occurrence of a strong earthquake, facilitating the passage of
emergency vehicles and contributing to the safety of the population.
1 INTRODUCTION
Increasing awareness of the threats posed by seismic events to critical transport infrastruc-
ture has led to the demand to seismically retrofit highway viaducts and other bridges to
improve their ability to withstand a strong earthquake. Continually evolving technology and
the improving evaluation and design abilities of practitioners have also contributed to the
demand—as have, of course, increasingly stringent national design standards.
In recent years, curved highway bridges (Figure 1) have become more widely used, as the
most viable option at complicated interchanges or river crossings. Curved structures are more
prone to seismic damage than straight ones, and may sustain severe seismic damage owing to
rotation of the superstructure or displacement toward the outside of the curve line due to the
complex vibrations that arise during strong earthquake ground motions.
A commonly adopted earthquake protection strategy consists of replacing the vulnerable
conventional bearings with seismic isolation devices such as lead rubber bearings (Figure 2),
as described below.
Figure 1. A curved highway viaduct. Figure 2. A lead rubber bearing in a bridge.
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208 Seismic analysis & design
2 SEISMIC ISOLATION
The main objective of a seismic isolation system is to increase the natural period of a structure.
However, rather than simply increasing the natural period to a high value, an efficient seismic
design also considers how energy dissipation capability can be increased and how lateral
forces can be distributed to as many substructures as possible. Bridges, and viaducts in par-
ticular, are ideal candidates for the adoption of such a seismic isolation approach due to the
ability to distribute lateral forces among multiple supports, and thanks also to the general
ease of installation and inspection of isolation devices.
An isolation system placed between the bridge superstructure and its supporting substruc-
ture is generally capable of increasing both flexibility and energy dissipation. Flexibility in the
horizontal plane will lower the frequency of the bridge, decreasing earthquake-induced accelera-
tion, while the energy-dissipating capacity of the seismic isolators will considerably reduce the
damaging energy exerted to the bridge piers. Moreover, when isolation devices are installed at
the tops of a bridge’s piers, the lateral force from the superstructure during a seismic event can be
distributed among all piers, avoiding the concentration of lateral forces at specific locations.
Among the different seismic isolation devices available, elastomeric isolators have found
wide application in bridge structures. This is due to their simplicity and their combining of
isolation and energy dissipation functions in a single compact unit. They provide a high level of
damping—a crucial aspect of seismic protection—to minimise the seismic energy flow to the
superstructure and to limit the horizontal displacements of the isolators (Mendez et al., 2011).
A particularly popular type of elastomeric isolator among seismic engineers is described
below: the Lead Rubber Bearing.
3 LEAD RUBBER BEARINGS (LRB)
As shown by Figures 3 to 8, a Lead Rubber Bearing (LRB) is similar to a normal elastomeric
bearing with anchor plates, but made using specially chosen elastomer and with a lead plug at
Figure 3. 3D illustration of a multi-directional
LRB, designed to allow movement in any hori-
zontal direction.
Figure 4. Cut-out view of a multi-directional
LRB, showing the lead core at the centre of the
elastomeric pad.
Figure 5. 3D illustration of a guided LRB,
designed to allow movements along one axis and
resist transverse forces.
Figure 6. Section through a guided LRB, show-
ing guide-bars at each side of the elastomeric
pad, and the pad’s lead core.
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