There are two distinct strategies to solve seismic problems: one, the conventional design approach, adds strength and ductility to the structure; the other, the seismic isolation approach, protects the structure by limiting the seismic attack, rather than resisting it. An approach said to be new to North America recently was performed on the Marquam Bridge in Portland, Ore. The system combines the two strategies to form a custom solution designed to meet the requirements of the designer and owner.
This approach, imported from Europe, is designed to create an efficient
system that is active against seismic forces, yet differs from the classic
"structural system," which resists vertical loads through the
use of traditional bearing arrangements. The system is comprised of seismic
devices that are installed to control the horizontal (lateral) actions of
an earthquake, freeing the bearings from these forces and the damage that
can occur. Hence, the bearings are all of the "free" or "multi-directional"
type. Additionally, specific seismic hardware is installed to provide temporary
restraints (shock transmitters) if strengthening is desired while using
a variety of products with high-energy dissipation capabilities to achieve
a desired base isolation.
The Marquam Bridge, which carries Ip;5 across the Willamette River at
Portland, is the first U.S. example of this philosophy as applied to a retrofit
project, according to its designers, FIP-Energy Absorption Systems (FIP-EAS),
Burlingame, Calif.
Built in 1963, the bridge comprises metal trusses carrying two levels of
roadway for a total length of 1,043 ft. Construction is of typical Gerber
girder configuration in which the two end girders have a cantilever beam
toward the center of the bridge, upon which the suspended central truss
is supported. The two end spans measure 301 ft 6 in.; the central span is
440 ft, of which 260 ft comprise the central suspended truss. The span rests
upon four reinforced concrete piers.
The Oregon DOT (ODOT) specified that the $8.5-million project:
The isolators are of the sliding type and combine a free sliding pot bearing
to transmit the vertical loads and a series of steel dissipating elements
to control horizontal actions. These base isolation devices were supplemented
by adding four shock transmitters at the expansion end of the suspended
span.
The isolators used for the expansion points at each end of the span were
unique in that they incorporated a shock transmitter that is active between
the superstructure and steel dissipating elements. This concept allows for
thermal and other service-load movement without engaging the dissipating
elements.
According to Byron West, manager of sales and marketing, FIP-EAS, the Marquam
project, like most retrofit jobs, is best accomplished when the seismic
goals are achieved, while at the same time eliminating the requirements
for expensive strengthening of the piers, maintaining serviceable pre-existing
bearings in their "as designed" configuration and leaving the
bridge with as few changes as possible from the original plans.
"That means original fixed points remain fixed, and expansion points
remain flexible," West said. "This isostatic configuration permits
elements of the bridge to function independently during normal service conditions."
To accomplish the objective of maintaining the existing bearing system,
which called for expansion bearings on the land-side piers and fixed bearings
on the two in the center, FIP-EAS utilizes sacrificial restrainers (shear
keys) at each pier-bearing location. These shear keys assure that normal
service loads, including wind, braking actions, and even moderate earthquakes,
do not unnecessarily stress the dissipating elements and displace the isolators.
Additionally, these restrainers serve to impede displacement in the isolators
at what were the "fixed" piers, and also limit transverse movements
at the "mobile" piers.
In the event of a design maximum earthquake, the sacrificial restrainers,
as the name implies, fail, and the shock transmitters at each end of the
bridge lock up. When locked, the Marquam becomes, in effect, a multispan
continuous girder (hyperstatic) structure, which has proven to be the most
effective when it comes to resisting earthquakes.
Once the design threshold is determined for the steel elements, which act
elastoplastically and have excellent dissipating capacities, the maximum
lateral forces transmitted to the piers can be controlled. Ultimately, all
piers remain within the elastic limits even in the worst earthquake, and
the longitudinal displacement of plus or minus 5 in. is achieved in accordance
with the design requirements.
According to Steve Starkey, ODOT structural engineer, "The earthquake
in Northern California in 1989 spurred our seismic retrofit program and
we were actively looking for creative approaches that would allow us to
meet predetermined standards. These standards, most importantly called for
retrofitting the superstructure to prevent pull-off. We also wanted a design
that would minimize the costs of any future retrofit, especially any requirements
to strengthen the substructure.
"The FIP engineers provided a satisfactory initial design proposal,
which we incorporated into our final plan."
The Marquam retrofit project is the first example in the U.S. of high damping-capacity
sliding isolators (largest isolator 5200 kips vertical load/715 kips lateral
load) in combination with shock transmitters. This successful marriage of
seismic hardware demonstrates how different technologies can be incorporated
to achieve design goals. Further, this can be done in such a way that superior
performance is provided for the lowest total cost.
According to Jim Keller, project manager for Mowat Construction Co., Vancouver,
Wash., the entire job took approximately two-and-a-half years, beginning
in February 1993 and concluding in December 1995. The actual bearing installation,
which was done at night to permit maximum traffic flow, was performed over
six months from April through September 1995.
"The placement of the devices went extremely smoothly," Keller
said. "We jacked up the bridge deck at each of eight bearing points
and lowered the bearings over the side. Virtually everything fit perfectly
except for some minor modifications that had to be made to locally fabricated
mounting brackets for the shock transmitters."
FIP-EAS' West said, "Given the ODOT specification and AASHTO design
recommendations, the Marquam project carries a distinctive American identity.
Test procedures were in accordance to AASHTO standards; pot bearings, anchorage
system and other important considerations, including the protective coating
applied to all exposed surfaces, met with AASHTO and ODOT standards."
During the preliminary stages of the project, a multimodal analysis showed the existing structure was incapable of resisting the design maximum expected earthquake. The bridge now can be reasonably expected to provide decades of service even in the event of an earthquake.
