Bend but don't break

Dec. 28, 2000
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:

  • Provide total seismic protection for the superstructure in the event of the design maximum expected earthquake in both the longitudinal and transverse (lateral) directions;
  • Minimize strengthening to existing structural elements such as piers, trusses and foundations;
  • Maintain the existing bearing system for the service loads (fixed points as fixed and expansion points as expansion); and
  • Limit the relative displacements to values established by the original designer.
The state defined "total seismic protection" as the entire structure remaining within elastic limits during the most severe expected seismic attack, i.e., any structural damages shall be avoided. To achieve all four objectives, ODOT selected FIP-EAS, a recently formed joint venture between Energy Absorption Systems Inc., Chicago, a manufacturer of highway safety hardware, and FIP Industriale, an Italian-based provider of seismic management technologies, for the project. Using its multidisciplinary approach, FIP-EAS designed a system that replaced the pre-existing vulnerable steel bearings with high energy-dissipating isolators.

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.

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