Suspending the "Big One"
Seattle-Tacoma plans 5,400-ft-long suspension bridge to withstand seismic forces
When most of the 45 million residents clustered along the West Coast refer to the “big one,” they are inevitably discussing a major earthquake along California’s infamous San Andreas Fault.
That is, of course, unless they are among the 2 million residents of the greater Seattle-Tacoma area. For these Pacific Northwest dwellers, the “big one” is a seismic movement between the Juan De Fuca Plate beneath the Pacific Ocean and the North American Plate. The “big one” could be capable of releasing as much as 16 times more energy than a major earthquake on the San Andreas Fault.
When engineers at the Washington State Department of Transportation (WSDOT) considered building a new parallel suspension bridge to supplement the existing Tacoma Narrows Bridge, providing a bridge with adequate seismic resistance was very much on their minds.
Birth of a bridge
In June 1993, the Washington State Legislature unanimously enacted the Public/Private Initiatives Act to attract private business investment for unfunded state transportation needs. The new Tacoma Narrows Bridge, now under construction, is a direct result of that initiative and subsequent legislative changes that modified project funding from private to public.
Since transportation capacity improvements across the Tacoma Narrows had been the focus of intensive studies during the last two decades, WSDOT evaluated proposed solutions and alternatives, including adding a lower roadway to the existing bridge or constructing a new parallel bridge.
After evaluating these alternatives, WSDOT issued a Notice-to-Proceed on Sept. 25, 2002, to design-builder Tacoma Narrows Constructors (TNC), a joint venture of Bechtel Corp. and Peter Kiewit Sons Inc., to design and construct a new parallel suspension bridge. The Joint Venture of Parsons/HNTB began developing project concepts with United Infrastructure of Washington and WSDOT in 1994 and led the total seismic analysis for the new bridge and its major components as part of a subcontract with TNC.
The new Tacoma Narrows Bridge will increase the capacity of Rte. 16 between the Seattle-Tacoma metropolitan area and the Olympic Peninsula by providing separate bridges for eastbound and westbound traffic. The $849 million design-build project includes the construction of the new bridge, seismic retrofitting and deck modifications of the existing bridge, construction of 2.5 miles of roadway approach work, constructing a new toll facility and the cost of toll operations.
When completed, the new bridge will be the first major suspension bridge built in the world under a design-build delivery method, as well as the longest span built in the U.S. since the Verrazano-Narrows Bridge was completed in 1964. The new parallel crossing features a 5,400-ft-long suspension bridge with a main span of 2,800 ft, a 1,200-ft side span on the Tacoma side and a 1,400-ft side span on the Gig Harbor side.
Other features of the new bridge include a superstructure with a unique orthotropic deck system that will be integral with each stiffening truss, steel floor beams spaced at 20 ft on-center, two stiffening trusses supported by suspenders spaced at 40 ft on-center and two 201?2-in.-diam. main cables spaced 78 ft apart. The substructure consists of concrete towers supported on deep-water dredged caissons and conventional gravity anchorages on both the Tacoma and Gig Harbor sides.
Beneath the Narrows
The bridge is located in the Puget Lowland Basin between the Olympic Mountains to the west and the Cascade Mountains to the east. Depth to bedrock in the basin can be up to 3,600 ft and is estimated to be 1,800 ft at the bridge site. Sediments over bedrock have been deposited by a number of glacial flows occurring in recent geological times. During the most recent ice age, the site was covered by an ice sheet estimated to be about 3,000 ft thick, which compacted and over-consolidated the soils beneath the ice to a very hard and dense condition. As the ice from the last glacial incursion receded about 13,500 years ago, medium dense to dense normally consolidated soils were deposited in depressions and flood plains.
Seismicity of the region is largely a consequence of the Juan de Fuca Plate’s slow but unrelenting thrusting beneath the North American Plate in a northeastward direction. Seismologists attribute earthquakes occurring in this region to three distinct phenomena: movement between the two tectonic plates themselves; deep (25- to 37-mile) earthquakes resulting from ruptures within the Juan de Fuca Plate; and shallower earthquakes from ruptures in the North American Plate.
While there are no historical written accounts of large earthquakes resulting from movement between the Juan de Fuca and North American plates, seismologists have found evidence that these large seismic events have a recurrence interval of about 600 years. Historical tsunami records from Japan indicate that the most recent major event occurred approximately 300 years ago.
More frequent, yet less intense, earthquakes occur as a consequence of ruptures within either the Juan de Fuca or North American plates. Examples of these seismic events include the magnitude 7.1, 1949 Olympia earthquake; the magnitude 6.5, 1965 Seattle-Tacoma earthquake; and the magnitude 6.8, 2001 Nisqually event, all of which had relatively deep epicenters. Their depth accounts, in part, for the light damage to buildings and highway infrastructure.
At the conceptual design stage of this project, TNC investigated both large-diameter drilled shafts and dredged caissons as possible foundations for the main towers. Although the original Tacoma Narrows Bridge is supported on dredged caissons, TNC thought modern drilled-shaft foundations might provide a more economical foundation. However, foundation studies indicated that drilled-shaft foundations would require clusters of 12- to 14-ft-diam. battered drilled shafts extending up to 400 ft below the bottom of the pile cap. Dredged caissons were therefore determined to be a better structural and more economical alternative.
The caissons measure 80 ft by 130 ft in plan view, 190 ft deep at the west tower and 216 ft deep at the east tower. Both caissons are begun as 18-ft-deep steel cutting edges, which were prefabricated at a shipyard dry dock and floated out into the Narrows. The body of the caisson was constructed out of concrete poured in place over water. Crews maintained positive buoyancy of the caissons at all times as they sank them in a precisely controlled manner to the bottom of the Narrows. Since reaching the Narrows floor, crews have continued to descend the caissons into the sea floor through controlled dredging operations. The east caisson will be sunk 67 ft and the west caisson 62 ft to the design bottom elevation into stiff glacial material.
Seismic risk and much more
Because of the substantial investment required for the new bridge, WSDOT specified performance-based seismic design criteria. The overall goal was to provide a bridge that would remain undamaged during frequently occurring earthquakes, and also be repairable after a more serious seismic event.
Design criteria stipulated that the structure must comply with specified performance levels for two distinct seismic levels: (1) the Safety Evaluation Earthquake (SEE), defined as a seismic event with a mean return period of 2,500 years (or “no collapse” criterion); and (2) the 100-year Functional Evaluation Earthquake (FEE or “elastic performance, no damage” criterion). For the SEE, no damage was permitted at anchorage blocks, the suspension cable system and the stiffening truss, except for secondary members. Minimum damage was permitted at caissons and anchorage deck and girders. Repairable damage was allowed at towers and secondary members of the stiffening truss. Expansion joints were the only bridge component where significant damage was permitted.
During the FEE, no damage was required for all bridge components.
In addition to damage limits, design criteria placed restrictions on permanent lateral displacements, or residual drift. For the FEE, no residual drift of any member was permitted. At the SEE level, design criteria limited transverse residual tower drift to 3 ft at the top of towers and placed an additional residual drift restriction of 2 ft relative to the top of the caissons to anywhere between the top of the caissons and the top of the towers. In the longitudinal direction, corresponding residual drifts were limited to 2 ft and 1 ft, respectively. Residual drift at the top of the caissons was limited to 1 ft in both the transverse and longitudinal directions.
Allowing damage at some bridge components as well as permitting some residual displacements, while improving the economy of the bridge, severely complicated the seismic analysis. Since damage limits were specified by limiting concrete and reinforcing steel strains as well as residual drifts, the analysis needed to be sophisticated enough to estimate these parameters after a seismic event. To accomplish this, the design criteria mandated that a nonlinear time-history analysis be used as the principle analysis tool. This analysis technique differs from the more typically applied linear, or elastic, analysis by providing the ability to determine and record the actual damage occurring during a particular seismic event in measurable terms.
Using an analogy involving a common wire shirt hanger obtained from a dry cleaner helps to explain the differences between these two analysis methods. If the shirt hanger is flexed slightly but springs back to its original shape without remaining permanently bent, the hanger has performed elastically. However, if the shirt hanger is bent too far, it will permanently yield. This behavior is termed nonlinear and is much more complicated to capture in analytical models than simple, elastic behavior.
Deciding on seismic events to shake the bridge is complex. The design criteria specified that three unique ground-motion records should be used when performing nonlinear time-history analysis for the SEE. Each of these records was further specified to have three orthogonal components, two horizontal and one vertical.
Only one three-component ground motion was required for the FEE. Unlike buildings, which typically have a relatively small footprint and can therefore be analyzed with a constant ground motion, long-span bridges have foundation elements at considerable spacing, and ground motions vary from one foundation to another. The design criteria therefore mandated that this variance, referred to as spatial incoherency, should be considered. To conduct this analysis, design criteria specified the use of the ADINA general-purpose finite-element program. This software is well-suited for this type of analysis and, in fact, also was specified by the California Department of Transportation for the seismic analysis of its toll bridge seismic retrofit program.
Caisson rocks but does not roll
The dredged caissons supporting the two main-span towers are quite massive. In fact, they contain almost 85% of the seismic mass of the entire bridge, including the superstructure. It was therefore no surprise when seismic analyses showed that the seismic response of the caissons would dictate the response of the entire bridge.
Before beginning elaborate analysis, seismic specialists on the design team discussed desirable seismic responses of the caissons. It was agreed that allowing the caissons to rock about their base would provide a very effective means to minimize seismic forces entering the caissons and propagating into the towers and the superstructure. Allowing caisson rocking would be a way of isolating the caissons from the full effects of the seismic ground motions, thus drastically reducing forces to the entire bridge.
The challenge with allowing rocking would be to devise a modeling technique that would mimic the actual rocking that may occur during seismic events. However, since criteria placed limits on residual displacements of the caissons and the towers, rocking effects on the glacial material below the caissons would need to be captured. While rocking would limit caisson forces, excessive rotation or sliding of the caissons would violate design criteria by leading to significant caisson residual displacements.
This modeling challenge was met by using arrays of one-dimensional nonlinear Winkler soil springs across the entire footprint of the caissons and along the sides of the caissons below the mudline. Before arriving at the appropriate strength and stiffness of the Winkler springs, the geotechnical engineer prepared full-continuum models of stand-alone caissons with soil represented by three-dimensional solid elements completely surrounding the caisson.
It was recognized that full soil-continuum models were much too complex to be included in global bridge models constructed to perform nonlinear time-history analyses. Therefore, one-dimensional nonlinear Winkler soil springs were derived from push-over analysis results conducted with the full soil-caisson continuum model. Push-over analysis results between stand-alone caisson models with the Winkler soil springs and the caisson-soil full continuum model also were compared and excellent correlation was found.
The nonlinear Winkler springs were capable of capturing elastic soil response under light loading and, under very large loads, would permanently deform, mimicking permanent settlement of the supporting soil. The Winkler springs also would not support any tension forces, allowing separation between the caissons and supporting ground during rocking induced from seismic ground motions. Using a modeling technique that accounted for permanent ground settlement provided the means to capture residual displacements in the caissons.
The nonlinear time-history analysis showed that caisson rocking was indeed occurring and substantially reducing seismic forces in the caissons and the rest of the bridge. Analysis results were then used to select the proper reinforcement and member sizes of the tower columns and struts.
It was determined that the residual drift limits required by the design criteria generally controlled the design of tower members and were more stringent than allowable concrete and reinforcing material strains specified by the design criteria. Forces in the superstructure suspension system, particularly suspenders, increased somewhat during seismic loading. However, no significant design modifications were required to the suspension system. The stiffening truss is adequate for seismic forces, with only selective strengthening necessary near the towers to comply with the design criteria of having no damage in the stiffening truss.v
WSDOT showed substantial foresight by taking the prudent course of specifying performance-based seismic design criteria. The resultant new Tacoma Narrows Bridge will be well-prepared to safely resist the “big one.”