Anaheim street bridge: gateway to the Pacific Rim.
Holmes & Narver (H&N) was contracted by the City of Los Angeles' Harbor Department for the design and engineering support during construction of this $25-million project in Wilmington. The contractor is Brutoco Engineering and Construction, Inc. The project is funded by the 1991 Intermodal Surface Transportation Efficiency Act (ISTEA), the City of Los Angeles, and the Ports of Los Angeles and Long Beach.
One of the driving forces of the project is the development of the Alameda Corridor, a $1.8-billion rail and highway transportation project designed to consolidate the operations of three freight railroad carriers onto one high-capacity rail corridor. The Alameda Corridor is designed to improve railroad and highway access to the ports of Long Beach and Los Angeles. Together these two ports comprise the San Pedro Bay Ports, the largest port complex in the United States and a gateway of Pacific Rim economic markets. The route, which is being constructed along Alameda Street, will eliminate all at-grade highway crossings of the railroad, while consolidating 90 miles of branch line tracks into one 20-mile corridor. The corridor is anticipated to be in operation by the year 2001.
Anaheim Street is a major circulation roadway for the ports of Los Angeles and Long Beach. Linked with three freeways - the Harbor Freeway (I-110), the Terminal Island Freeway (SR 47), and the Long Beach Freeway (1-710) - Anaheim Street carries a significant volume of truck and commuter traffic.
A new bridge is being constructed to replace an existing reinforced concrete bridge, which was built in 1927. The existing bridge was too low, too narrow, significantly deteriorated, and the spacing between foundations insufficient to accommodate passage of the Alameda Corridor below.
The new bridge is 1,344 ft long and 94 ft wide. It will span the existing Dominguez Channel and the proposed Alameda Corridor rail lines and provide clearance for additional rail lines in the future. The new structure consists of nine spans of cast-in-place post-tensioned concrete constructed in two halves.
By constructing the new bridge in two stages the engineers were able to maintain traffic on Anaheim Street during construction. In Stage 1 the street was reduced to two lanes and shifted onto one side of the existing bridge. The bridge was cut longitudinally down the middle and half of the superstructure was removed. All the substructure remained intact to maintain the structural integrity of the remaining portion of the bridge carrying traffic. Half of the existing bridge was used for traffic in Stage 1 while half of the new bridge was used during Stage 2. This kept traffic circulating on this critical corridor during construction to minimize disruptions to trucking activity at this busy port complex.
The new bents were constructed between the existing foundations, and the first half of the new superstructure was erected. The traffic was shifted to the new bridge in July 1996 and demolition commenced on the remainder of the existing bridge. The second half of the bridge will be essentially a mirror image of the first, and will be connected at the median by a closure pour.
Work had to be carefully coordinated with utility companies to relocate and protect a large number of oil, gas, water, telephone, and electrical lines, which were attached to the exterior of the existing bridge. During reconstruction, all these utilities were moved, also in stages, and reconstructed in the internal cells of the new box girder. Provisions were also made for future utilities in the space between the box girders at the median.
Built to Earthquake Criteria
The new bridge is designed to meet both Caltrans and Port of Los Angeles earthquake criteria and employed the most recent design recommendations established by Caltrans following the 1994 Northridge earthquake. Prior to the Northridge earthquake, prevention of earthquake damage had focused on the reinforcement of columns and foundations, increased hinge restraint, and improved seat width at hinges and abutments.
The Northridge earthquake and subsequent research revealed potential weaknesses in bridge superstructures as well. Therefore, much of the newest criteria involves reinforcement of bent caps and superstructures near the column connections.
Caltrans criteria requires that a bridge remains ductile and does not collapse under conditions of a "maximum credible earthquake" (MCE). The loading imposed by the MCE is related to proximity to an active fault, the maximum amount of energy the fault is able to generate, and the soil conditions at the site.
The City of Los Angeles Harbor Department criteria imposes additional requirements based on a desire to limit damage to the structure in lower level earthquakes, which have a greater probability of occurrence in a given time period. For many components, the Caltrans criteria took precedence. In other components, the Port criteria was more stringent. The most significant difference in new designs is a dramatic increase in the density of reinforcement in the bent caps and in the superstructure members framing into the bents. Both the City of Los Angeles and Caltrans were involved in the review and approval of the final design.
From the ground up, the bridge appears similar to many other bridges in California: a cast in place, post-tensioned box girder structure. The difference is below the ground surface.
Due to the potential massive soil liquefaction at the site, the bridge is founded on groups of cast-in steel shell concrete piles that are 42 in. in diameter and up to 140 ft long.
Liquefaction occurs under conditions where soil is composed of medium to loose sand and a high water table is present. Vibrations caused by an earthquake can turn this type of soil to quicksand. It is not unusual for there to be isolated layers of liquefiable soils interspersed within more stable layers of soils at a site; this is a condition that can be readily addressed with minor modifications to the foundation design.
In this case the top 50 ft of soil is subject to liquefaction. During a seismic event, these soils would provide little resistance to lateral or vertical foundation movement. Therefore, the bridge foundations had to be designed almost as if the soil were absent altogether.
A variety of solutions were considered. Most involved soil remediation. However, due to the silt content of the insitu soils, the proximity to existing facilities, and the acreage of the site, all these alternatives proved to be either unfeasible from a constructibility standpoint or prohibitively expensive.
Various alternative foundation designs were also evaluated, including large foundations supported by many standard size piles. However, the new foundations where limited in size because they were to be located between existing foundations to avoid interference with the existing piles of the original bridge, which were to remain. It was also determined that this approach would not have provided adequate lateral load capacity during the design seismic event.
The solution to the problem of providing a strong bending member at a realistic cost was to support the foundations on clusters of 42-in. diameter concrete piles cast in driven steel shells. In total, 192 of these large diameter steel shells were used to support the structure.
Working in the Flood Channel
Field conditions also pose a number of construction and scheduling challenges. The Dominguez Channel is a seasonal flood control channel, which is maintained by the Los Angeles County Public Works Department. During the winter the channel capacity cannot be reduced, so construction in the channel for cofferdams, falsework, and other obstructions cannot occur. During the dry summer, the capacity can be reduced to specified levels, which allows access to the channel for equipment and excavation.
To meet the flood control schedule without delaying the project another year, the steel shell pilings, which have a five-month procurement lead time, had to be pre-ordered and purchased prior to awarding the construction contract. Not having the "luxury" to perform a pilot study to confirm that the pile lengths as designed would be correct in the field, additional sections of the steel shells were also pre-ordered in case the piles needed to be lengthened.
In addition, the precise load-bearing capacity could not be confirmed until the piles were driven using pile-driving analyzers. Additionally, some pilings were redriven with the concrete in place to further verify loading capacity.
To install the foundations in the channel, first a cofferdam was built and pumped dry. The open ended steel shells were driven through the liquefiable layers into the firm soil below using a Delmag D80-23 pile hammer - the largest terrestrial pile hammer available. The shells were augered out, a concrete seal course was placed, and the piles were drained and cleaned. Finally, a reinforced concrete core was installed for attachment to the footing.
Temporary extensions were used at the top of the piles to maintain the equilibrium of water levels within and outside the piles and to prevent the soil plug at the bottom from dislodging prior to placement of the seal course. In addition, the site is impacted by a county freshwater recharge well, which increases the hydraulic head around the piling. Throughout the construction process, work is being closely coordinated with the flood control district and the contractor to assess these dynamic conditions and identify the best way to proceed while maintaining the schedule.
The City of Los Angeles Cultural Affairs Commission must review and approve the esthetic details of any significant civil engineering project in the city. The bridge's immediate surroundings are highly industrialized, so the esthetic theme had to be subtle and economical yet provide the structure with sufficient character.
The general configuration of the bridge is straightforward. Typical in California, a standard cast-in-place post-tensioned concrete box girder bridge is proven to be economical, appropriate for the loading conditions, and easy to maintain.
Taking a cue from the Port of Los Angeles' logo, the designer incorporated a wave pattern into the exterior of the concrete bridge barriers and added surface textures to the flat faces of the retaining walls and abutments. Plastic form liners were used to cast these textures into the concrete. Sloped exterior girders, octagonal columns, and landscaping on the slopes contribute to an attractive, cost-effective design that fits well into its surroundings.
Today, approximately 40 percent of the container traffic through ports of Los Angeles and Long Beach moves across the nation by railroad. Within the next decade, this figure will increase to over 50 percent. Completion of the Anaheim Street Viaduct Widening project will contribute to the long-range expansion plans of the San Pedro Bay Ports, an integral part of the overall development strategy necessary for southern California and the nation to gain the full economic benefits of Pacific Rim trade.
GREG BROWN Director Bridge Engineering, Holmes & Narver, Inc. Orange, California
JAMES KINKOR Project Manager, Los Angeles City Harbor Department Los Angeles, California
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|Author:||Brown, Greg; Kinkor, James|
|Date:||Feb 1, 1997|
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