A beloved 1921 Parker truss bridge in Healdsburg, California, underwent an extensive rehabilitation and seismic retrofit so that the community could continue to use the historic crossingNESTLED among the Dry Creek, Alexander, and Russian River valleys of northern Sonoma County, California, the city of Healdsburg is in the center of one of the most diverse and beautiful wine regions in the state. For almost a century, drivers entering or leaving Healdsburg crossed the picturesque Russian River on a historically significant steel truss bridge, which was completed in 1921. The crossing, now known as the Healdsburg Avenue Bridge, is a two-span, 437 ft long steel Parker truss bridge, a nearly extinct bridge type patented by Boston civil engineer Charles H. Parker in 1870. This type of bridge is characterized by a top chord that is shaped in a curve. In 2011, the bridge was listed in the National Register of Historic Places.
The bridge was designated as part of the new north-south U.S. Route 101 in 1926, but in the 1960s the California Department of Transportation (Caltrans) realigned 101 to a location just west of Healdsburg. That relocation included a concrete freeway bridge across the Russian River about 0.5 mi downstream of the original alignment. At this time, Caltrans also passed ownership of the truss bridge to Sonoma County. In 1980, ownership was passed to the city of Healdsburg when the city expanded and annexed a portion of land to the east of the Russian River. Caltrans reports that at that time city officials made attempts to replace the bridge because of its low National Bridge Inspection Standards (NBIS) inventory rating, which is a measure of structural adequacy established by the Federal Highway Administration (FHWA). However, outcry from local citizens led to the decision to keep the bridge in use but to limit the weight of vehicles crossing it to 17 tons.
After the magnitude 6.9 Loma Prieta Earthquake in 1989, emergency legislation was enacted in California that established the Local Seismic Safety Retrofit Program using federal transportation funds. The intent of the program is to identify and address the seismic deficiencies of local (city- and county-owned) public bridges within California. Over the course of the next few years, the Healdsburg Avenue Bridge was identified as one of more than 1,200 bridges in the state with deficiencies that needed to be fixed through seismic retrofitting. Unfortunately, the city lacked the necessary matching funds-11.5 percent of the total costs-that were required to receive federal funding toward the remainder. So the seismic retrofit was postponed until funds could be secured. In 2006, California voters approved a statewide measure, the Highway Safety, Traffic Reduction, Air Quality, and Port Security Bond Act of 2006, also known as Proposition 1B, establishing a $125-million Local Bridge Seismic Retrofit Account (LBSRA) specifically to provide the matching funds required for such retrofits. The city subsequently applied for and was awarded LBSRA funds to move forward with addressing the Healdsburg bridge issuesIn 2010, the city of Healdsburg selected Roseville, California-based Omni-Means Ltd. as the prime civil engineering consultant to prepare engineering and environmental studies to rehabilitate, replace, or relocate the Healdsburg Avenue Bridge. Cornerstone Structural Engineering Group Inc., based in Fresno, California, was added to the team to provide structural engineering support for the evaluation of bridge repair alternatives. Ultimately, Cornerstone prepared the final structural design construction documents and provided construction engineering support services for the repair and strengthening requirements for the bridge.
The original Healdsburg Avenue Bridge's concrete deck carried two lanes of traffic over a narrow (by today's standards) roadway that was 19 ft 8 in. wide. The bridge's superstructure consisted of twin parallel steel trusses separated by 24 ft. (See the figure on page 62.) These are called "full-through" trusses because of the overhead horizontal bracing frames that connect them, and they spanned 200 ft from the abutments on either side of the river to a central pier located in the middle of the water. The top chords of the trusses were constructed using a composite of riveted steel members, while the bottom chords and web (diagonal) members were cast-steel eyebars connected by 4 in. diameter steel pins. The trusses extended approximately 20 ft above the concrete bridge deck, but the clearance from the deck to the overhead bracing frames before the rehabilitation and seismic retrofit work was only 14 ft 10 in. Because of the low clearance, the bracing frames had been hit numerous times over the years by passing trucks. Before the rehabilitation and retrofit work, no traffic barriers existed to protect the trusses themselves from vehicular impact; the existing 9 in. tall concrete curbs provided minimal protection. On the exterior edges of the trusses, away from the roadway, were two 5 ft wide sidewalks protected by a latticed steel railing. The bridge substructure consisted of two concrete abutments and the concrete center pier, both supported by Douglas fir timber piles embedded in pile caps.
In 2010, after nearly a century in service, the bridge carried an estimated 7,000 vehicles per day. With its narrow deck width and substandard vertical clearance, the bridge was considered functionally obsolete, according to the NBIS rating. To keep the bridge, the FHWA required that the overhead bracing frames be raised to provide a minimum 15 ft vertical clearance above the bridge deck and that a steel barrier rail located between the concrete curb and the bridge trusses be added to protect the trusses from vehicle impacts. Additionally, the FHWA required that the bridge be strengthened to support a minimum load equivalent to a 36-ton truck, which is designated by the American Association of State Highway and Transportation Officials as an HS-20 loading. The requirement to more than double the weight allowed on the bridge was a seemingly insurmountable task and drew into question the feasibility of rehabilitation versus a complete bridge replacement.
The FHWA also required the city to make a long-term commitment to perform regular preventive maintenance because the existing steel bridge would require more costly maintenance than a modern concrete bridge. To support this goal, the design team prepared a maintenance matrix early in the project that identified the estimated costs and recurrence intervals of routine and specialty maintenance items ranging from the cleaning of deck drains to the replacement of the paint system. The costs for this maintenance work were then annualized. The city of Healdsburg passed a resolution committing to providing a long-term funding source for preventive maintenance of the bridge.
In addition to the bridge's vehicular load limitations, substandard height and width, and seismic vulnerabilities, its foundation was in need of repair. The center pier in the middle of the river was supported by timber piles that were partially exposed because of scour. With the possibility that scour could reach a depth of 16 ft during floods from a 100-year storm, the piles could be exposed, which could cause the bridge to collapse. Additionally, previous studies found that these pier piles would fail during a major seismic event. Replacement of the center pier was required.
In March 2010, Cornerstone performed a field investigation to confirm basic geometric elements of the bridge such as member sizes and truss geometry, and particularly the locations of the connecting pins along the truss's top chord. These were used to make independent calculations to confirm the capacity of the bridge to carry truck loading. The previous load-rating calculations determined that the reason for the bridge's inability to carry HS-20 loading was a deficient top chord. A substantial reduction in the capacity of the top chord was thought to be caused by the dead-load bending moments in the steel truss chord members. The field investigation and subsequent analysis by Cornerstone, however, determined that the slightly offset locations of the top chord connecting pins relative to the centerline of the top chord members, as called for in the original design, served to produce secondary bending moments in the top chord that were opposite in direction to the dead-load bending moments. This beneficial location of the connecting pins served to increase the strength of the chords, making the HS-20 target design loading for the rehabilitation feasible with only the addition of steel cover plates to the webs of the channels that composed the built-up top chord compression members.
With the viability of a rehabilitation plan confirmed, the city council voted in September 2010 to preserve the existing structural rehabilitation with a seismic retrofit.
Over the next two years the design team focused on preparing documentation to comply with the National Environmental Policy Act and California Environmental Quality Act at the same time as they developed the rehabilitation and seismic retrofit strategies. Throughout this process, proposed details were provided to the California State Historic Preservation Office (SHPO) for confirmation that the changes would not impact the historic fabric of the structure, which in this instance was primarily determined to be the steel truss, the latticed pedestrian railing, and the art deco style of the center pier wall.
The seismic retrofit strategy included multiple elements. Lead-rubber base-isolation bearings were placed below the ends of the trusses at both abutments and at the top of the center pier to reduce transverse seismic loading to the bridge superstructure. Bottom truss chord splices and other supplemental steel members were added below the bridge deck to provide continuity of the seismic load path and to ensure the proper transfer of seismic loads to the foundations.
Base isolation of the bridge was a key component to saving the existing structure. The custom-built bearings were fabricated by Seismic Energy Products of Athens, Texas, to meet the design specifications needed to reduce seismic forces below allowable stresses in the truss's current configuration while keeping seismic deflections to a manageable level. Had base isolation not been used, significant strengthening of the highly visible upper chord bracing and portal frames would have been required, which would have had a negative visual impact. Bottom truss-chord splices were designed to be recessed within the flanges of the bridge deck's floor beams to minimize visual impact, and steel framing was located at the abutment and pier bearings to keep it hidden from view from beneath the bridge at nearby Healdsburg Veterans Memorial Beach, located just downstream of the bridge.
The rehabilitation strategy of the superstructure included strengthening key structural elements such as the end portal frames and upper chord members with new steel cover plates, restoring the badly corroded floor beams and increasing stringer capacities by installing new flange cover plates, and installing headed studs to the tops of the existing floor beams and stringers to make these members act in composite with the new bridge deck. Additionally, the rehabilitation strategy for the superstructure included using heat to straighten members that had been damaged by vehicle impacts and replacing the normal-weight concrete deck with a fiber-reinforced lightweight concrete deck.
For the foundation rehabilitation, scour and seismic concerns at the central pier were resolved by replacing the existing pier with a "super bent" design, which was the most cost-effective option. (See the figure on page 63.) This replacement design allowed for two 7 ft diameter cast-in-steel-shell piles to be installed just beyond the edges of the existing deck and a bent cap to be built around the existing pier to enable the steel truss spans to remain in place throughout construction. After the new elements obtained design strength, the existing pier wall was demolished, and the entire weight of the steel truss bridge was transferred to the new 50 ft span pier-cap beam. The space between the columns was then filled in with a new pier wall that was designed in the same art deco style as the original to preserve the bridge's architecture. In addition, the new pier was designed to withstand seismic loading as well as local and degradation scour that would result if the downstream Veterans Memorial Beach dam were to be removed.
The work also included devising solutions for issues caused by the existing railing and pedestrian protection measures that had been implemented after the bridge's original construction. Before rehabilitation, a chain-link fence had been installed on the bridge to prevent people from jumping from the bridge deck into the river below. But this fence blocked the view of vehicles turning onto and off of the bridge from Front Street. Additionally, the existing steel lattice pedestrian railing did not meet current design standards for railings and had to be replaced. To resolve these issues, several railing concepts were developed, including an innovative new steel cable railing that preserved the historic lattice railing and enabled the removal of the chain-link sections. These changes improved drivers' oblique lines of sight. After reviewing 3-D visual simulations prepared by Omni-Means, SHPO concurred with the selection of the new cable railing system. It was also decided to reconstruct the intersection adjacent to the bridge to improve drivers' turning radii and add a signal to further improve the safety of the intersection.
At the conclusion of the environmental review process, several regulatory permit restrictions were placed on the construction of the project. These restrictions included mitigation measures to protect endangered fish species within the Russian River and required use of vibratory driving for the installation of temporary sheet piling. Permanent piles were required to be driven inside dewatered cofferdams while the construction team performed hydroacoustic monitoring to ensure that underwater sounds did not exceed established thresholds and thus endanger the steelhead and salmon known to swim in the river. All operations within the channel were limited to being performed between June 15 and October 15.
Another restriction was that the commercial and public uses of the Russian River's eastern shore needed to be maintained during construction. On this side of the river, to the north of Healdsburg Avenue, a local kayak and canoe guided tour operates, and interruption to its business during the summer would be financially disastrous to the company. Additionally, Sonoma County operates the Veterans Memorial Park and adjacent beach, located to the south of Healdsburg Avenue, during the summer. Maintaining these operations required providing surface water access through the construction site below the bridge and required that pedestrian and bicycle traffic across the bridge be maintained during construction. Because of the hazardous nature of the lead paint abatement and removal operation required as part of the superstructure rehabilitation, an option to provide access around the bridge via shuttle service was also specified in the contract documents.
Before beginning preliminary design, a more detailed structural investigation was performed in August 2012 with the assistance of Caltrans staff, who operated one of its under bridge inspection trucks (UBITs). The information collected during this investigation was crucial to the final design because the six pages of as-built plans that were available for the bridge, while accurately identifying member sizes, did not contain any details about the numerous riveted steel connections. The Caltrans inspector took photos of floor beam, stringer, and truss pin connections below the deck using the UBIT and noted any signs of corrosion. The inspector also took photos of all four sides of every truss member and its above-deck connections for use in later verifying the adequacy of the riveted connections. The investigation also noted the numerous areas where steel members had been deformed, and sometimes torn, by vehicle collisions. In many locations, truss diagonals were also found to be loose, a situation that called into question the load distribution between the parallel trusses' diagonal membersThe final design began in September 2013 and was completed by February 2014. During this process the city decided to add municipal utilities-electrical service and three wet utilities consisting of water, sewer, and recycled water-across the bridge to provide for future city growth. Because these were major trunk lines serving the growing communities south of the Russian River, the utilities needed to remain in service after a major seismic event. This performance objective was complicated by the selection of the seismic isolation bearings that were anticipated to allow bridge movements of up to 18 in. in any horizontal direction during a seismic event.
Meeting both the seismic and utility performance objectives required the creative use of expansion-deflection fittings, which are essentially large telescoping ball joints installed in line on the pipes. These were used in all wet utility pipelines to allow for seismic movements at the pier and inside both abutments without severing the pipes. While the expansion-deflection fittings were originally planned to be located outside the abutments, the city requested that they be hidden from view within enclosed bin abutments. To accomplish this, the concrete bins within the bin abutments were reconfigured, and slotted openings at the front faces of the abutments were added.
The project went out to bid in April 2014, with an estimated construction cost of $12 million, including $2 million for supplemental work. This supplemental work budget was established to account for the possibility that some steel beams below the bridge deck that could not be seen until the concrete was removed might be corroded and need to be replaced. Granite Construction Company was awarded the construction contract and issued a notice to proceed three months later, in July. The city of Healdsburg contracted with Omni-Means and Cornerstone to perform construction management services, thereby maintaining continuity between the design and construction management teams.
The first order of work during construction was lead abatement. Instead of providing shuttle service to those pedestrians and bicyclists who wanted to cross the bridge during the lead-abatement operations, Granite Construction opted to provide safe open-air passage for them by establishing a negative-pressure, double-baffle containment system around the affected elements of the superstructure. The contractor also frequently tested the air, water, and soil outside the containment for the presence of lead. In addition to these measures, the blast media that were used to remove the lead paint were treated with a chemical compound that neutralized the lead primer on the bridge. This strategy allowed the paint that was removed from the bridge to be reclassified as a nonhazardous waste product. A specialty lab confirmed the successful neutralization of the lead by testing the removed material prior to its removal from the job site. Concurrent with the lead-abatement operation, Granite Construction strengthened the upper chords by installing strengthening plates to the chord webs.
After successfully removing all lead paint and applying a new primer base coat to the steel, the next order of work was to remove the existing bridge deck and complete the rehabilitation and retrofit work below it. A significant amount of corrosion was discovered on the exterior line of floor stringers after the deck was removed. It was determined that over time the deck drains had clogged, leading to standing pools of water forming after every rain event. This water pooled at the curb line directly above the exterior line of stringers, which saturated the concrete and formed pack rust. Pack rust had destroyed nearly half the stringers' flange thickness and left the underlying steel brittle. Several options were considered to strengthen the flange; however, these options would be prone to fatigue. Because of this, the team decided to entirely replace the exterior stringers. Granite Construction was able to source S-shaped steel sections that were virtually identical to the historic sections.
The sidewalk brackets were also found to have pack rust; however, the majority of the brackets were allowed to remain in place as the design called for them to be strengthened with new cover plates. Crews also discovered that the steel eye. bars, which form the lower chord members, had begun to form pack rust in many connection locations. These steel eyebars have been known to be vulnerable to cracking and sudden failure, so particular attention was given to their condition. To treat these joints, the team opted to adopt the Washington State Department of Transportation's procedures, which included the application of a rust-inhibiting, epoxy-based penetrant sealer that permeates, binds with, and seals rust in place. After that application, the team caulked the joints on the top and sides to prevent further intrusion of water but allowed an opening at the bottom for evaporation and drainage of any condensate that might form within the joint.
After completing the below-deck work, construction of the fiber-reinforced lightweight deck was completed in stages to allow pedestrians and bicyclists continuous access during construction. Concrete deck cracking was a major concern because of the bridge's relatively flexible truss and the desire to use lightweight aggregate in the new deck, so a lightweight deck concrete mix that incorporated fiber-reinforced polymer strands was used. Formwork for the new deck incorporated thickened areas and holes to allow penetration of the truss members that matched the historic dimensions of these elements. Granite Construction took a great deal of care to accurately re-create the original concrete deck.
After the installation of the new deck, it took one week to straighten, using heat, all the previously damaged steel members on the truss.
The heat straightening also included "tuning" the truss diagonals to ensure that the truss loading was shared equally by parallel diagonals. This tuning process consisted of applying a small lateral load to the truss diagonals to deflect them, then releasing the diagonals to observe the oscillation rate, similar to tuning a guitar string. Because the parallel eyebars have identical cross-sectional properties, theoretically they will oscillate at the same rate when under the same tensile loading. Using the undamaged eyebar as a reference, the damaged eyebar was straightened until the oscillation rates of the parallel members were about equal.
After these major operations, Granite Construction shifted focus to finish painting and restoring the aesthetic aspects of the bridge. A stone dedication plaque dated 1921 that had been embedded in the original concrete pedestrian railing at the east abutment was restored and recast into the new railing. A local museum was in possession of the sister plaque that had originally been cast in the railing's west abutment and removed so that a sidewalk could be installed. This plaque was restored and recast into the new concrete pedestrian railing at the west abutment.
The existing pier light posts and lighting were also restored. Although these lights were removed at some point, light posts at the abutments were clearly visible in historic photos of the bridge, and their anchor bolts were apparent in the original concrete pedestrian railing. Historic photos and measurements of the pier light posts were used to custom-fabricate new light posts at both abutments, and a state-of-the-art LED lighting system was installed along the length of the bridge.
Concurrently with the steel painting and aesthetic enhancements, pier construction was completed during the narrow window of time available during the summer of 2015.
Through close collaboration between Granite Construction, Omni-Means, Cornerstone, and the city of Healdsburg, the bridge was successfully reopened to vehicular traffic on schedule and within budget. A formal dedication ceremony was held on August 13, 2016, to coincide with the annual Healdsburg Water Carnival held at the Veterans Memorial Park. It was a redletter day for sustainability, because almost all the existing bridge materials that had served the community for almost a century were now rehabilitated and repurposed to continue to serve the community for many years to come.
engrmujahidshafie@gmail.com
engrmujahidshafie@gmail.com
