By BEN HAWTHORNE P.E., S.E., M.ASCE
On June 8, 2018, more than 100 years after its namesake was born in the Little Harbor neighborhood of Portsmouth, New Hampshire, residents and dignitaries from New Hampshire and Maine gathered to celebrate the opening of the new Sarah Mildred Long Bridge, located just down the street. The innovative structure features stacked precast-concrete approach spans flanking a 300 ft long, 4 million lb steel and concrete vertical lift span supported by four independent precast-concrete lift towers.
The Sarah Mildred Long Bridge connects the cities of Portsmouth, New Hampshire, and Kittery, Maine. It carries a vital interstate regional road as well as a single-track freight rail link. The bridge is one of three crossings within a 1¼ mi stretch of the Piscataqua River-the Memorial Bridge is to the south, and the Piscataqua River Bridge is to the north. Appropriately nicknamed the Middle Bridge, it carries the U.S. Route 1 Bypass, provides access to and from the Portsmouth Naval Shipyard in Kittery, and serves as a critical backup route for the Piscataqua River Bridge, which carries Interstate 95 and commercial traffic.
The new bridge replaces the original structure, which was named the Maine-New Hampshire Bridge upon opening in 1940. Decades later, it was rechristened the Sarah Mildred Long Bridge in honor of Long, who had been an employee of the Maine-New Hampshire Interstate Bridge Authority for 50 years at that point.
The original bridge was a bilevel, steel truss structure with a two-lane roadway on the upper level and a singletrack freight rail line below. By the early part of this decade, the bridge was in poor condition structurally and operationally, sometimes stalling in either the lowered or raised position. Vehicular traffic was impacted not only by reliability concerns but also by the frequency of required openings. A retractable span for small maritime vessels was located along the Kittery approach, but the main span required many openings per day, bringing traffic to a halt each time.
The opening of the original lift span was skewed to the river channel, which reduced the usable width. Larger vessels that had tugboats attached were unable to pass through the bridge opening, which created a potentially hazardous condition. This problem was exacerbated by the piers, which offered no protection for the lift-span towers. In 2013 this risk was highlighted when a large freighter became unmoored and drifted into the lift-span pier.
Concerns regarding the bridge's maritime use and future economic effects were so high that replacing the bridge had become a necessity. It was decided that the Maine Department of Transportation (MaineDOT) would manage the design and construction of the new bridge, while the New Hampshire Department of Transportation (NHDOT) would assume responsibility for the daily operation of the span.
The design was performed by the joint venture team of New York City-based Hardesty & Hanover (lift bridge) and FIGG Bridge Engineers Inc. (approach spans), a company that has its headquarters in Tallahassee, Florida. MaineDOT elected to pursue the construction manager/general contractor (CM/GC) project-delivery method. Pittsfield, Maine-based Cianbro was the contractor for the design and construction phases. The CM/GC design process began in late 2012 and was completed in September 2014. Construction started in the winter of 2014-15 and was completed in early 2018. The bridge officially opened to traffic on March 30, 2018. The total cost was $165 million.
The replacement design needed to support vehicular and railroad traffic, accommodate maritime users, and reduce the impact on the traveling public. Additionally, the new span had to improve navigation through the channel and mitigate the effects of vessel allision. These design goals had to be delivered on an aggressive schedule. The deadlines became even more critical after a mechanical failure stranded the original lift span in the raised position, closing traffic on the bridge 10 weeks earlier than expected.
To improve channel navigation, the bridge was constructed upstream of the existing bridge on a new alignment, which reduced the bridge skew from 25 to 15 degrees relative to the centerline of the channel. The lift-span pier caps were designed to be skewed to the bridge and to be parallel to the channel in order to increase the channel width relative to the span length.
A study of the lift span's opening requirements concluded that most of the openings were for vessels less than 50 ft high. To gain additional vertical clearance, the design team proposed a single-level, dual-use lift span that would support rail and vehicular traffic. The span would typically be seated at the roadway level, offering 57 ft of clearance but could be lowered to carry trains as necessary. The added height decreases lifts by 68 percent, which is feasible because of the unique circumstances of rail usage on the bridge. The U.S. Navy is the sole user of the rail line; the tracks are used less than 10 times per year, and the times are prescheduled. MaineDOT and the navy were receptive to the innovative span design concept. With the project goals set, concept development began.
The CM/GC team solicited feedback on the bridge's aesthetics from the surrounding communities by holding public design workshops, ultimately securing community acceptance. Common themes included modern maritime architecture and local relevance. The lift span was designed to have a sleek, structural-steel box girder superstructure, and the aesthetic design of the towers, arguably the most prominent element of any lift bridge, draws heavily on the local Piscataqua region. The decision to expose the main counterweight sheave at the top of each tower was based on a halyard lock used on top of sailboat masts. Uniquely local architectural elements-lighthouses, textures, and proportions-were referenced to accentuate the scale of the towers.
Soil conditions at the channel bottom were not conducive to traditional pier-protection solutions. For the existing bridge, shallow overburden above rock necessitated expensive fendering systems with little resistance to overturning. The CM/GC team nixed using a fender system that would be used infrequently and instead opted to put that cost back into the permanent load-carrying elements. The lift-span substructure was designed with additional capacity to accommodate direct vessel impacts.
The design approach was based on a kinetic energy method, which determines the amount of energy that needs to be absorbed by the tower foundations. The tower foundations were sized to absorb the impact energy while sustaining acceptable deformation, which was defined as the maximum permissible deflection the tower foundations could undergo without degradation, permanent deformation, or hindrance of mechanical component operations.
The criteria governing substructure design were based on extreme event-load combinations from vessel impact, which were established after a river user survey. From that survey, the controlling design vessel was determined to have a deadweight of 500,000 metric tons operating at a speed of 4 knots. The estimated equivalent static impact forces ranged from 7,380 kips for an unloaded vessel to 12,000 kips for a loaded vessel.
The movable aspect of the towers presented unique challenges during foundation analysis and design. The stringent tolerance requirements for movable operation do not allow for a typical, nearcollapse substructure design approach during ship-impact analyses. Also, the hydraulic study determined channel degradation and contraction scour were likely to result in the complete erosion of surficial soil deposits. Therefore, foundations for the lift tower were designed to rely solely on the bedrock, which underlies the surficial soil deposits, resulting in large, stiff substructure elements.
The foundation configuration was dictated by the lateral load anticipated during a vessel impact. The team used FB-MultiPier finite element analysis software (developed by the Bridge Software Institute, of Gainesville, Florida) to establish foundation configurations. The model captured nonlinear rock responses to analyze rock-shaft interaction during vessel impact loading. These responses were then used to determine the tower foundation system's potential capacity to withstand impact from errant vessels. The final lift-span foundation arrangement comprises 15 ft deep pier caps on eight 10 ft diameter drilled shafts with 35 ft rock sockets.
To avoid dense/congested rebar cages, the drilled shafts were designed with a 1 in. thick permanent steel casing that was sealed into the top of the bedrock as a structural element. As a result, the construction details called for sealing permanent casings in rock, with embedment reaching up to 5 ft, and 4 ft of casing embedment into the pile cap to address the concentration of maximum moments at the top of the rock socket and at the bottom of the pile cap. The permanent steel casing improved bending/flexural strength as well as the shear resistance of the drilled shafts. In addition, the confinement provided by the perma-nent steel casing and rock socket meant additional shear reinforcement was unnecessary.
Additional Insight: Approach Structures
The in-water work was complicated by the Piscataqua River's swift currents and 8 ft tide changes. To provide easier access and limit the amount of work done by barge, a temporary trestle was constructed from each embankment to the tower pier locations. The trestle provided access for machinery, workers, and supplies during the construction of the foundations.
The pier caps are 125 by 65 ft in plan. The pier cap form comprised precast tub sections, which were flooded during placement but before posttensioning. Once all segments were in place and posttensioned, the tubs were sealed, pumped, and kept dry.
The reinforcement per pier totals 500 million lb, and more than 3,000 cu yd of concrete was placed in each tub.
The lift span is supported by four 200 ft tall, independent, precastconcrete segmental lift towers-the first of their kind in the country. The counterweights are connected to the lift span by wire ropes draped over 20 ft diameter sheaves located at the tower tops. The towers are closed concrete structures constructed from precast concrete segments, posttensioned together. The tower section is a closed box that is structurally rigid, requiring no external bracing. Often lift towers comprise open frameworks that are exposed to weather conditions, but the closed towers offer the advantage of protecting the counterweight and ropes.
Individual tower segments were fastened with posttensioned bars, and the entire tower system was posttensioned with tendons anchored to each tower top, passing through the pier cap. The size of the tendons and the location of the anchorages were carefully selected to counterbalance the overturning moments due to the eccentricity between the center of the concrete segments and the center of the sheave supports.
Cast-in-place towers that used self-climbing formwork were originally considered during design. However, precasting the tower segments was suggested during the CM/ GC phase. During final design, a site adjacent to the bridge became available for casting segments, making precast construction a viable option. Design plans were immediately developed to include a precast, segmental tower alternative. Both options were priced by the CM and an independent cost estimator, and the precast option was selected as the preferred alternative.
Casting was performed by Cianbro using metal formwork to ensure dimensional stability. Each segment was an 8 ft deep closed concrete box. The segments were match-cast to ensure closely fitting joints. The rebar cages were pretied and lifted into place, which kept tower erection on schedule. The contractor was able to cast up to one segment every three working days and place as many as three segments per day.
The erection tolerance for the towers was specified to not exceed 1 in. out of vertical per 100 ft of tower height. Adjustments during segment casting and shimming during installation achieved the required verticality. The machinery and electrical rooms span between the towers. The machinery room roof is at the railroad-track-level deck, and the electrical room roof is at the roadway-level deck. The tower span railroad level includes a ballasted deck with continuous rail into the approach span. All tower work was performed in advance of the lift span float-in.
Lateral loads on the lift span transmit load to the lift towers through a lateral-force-resisting system comprising lock bars, centering devices, span guides, and bearings. Lateral forces acting on the lift span include seismic, vessel allision, braking, traction, wind, and forces from machinery.
The lock bars were designed primarily to resist operating machinery forces at the roadway-seated level in accordance with the movable bridge design specifications of the American Association of State Highway and Transportation Officials (AASHTO). The lock bars were driven into position by an actuator housed in the electrical room. There are no lock bars at the railway-seated level, and a design exemption was granted for this condition, which was justified by the additional span-heavy imbalance at the rail level and low train speeds.
The centering devices were aligned for transverse force resistance as well as for span positioning at the roadway- and railway-seated levels. Centering devices provide 1/8 in. maximum total clearance (1/16 in. on each side in the ideal aligned position) with the span in the roadway-seated position. This reduced clearance is a secondary level of centering. The initial span centering occurs at the span guides, which are tapered as the span approaches critical clearance.
Centering devices provide tight clearance (0 in.; +1/32 in., -0 in.) with the span in the railway-seated position. This alignment is necessary to eliminate span-centering forces through the miter rails. The centering devices were designed to account for all primary lateral forces that could be transmitted after slip of the lift-span bearings.
Highway bearings were fixed at one end of the lift span and expansion rocker bearings were used at the other. The rocker bearings were suspended from the underside of the lifting girder and bear on a strike plate mounted on the retractable bridge seats. Fixed bearings include a receiver mounted to the underside of the lifting girder and a base weldment attached to the retractable bridge seat that fixes the bearing in the longitudinal and lateral directions. The railway bearings on each end are plate rocker bearings, with the expansion end fixed through lateral bearing plates to resist traction and braking forces.
The lift bridge main span is a multibox steel-girder system with a composite steel plate and concrete deck. The 300 ft long span is supported at each end by steel lifting girders that transfer span loads to the substructure through wire rope attachments on each end and through multiple sets of bearings mounted to the underside of the girder. The total load to lift, which includes all permanent loads from the girder span and lifting girders, is 4 million lb.
The lift-span structure was designed in accordance with the
MaineDOT Bridge Design Guide
AASHTO LRFD Bridge Design Specifications
AASHTO LRFD Movable Highway Bridge Design Specifications
, and the
Manual for Railway Engineering
from the American Railway Engineering and Maintenance-of-Way Association (AREMA). The global design of the box girder system was governed by the larger AREMA rail loads, while certain secondary elements and details were dictated by the AASHTO codes.
The lift span has a 38 ft, 7 in. wide roadway, comprising two 12 ft lanes of vehicular traffic, two 4 ft, 6 in. shoulders that bicyclists can use, and a 5 ft, 7 in. center median, which houses the railroad track. The live load used for the Strength I limit state is the MaineDOT-modified live load, which consists of the standard HL-93 live load with a 25 percent increase in the design truck axle loads.
For railroad traffic, the navy stipulated three design loads: two current train car arrangements and a proposed future load. The governing load induced a maximum moment in the span that was approximately 83 percent of the Cooper E80 bending moment.
The selected design load for the lift-span girder was therefore a Cooper E80 load traveling at 25 mph. Based on the difference in actual loading conditions and the design loads, the lift-span girder was optimized to have a utilization of 99 percent for the full Cooper E80 load at the design speed. This approach accommodates potential load increases and does not unnecessarily burden the span-operating system and structure with excess weight. Every pound of span weight is offset by roughly the same counterweight.
MaineDOT stipulated that a raised median that incorporated the running rails would not be permitted because of maintenance and safety concerns. Including a recess in the orthotropic top plate to accommodate the tracks added complexity and cost to the fabrication and introduced fatigue-prone details. Instead, the top plate was lowered uniformly across the width of the structure, and a full-depth reinforced-concrete deck was included to embed the running rails. The additional stiffness and section area provided by the orthotropic ribs were no longer necessary, and the deck fabrication details were greatly simplified. The additional weight of the concrete deck was offset by reducing the steel cross section to keep the lift load manageable. To keep the weight down, an integral wearing surface was used and the deck concrete mix used was a lightweight, low-permeability substance with a strength of 5 ksi. MMFX Technologies' chromium-reinforced concrete was used in the deck for its anticorrosion properties. MMFX's parent company, Commercial Metals Company, is headquartered in Irving, Texas.
The embedded-rail system includes a direct-fixation rail at each end, with the remaining length designed as a "floating" track cast into the concrete median. Each rail rests on an elastomeric attenuator pad placed within a rubber rail boot. The boot and pad sit on a polyurethane grout pad, and the deck concrete was cast directly against the boots. The attenuator pads and boots improve load distribution, reduce vibrations, and control rail movement to minimize force transfer into the adjacent concrete. The median concrete has a stamped faux brick finish dyed red to delineate it from the driving surface.
Determining the structure depth was a challenge because of competing desires for maximum navigation clearance and minimized approach grades. Also, during the CM/GC process, Cianbro provided feedback to the design team that lift-span costs could be significantly reduced by opening bidding to inland fabricators. To meet this requirement, all elements had to be shippable by truck, rail, or barge. The limiting depth of any section was set at 14 ft, and the limiting width was 12 ft. The structure design evolved to be subdivided transversely and longitudinally into 12 shippable segments.
Considering all the requirements and recommendations presented throughout the CM/GC process, the final lift-span structure evolved to consist of two adjacent steel boxes with vertical web plates and a shared continuous top plate with composite concrete deck. The bottom flanges of the box girders vary in thickness, from 1¼ in. at span ends to 21/8 in. at midspan. The deck plate is 2 in. thick below the track and 1 in. thick elsewhere.
The main boxes were aligned so that the interior webs were located directly below each rail track. This alignment eliminated the need for supplemental track support structures and ultimately reduced the span weight. The exterior web spacing was dictated by shipping-width restrictions and was located 11 ft center to center of the interior web. The roadway width extends beyond the exterior web, and the overhang loads are supported by a longitudinal fascia box beam spanning between transverse cantilever brackets spaced 12 ft apart on center.
AREMA rail loads are carried directly by the main longitudinal-load-resisting elements, while AASHTO vehicular loads act transversely on the deck. The composite steel plate and the concrete deck were designed as a continuous transverse strip spanning from fascia to fascia.
The proximity of the adjacent box sections prohibited the use of live-load distribution factors, and the structure was analyzed as a single unit that supports vehicular and rail live loads. The adjacent boxes are connected through the shared top plate; full-height, bolted, intermediate diaphragms; and the lateral bracing between bottom flanges. Discrete boxes were braced to develop shear flow across the entire cross section and provide torsional stiffness. The section was assessed for vessel allision damage, including mast- and deckhouse-impact scenarios.
The box girder webs were stiffened longitudinally and transversely. Interior bracing was provided within the box girder elements at each transverse diaphragm and cantilever bracket connection location. The top plate includes a longitudinal stiffener that was needed during deck placement.
The aerodynamic shape of the original design, which was well received by the public, was maintained by the use of fiber-reinforced plastic fairings, which were attached to the edge of the steel deck plate and at the exterior edge of the bottom flange. These fairings also reduce lateral wind pressures and provide protection from the elements and salt-laden runoff.
The box girder bottom flanges and the lower portion of the webs are fracture-critical members. Lift-span inspection access is enabled by various points of entry. The interior of the box girders is accessible through hatches in the lifting girder web as well as via doors in the electrical room walls. The exterior face of the box girders is accessible via the lifting girder's mounted-access platforms and using ladders leading to a door in the fairings. Within the fairings is a fiberglass walkway that runs the full length of the bridge that is supported on one side by the fairings themselves and on the other by brackets bolted to the box web. Ladder support rails were included to aid with inspection. The fairings also support the navigation lights and include an access point for service. The center between the boxes is accessible via a ladder attached to the pier top when the span is in the lower seated position. The steel grating platform is full length, and each intermediate diaphragm includes a void for access.
The lift-span girder, which is a variable-depth, noncomposite, single-web girder, is attached to the lifting girder with bolted connections at all webs and flange plates. The lifting girder transfers the full span weight to the lifting ropes and supports the dead weight as a simple span between the rope connections. Live loads are transmitted through the lift-span girder webs, into the lifting girder, and through the bearings. Span guides and centering devices were mounted to the lifting girder face.
The lift-span sections were shipped to the project site by rail and barge and assembled on a single large barge that was ultimately used to float the structure into place. After all the bolted connections were fastened, transverse and longitudinal complete penetration deck splice welds were completed.
Once field-assembled, but prior to placing the concrete deck, the lift span was floated into place. The navigational channel was closed for 10 days to allow for the float-in, concrete deck placement, and installation of counterweight ropes, centering devices, guides, and joints as well as functional testing of the bridge controls.
The timing of the float-in was coordinated with high tide so that the span could be floated in above the railroad bearings and seated on them; the barge was floated out once the tide lowered. The span could only be moved at slack tide so that the tugs could control the barges. The clearance between the lift girders and the towers during float-in was approximately 1 ft on each side. Cianbro orchestrated the float-in using a three-barge system: a span barge, which supported the structure; a fixed-guide barge; and a push barge.
The operating machinery is located at the base of the towers. This recent innovation to the movable bridge industry means quicker construction, lower cost, and easier access for maintenance. The machinery was placed in the towers, machinery room, and electrical room before tower erection was complete. This allowed the contractor to work on the machinery components and the structural components concurrently.
The machinery room houses the motors and reducers that drive the operating drums, which are in each tower. A single 150 hp motor drives the span. To provide a fully redundant system, two motors were installed; they alternate operation of the span with each lift.
There are two uphaul and two downhaul ropes connecting each drum to each counterweight. The uphaul ropes pull the counterweight down. The downhaul ropes travel over a deflector sheave at the tower top, connect to the top of the counterweight, and pull the counterweight up. Each counterweight is connected to a corner of the lift span by way of 10 counterweight ropes over the counterweight sheave, located at the tower tops.
The control room provides maximum visibility to the navigable channel and roadway. To facilitate construction, the control room was fabricated from precast concrete and cantilevered off the tower segments using posttensioned connections.
A unique feature of the bridge is that the lift span lowers from the roadway position to the railway level. To allow for the roadway bearings to clear the path for the lift span to lower, actuator-controlled retractable seats were incorporated into the design. The seats are large, trapezoidal, structural-steel elements that pivot about bearings mounted to the lift tower structure. The seats are pushed by the actuator into the open position-parallel to the span-to provide a cantilevered bearing seat in the roadway position.
The retractable seats on the fixed side enable a pin receiver connected to the lifting girder to perfectly align with a 5 in. diameter pin on the retractable seat every time. On the expansion side, the retractable seat position also had to be repeatable, but the bearing detail allowed for a greater tolerance because of the expansion and contraction at that end.
The control system, which is operated by touch screen, was more complex because of the two seated positions as well. There are two sets of operations: one to raise the span for ship passage from the roadway-seated position and another to lower the span to railroad position for train passage. For railroad operation, the span lifts 2 ft, the seats rotate out of the way, and the span lowers to railroad level. This sequence is reversed to return to the roadway position. However, opening for ship passage is more traditional.
The new Sarah Mildred Long Bridge improves vehicular, rail, and navigable travel between Maine and New Hampshire. The challenges of coordinating different disciplines, site conditions, owner and contractor input, and meeting an aggressive schedule and budget were overcome by using innovative solutions throughout the design and construction processes. The result is a beautiful structure that will last into the next century.
Ben Hawthorne P.E., S.E., M.ASCE, is a senior associate and senior project engineer at Hardesty & Hanover's, New Haven, Connecticut, office
PROJECT CREDITS Design and construction Maine Department of Transportation (MaineDOT), Augusta, Maine Owners New Hampshire Department of Transportation, Concord, New Hampshire, MaineDOT Lift bridge design (span, towers, foundations, substructure) Hardesty & Hanover (New York City and New Haven, Connecticut, offices) Mechanical, electrical, and architectural design Hardesty & Hanover Approach span design FIGG Bridge Engineers Inc., Tallahassee, Florida Roadway design Sebago Technics, South Portland, Maine Geotechnical analysis GZA, Portland, Maine Contractor Cianbro, Pittsfield, Maine
Civil Engineering, November, 2019, © American Society of Civil Engineers. All Rights Reserved