Juan Valenzuela, PE, And Edward Severino, PE 2023-09-01 14:26:48
A curved flyover bridge in Central Florida required a quick redesign, opening the door for a steel option that eventually exceeded state and public expectations.
WEKIVA PARKWAY fills a gap of sorts.
The 25-mile expressway, also known as State Road 429 (S.R. 429), is well on its way to completing Central Florida’s beltway. Weather permitting, the final link in what will ultimatley be a 54-mile beltway around greater Orlando will open in December. The parkway will provide travel options while also helping protect the natural resources surrounding the Wekiva River, a National Wild and Scenic River and Florida Outstanding Waterway.
Developing and building the 25-mile stretch of road has been a forward-thinking collaborative effort led by the Florida Department of Transportation (FDOT), the Central Florida Expressway Authority (CFX), and Florida’s Turnpike Enterprise, with the latter handling the toll operations for the FDOT sections. The vision for the project grew from 30 years of extensive engagement with national, state, and local government leaders, advisory committees made up of environmentalists, community and business leaders, and thousands of area citizens.
The parkway has been heralded as a shining example of transportation planning through an environmentally sensitive area, and its development includes more than 3,400 acres of land for conservation set aside to protect the Wekiva River Basin and new wildlife bridges for improved habitat connectivity. It also alleviates traffic congestion in the vicinity and provides improved evacuation time during severe weather emergencies such as hurricanes.
Realigned and Reconstructed
One of the crucial steel-framed components of the parkway is Section 3B—the westernmost portion of the project—which includes the realignment and reconstruction of S.R. 46 in Lake County. The project converted the previous diamond interchange at U.S. 441 over S.R. 46 to an at-grade intersection by providing a new flyover bridge, which opened in September 2020, to accommodate the heavy traffic volumes from southbound U.S. 441 as vehicles enter the S.R. 46 corridor and connect to the Wekiva Parkway.
The project’s role as a gateway to the City of Mount Dora necessitated innovative considerations regarding the layout and aesthetics of the interchange. Mount Dora’s height restrictions required the lowest possible profile for the proposed interchange, and in adhering to them, the project avoided the typical three-level version. The concept plans from the study phase called for a 1,593-ft-long flyover eccentric to the at-grade intersection.
In the diamond interchange configuration prior to construction, U.S. 441 was at the original grade atop a hill and S.R. 46 was in a cut section under U.S. 441. The long flyover from the study phase crossed over U.S. 441 and S.R. 46 at a higher elevation. In the final design phase, though, the engineering team proposed spanning directly over the intersection to shorten the flyover bridge’s length rather than be eccentric to the intersection. Then U.S. 441 was sunk to match S.R. 46, which lowered the bridge’s profile to minimize impacts on adjacent residences and businesses. A steel trapezoidal box (tub) girder option with integral pier diaphragms further reduced the structure depth.
The construction sequence required removing the previous diamond interchange bridges, with traffic detoured onto one of the previous at-grade ramps. This significantly improved the safety of the construction workers and provided more opportunities for temporary shoring tower locations.
Traffic modeling showed a single-lane flyover bridge would provide a satisfactory level of service in the design year (20 years after construction), but FDOT agreed a two-lane bridge would better provide for future growth beyond the design year when considering how site constraints would impact future bridge widening. As such, the bridge was designed with two 12-ft lanes superelevated at 2.6% for the 35-mph vehicle design speed. The outside shoulder is 10 ft wide, and the inside shoulder is 11 ft wide due to the sight distance around the inside of the curve. The inside shoulder is wider than the standard 6-ft width to increase horizontal sight distance on this flyover ramp bridge.
The bridge has 42-in.-tall TL-5 traffic barriers on both sides in lieu of the standard 32-in.-tall TL-4 barriers used on most bridges in the state. This was due to the tight 500-ft radius and projected truck traffic of 10%, which warranted taller barriers. In addition, the taller barriers required the cantilevered portions of the bridge deck to be thicker and more robust when compared to the TL-4 barriers.
Structural Design Considerations
Curved concrete and steel box girder alternatives were developed during the design phase, but the concrete box girder alternative was ultimately removed from the bid documents due to a shift in FDOT policy on post-tensioning corrosion-protection methods. The policy change occurred during right-of-way acquisition, which did not leave adequate time to redesign the concrete alternative.
Since the project constraints included a tight radius and the profile necessitated integral diaphragms on the single-column piers, it was clear from the start the bridge’s deck area would have a higher pounds of steel per square foot than more traditional bridges without integral pier diaphragms. With the added consideration of the bid documents—including the curved post-tensioned concrete superstructure alternative—the net differences were evaluated between using more traditional end span lengths (75% to 80% of the 231-ft main span) for the steel alternative versus matching the end span lengths from the concrete superstructure option (50% of the main span length). Because the bridge superstructure constraints already required a high degree of torsional and longitudinal stiffness, there would not be a significant increase in member sizes and fabrication costs if the end span lengths were shortened to match the concrete alternative.
The bridge has 60 lb of steel per sq. ft (about 683 tons total), and the cost for the structural steel pay item in the bid documents (which included only fabricated structural steel elements) was about $4,000 per ton, based on the winning bid. The total cost for the flyover bridge was $4.2 million, which amounts to about $186 per sq. ft of bridge deck when the project was bid in June 2017.
The steel bridge’s stiffness was sufficient to avoid any uplift on the bridge by the inherent stiffness of the external bracing and internal longitudinal girder quasi-box bracing behavior, along with the integral box-shaped pier diaphragms. The reduced long-term maintenance costs associated with the longer end spans was also considered. Therefore, the final design phase proceeded with the steel superstructure alternative matching the bridge length of the concrete superstructure option (120-ft-long end spans and a 231-ft-long main span for a total bridge length of 471 ft), which allowed just one set of roadway plans to accompany the two bridge options (the structure depth of both options was the same, resulting in one profile for both alternatives).
The innovative design demonstrated that the end span lengths of a bridge can be reduced by using the torsional and longitudinal stiffness when the bridge has similar constraints. Grade 50W weathering steel was used throughout most of the superstructure, resulting in reduced maintenance needs, and HPS 50W steel was used for the integral diaphragm members, which were designated as Nonredundant Steel Tension Members (formerly fracture-critical members, or FCM) in the plans due to the higher toughness and improved weldability.
The challenging internal and external flexural, warping, torsional, distortional, shear, and St. Venant torsion shear stresses were resisted by internal cross bracing and stout lateral WT section horizontal truss bracing, contributing to the system stiffness. The transverse integral diaphragms were designed to be composite with the deck in two-way bending with principal stress checks, even within the negative moment region as required per AASHTO LRFD Bridge Design Specifications (store.transportation.org), and were supported on two eccentric polytetrafluoroethylene (PTFE)/pot bearings as part of a system of fixed and guided bearings for translations and rotations in the direction of movement. The movement was further accommodated by the strip seal expansion joints at the beginning and end bridge.
The integral steel box pier diaphragms required the longitudinal box girders to align geometrically and provide continuous access for maintenance and inspection throughout the limits of the structure. Therefore, the walls of the pier diaphragm box girder required 22∕3-ft-wide by 3½-ft-tall openings at the intersections with the longitudinal tub girders and along the length of the pier diaphragms. The openings were strategically located so they could be reinforced by the jacking stiffeners at the sides in case the bearings ever needed to be replaced.
The box girder internal cross bracing and horizontal truss members required accommodating the interruption for the continuous diaphragm top flange, the jacking and bearing stiffeners, as well as the adjacent field splices that allowed the diaphragm boxes to be shipped in one piece. The internal stresses from the shortened end span arrangement were balanced by the stiffness of the integral box diaphragm, where longitudinal and transverse bending necessitated the need to check the principal two-way stresses in the diaphragm top flange. The connections, often with eccentric bolt groups, were designed for applicable limits states: tension and shear yielding and rupture, bearing and tearout, block shear rupture, and Whitmore sections for tension member gusset plates, among others. The integral diaphragms were designed by hand, while the longitudinal girders were modeled using a 3D grid model.
FDOT classifies the flyover as a “major bridge,” which required an independent peer review with oversight and review from the FDOT Central Office. The peer review’s intent was to produce an independent design using different 3D modeling software than the original design to verify the adequacy of the modeling methods and the assumptions used to create the models.
When it came to the deck pour, the sequence was designed to minimize the potential for cracking in the deck caused by subsequent pours. The first three deck pour limits were within the positive moment regions at the ends and in the middle of span 2, away from the tension and stress reversal regions, and enough deck area was poured in the first two end pours to anchor the end spans and avoid uplift. The subsequent pours were the negative moment regions.
Fabrication and Shop Assembly
The flyover bridge was fabricated with ten tub girder sections and two pier diaphragm box girders. Since it uses weathering steel, all complete joint penetration (CJP) welds were required to have the last two layers of the weld made with a weathering consumable. The CJP butt splice welds were internally filled with typical non-weathering Lincoln Electric’s L61/960 SAW (submerged arc welding) weld wire and flux combination. All CJP welds were then capped with two passes welded with Lincoln Electric’s Lincolnweld LA75/960 SAW wire flux combination—the weathering welding consumable—to ensure that the welds would have the same corrosion resistance and aesthetic patina as the base metal on the rest of the bridge.
The construction specifications in the contract documents specifically required a full bridge fit-up. To assemble the individual steel girder components as they would be configured in final erected position, fabricator Tampa Steel Erecting Company altered its typical standard shop assembly procedures to accommodate the integral pier diaphragms.
In lieu of starting the laydown process at one of the end bents and progressing to the other, the laydown started with each one-piece pier diaphragm section and progressed towards the end bents. Careful staging of components was essential for proper tub girder-to-pier diaphragm fit-up. The gap between the ends of abutting girders and pier diaphragm stubs was held to no more than ¼ in. A 100-ton Mi-Jack travel lift maneuvered the tub girders and pier diaphragms into the correct positions.
The tub girder weights ranged from approximately 36.5 tons to 78 tons, with lengths ranging from 51 ft to 123 ft. Pier diaphragms weighed about 59 tons each, with an overall length of 38.8 ft. Each pier diaphragm had two bearings, located 8 ft apart. These bearing points were set first, then girders were brought in from each side, trimmed to length, and bolted together with match-drilled splice plates. The final survey tolerances of the bearing locations had to be within ⅛ in. for each direction.
Coating Process
After the laydown assembly, the components were disassembled and prepared for blasting and coating. The entire exterior and interior of all the girders and pier diaphragm boxes were blasted to an SSPC SP10 near white metal blast, removing mill scale and contaminants and achieving an even patina of the weathering steel.
The bridge’s exterior was left bare, and the interior was coated with a single layer of Carboline’s Carboguard 893 Epoxy coating. In lieu of the standard FDOT requirement for a two-coat system (inorganic zinc and epoxy) for the interior of steel tub girders, this project was approved by FDOT for a single coat of the 893 Epoxy. That provided an illumination-friendly surface to assist maintenance teams with future inspections and select the proper coating that would provide the best value to Florida residents. Removing the requirement for the inorganic zinc coating and allowing a single interior coat saved time, labor cost, and material.
Girder Handling and Delivery
The tight 500-ft radius required careful planning for lifting, handling, and shipping of the highly curved tub girders to avoid rollover. It was critical to determine the longitudinal centerline of gravity for establishing rigging pick points and trucking bolster support points so the girders would stay balanced during lifting and shipping operations. Tampa Steel created temporary supports at the shop, including free-moving tilt tables, and then set the ends of each girder on the tilt tables with its 100-ton Mi-Jack mobile crane. Each end of the girder sections was then shifted transversely until the balance points were found and marked on the girders. The points were used to center the girder sections under the crane hooks and on the trucking equipment.
The increased lengths and weights of the oversized loads required the trucking routes to be carefully planned and coordinated with FDOT, whose analysis explored clearances, structures to be crossed, and load-limit restrictions. Both lead and follow escort vehicles were required for each load that was delivered. Gross vehicle weights, axle spacings, widths, heights, and travel times were planned in detail by Gator Transport, Inc., and then coordinated with FDOT, which ultimately granted the oversize load permits to ship each of the fabricated sections.
Social and Economic Considerations
The structure incorporated intersection signing and signals on the bridge to supplement the mast arms for the intersection beneath the bridge. A permanent bat abode was mounted between the box girders to be inhabited by the environmentally protected bats that were present in the previous bridge. The bats were humanely excluded from the original bridge prior to construction, and the abode had to be located in a portion of the bridge with reduced live load vibrations (near a support). It consists of 4-ft by 8-ft sheets of plywood with ¾-in. spacers, which provide the bats with crevices for roosting.
As far as aesthetics, the box girders met Mount Dora’s requirements and have a rustic weathering steel look that met the Wekiva Parkway Aesthetic Master Plan requirements. The color schemes and textures with recessed form liners were also a requirement on the columns and the scallops on the outside face of the traffic railings.
The Mount Dora City Commission and FDOT did not want the interchange to feel like an interstate roadway in residents’ backyards. Not only did this structure eliminate the need for a three-level interchange, but it also further reduced the profile by up to 10 ft with the use of an innovative integral diaphragm with trapezoidal box girders when compared with a more traditional use of steel plate girders supported on top of hammerhead concrete pier caps. The shorter and more compact bridge is significantly less intrusive to the adjacent residents, yet still provides full functionality of the original concept from a traffic perspective.
The completed project represents an elegant solution that satisfies the many different project constraints. All told, the construction cost was $32.8 million, the estimated construction duration was 850 days, and the total continuous structure length of 471 ft is nearly 70% shorter than the 1,593-ft flyover eccentric to the at-grade intersection from the study phase concept plans.
The project will serve as a valuable case study that illustrates a better solution than more traditional interchange configurations when the level of service is acceptable. The revised geometry from the study phase simplified the project and allowed the use of a simpler mechanically stabilized earth (MSE) embankment, which shaved nine months off the construction schedule, saved roughly $3 million, and greatly reduced impacts on the adjacent residential properties in Mount Dora.
The opinions, findings, and conclusions expressed in this article are those of the authors and not necessarily those of the Florida Department of Transportation or the U.S. Department of Transportation.
Owner
Florida Department of Transportation
General Contractor
GLF Construction Corporation, Miami
Bridge Prime Designer and Structural Engineer
Florida Bridge and Transportation, Orlando
Bridge Independent Peer Review Engineer
BCC Engineering, Miami
Prime Design Consultant and Design Project Manager
Moffatt and Nichol, Lake Mary, Fla.
Construction Engineering Inspection (CEI)
RK&K (Rummel, Klepper and Kahl), Orlando
Erection Plan Drawings
McElhanney Consulting Services, Tampa
Steel Team Fabricator and Erector
Tampa Steel Erecting Company , Tampa
Detailer
Tensor Engineering , Indian Harbour Beach, Fla.
Juan Valenzuela (jvalenzuela@flbridge.com) is a vice president and served as the structural engineer of record for the flyover project, and Edward Severino (eseverino@flbridge.com) served as the senior structures engineer. Both are with Florida Bridge and Transportation, Inc.
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