By: Vincent T. Gastoni, P.E.
The Minnesota Depart of Transportation (Mn/DOT) is replacing the 60-year-old structurally deficient Highway 61 arch truss bridge over the Mississippi River in Hastings, Minn., with an innovative record-breaking, free-standing arch structure designed for a 100-year service life.
With a main span of 545 ft, the new free-standing tied-arch bridge will set a North American record for this structure type. The winning team of Lunda Construction, Black River Falls, Wis., and Ames Construction, Burnsville, Minn., with designer Parsons, Pasadena, Calif., successfully proposed this innovative alternative through Mn/DOT’s design-build best-value selection process. This bold, elegant structure will provide a highly redundant, cost-effective community icon for the next 100 years in this historic river town and scenic recreation area.
Showing what it’s made of
Carrying the highest daily traffic volume of any two-lane highway in Minnesota, the existing bridge is in the lowest 1% of the state’s bridge inventory for overall sufficiency. Originally programmed for long-term replacement, Mn/DOT accelerated the project through its design-build program following the Minnesota legislature’s work involved in passing a funding bill to repair/replace fracture-critical bridges. The Mn/DOT design-build request for proposals (RFP) identified a new four-lane crossing of two acceptable structure types: a basket handle tied arch and a diamond tower cable stay. The RFP also allowed the design-build teams to propose alternatives through a confidential alternative technical concepts (ATC) process.
Of the three proposing teams, the Lunda/Ames team, with a construction bid of $120 million, produced the best-value combination of a technical solution at the lowest cost. The tied-arch main span utilizes steel-box ribs, a network hanger system and a post-tensioned concrete tie-girder framed into the main river piers.
The new structure will be located adjacent to the existing bridge, with the main-span arch being assembled up river, then floated in by barge and lifted into position. The approach structures consist of a 5-ft-thick, five-span post-tensioned concrete solid slab on the south and a five-span precast pretensioned concrete (PPC) beam and deck structure to the north. The north approach PPC beams set a Minnesota record, at 174 ft long, and served as the basis for Mn/DOT’s new statewide long-span PPC MW Beam Standards. Traffic will be maintained on the existing bridge throughout the duration of the project.
During the procurement, Mn/DOT’s project manager, Steve Kordosky, determined that the Lunda/Ames free-standing arch alternative design met the context and intent of the project based on Mn/DOT’s earlier environmental assessment, preliminary engineering and visual quality coordination with the community. However, this still presented a significant project risk.
As the design and construction started, the first challenge of the Lunda/Ames team was to obtain the communities’ acceptance for a structure they had not seen. The Lunda/Ames project architect, Bradley Touchstone, and Mn/DOT architect, David Hall, wasted no time in resolving this risk. Consensus was quickly achieved through the visual quality team process, which involved stakeholder and community representatives for the record-breaking structure that is sure to become a 21st century icon for Hastings.
A nice rib
More than a year into the project, the design has been completed and construction is well under way. Lunda/Ames Project Manager Doyle Honstad has faced some adversity in building this project. An unusually wet fall of 2010 and a wet winter, spring and early summer of 2011 have impacted river pier construction. The Minnesota government shutdown in July 2011 has further impacted the schedule, pushing back steel fabrication. Project leaders are examining methods of construction acceleration to make up for lost time.
Mn/DOT’s RFP included specialized performance requirements for redundancy, durability, aesthetics and maintenance. To ensure the design-build team proposals would meet these criteria and avoid potential delays, Mn/DOT instituted an innovative, confidential pre-accepted element (PAE) procurement process. This process required the design teams to submit their method and approach to redundancy, scour/hydraulics, fixity/expansion and analysis methods for approval ahead of the proposal submittal. For the design team, PAEs were very useful in communicating the structural adequacy of the proposed design and getting the owner’s full buy-in ahead of the project initiation, allowing for a smooth startup and providing a framework for swift design approvals.
The innovative design presented significant challenges. The arch rib, network hanger system and framed concrete tie girder create a highly complex, interdependent and nonintuitive structural system. Timely design and plan approvals were critical to the schedule and a challenge when combined with the owner’s rigorous design criteria for a structure type it had not anticipated and a design-build schedule that required foundation construction within months of project startup. The Parsons design team, led by Martin Furrer, was uniquely qualified for both challenges, having completed some of the nation’s most complex design-build projects, such as the Christopher S. Bond Bridge in Kansas City, Mo., and the Audubon Bridge in Louisiana.
The Lunda/Ames team’s final selection of bridge components was driven by Mn/DOT’s best-value process. Redundancy, durability and maintenance criteria drove the selection of the team’s free-standing rib, post-tensioned concrete tie and steel floor system. Additionally, all structural tension members were required to be load-path redundant to eliminate fracture-critical bridge elements on the structure.
The steel free-standing arch rib provides an easily maintained, aesthetically pleasing element that simplified fabrication cost, as each arch can be assembled independently. Mn/DOT specified additional criteria to this alternative, including a minimum factor of safety against arch bucking of 2.0 at the strength limit state and limiting the lateral deflection to R/300 (where R is the rise of the arch rib). Trapezoidal in shape and up to 8 ft deep at its base, the lateral stability of the rib is provided by its heavy section modulus, which resists any out-of-plane forces. The design team used detailed 3-D finite-element models to study stability and force interactions. As part of its internal peer review, Mn/DOT’s design manager, Dave Dahlberg, engaged Dr. Ted Galambos, emeritus professor of structural engineering at the University of Minnesota, to review the Parsons design as it progressed. Galambos’ validation of the design team’s original design approach and project-specific criteria proved very beneficial to Mn/DOT, as current American Association of State Highway & Transportation Officials (AASHTO) specifications do not clearly address free-standing arch design.
The solid cast-in-place, post-tensioned concrete tie-girder was selected due to its low maintenance and high durability. It also proved to be cost-effective because it is poured in place, eliminating any fit-up issues with the arch rib. Durability, maintenance and cost savings also were enhanced by framing the tie girder into the pier foundations and eliminating the need for bearings. Multiple bonded tendons provide redundancy and net compression under all service level loads with net zero tension under a loss of up to 25% of the tendons.
The concrete deck is supported by a series of steel floor beams and stringers and provides a lightweight, highly redundant, durable floor system. Floor beams are located at each hanger point and fully framed into full-depth stringers to form a grillage. This provides a highly redundant system that allows loads to redistribute through connected members in case any component should fracture. The floor system also is designed to remain within strength and serviceability limits under the loss of any one element. Additional detail redundancy was provided through the maximum use of bolted connections.
The bridge also is designed for the loss of any hanger, with all hanger connection plates utilizing bolted connections to eliminate the potential for crack propagation and allow for future replacement. The network hanger configuration and proper detailing of individual hanger connections provides an efficient distribution of hanger forces through the system in the event of the fracture of any individual hanger or connection.
Three-dimensional global modeling was used to analyze all aspects of the structural behavior. Specialized erection sequencing of the concrete tie girder post-tensioning was required to incrementally balance the construction of the tie girder and deck loads against the effects of creep and shrinkage to maintain the arch rib in a complete state of compression. Time-history analysis was used to investigate the effect of floor system fracture cases and hanger loss.
The main river piers are concrete delta-style frames with the tied-arch superstructure fully framed into the pier through the knuckle connection. The stiffness of the foundation system was then integral to the overall force effects in the structure. The north pier is located in 190 ft of soft soils overlaying rock and supported on unfilled 42-in. driven steel pipe piles. Drilled shafts were investigated early but were not cost-effective, impacted the schedule and presented a risk to the existing bridge due to potential caving effects. Statnamic pile load testing was used to validate the vertical capacity and lateral performance of the 42-in. piles. The south pier footing is close to the rock surface; however, the rock was deeper, more sloped than expected, and the originally planned spread footing was changed to short drilled shafts during the final design. Dan Brown & Associates provided the team with geotechnical analysis and recommendations.
As with all complex bridges, the erection method is an integral part of the overall design and analysis of the structure. Because in-place erection was not practical, significant consideration was given to off-site erection and final placement methods early in the design development procurement phase. Ultimately, for both schedule and safety reasons, the team elected the innovative approach to build off-site, move the arch onto barges using self-propelled motorized transports (SPMTs), barge the arch into place and lift it into position with strand jacks. A temporary tension tie and bracing system will be used to facilitate the erection and minimize weight. Once in place, the concrete knuckles and tie can be placed and sequentially post-tensioned. After the deck and barriers are placed, final hanger adjustments are made.
Mn/DOT determined that an extended 100-year service life was necessary for this new, modern structure on a critical transportation link. To achieve this goal, the Lunda/Ames team incorporated proper detailing, durable materials, detailed quality control and easy inspection and maintenance into the design through a detailed and quantified corrosion protection plan. Durability was enhanced with stainless steel rebar in the main span deck along with the Lunda/Ames team’s added-value features: project-specific high-performance concrete specifications for mix designs, testing and mass concrete; enhanced steel-fabrication quality practices; and a galvanized, grease-impregnated hanger system coated in noxyde paint. This also will be the first U.S. bridge project to incorporate computed radiographic testing in steel fabrication for the nondestructive testing of welds.
Moving right along
Construction of the new bridge began shortly after receiving notice to proceed in July 2010. The first order of work included the ground improvements at the north end of the project. A load-transfer platform of piling and geofabric layers was installed to prevent settlement of the new approach embankment, which overlays the deep, soft soils.
By August, piles and footings for the north approach piers were being installed, and excavation of the south river pier had begun. The work continues, with all the river bridge piling in place and most of the footing work finished. The south river pier was completed in October 2011, with the north pier close behind. The approaches also are taking shape, with superstructure work ready to begin by late fall.
The joint venture and Mn/DOT are currently working toward a summer 2013 traffic opening. After the new bridge is open for traffic, the existing structure will be demolished and all final connections will be made. Full project completion is scheduled for spring 2014.
About The Author: Gastoni is a principal bridge engineer and project design manager with Parsons.
