In April 2004, a scanning tour of Japan, the Netherlands, Belgium, Germany and France was made to obtain information about bridge construction methods being used in these countries to minimize traffic disruption, improve work-zone safety, minimize environmental impact, improve constructibility, increase quality and lower life-cycle costs.
The scanning tour was sponsored by the Federal Highway Administration, the American Association of State Highway & Transportation Officials and the Transportation Research Board.
During the tour, several different methods that can be used to remove existing bridges and move complete bridges into place were observed. These methods allow a new bridge to be built at one location near or adjacent to the existing structure and then be moved to its final location in a short period of time. Construction can, therefore, take place in an environment where construction operations are completely separated from the traveling public.
These methods reduce traffic disruption times from months to days or hours, restore the use of existing highways in significantly less time, improve work-zone safety, minimize environmental impact and improve constructibility.
Although the concept of building off line is used for railway bridges in the U.S., the concept of building highway bridges off line and then moving them into place needs to be developed for greater use nationwide. Various systems for moving bridges and their components were identified during the tour and are described in this article.
Propelled to travel
Of particular interest to the team for lifting, driving and positioning bridges were the computer-controlled self-propelled modular transporters (SPMTs). A single SPMT has either six or four axle lines. Each axle line consists of four wheels arranged in pairs and can support a maximum of 33 tons when ground conditions permit. Each pair of wheels can pivot 360° about their support point. As a result, an SPMT has complete freedom of movement in all horizontal directions.
Through its hydraulic suspension system, the SPMT equalizes loads on each axle even on irregular surfaces. The bed of the SPMT can be raised by 24 in. and tilted in both directions to maintain a horizontal bed on an inclined surface. Grades as steep as 8% have been climbed, but the maximum grade depends on site-specific friction. Vertical lifting equipment can be mounted on the SPMT platform if required.
The SPMT is self-propelled and can be coupled longitudinally and laterally to form multiple units all controlled by one driver. The driver walks with the units and carries a controller connected to the units by an umbilical cord. The controller has four basic commands: steering, lifting, driving and braking. The SPMTs can be transported to the bridge site on normal flat-bed trailers or shipped in flat-rack containers. The units are used throughout Europe and have been used on a few bridges in the U.S., including the Lewis and Clark Bridge in Washington state; the Wells Street Rapid Transit Viaduct in Chicago; and the Third Avenue Bridge in New York.
The scanning team visited a railroad bridge in France that had been moved into place using SPMTs. The bridge was a four-span structure across a new highway in Normandy. The bridge was constructed on a concrete slab. It was then moved 146 ft into position using SPMTs.
To accomplish the move, pairs of temporary concrete beams were cast between the three piers. SPMTs then lifted the beams and moved the bridge. After the bridge was positioned, the beams were cut into sections to reduce their hauling size and weight.
To accomplish the bridge placement, the railroad track was closed for 48 hours. Total time for moving the 2,200-ton bridge was eight hours. The average speed of travel was 7.9 in./min.
In relocating bridges using SPMTs, the following factors need to be considered:
- Loads and reactions imposed on the structure during moving are different from those when the bridge is in its final position and need to be considered as part of the original design;
- Geometric tolerances must be strict enough to avoid excessive stresses on the bridge yet reasonable enough to permit an optimum speed of movement;
- Specific geometric distortion tolerances for moving must be specified with appropriate penalties for exceeding them;
- Geometric distortion must be monitored during the moving operation;
- Temporary structures are needed to support the bridge before and during the move;
- Ground-bearing capacity needs to be considered;
- SPMT owners should be included in the initial planning process to ensure a cost-effective approach;
- SPMT owners with appropriate expertise and experience should be specified to do the move as subcontractors to the prime contractor; and
- Bonuses for early completion and penalties for late completion should be included in the contract.
Skid’s mark
The scanning team visited a bridge in France that was built to be skidded into place. The bridge was a four-span, 3,600-ton structure across a new highway. At the time of the site visit, construction of the bridge adjacent to the railroad track was almost complete. The bridge was then slid from its construction location to its final position. To accomplish the move, the bridge was built on two foundation slabs, one on top of the other. The top slab was connected to the piers. The base slab provided a foundation for building the bridge and a sliding surface for the upper slab.
The bridge was moved into its final position by sliding the top slab over the bottom one. To reduce friction between the two slabs, a waxed and greased plastic membrane was placed between them. Bentonite was pumped through tubes in the top slab to the interface to act as a lubricant and to fill voids in the soil as the top slab slid off the base slab onto the ground. The same tubes were later used for grouting underneath the top slab when in its final position.
Ready to launch
The force to slide the bridge was provided by four strand jacks. These pulled on tendons anchored at the leading edge of the base slab and pushed on the trailing edge of the top slab. Each tendon consisted of 37 0.6-in. diam. prestressing strands.
Incremental launching is a technique in which complete bridges or superstructure components of bridges are built behind an abutment and then incrementally launched from the abutment across the pier tops. The method is suitable for building bridges across wide, deep valleys and has been used for both straight and curved bridges. In Japan, the same technique was used to add a second level above an existing highway.
The Arimatsu Viaduct consists of two side-by-side six-span continuous steel-box girder bridges with orthotropic decks and runs above Rte. 23, a major highway between Tokyo and Kobe.
The viaduct has a length of 2,150 ft and a weight of 13,200 tons. The longest span length is 427 ft. The superstructures for both bridges were assembled on falsework in span-length increments at the end of the viaduct and launched longitudinally above Rte. 23 using a special automated launching system.
Each of the six spans had to be launched within a 12-hour window between 8 p.m. and 8 a.m. The bridges were launched side by side. The automated launching system used a centralized control system to maneuver 100 jacks including 56 synchronized jacks each with a 550-ton capacity. The synchronized jacks were used to control the up and down movement, left to right directions and height differences. A course-correction device was provided at each bent to maintain a gap of 1.6 in. between the two bridges during the launch.
At St. Pierre du Vauvray in France, an original method of laterally launching was used to eliminate a grade crossing and provide a road underpass with only a 22-hour interruption to train traffic. The contractor excavated a large pit adjacent to the railroad tracks and built a reinforced concrete box culvert in the pit. Then, 1,635 cu yd of soil was excavated from beneath the railroad tracks. The excavation was then sealed to form a cofferdam and the excavation flooded. The 950-ton culvert was floated into position. This method is mainly useful where there is an ample supply of water.
Separating the two
The advantage of building a complete bridge off line is that construction can take place in an environment that is completely separate from the traveling public. This reduces traffic disruption times and lane closures, improves work-zone safety, improves constructibility and lowers life-cycle costs.
The method selected to move the bridge to its final position depends on site-specific conditions. It was observed in Europe that the use of SPMTs offers the advantages of more flexibility and better directional control than the other methods to move bridges into place.
The author of this article acknowledges the scanning team members who participated in the tour and contributed to the final report and its finding, particularly Mary Lou Ralls (formerly of the Texas DOT) of Ralls Newman LLC and Benjamin M. Tang of the Federal Highway Administration, who co-chaired the scanning tour.
