With advances in material technology, bridge designers are able to increase the efficiency of structural components and design longer and wider spans, but these designs require innovative construction solutions. Clients, on the other hand, are imposing more and more stringent schedule milestones, forcing contractors to seek solutions that save time and money. The bridge construction industry must find 'out of the box' technical solutions for the erection of superstructures that have been specifically designed for a certain project in order to increase productivity.
Erection trusses have now been in use for over 40 years, having originated in Europe, and gradually expanded into markets across the rest of the world. As projects become more schedule-driven and technically complex, and as the specialised equipment sector finds safer, more efficient ways of erecting bridges, the initial up-front costs can be economically justified and the equipment regarded as essential to the success of the project.
One example of innovative problem solving is the High Five project in Dallas, Texas, which involves construction of a five-level segmental balanced cantilever interchange. Due to the access problems associated with the conventional delivery and erection of precast segments, the flyovers were designed to be cast-in-place. Both schedule and budget indicated that a precast segmental solution was preferable, and the development of appropriate equipment for the construction process allowed a redesign of the scheme to accommodate the precast solution.
Now the bridge is being constructed using the precast segmental balanced cantilever method, and is built by means of the special launching carrier. This equipment is capable of lifting the segments up from the pier locations, on either side of the pier, and moving along the erected deck, to transport the segment to its final location, or lift it over the deck and travel to the other side of the cantilever for final erection.
The carrier is on steering, rubber tyres which allow it to travel on the deck, and handle the tight radius of curvature of the ramps. Due to the super-elevation, the carrier is equipped with a dynamic, continuously auto-levelling system and once positioned, it supports itself on a jacking system and is tied down to the deck for the erection of the segments.
Present production rates allow two to four segments to be erected per day in a single shift, depending on the post-tensioning layout. Relocation of the carrier from a finished cantilever to the pier head of a new cantilever takes a total of three days and involves the equipment being dismantled and reassembled into four main components. All is done with the use of a 100t crane positioned near the pier. The carrier has the flexibility to lift up segments from any location underneath the erected deck, thus greatly reducing disruption and detours to existing traffic below the new interchange.
Construction of the High Five Interchange in Dallas is being carried out by general contractor HB Zachery, with the casting and erection being subcontracted to a joint venture of Rizzani de Eccher USA and HB Zachery. Supply and design of the erection equipment is by Italian specialist manufacturer Deal.
Two further examples of lifting and erection equipment are presently in use on the Taiwan High Speed Rail contract C215, erecting some 20km of deck made up of 602 full bridge spans, each weighing 700t, and the Milan-Bologna High Speed Rail project in Italy, a contract consisting of about 12km of precast full spans weighing 700t and measuring 31.5m long.
On both projects, special carriers were custom-designed and built to lift up the 700t spans from a casting yard located on the bridge abutment, transport the concrete beams over the erected deck, self-launch both the equipment and the concrete beam over and onto the support beam, launch forwards the support beam, and lower the concrete beam into its final position. On both projects, the erection of two concrete beams is being achieved each day on a regular basis.
In this instance, the concrete beam carrier and the launcher are the same piece of equipment, and the carrier travels at a speed of about 4km/h on the erected deck. The main support/launching beam is left on its support legs on the piers of the span to be erected while the traveler returns to the casting yard to load and transport the span back to the final location. Once back in position, the carrier is attached to the support beam, and launches itself over the support beam until the concrete beam is over its final position.
At this point, the carrier launches the support beam forward, leaving the span open for the concrete beam to be lowered into its final position. The concrete beams may even be equipped with bearings, having them fixed to the underside of the beam, in order to remove the installation of the bearings from the general construction cycle. Construction work on the Taiwan High Speed Rail project is being carried out by main contractor Obayashi-Fu joint venture, with casting and erection subcontracted to Rizzani de Eccher/VSL joint venture. The erection equipment has been supplied by Deal. On the Milan-Bologna High Speed Rail project, the general contractor is Pizzarotti, using erection equipment also supplied by Deal.
A very similar carrier is presently being used on the skyway of the Oakland Bay Bridge in San Francisco, USA, for moving segments from the long-line casting beds, to storage, and then from storage to the barge for delivery to site. The segments have a maximum weight of 750t, 26.6m wide, 8.6m high, and 8m long and are believed to be currently the largest segments ever precast. General contractor Kiewit is using erection equipment from Deal.
Finally, a more conventional piece of equipment is in use on the Hathaway Bridge in Panama City, Florida. This launching truss has two 200t lifting winches, and is erecting 100m spans over water. The segments are variable, with a width of 25m, the largest being 5.5m high with a weight of about 190t. The project is a design/build contract, and the size and length of the spans were governed by the limits of size and weight of the launching truss. The segments, including the pier segment, are transported by barge and hoisted up and erected with the launching truss. The truss is equipped with a stabilising arm, which is relocated every two to three segments, and acts as a safety feature to hold up the cantilever in case a segment is dropped.
General contractor for the Hathaway Bridge is Granite Construction, with casting and erection subcontracted to a joint venture between Rizzani De Eccher USA and Granite. Erection equipment has been supplied by Deal.
These examples clearly show the advances made and the sophistication of lifting equipment currently available in the bridge construction industry. Such special custom-designed solutions have become an integral part of this type of bridge construction, and are capable of stretching the imagination and limits of conventional equipment.
Riccardo Castracani works for Rizzani De Eccher.
