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Bridge Construction

Manual on Use of Self-Propelled Modular Transporters to Remove and Replace Bridges

Chapter 2. Benefits and Costs

2.1. Impacts of Conventional Construction

Removing and replacing a bridge on its original horizontal alignment using conventional methods typically requires closing the bridge to traffic for 6 months to more than a year, diverting traffic that previously used the bridge to a detour route during that time. The traffic that passes underneath a typical bridge during conventional construction is also disrupted with multiple rolling roadblocks, numerous lane closures, and crossovers during the extended onsite construction period. Conventional in-place demolition of an existing bridge that crosses a high-traffic-volume roadway is typically done at night to limit traffic disruption, with restrictions on hours of work, for example, from 10 p.m. to 6 a.m., further reducing the contractor's production.

2.2. Benefits of SPMT Bridge Moves

The ability of SPMT technology to move bridges in minutes or hours can significantly reduce traffic disruption, restore the use of existing highways in significantly less time, improve work zone safety, minimize environmental impact, improve constructability, and lower life-cycle costs. Particularly attractive are the reduced onsite construction time, improved quality of construction, and increased contractor options that are possible when SMPTs are used to move new prefabricated bridges into position.

2.2.1. Reduced Onsite Construction Time

Using SPMTs to install bridges offers the shortest possible onsite construction time. Disruption to traffic using a bridge under construction and a highway crossing underneath the bridge can be decreased from months to minutes or hours. The decreased onsite construction time also improves work zone safety for both motorists and construction crews. Access to schools and hospitals and access by emergency response services are also affected less with reduced onsite construction time. In addition, shorter construction times improve public perception of service.

2.2.2. Improved Quality of Construction

Improved quality with longer service life and reduced maintenance can be achieved in prefabricated bridge systems built offsite and quickly moved into position with SPMTs. Quality is enhanced for several reasons. First, the bridges are built in the controlled conditions of the staging area, allowing continuous operations without the need to accommodate traffic. Second, the bridge can be built as a unit in the staging area and moved as a unit to the bridge site, reducing the number of deck joints typically required in prefabricated bridge systems. Bridge owners in Europe have demonstrated this with their multiple-span continuous bridges, both with and without substructures, constructed offsite and moved into final position with SPMTs. Third, work can begin early in the staging area, independent of onsite construction status, allowing adequate time for proper construction of the system and proper curing of the concrete to ensure good long-term performance. These advantages can lead to improved reliability in achieving the desired 100-year service life compared to conventional construction.

2.2.3. Increased Contractor Options

The SPMT driving method increases the contractor's options. Its use allows the new bridge to be assembled at an offsite location and quickly driven in any direction to its final location. Precision movement to position the bridge within a fraction of an inch is possible with SPMTs. Driving also eliminates many of the issues related to overhead height restrictions that impact crane lifting operations, and the supported SPMT loads provide added safety assurance relative to suspended crane loads.

Benefits to contractors also include being able to use conventional construction methods to build spans at staging areas near the installation site. The contractor further benefits from the flexibility of being able to work on the bridge as many hours as needed during the day, as well as from the improved safety and reduced risks of working at a lower height away from traffic. In addition, since foundation and possibly substructure work is being done onsite concurrently with prefabricated construction activities at the near-site staging area, an onsite delay (for example, because of unexpected utilities) will not impact construction at the staging area. Also, since more of the work is done offsite, fewer workers are required over or near traffic, further enhancing safety. Fewer and shorter lane closures also improve safety since any deviation from normal traffic patterns can result in accidents.

2.3. Costs of SPMT Bridge Moves

2.3.1. Initial Construction Costs

Total initial cost for using SPMTs can range from $50,000 to more than $500,000, depending on the location and requirements of the job; see 7.1 and 7.2 in "Case Studies" for cost breakdowns on two projects. While using SPMTs requires significant mobilization costs to get the equipment to the site and set up, substantial offsetting savings can be obtained by their use on a project. The cost for using SPMTs should be weighed against these savings to determine the net cost increase or reduction.

Initial cost savings that can result from using SPMTs to quickly move bridges include reduced costs because of fewer maintenance-of-traffic setups, shorter hours for off-duty law enforcement officers required for fewer rolling roadblocks, reduced switching of manpower from day to night (typically saving two hours per night shift plus gaining back the day/night/day shift change loss of the next day), and elimination of double handling and permits required to haul off the old beams.

Savings are also obtained through elimination of the need to build, maintain, or repair a detour or temporary structure that could otherwise be required for an extended closure period. Savings can also be obtained by delivering accelerated construction projects using a smaller owner and contractor workforce. Cost savings not reflected in the bid price include savings related to the owner's reduced construction engineering and inspection requirements because of the accelerated construction schedule.

Contractors are also recognizing initial cost savings through reducing overhead, labor, and equipment costs; reducing liability insurance premiums and equipment rental time; and completing projects quickly and bidding more projects without having to exceed their bonding limits. The incentives that can be made and the increased number of projects possible because of the speed of installation can also result in increased contractor profits compared to profits made through conventional construction methods.

Cost savings during the service life of the bridge should also be considered. The improved quality possible with this method can result in better long-term performance and reduced maintenance needs.

See the "7.1.1 I-4/Graves Avenue" case study for construction cost numbers for that project.

2.3.2. Delay-Related User Costs

Bridge closures, lane closures, detours, rolling roadblocks, and their resultant traffic buildups and stoppages wreak havoc on reliable travel times. When delay-related user costs are included in cost analyses for moderate and high traffic volume locations, accelerated construction projects are always the best-value solution.

Significant reductions in delay-related user costs can be achieved using SPMTs to move bridges. No other technology offers shorter onsite construction times than can be achieved with the use of SPMTs to move prefabricated bridges.

See the "7.1.1 I-4/Graves Avenue" case study for delay-related user cost numbers for that project.

Updated: 08/21/2013
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