Manual on Use of Self-Propelled Modular Transporters to Remove and Replace Bridges
Chapter 3. Planning
The owner should design the project for the SPMT driving installation method to ensure the most efficient and cost-effective use of SPMTs to move bridges. The contract documents, however, may be developed for a conventional construction method, with additional wording that allows an alternative bridge installation method. This additional wording in combination with incentives and disincentives for early and late completion, respectively, and other contracting strategies for accelerated construction can be alternative means of achieving the owner's reduced construction timeline.
Suppliers of SPMT equipment and related engineering services should be included in discussions from the initial planning stages of a bridge project. Their inclusion will facilitate the development of contract plans and specifications that address the necessary details related to bridge movements to ensure successful projects and a cost-effective approach. If Federal funds are included in the project, the SPMT company that works with the owner in the planning stage will not be allowed to bid on the project because of Federal regulations on fair trade practices unless all SPMT companies are invited to participate in the discussions.
3.1. Project Selection Criteria
Using SPMTs to remove or install a bridge may be advantageous for a variety of reasons. Two primary criteria for initial consideration of SPMTs are the following:
- Need for minimal closure time to limit traffic disruption. The need for rapid onsite construction to get the bridge or the underneath roadway back in service to limit traffic disruption is a primary criterion for the use of SPMTs because their use provides the least possible closure time.
- Availability of a location for offsite bridge construction. The use of SPMTs requires a staging area for offsite construction of the bridge. A feasible route from the site to the bridge location must also be available.
Project conditions that make SPMT technology attractive are the following:
- The bridge or underneath roadway has high traffic volume.
- The bridge or underneath roadway is on an emergency evacuation route.
- The bridge is over a railroad or navigable waterway.
- Schools or hospitals are accessed via the bridge or underneath roadway.
- Commerce is negatively impacted by the construction.
- Overhead or adjacent work space constraints such as power lines or hurricane barriers prevent the use of conventional construction with cranes.
- Air or noise quality constraints limit the type or timing of construction activities.
- Endangered species on the site limit the timeline for construction activities.
- Weather constraints such as cold weather limit the length of time for construction activities.
Using SPMTs requires significant mobilization costs to get the equipment to the site and set up for bridge moves. The cost of using SPMTs should be weighed against the reduced costs that result from the significantly reduced onsite construction time to obtain an accurate net cost increase or decrease. Owners considering the use of SPMTs on a project should discuss cost implications with their local contractors during the planning process.
A decisionmaking framework on the FHWA Web site (www.fhwa.dot.gov/bridge/prefab/) can assist owners in determining whether prefabricated bridges will benefit their specific project.(3) The framework discusses the many issues that may make prefabricated construction the solution of choice.
Using the decisionmaking framework and this manual in combination with analysis of delay-related user costs should provide the guidance that bridge owners and other bridge professionals need to understand the technology, determine whether using SPMTs will benefit a specific bridge project, and develop contract documents that incorporate the technology.
3.2. Traffic Considerations
High traffic volumes, for example, 40,000 or more vehicles per day, may justify the use of SPMTs to quickly move bridges. The impact of detours, lane closures, rolling roadblocks, and crossovers on traffic flow should be considered. The high traffic volumes may be crossing the bridge being replaced or traveling underneath.
The owner should develop contracting strategies and traffic control plans that are consistent with the traffic control windows and construction scheme in the contract documents. The dates and time limits for bridge closure windows should be clearly specified in the contract.
3.2.1. Traffic Impact Comparison Between Conventional and SPMT Construction
The following bridge construction activities are not allowed over active traffic:
- Bridge demolition
- Beam erection
- Deck form erection and removal
- Concrete deck pouring operations
Bridge demolition and beam erection typically require full closure of the underneath roadway. For conventional construction, deck form erection and removal and concrete deck pouring operations typically require traffic to be moved from underneath these operations using lane shifts, lane closures, or both. Lane closure restrictions for a specific site may require construction operations to be performed only at night, increasing labor and traffic control costs and decreasing production rates.
Table 1 compares traffic impact of conventional construction methods to SPMT construction methods for bridge demolition and beam/span placement. Deck form erection and removal and concrete deck pouring operations require additional traffic impact for conventional construction; no traffic impact results from these operations with SPMT construction methods because they are performed at the staging area away from traffic.
Table 1: Traffic impact comparison of conventional and SPMT construction methods.
|Construction Operation||Duration of Traffic Impact||Traffic Control Scheme|
|Bridge demolition||2-3 days per span removed||Traffic detour, Interstate crossover|
|Beam placement||Simple-span beam: 25-30 minutes per beam placed||Traffic detour, Interstate crossover or rolling roadblock|
|Continuous steel: 45-90 minutes per field section placed and bolted||Traffic detour, Interstate crossover|
|Construction Operation||Duration of Traffic Impact||Traffic Control Scheme|
|Bridge demolition||25-30 minutes per span removed||Traffic detour, Interstate crossover, rolling roadblocks|
|Span placement||Simple span: 2-3 hours per span placed||Traffic detour, Interstate crossover|
|Continuous steel span: 2-3 hours per unit placed||Traffic detour, Interstate crossover|
3.2.2. Contracting Strategies and Delay-Related User Costs
During the planning process the owner should determine contracting strategies that will encourage bid competition. For example, the incentives/disincentives should be large enough to make the offsite fabrication and move an attractive option to the contractor and give the contractor a financial reason to buy in to the owner's reduced construction timeline.
Delay-related user costs should be calculated to determine appropriate funding levels for contracting strategies. Owners use various user cost models. The Florida Department of Transportation (FDOT) uses a modification of the Arizona Department of Transportation (ADOT) user cost model.(4) An example user cost spreadsheet is in Appendix A.
3.3. Site Requirements
Sufficient land area (staging area) for bridge assembly should be available for SPMT construction methods. The staging area should be within a suitable distance from the bridge site. A feasible path must be available from the staging area to the bridge site, or from the staging area to barges that can go to the bridge site.
The ground surface at the staging area and along the path should be relatively flat and have adequate ground bearing capacity or steel plates provided to resist the loads.
3.4. Efficient Use of SPMTs
SPMT mobilization and remobilization costs can be significant. The scheduling of construction activities should provide efficient use of SPMTs to minimize their downtime on the project and the length of time they are required for the project.
Many bridge replacement projects along high-volume roadways require phased construction because of right-of-way costs, requiring remobilization of the SPMTs to place the second-phase bridge. Concepts that allow use of the SPMTs for other nonuser-critical spans on the project can help pay for their mobilization costs and other costs such as temporary shoring for composite dead load systems. For example, the SPMTs could be used to remove other bridges that will be replaced on the project but are not on the critical path.
3.5. Project Staffing
The delineation of responsibilities among the owner, the contractor, and the contractor's subcontractors (for example, the SPMT company) should minimize project risk by assigning each task to the party most capable of handling it successfully. Doing so reduces the potential liability of all parties and helps ensure a successful project.
All parties in the project should have an adequate understanding of the overall project and their roles to successfully complete the project. Responsibilities of each party must be clearly defined, including where each responsibility begins and ends.
Personnel responsibilities include the following:
- Owner-Provides qualified staff empowered to make prompt decisions and be available during all active hours of contractor work.
- Contractor-Provides qualified engineering and construction staff for the required services related to removing existing bridges and prefabricating, lifting, moving, and erecting new bridges.
- SPMT company-Provides qualified engineering and construction staff for the required services that are the SPMT company's responsibility related to removing existing bridges and lifting, moving, and erecting new bridges.
The owner should consider prequalification of contractors and SPMT companies to ensure that qualified staff members are available for the project.
3.6. Coordination with Other Entities
Coordination with other entities should begin as early in the project as possible to avoid delays to bridge moves. Activities should be initiated to ensure that at least one feasible staging area is available and to obtain required permits early (for example, U.S. Coast Guard permits for transport in navigable waters). Other coordination needs may include railroad agreements and utility relocations.
3.7. Outreach and Communication
Communication with the public should begin in the early planning stages of the project to ensure understanding of the impact on traffic during construction.
The owner's public relations staff will take the lead for media relations activities related to the project, such as disseminating news releases. Its district construction staff will control any public tours of the project, including arranging a safe area for viewing and issuing instructions/announcements at the jobsite. If the bridge moves are the State's first, the State bridge engineer should be approached about holding a demonstration workshop in conjunction with the moves.
The owner may choose to set up cameras for time-lapse photos or use nearby traffic management cameras if available. The owner may also hire a roaming videographer to document the bridge moves, maintaining ownership of all film rights and possibly producing clips for postevent release.
3.8. Contingency Plans
The contractor should have contingency plans for unplanned incidents, including backup equipment in case of mechanical failure and a reserve detour route for extended closures. The SPMT company should anticipate out-of-service conditions such as flats and failures and develop contingency plans accordingly to ensure that the operator can manually coordinate the SPMT units as needed to complete the bridge move.