TWM Contributors: David Holdener PE, SE and Matt Joost, PE, SE
Date: May 2021
How Construction Phase Engineering Services Benefit Bridge Contractors
Construction phase engineering is a term our engineers use to describe the services that our structural engineering group provides to heavy highway/bridge contractors. Although these items are typically temporary in nature, they can be very complex and critical to the successful completion of a construction project.
While we perform a wide variety of construction phase engineering services, seven key components should be considered in bridge design, including:
- Jacking Plans | These are required in instances where bridge bearing replacements are necessary; they can be as straightforward as placing a jack on the existing concrete seat and lifting the bridge or include other more elaborate plans that require a temporary support structure below where cap space or vertical clearance restricts jack placement. When lifting steel structures, it’s important to take into consideration the need for steel bearing stiffeners at the location of the jack.
- Demolition Plans | Bridges over railroads or some bridges that are staged require demolition plans prepared by an engineer. These plans detail the sequence of removal and can vary widely in scope and detail. The simplest of these is a basic sequence describing the structure removal, typically involving the use of an excavator with a breaker pulverizing the existing structure. The most complex require analysis of the existing bridge for each stage or removal, rigging details, and other items typically considered for an erection plan.
- Erection Plans | We have provided numerous erection plans for contractors detailing the sequence of girder construction: these vary from curved girder erection plans to simple erection plans over railroads. The more complicated plans require analysis of the partially built bridge structure at all stages of girder construction. Construction loadings, in particular wind, need to be considered. Temporary bracing, hold cranes, and temporary towers are often needed to ensure stability of the structure. Plans we prepare show stages of erection along with crane placement, girder staging areas, and rigging components used for the girder lifts.
- Cofferdams | In order to construct bridge substructures in rivers or streams, cofferdams typically consist of steel sheet piles that are driven around the perimeter to enclose the construction and give the contractor a dry area to work. These structures are comprised of the steel sheets that form a zig-zag pattern in plan and may include steel wales and struts to provide additional support. Cofferdams that are staged can be challenging as adequate space between the stage construction and stage removal lines can be limited.
- Temporary Shoring or Falsework | Reinforced concrete slab bridges are cast-in-place structures that require shoring during construction. We have engineered custom framed structures comprised of steel beams for support of the wet concrete. These plans require consideration for the load of the concrete paver on the edge form and exterior beam. The shoring deformation under load needs to be compatible from beam to beam for the constructed slab thickness to be uniform.
- Temporary Soil Retention | Retention, typically consisting of driven sheet piles, is required for staged structures behind abutments or existing culverts. Retention can also be necessary in order to construct MSE walls or make excavations for construction of substructures in proximity to existing railroad tracks. Height of retention, soil properties, staging width, and skew are some factors that can play a large role in the retention design and details. Rock or glacial till can prevent adequate toe of the sheets and needs to be considered during design development.
- Accelerated Bridge Construction (ABC) Methods Including Bridge Slides and Launches | If your structure is moving, something is typically wrong. For these projects we’re intentionally moving the bridge from point A to point B. The contractor’s preferred system, construction tolerances, and structure deformations need to be carefully considered during design.
Working directly with contractors broadens our perspective as bridge designers and guides the design process for constructability. By collaborating with contractors prior to the bid, our Structural Group can help determine if their preferred system will work. We also size components to assist with their bid – frequently adapting our engineered solutions to components that contractors frequently have available lying around in their yard. Pile cutoffs from old jobs are a favorite.
Expertise in this important phase of construction engineering is a key fundamental for success. From evaluation through execution, TWM can provide the level of services your construction project needs.
Find out more about TWM’s Construction Phase Engineering and Structural Engineering.