Designing for Precast
Precast duct bank can be a useful means of increasing productivity and reducing costly delays. The greatest impact may come from utilizing a hybrid system of both precast and cast-in-place duct bank. In the following sections, specific use-cases are outlined where using a hybrid cast-in-place and precast design approach can provide the most benefit to your project.
Use RED-E-DUCT to Minimize Formwork
Formwork can be a labor and time intensive aspect of a cast-in-place duct bank, however, by designing for precast it is possible to greatly reduce the time and labor required to form and build electrical duct banks
Easily ACCOMMODATE Peel Outs
It is often useful to incorporate cast-in-place sections to take advantage of the benefits of both methods of construction. Precast duct bank can be placed strategically to act as an anchor point for cast-in-place formwork. If a cast-in-place section is required, precast can be used on either end of the cast-in-place section so that formwork can be installed directly to the precast sections, minimizing the bracing and reinforcing required to form an equivalent cast-in-place system. This method can be used at manhole connections, peel-outs, stub-ups, and changes in vertical elevation.
Utilizing standard configurations not only helps to simplify the design process, it can also lead to increased flexibility on the project site. Forterra offers a wide variety of standard configurations with custom configurations available, however, minimizing the number of unique configurations allows for the most flexibility on the project site. Construction teams can inventory Red-E-Duct sections on site and simply pull from inventory any time a specific part is required. This allows installation crews the ability to utilize any inventoried part number where it is required regardless of the original installation sequence. If a change in the construction sequence occurs due to weather, equipment issues, scheduling conflicts, or any other reason, installation crews have the flexibility to move to new areas and stay productive. The ability to install precast sections quickly and backfill immediately also allows areas that may not have been candidates for construction to fit into tight schedule windows to keep your project moving forward.
Primary Benefits of Standardization
- Increased Construction Flexibility
- Simplified Design Process
- Reduced Costs
Horizontal and Vertical Stacking
Standard Red-E-Duct sections can be stacked in either horizontal or vertical orientations to create larger and more complicated geometries while maintaining all of the benefits of standardization.
Stub-Ups may also be stacked in a similar manner.
Contact the Red-E-Duct engineering team to learn more
RED-E-DUCT conduits are surrounded by proprietary high strength thermal concrete that provides a stable environment that protects expensive high voltage cables and ensures efficient system operation at safe temperatures throughout the expected design life.
Thermal performance can be an important aspect of many duct bank designs. RED-E-DUCT offers superior thermal performance and consistency. Each section is produced in a quality-controlled environment ensuring target Rho values are achieved for each section. Forterra performs thermal resistivity testing daily to verify concrete performance. Third party resources can also be used to support mix design verification and if required, performance testing at intervals throughout the project.
RED-E-DUCT utilizes high-strength concrete in all mix designs, ensuring performance equals or exceeds cast-in-place reinforced duct bank. If a design case requires extreme performance, the RED-E-DUCT engineering team will work with you to ensure all project requirements are met.
All RED-E-DUCT mix designs utilize full integral color that meets or exceeds most project dye requirements.
Common Design Examples
When a stub-up conduit assembly is constructed on site, the conduit must be carefully positioned and braced for concrete encasement (cast-in-place) to ensure that when cables are installed, they end up in the correct location. When a precast stub-up is used, the time consuming and difficult work required to correctly position conduit elbows is eliminated. RED-E-DUCT precast stub-ups are quickly and easily set into position. All conduits encased within the precast assembly are precisely located ensuring cables will be routed to the planned position.
Using cast-in-place in areas where tight tolerances are required allows for maximum flexibility on the job-site. Designers can take advantage of this flexibility by locating cast-in-place sections at peel-outs. By using a cast-in-place section at a peel-out, a designer can take advantage of precast construction up to a peel-out location while also providing the flexibility of cast-in-place construction at the peel-out location. Precast can be used on either end of the peel-out allowing for a small closure pour with minimal forming.
Designing for precast between structures can often be challenging. Field tolerances for the placement of structures and joint growth can sometimes mean that a perfectly designed system cannot be realized in the field. In cases of designing between manholes or structures, it is useful to add a cast-in-place section in at least one segment of the duct bank run. A cast-in-place section can allow for tolerances in the placement of manholes, pouring of foundation walls/slabs, or any joint growth that may occur when installing precast between structure.
Changes in Elevation
When a change in elevation is required for a duct bank design, cast-in-place sections may be best suited to provide maximum construction flexibility on-site. Elevation changes can be approached in the same manner as peel-outs allowing crews on-site to realize the benefits of precast while maintaining the flexibility of cast-in-place.
RED-E-DUCT can increase construction flexibility and improve schedule significantly when incorporated into road crossing designs. Precast sections can be installed, backfilled, and the crossing can be open to traffic within hours of opening the trench. All RED-E-DUCT sections are rated for HL93/HS20 loading.
Rail crossings can create considerable burden to designers and construction teams. In many cast-in-place designs, significant soil cover below the sub-ballast can require additional excavation and may require shoring or trench boxes, adding time and cost. With RED-E-DUCT, soil cover can be reduced to just 6” of cover below the sub-ballast for most applications resulting in significant construction cost and schedule savings. The figure below highlights the advantages of RED-E-DUCT over traditional cast-in-place designs when used in rail applications.
RED-E-DUCT Design Resources
3d Design and Modeling Files
Our RED-E-DUCT design and layout teams commonly work in digital environments. Electronic files (.dwg or .step) are easily shared and may be integrated with plant design software like Bentley or Smart Plant. With Forterra furnished digital RED-E-DUCT component files, plant electrical designers can easily import precast duct bank models into plant design software. Alternatively, if provided with customer exported digital files, Forterra can lay out an equivalent precast duct bank system and recommend solutions for specific project applications. The transfer and integration of electronic files has demonstrated the ability to significantly reduce the time required to review and approve design and layout drawings.
Review and approval process can vary by project and often require more than one submittal. By utilizing standard configurations within your design, drawings can be submitted well in advance of final design. Once approved, production can begin with final quantities to follow as the design matures.
Layout drawings are often a valuable tool for site construction. Typical features of a layout drawing include precast part and drawing number identification, start and end coordinates for precise placement of precast sections, bill of materials for individual runs, and other beneficial information. Forterra offers and can provide layout drawings according to specific project requirements.