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  • Writer's pictureRaj Singh

Constructability in Bridge Design #1 - General

Updated: Mar 14

In the planning and design of bridges, it's crucial to incorporate construction knowledge and experience to enhance practicality, efficiency, and quality. This approach, known as constructability, involves the early and ongoing participation of staff with expertise in bridge construction and constructability. Key players include fabricators, erectors, bridge contractors, and experienced bridge design and construction engineers. The main reason to consider constructability during design is to facilitate easier construction using approved or conventional practices. This approach improves construction quality and reduces claims and litigation. As a result, the construction process becomes smoother and more efficient, leading to lower construction costs.

Consider a scenario where without a constructability review, one proceeds with a design that calls for a specialized, custom-made component. This component is not only expensive to produce but also requires a long lead time for fabrication. Furthermore, the installation of this component is complex, necessitating additional training for the construction crew and potential delays in the construction schedule. There's also a higher risk for installation errors, leading to potential disputes or claims. However, with a timely constructability review, the team identifies these issues before construction begins. The review recommends replacing the custom component with a standardized part that's readily available and easier to install. The change results in a significant reduction in both cost and duration of the construction process. Furthermore, because the installation is more straightforward, the quality of construction improves, reducing the potential for errors and subsequent claims or litigation. Thus, the constructability review leads to a smoother, more efficient construction process with improved outcomes.

The Deh Cho Bridge project in the Northwest Territories serves as an extreme example of this. The first design included several unconventional elements, such as a plate girder transformed into a truss with void cut-outs in the web, and cables with custom-designed connections using PT bars. As expected, this design faced approval issues from the owner's engineer due to various deficiencies when evaluated against provisions in the applicable design standards. Given the availability of conventional design solutions and pre-approved off-the-shelf alternatives, the proposed design was seen more as an invention, leading to a complete redesign of the bridge's superstructure. This caused a delay in the project and a significant increase in overall project costs.

Comparing the original design and re-design of Deh Cho Bridge Truss

A few guiding principles can help streamline constructability in design:

KISS - Keep it simple silly

Simplicity: Resist the temptation to make the design more complicated than necessary. Configure the bridge to enable efficient construction and select details that require less time, resources, and temporary construction activity. In bridge design, the principle of simplicity advocates opting for a standard girder bridge design over more complex structures like arch or cable supported, unless necessary due to factors like span length or aesthetic requirements. This reduces the number of unique parts and construction methods required. It can also involve choosing a construction method that requires less temporary support structures, which can save time and resources. For example, instead of constructing falsework to support a bridge during construction, using a cantilever construction method should generally be more efficient and require fewer resources unless the local construction industry prefers a particular methodology.

Think modular - Think Lego

Standardization: The principle of Standardization in constructability emphasizes the importance of using modular components and standard details to achieve economic and efficient construction. Greater savings can often be found in streamlining the construction process than in optimizing material usage. Evaluating preassembly concepts can significantly facilitate the fabrication, transportation, and installation of components.

In the construction of the Deh Cho Bridge and the Fort Nelson Bridge in Northern BC, the design process prioritized modularity. The use of shop-manufactured modular components was maximized, which sped up the on-site assembly and installation process. For example, the Deh Cho truss was designed as a "Lego" system that is easy to fabricate and assemble. The chord member geometry remains consistent across the bridge's length. Varying force effects are managed by altering the steel strength and, when required, by modifying the chord "I" section with added side plates. Steel grades of 350 AT and 485 AT are specified for the truss. Open profiles were chosen for the truss members to ensure easy access and assembly.

These projects took into consideration the extreme northern conditions of the sites. Details were incorporated into the superstructure structural steel components that enabled an advantageous erection methodology of an incremental launch. This scheme not only minimized environmental impact but also eliminated the need to work from either an ice bridge or a seasonal work bridge due to ice breakup. The result was bridges of higher quality, with improved durability and reduced maintenance requirements. These projects exemplify how standardization and the use of preassembly concepts can streamline the construction process, enhance the quality of the final product, and lead to significant cost savings in both the short and long term.

Call the experts to bridge the knowledge gap

Seeking Expertise: In the early conceptual design phase, it's crucial to engage local suppliers and contractors who have extensive project experience. They can provide valuable insights that can steer the project in the right direction. For instance, a precaster such as Conforce Structures, which specializes in precast concrete girders, can provide expertise on girder selection and details, taking into account considerations for shipping and erection.

Erectors, such as E.S. Fox Limited, N-Force, and fabricators such as Central Steel, can offer valuable insights into material selection, component modularity, and effective erection techniques during the planning stage. Incorporating their practical experience in transportation logistics, erection strategies, and constructability at an early stage can lead to cost-effective, practical, and sustainable designs that are easier to maintain.

By incorporating these principles, the bridge design can be optimized for practical implementation, leading to a smoother construction process, reduced costs, and an overall improved quality of the final product. Incorporating constructability into your bridge project is a proactive approach that can save time, money, and resources. The 2010 article below on the Deh Cho Bridge from Bridge Design and Engineering Magazine - Issue #58 provides insights on the bridge redesign process that resulted in material savings and improved constructability.

BDE Bridge Deh Cho Remote Control
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