The Art of Span Layouts for Concrete Segmental Transit Viaducts
- Saqib Khan
- Jun 3
- 6 min read
A well-chosen span arrangement isn’t just a geometric exercise—it’s a balancing act that directly influences cost, constructability, long-term performance, and durability. In our last post, we explored the Pre-Compensation Force Method (PFM)—a creative engineering technique used to offset internal stresses caused by creep and shrinkage in segmental bridges. But here’s the catch: PFM only works effectively when span layout and closure joints are smartly positioned.
The reason lies in how internal forces develop over time. PFM introduces a deliberate counterforce just before closure to preemptively balance expected stresses. However, if closure joints are poorly located—such as directly at midspan—or if span lengths are irregular and induce differential stiffness, the counterforces may become misaligned with actual stress paths. This can lead to cantilever tip uplift, distortion, or residual stress concentrations. Effective use of PFM requires geometric strategies that support symmetrical behavior, predictable moment zones, and constructible stitch points. In this post, we zoom out to examine how to achieve that, starting with the ideal layout when no external constraints exist.
The Ideal Starting Point: Uniform, Short Spans
In situations without geometric, environmental, or foundation constraints, the most economical and structurally straightforward solution is a uniform short-span layout—typically using spans of 30–40 meters constructed span-by-span. This modular approach maximizes repetition, standardizes fabrication and erection, and simplifies analysis and detailing. Systems like the Evergreen Line in Metro Vancouver demonstrate how effective this can be in straight corridors with minimal land impacts. These layouts are not only cost-effective but also reduce long-term internal force buildup by minimizing superstructure continuity over large distances.
Majority of the Evergreen Line in Metro Vancouver uses a uniform, short-span layout with repetitive piers and segments—ideal for span-by-span construction. This modular approach supports fast erection, consistent fabrication, and low long-term internal stress buildup.
Balanced Cantilever Strategy: Symmetric Layouts (L₁–L₂–L₁)
When longer spans are needed—such as over rivers, highways, or existing infrastructure—balanced cantilever construction becomes the preferred method. In these cases, a symmetric arrangement like L₁–L₂–L₁ is structurally advantageous. It enables balanced cantilevering from both piers, minimizes differential displacements, and reduces the complexity of construction staging. For optimal performance, the side-span to main-span ratio should lie between 0.75 and 0.85. Ratios below 0.6 can lead to uplift at the ends or require temporary tie-downs, while ratios above 1.0 reduce the efficiency of the central span. This ratio range also improves the behavior of the structure under long-term creep, shrinkage, and thermal effects—especially when used in conjunction with the Pre-Compensation Force Method.
Not all symmetric layouts are selected purely for their structural benefits. In many cases, symmetry emerges as a constraint-driven necessity. For instance, the Rach Chiec Bridge on the Ho Chi Minh City Metro features a 63m–105m–63m configuration to span the Rach Chiec River without intermediate supports. While the layout fits the L₁–L₂–L₁ pattern, it was shaped by the need to clear the waterway with a single span. The shorter side spans enabled cantilevering from each pier without the need for temporary falsework in the river. Despite being driven by site conditions, this layout provided the symmetry needed to effectively implement PFM and balance long-term internal forces.
The Rach Chiec Bridge in Ho Chi Minh City features a 63m–105m–63m span layout that clears the river with a single central span. Though constraint-driven, its symmetric configuration enabled balanced cantilever construction and effective use of the Pre-Compensation Force Method (PFM).
A more deliberate application of symmetric layout is seen on the Yamuna Bridge in Delhi Metro Phase IV, which follows an L₁–L₂–L₂–L₁ span arrangement (60m–80m–80m–60m). This configuration provided balanced cantilever staging, improved force distribution, and smoother construction over a wide river. The symmetrical behavior also enhanced the performance of PFM by allowing internal pre-compression forces to be distributed evenly between piers.
The Yamuna Bridge in Delhi Metro Phase IV follows a 60m–80m–80m–60m symmetric span layout. Closure joints were deliberately staggered to align with moment reversal zones, improving PFM effectiveness and minimizing long-term stress concentrations.
When the Site Dictates: Asymmetric Layouts (L₁–L₂–L₃)
By contrast, asymmetric layouts such as L₁–L₂–L₃, where span lengths differ on either side of the central span, are generally avoided unless dictated by unavoidable site conditions and obstacles. These might include tight horizontal curves, staggered pier locations, utility conflicts, or irregular foundation spacing. While asymmetric layouts increase design complexity and require more careful management of cantilever lengths and closure staging, they are sometimes necessary for constructability in constrained environments.
The Gokulpuri Flyover on the Delhi Metro is a good example. With a layout approximating 64m–90m–70m, the bridge was designed to accommodate a tight radius and skewed pile foundation at one pier. Another example is the Mithi River Bridge with an arrangement of 75m - 125m - 70m where the asymmetrical arrangement where site constraints limited placement locations of foundations. While not ideal from a structural balance standpoint, the layout allowed the bridge to be built within a narrow, constrained right-of-way.
The Gokulpuri Flyover on the Delhi Metro features an asymmetric span layout of approximately 64m–90m–70m. This configuration was adopted to navigate tight horizontal curvature and skewed foundation geometry, demonstrating how structural compromises are sometimes necessary in dense urban environments.
Fine-Tuning Performance: Staggered Closure Joints
Another critical factor in the success of long-span segmental viaducts—and especially in the performance of PFM—is the location of closure joints. Although it's common to place closure joints at midspan, this location coincides with peak bending moments under service conditions and long-term effects. As a result, it can lead to stress concentration and reduce the effectiveness of internal force balancing.
A more refined approach is to use staggered closure joints, placing them closer to zones of moment reversal—typically around the quarter-span or one-third span locations. This allows the closure to occur in regions of lower internal force, reduces residual stress buildup, and improves compatibility with pre-compensation techniques.
On the Yamuna Bridge, designers applied staggered closures to minimize tip uplift during the stitching of cantilevers and to better align jacking forces with structural stiffness. This approach resulted in improved long-term performance, reduced cracking potential, and smoother stress transitions across joints.
The Trade-offs Behind Every Good Layout
Designing a good span layout is always a balancing act. There is no perfect configuration that maximizes all performance objectives simultaneously. Fewer joints improve durability and reduce maintenance demands—but they often require longer spans, which increase segment weight and complicate construction. Fewer piers reduce land use and foundation costs, but result in more demanding superstructures. Conversely, shorter spans are easier to build and allow for lighter structures, but introduce more joints and substructure elements. And while balanced stiffness enhances long-term performance under creep and shrinkage, it can be at odds with geometric constraints or erection sequences.
These realities mean that bridge designers must adopt a system-level perspective—evaluating each project’s unique conditions and making intentional trade-offs between performance, cost, constructability, and service life. This is where a thoughtful design philosophy becomes essential.
At Spannovation, our span layout strategy for transit viaducts is built around four core principles. First is Span Length Strategy—prioritizing longer spans only at constrained points (e.g., crossings), while maintaining shorter spans elsewhere to reduce self-weight and simplify construction. Second is Stitch Point Optimization—placing closure joints in locations that align with natural force reversals rather than simply geometric midpoints. This helps reduce long-term moment buildup and improves the effectiveness of techniques like the Pre-Compensation Force Method. Third is Substructure Modularity—aligning pier spacing with segment lengths to facilitate repeatable casting and efficient erection. Finally, in seismic regions, we emphasize Continuity vs. Redundancy Trade-offs—grouping spans strategically to maintain structural resilience without compromising force management or constructability.
Final Thoughts: Layout as the Foundation of Performance
Whether chosen for economy, constructability, or necessity, every span layout reflects a series of calculated decisions. Symmetric layouts with well-proportioned spans and staggered closures provide the best foundation for long-term structural performance—particularly when paired with innovations like PFM. When constraints demand asymmetric or unconventional layouts, the key is to adapt with intention, using modeling and detailing to mitigate trade-offs and preserve durability.
Ultimately, the span layout is not just a starting point—it’s the backbone of the entire viaduct system. Get it right, and everything that follows—from erection staging to lifecycle maintenance—becomes simpler, more predictable, and more resilient.
📝 About the Authors
Varun Garg is Lead Bridge Engineer at Spannovation Consulting India Pvt. Ltd. with over 15 years of experience in the analysis, design, and construction engineering of complex bridge structures. He specializes in long-span segmental bridges built using cantilever construction methods, with a focus on transit and highway infrastructure across India and Southeast Asia.
Saqib Khan, P.Eng. is Principal Engineer at Spannovation Consulting Ltd. in Vancouver, Canada. With over 24 years of experience in bridge design, construction engineering, and seismic retrofit, he is a recognized expert in performance-based seismic design and deep foundation systems. Saqib has led the delivery of major infrastructure projects across North America and Asia.
This blog series is inspired by our technical paper in the December 2024 publication of ‘The Bridge & Structural Engineer’ by Er. Varun Garg and Saqib Khan, P.Eng.
“Optimizing Long-Span Segmental Bridges for Mass Transit Using the Pre-Compensation Force Method”
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