Cracking the Code: What is the Pre-Compensation Force Method for Segmental Bridges?
- Varun Garg
- May 2
- 4 min read
Ever stood on a diving board? The further out you go, the more it deflects. Now picture building a bridge the same way—extending concrete segments from piers, cantilevering out until they meet mid-span. That’s how most long-span metro bridges are built using cantilever construction. But unlike diving boards, these bridges become continuous structures once the closure segment is cast—and that’s when things get tricky.
Concrete shrinks and creeps over time. Once the cantilevers are stitched together, that time-dependent shortening can’t happen freely anymore. Instead, it builds up as restraining internal forces, pushing back against the piers and piling stress into the foundations.
The Engineering Paradox
Here’s where engineers face a familiar cycle:
“Shrinkage and internal forces are stressing the foundations. Let’s add more piles.”
“Now it’s stiffer, so the forces got worse. Add even more piles?”
“Why are inner piles overloaded and outer ones barely used?”
It’s a structural feedback loop—and not a cheap one. Without intervention, substructures balloon in size while material efficiency declines.
The Pre-Compensation Solution
The Pre-Compensation Force Method (PFM) breaks this cycle by introducing counteracting forces before the problem fully develops.
Rather than react to time-dependent effects like shrinkage, engineers anticipate them. Just before the closure segment is cast, a calculated force is applied outward at the cantilever tips—effectively “pre-loading” the structure with internal forces that will balance the long-term effects once the bridge is continuous.
Think of it as a strategic nudge: not resisting future forces with brute strength, but neutralizing them in advance. The result?
More balanced pile reactions
Reduced internal stresses
10–20% savings in substructure materials and construction effort
How It Works on Site
Concrete “reaction blocks” are cast into the end segments. Prior to closure, synchronized hydraulic jacks are positioned between the cantilevers. The pre-calculated force is gradually applied and held steady, with deflections and strains carefully monitored.
While the force is held, the closure segment is cast using high-early-strength concrete. Once cured, the jacks are removed—and the bridge is now “tuned” for efficiency and enhanced long-term performance.
Real-World Results
PFM is already proving its value on major metro bridge projects. Below are real-world examples where the author(s) have directly contributed to its successful implementation:
Rach Chiec Bridge (Ho Chi Minh City Metro): A 4500 kN pre-compensation force reduced governing pile reactions by 15%—allowing piles to be shortened by 12 meters and saving weeks of construction time.
Yamuna Bridge (Delhi Metro): On this four-span, 280 m continuous module, engineers applied a 3000 kN force to keep foundations viable—averting what would have otherwise been a cost-prohibitive design.
Mithi River Bridge (Mumbai Metro): One of the toughest applications—curved alignment with a 210 m radius and rigid, rock-socketed foundations. A 5000 kN force made the design feasible and reduced substructure strain.
Finding the Sweet Spot
PFM isn’t one-size-fits-all. Applying too much force can overstress segments or cause unwanted deflections. Too little, and you miss the optimization benefits. For the Mithi Bridge, engineers tested pre-compensation forces from 3000 kN to 7000 kN. The final choice—5000 kN—was calibrated to structural response. But even then, the bridge’s curvature introduced an unexpected 86 mm of transverse deflection.
The fix? Plan pre-camber—a built-in geometric correction to absorb that twist once construction forces settle. These are the challenges that define modern bridge design—where precision modeling meets on-site adaptability.
Key Takeaways for Bridge Professionals
✅ PFM offers a practical engineering alternative to overbuilding foundations
✅ Expect 10–20% reduction in substructure and pile requirements
✅ Especially useful for tight curves, skewed alignments, and height-constrained sites
✅ Implementation needs advanced modeling but uses standard hydraulic equipment
✅ Suitable for transit viaducts and highway bridges with integral piers
Coming Up Next
We’ll take a closer look at real-world applications of this method:
Rach Chiec Bridge: Making it work in challenging Vietnamese soils
Yamuna Bridge: Managing a four-span continuous structure
Gokulpuri Flyover: Navigating curves, skews, and construction complexity
Mithi River Bridge: Where pre-compensation meets 3D geometry
📝 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”