Application of Pre-Compensation Force in Segmental Bridges
- Varun Garg
- 7 hours ago
- 3 min read
The success of the Pre-Compensation Force Method (PFM) in segmental bridge construction relies not only on its precise design but also on its real-time validation during field execution. The Yamuna Bridge on Delhi Metro’s Phase IV corridor offers a practical demonstration of how elevation monitoring, feedback loops, and adaptive engineering decisions drive effective force application.
The accompanying sketch illustrates vertical elevation differences between cantilever tips at different construction stages. These are not horizontal gaps, but changes in height that reflect how each cantilever segment behaves when subjected to pre-compensation forces.
Assessing and Fine-Tuning Cantilever Displacements
During the application of pre-compensation forces in segmental bridge construction, it is common to observe differential displacements between adjacent cantilevers due to variations in stiffness and construction sequence. These elevation differences are closely monitored in the field and compared to model predictions to validate alignment and identify any required corrections.
For instance, at Stage 4 of the Yamuna Bridge construction, pre-compensation was applied between two cantilevers—one already stitched into a stiffer, unified segment and the other still acting independently. This stiffness asymmetry led to unequal vertical deformation:
The flexible cantilever experienced a greater upward deflection (66 mm)
The stitched, stiffer side moved only slightly (13 mm)
This mismatch was not due to horizontal separation, but rather a rotational response of the more flexible cantilever, resulting in an uneven tip elevation and slight tilt. Such behavior is common in staged construction, particularly when dead load assumptions differ from field conditions. To address this, engineers use field-tuned solutions such as strategically placed counterweights (kentledge) to locally adjust tip deflections and reduce level differences before casting the stitch segment. This fine-tuning ensures that the final structure aligns within tolerance and performs as intended.
Fine-Tuning with Counterweight
While analytical models had assumed symmetric dead load conditions, field measurements revealed unexpected imbalances. To locally address this, we recommended applying kentledge (counterweight) to the tip of P193:
Initial 50-ton load was distributed too broadly, reducing effectiveness
Load was increased to 75 tons to restore deflection goals
This led to minor lateral movement at P193 (6 mm), prompting fine-tuned redistribution:
30 tons were removed from one side
15 tons were re-applied on the P194-facing side
This adjustment brought the level difference between P193 and P194 down to just 4 mm, within acceptable stitching tolerance.
Why Not Just Apply More Force?
Increasing the pre-compensation force might seem like a straightforward solution to close level gaps—but it wasn’t appropriate here due to:
Structural limits on reaction block and substructure capacity
Design optimization constraints: pre-jacking forces had been carefully balanced during design to avoid over-stressing
Load assumption corrections: once dead load imbalance was identified as the issue, kentledge offered a localized solution without altering global force application
Field Engineering & Decision Support
Recognizing the complexity of on-site behavior, our engineering team visited the site directly, reviewed conditions, and worked alongside the contractor to assess real-time behavior. Based on this hands-on evaluation, we:
Reviewed how the structure was behaving relative to the model
Advised on counterweight placement
Monitored behavior during PFM application, with contingency thresholds in place to halt loading if deflections exceeded safe limits
This collaborative, responsive approach ensured the PFM process remained within the safe envelope, while allowing sufficient correction for geometric alignment prior to closure pour.
📝 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”