Impact of Tendon Placements on Shear Lag Phenomenon in PSC Box Girders

Abstract

PSC box girders are extensively employed in Modern bridge construction because of their efficiency in structure, torsional rigidity, and durability. Shear lag, however, tends to produce non-uniform stress distributions over the flanges, resulting in local stress concentrations, compromised flexural effectiveness, excessive deflection, and possible cracking. This research focuses on the effect of tendon position on shear lag behavior in a two-span continuous PSC box girder. Six tendon geometries were compared under uniform geometry, material response, and typical bridge load conditions. Results show that the optimized tendon layout enhances uniformity of stress with 35% reduction in shear lag, along with 26% and 21% reductions in peak bending moments and maximum deflections, respectively. Placement of tendons close to critical web-flange connections improves the transfer of loads, structural rigidity, and performance of the bridge. The conclusions give the advice in designing lighter, shorter-span, and more performing PSC box girders.

Introduction

Prestressed Concrete (PSC) box girders are widely used in modern bridge construction due to their high stiffness-to-weight ratio and torsional rigidity. However, they are prone to shear lag—a phenomenon where longitudinal stresses are unevenly distributed, causing stress concentration near the webs, inefficiency in flange usage, and cracking under service loads.

2. Research Gap

While shear lag behavior and geometry influences have been studied, limited research exists on how tendon layout specifically affects shear lag in PSC box girders. This study aims to fill that gap by systematically analyzing different tendon configurations.

3. Objectives

The research aims to:

1. Model PSC box girders with various tendon layouts using Finite Element Analysis (FEA).

2. Evaluate how tendon positions affect stress distribution, bending, and deflection.

3. Compare layouts to identify shear lag reduction patterns.

4. Recommend optimal tendon configurations for improved performance.

4. Methodology

• A two-span continuous PSC box girder (45 m span, 14.8 m wide, 3 m deep) was used for all simulations.

• Material properties and total prestressing force (29,000 kN) were kept constant.

• Six tendon configurations (uniform and varied) were tested, changing only the position and distribution of prestressing tendons.

Models Overview:

• Model 1 (Reference): Uniform 1000 kN tendons throughout.

• Model 2: Concentrated prestressing (14,500 kN) near web-flange junctions.

• Models 3–6: Asymmetric or variable force distributions (ranging 500–4000 kN), exploring different positioning strategies.

5. Analysis Parameters

The study considered:

• Stress distribution across flanges.

• Peak shear lag values.

• Bending moments and deflections.

• Transverse shear forces.

• Overall efficiency and serviceability.

6. Results & Discussion

The results demonstrated that:

• Tendon placement significantly affects shear lag and longitudinal stress behavior.

• Strategically concentrated tendons (e.g., near web-flange junctions) improved stress uniformity and stiffness.

• Some configurations reduced deflection and improved load distribution.

• Non-uniform and optimized layouts performed better than the standard uniform layout (Model 1).

Conclusion

This study evaluated the influence of tendon placement on shear lag, bending moments, displacements, and transverse shear in PSC box girders. Key findings include: 1) Shear Lag: Model 2 reduced FDC by 35.1% compared to Model 1, while Model 5 increased shear lag by 6.3%, identifying configurations to avoid. 2) Bending Moments: Sagging moments decreased up to 4.66% (Model 2), and hogging moments reduced by 1.6–2.87%, highlighting improved longitudinal stress distribution. 3) Displacements: Maximum deflections were minimal (<0.004?m), ensuring serviceability. 4) Transverse Shear: Variations were negligible (–0.96% to +0.22%), indicating stability under applied loads. Overall, Model 2 provides the most balanced performance, demonstrating that optimized tendon placement enhances stress distribution, structural efficiency, serviceability, and long-term durability of PSC box girders, emphasizing its importance in prestressed bridge design.

References

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Copyright

Copyright © 2025 Sanika S. Patil, Prof. Roshni John. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.