Introduction: Addressing Critical Safety Gapsin Bridge Engineering
Precast, pretensioned concrete girders are a cornerstone ofmodern bridge construction, particularly in China and across the globe, due totheir ability to enhance structural safety and mitigate common issues likeprestress loss and strand corrosion. Despite their widespread use, significantgaps remain in the understanding of their mechanical behavior under adverseconditions, such as strand breakage, leading to potential safety hazards.Published in 2025, this study targets these deficiencies by focusing on a 35-mlong precast, pretensioned high-strength concrete I-girder with intentionallyinduced double broken strands. Through a combination of experimental testingand theoretical modeling, the research aims to deliver actionable insights intobending performance, crack propagation, and failure characteristics. Theultimate goal is to inform improved design practices, maintenance approaches,and longevity assessments for bridges, ensuring safer infrastructure for publicuse.
Literature Review: Comprehensive Evolution ofResearch on Pretensioned Girders
The body of research on precast, pretensioned concretegirders has grown substantially over the past decade, addressing variousstructural aspects but leaving certain critical areas underexplored:
- 2016-2017 Contributions: Han et al. (2016) introduced amodified thick-walled cylinder model to estimate elongation in precast,pretensioned concrete members, though it lacked practical simplification.Mantawy et al. (2016) conducted field tests demonstrating excellent seismicperformance in pretensioned columns, yet noted challenges in visuallyinspecting reinforcement post-damage. Salazar et al. (2017) achieved a 35%reduction in material usage by employing 18 mm diameter strands in girders,while Guo et al. (2017) proposed the Paris model to analyze fatigue crackexpansion rates in concrete beams reinforced with prestressed carbon fiberplates.
- 2018-2019 Developments: Wang et al. (2018) modeledelongation length and bond stress in corroded strands of pretensioned beams,and Dang et al. (2018) developed a reliable technique using free-end slip (FES)to quantify elongation. Naji et al. (2018) analyzed shear capacity throughexperiments, finding measured values exceeding theoretical predictions perAASHTO LRFD 2017. Jayaseelan et al. (2019) reduced camber by 72% using fullytensioned top strands in precompression zones, and Williams et al. (2019) criticizedconservative shear strength estimates in AASHTO LRFD 2017 and ACI 318-14,advocating for broader testing. Yan et al. (2019) identified strand count andcorrosion length as key factors in bending capacity degradation, though bondand shear failures were overlooked.
- 2020-2022 Advances: Alirezaei et al. (2020) testedend-region reinforcement methods to control crack width and stress in steelgirders. Honarvar et al. (2020) used probabilistic stress analysis in concretebeams, highlighting thermal effects exceeding AASHTO limits. Lee et al. (2022)modeled prestress loss in steel strands due to thermal effects (5.2%-5.5% losspost-release). Al-Omaishi (2022) noted AASHTO LRFD’s failure to account forbeam configuration in creep and shrinkage losses. Babarinde (2022) advocatedfor ultra-high performance concrete to enhance crack resistance, recommendingprestress limits of 139.7 MPa to control crack width. Xiong et al. (2022) foundUHPC beam-column joints matched reinforced concrete in seismic performance.
Despite these strides, research on the bearing capacity andcracking mechanisms of girders with broken strands remains sparse, particularlyunder bending loads. This gap motivated the current full-scale experimentalstudy to provide a deeper understanding of failure patterns and structuralresilience.
Experimental Setup: Detailed Full-Scale Modelof a 35-m I-Girder
The study executed a meticulously designed full-scaleexperiment on a 35-m long precast, pretensioned high-strength concreteI-girder, specifically engineered with double broken strands to simulatereal-world damage scenarios. Conducted in a controlled industrial testingenvironment, the setup included:
- Structural Configuration: The I-girder, typical ofhighway bridge applications, featured a cross-sectional design optimized forload distribution, with prestressed strands strategically placed to mimicstandard construction practices. The double broken strands were introduced atcritical midspan locations to assess worst-case bending stress scenarios.
- Loading Mechanism: Symmetric bending loads were appliedusing hydraulic actuators positioned at two points along the span, replicatingthe load distribution experienced by bridge girders under traffic conditions.Load increments were carefully controlled to monitor progressive damage.
- Measurement Instrumentation: Advanced tools such asstrain gauges were attached to steel bars and prestressed strands to capturestress variations. Displacement sensors measured vertical deflection atmultiple points, particularly at midspan, while high-resolution cameras andcrack detection software tracked crack initiation, propagation, and width.
- Assessment Parameters: The experiment focused ondeformation evolution (tracking overall structural response), force analysis(stress distribution in reinforcement), crack development (pattern and severityunder load), and mechanical performance (bending stiffness, crack resistance,and ultimate capacity).
This rigorous methodology provided a granular understandingof the girder’s behavior under realistic, high-stress conditions, closelysimulating the challenges faced by operational bridges.
Key Findings: In-Depth Analysis of BendingPerformance and Failure Characteristics
The experimental results yielded comprehensive insightsinto the girder’s performance under bending loads, highlighting both strengthsand vulnerabilities:
- Vertical Displacement Metrics: The maximum verticaldisplacement at midspan was recorded as significantly lower than the allowablelimit specified in design standards, indicating robust structural integrityeven with compromised strands. This suggests that the girder retainssubstantial load-carrying potential despite localized damage.
- Crack and Bearing Capacity Factors: The crack factorreached 1.31, reflecting adequate resistance to crack propagation, while thebearing capacity factor of 1.54 demonstrated exceptional ability to sustainloads beyond expected thresholds, ensuring safety margins.
- Bending Stiffness Degradation: Post-cracking, the bendingstiffness of the composite girder decreased by approximately 70%, a significantreduction attributed to the loss of cross-sectional integrity at cracked zones.This degradation was most pronounced near the midspan, where bending momentswere highest.
- Detailed Crack Analysis: Numerous cracks emerged in theconcrete, primarily initiating at the bottom flange under tensile stress andpropagating upwards. These cracks led to excess stress redistribution to thesteel bars and remaining prestressed strands, yet the maximum crack widthsobserved during loading were substantially smaller than theoretical predictionsoutlined in the Specifications for Highway Reinforced Concrete PrestressedConcrete Bridge Culverts (JTG 3362–2018). This discrepancy suggests thatcurrent design codes may overestimate crack severity in such girders.
- Holistic Performance Evaluation: Despite the stiffnessreduction and crack formation, the girder exhibited optimal bending stiffnessfor its design purpose, sufficient crack resistance to prevent catastrophicfailure, acceptable ultimate bending capacity to handle overloads, and aductile failure mode characterized by gradual deformation rather than suddencollapse. This ductility ensures that warning signs (e.g., visible cracking anddeflection) are evident before failure, enhancing safety.
These findings collectively indicate that even with doublebroken strands, the I-girder maintains a high level of performance, providingreassurance for its application in bridge construction while identifying areasfor design refinement.
Theoretical Analysis: Advanced Modeling ofBending Capacity and Stiffness
To complement the experimental data, the study developed asophisticated analytical model aligned with the Code for the Design of HighwayReinforced Concrete and Prestressed Concrete Bridges and Culverts (JTG3362–2018) to predict bending capacity and stiffness under various conditions.Key components of the theoretical framework include:
- Bending Capacity Calculation: The model incorporatedmultiple parameters such as the significant factor of structure, design bendingmoment, ultimate compressive strength of concrete, yield strength of steelbars, and ultimate tensile strength of steel strands. Equations (1) and (2)from the study accounted for cross-sectional areas of steel bars and strands,web width, effective height, and distances between force points and edges,providing a comprehensive capacity estimate.
- Bending Stiffness Degradation: Post-cracking stiffnessreduction was quantified using a degradation coefficient, with the modelindicating a 70% stiffness drop in cracked sections (Equations 5-6). Theequivalent bending stiffness was derived from midspan deflection undersymmetric loading, factoring in load, span, and deflection (Equation 4).
- Crack Width Prediction: Maximum crack width wascalculated based on steel bar stress, elastic modulus of steel, and effectivereinforcement ratio (Equation 7). Notably, experimental crack widths wereconsistently smaller than theoretical values, suggesting that the girder’sactual performance exceeds code-based expectations.
This theoretical framework offered a predictive tool forgirder behavior, though the experimental results highlighted greater resiliencethan anticipated, indicating potential for recalibrating design assumptions tooptimize material use without compromising safety.
Implications for Bridge Engineering:Comprehensive Design and Maintenance Recommendations
The study’s findings carry profound practical implicationsfor enhancing the safety, durability, and efficiency of bridgeinfrastructure:
- Design Optimization Strategies: Engineers shouldintegrate higher crack resistance and stiffness considerations into girderdesigns, particularly for scenarios involving potential strand breaks. Thiscould involve increasing strand redundancy or using higher-strength materialslike UHPC in critical zones. Additionally, design codes might be updated toreflect the observed discrepancy between theoretical and actual crack widths,allowing for more efficient designs.
- Proactive Maintenance Approaches: Maintenance protocolsshould prioritize early detection of strand damage through non-destructivetesting methods (e.g., acoustic emission or ultrasonic testing) to preventprogressive stiffness degradation and crack propagation. Regular inspectionsshould focus on midspan areas most susceptible to bending stress.
- Enhanced Damage Assessment Techniques: The study’smetrics, such as crack width, vertical displacement, and bearing capacityfactors, provide reliable benchmarks for assessing girder health under load.These parameters can be incorporated into structural health monitoring systemsto provide real-time data on bridge condition, enabling timelyinterventions.
- Sustainability and Cost-Efficiency: By understanding theductile failure behavior and residual capacity of damaged girders,infrastructure managers can avoid premature replacements, extending servicelife and reducing costs while maintaining safety standards.
These recommendations collectively aim to bolster theresilience of precast, prestressed concrete bridges, addressing failure risksproactively and ensuring long-term performance in diverse operationalenvironments.
Conclusion: Pioneering Safety EnhancementsThrough Rigorous Research
This landmark study successfully dissected the bendingperformance of a precast, pretensioned high-strength concrete I-girder withdouble broken strands, demonstrating its remarkable capacity to withstandsignificant loads with a ductile failure mode that prioritizes safety. Byintegrating full-scale experimental testing with advanced theoretical modeling,the research addresses critical gaps in understanding mechanical behavior,crack evolution, and failure mechanisms under bending stress. The results affirmthe girder’s suitability for demanding bridge applications while deliveringactionable insights for refining design standards, enhancing maintenancepractices, and strengthening safety protocols in bridge engineering. This worksets a precedent for future studies to build upon, fostering safer and moredurable infrastructure worldwide.
Key Takeaways:
- A 2025 study in Scientific Reports conducted a full-scaletest on a 35-m precast, pretensioned concrete I-girder with double brokenstrands to analyze bending performance.
- Experimental outcomes revealed vertical displacement wellbelow standard limits, with impressive crack and bearing capacity factors of1.31 and 1.54, respectively.
- Post-cracking bending stiffness declined by 70%, yetobserved crack widths were significantly smaller than theoretical predictionsper JTG 3362–2018, indicating greater resilience.
- The girder showcased optimal bending stiffness, robustcrack resistance, substantial ultimate bending capacity, and a ductile failuremode, ensuring safety under distress.
- Detailed recommendations advocate for enhanced designfocusing on crack resistance, early detection in maintenance, and advanceddamage assessment techniques to maximize bridge longevity and safety.
