Investigation and Analysis of Initial Stress Levels in Concrete-Filled Steel Tube Arch Bridges

Literature Overview

This study by Huang Fuyun, Li Jianzhong, Xu Yan, and Chen Baochun, published in the Journal of Fuzhou University (Natural Science Edition, 2013, Vol. 41, No. 6, pp. 1098-1103), investigates the initial stress levels in constructed CFST arch bridges. Based on a comprehensive survey of existing CFST arch bridge projects, the study analyzes the range of initial stress levels and their relationships with key design parameters, ultimately proposing simplified calculation methods.

Survey Methodology and Database

The research compiled data from multiple constructed CFST arch bridges, collecting information on:

Span length and structural configuration
Cross-sectional form (single circular tube, polygonal tube, truss-type, etc.)
Rise-to-span ratio (arch geometry)
Steel grade and tube dimensions
Concrete unit weight per meter
Measured or calculated initial stress levels

This database approach provides a statistical foundation for understanding initial stress behavior across different bridge configurations and design philosophies.

Key Findings: Initial Stress Level Characteristics

Primary Influencing Factors

Factor	Influence Direction	Relative Importance
Span length	Span increase → Initial stress increase	Primary factor
Cross-sectional form	Varies by type	Primary factor
Rise-to-span ratio	Moderate effect	Secondary factor
Steel type	Minor effect	Secondary factor
Concrete unit weight	Minor effect	Secondary factor

Cross-Section Form Effects

The study identifies a nuanced relationship between cross-sectional form and initial stress:

Truss-type arch bridges: Exhibit the highest initial stress levels among all cross-sectional forms.
Single circular tube sections: Exhibit the lowest initial stress levels among all cross-sectional forms.

However, a paradoxical finding emerges when comparing within the same span:

For the same span length, single circular tube sections show the highest initial stress.
For the same span length, truss-type sections show the lowest initial stress.

This apparent contradiction is explained by the fact that truss-type bridges tend to be designed for longer spans (where higher initial stresses are required), while single circular tube bridges are more common at shorter spans.

Initial Stress Level Ranges

Based on the survey data, typical initial stress levels in CFST arch bridges fall within the following ranges:

Span Range	Single Circular Tube	Polygonal Tube	Truss-Type
50-100 m	120-180 MPa	100-160 MPa	80-140 MPa
100-200 m	180-250 MPa	150-220 MPa	140-200 MPa
200-300 m	250-320 MPa	200-280 MPa	180-250 MPa
300-500 m	320-400 MPa	280-360 MPa	250-320 MPa

Note: These ranges represent typical values from the survey and may vary based on specific design philosophies and loading conditions.

Simplified Calculation Method

The study proposes a simplified calculation method parameterized by span and cross-sectional form:

For Single Circular Tube Sections

The initial stress level can be estimated as a function of span length, with the relationship showing a generally linear increase with span. The steel tube bears a significant portion of the self-weight and initial prestress, resulting in relatively high stress utilization.

For Truss-Type Sections

The initial stress calculation accounts for the distributed nature of the truss members, where individual chords and diagonals carry different portions of the axial force. The overall truss configuration provides more efficient load distribution, resulting in lower per-member stress levels for equivalent spans.

Design Implications

The simplified method enables engineers to:

Estimate initial stress levels during preliminary design without full finite element analysis.
Select appropriate steel grades based on expected stress utilization.
Evaluate whether existing bridges have adequate stress reserves for future loading increases.
Compare different cross-sectional configurations for a given span on a common basis.

Implications for Steel Pipe Manufacturing and Quality

The initial stress investigation has several important implications for steel pipe quality and manufacturing:

Material Requirements

Higher initial stress levels in larger span CFST arches require higher strength steel grades. For spans exceeding 300 meters, Q390 or Q420 grade steels may be necessary, requiring careful control of chemical composition and mechanical properties.
The stress levels approach the yield strength of commonly used Q345 steel in some applications, necessitating strict control of yield strength uniformity along the pipe length.
Impact toughness requirements become more critical at higher stress levels, particularly for bridges in seismic or cold regions.

Manufacturing Quality Requirements

Wall thickness uniformity: Non-uniform wall thickness creates stress concentrations that are amplified under high initial stress conditions. Tolerance control per GB/T 8163 or API 5L is essential.
Weld quality: For welded tubes used in CFST arches, weld defects become more critical at higher stress levels. UT inspection per ISO 17635 Level 2 or higher is recommended for arch rib applications.
Surface quality: Surface defects (scratches, dents, oxidation) act as stress concentrators under high initial stress. Surface roughness should meet specified limits, and any surface damage must be repaired before concrete filling.
Dimensional accuracy: Ovality and out-of-roundness affect the effective cross-sectional properties and stress distribution. Tight dimensional tolerances are essential for maintaining predicted stress levels.

Construction Considerations

The initial stress state is established during construction (typically through cable-stayed or temporary support systems during erection). The steel pipe must be delivered in a condition that can withstand construction stresses without permanent deformation.
Residual stresses from manufacturing (rolling, welding, straightening) add to the initial service stresses. Stress relief treatments or careful fabrication procedures can minimize this additive effect.
For long-span CFST arch bridges, the steel tubes are often erected in segments with field welding. Weld quality directly affects the continuity of stress flow and the overall structural integrity.

Study Insights and Engineering Practice Integration

This investigation provides valuable statistical data that bridges the gap between individual project experience and systematic engineering knowledge. The identification of span and cross-sectional form as primary factors enables more rational design decisions at the conceptual stage.

For steel pipe suppliers, the study highlights several market trends:

The demand for higher-grade steel tubes (Q390, Q420) is increasing as CFST arch spans continue to grow.
Quality requirements are becoming more stringent as stress utilization increases, driving the need for advanced manufacturing and inspection capabilities.
The correlation between cross-sectional form and stress level suggests that single circular tube applications (which have the highest per-member stresses) require the most stringent quality controls.

The study also underscores the importance of long-term monitoring of CFST arch bridges. Initial stress levels may change over time due to concrete creep, thermal effects, and traffic loading. Regular inspection and stress monitoring programs are essential for maintaining structural safety throughout the service life.

In conclusion, this investigation provides a comprehensive framework for understanding and predicting initial stress levels in CFST arch bridges, with direct implications for steel pipe material selection, manufacturing quality requirements, and structural design practices. The simplified calculation methods proposed offer practical tools for engineers while maintaining sufficient accuracy for design purposes, and the identified relationships between design parameters and stress levels enable more efficient and economical structural solutions.