Parameter Analysis of In-Plane Ultimate Bearing Capacity of CFST Single Circular Tube Ribbed Arch

Literature Overview

This 2004 paper by Chen Baochun and Qin Zebao, published in the Journal of the China Railway Society, presents a parametric analysis of the in-plane ultimate bearing capacity of concrete-filled steel tube (CFST) ribbed arches using double nonlinearity finite element analysis. The study examines the influence of rise-span ratio, slenderness ratio, steel ratio, and loading method on the nonlinear behavior and ultimate capacity of the arch structure.

Core Technical Content

The research focuses on CFST ribbed arches, which are structural systems used in railway bridges, industrial buildings, and other large-span structures. The arch ribs are constructed using single circular steel tubes filled with concrete, taking advantage of the composite action between the steel tube and the concrete core.

Parametric Study Variables

The parametric analysis investigates the following key variables:

Nonlinear Analysis Approach

The double nonlinearity finite element analysis accounts for:

• Geometric nonlinearity: large displacements and rotations, P-delta effects
• Material nonlinearity: elastoplastic behavior of steel and concrete, concrete crushing and steel yielding
• Contact nonlinearity: interaction between the steel tube and the concrete core

The analysis simulates the entire loading process from the initial elastic stage through the plastic stage to the ultimate failure, providing a comprehensive understanding of the structural behavior.

Technical Interpretation

Rise-Span Ratio Effect

The rise-span ratio (f/L) is a fundamental geometric parameter that determines the thrust and bending moment distribution in the arch. A higher rise-span ratio results in:

• Greater horizontal thrust at the supports
• Reduced bending moments in the arch ribs
• Higher ultimate bearing capacity due to the increased axial compression component

However, a higher rise-span ratio also increases the material usage and may require more robust support structures. The optimal rise-span ratio must balance structural efficiency with economic and practical considerations.

Slenderness Ratio Effect

The slenderness ratio (L/i) is a critical parameter for the stability of the arch ribs. A higher slenderness ratio increases the susceptibility to buckling, which reduces the ultimate bearing capacity. The CFST construction provides inherent stability due to the concrete core, which increases the moment of inertia and provides lateral support to the steel tube. However, for very slender arches, the buckling capacity may still be a governing design consideration.

Steel Ratio Effect

The steel ratio (ρ) represents the proportion of steel in the composite section. A higher steel ratio increases the ultimate bearing capacity by providing greater tensile strength and ductility. However, the concrete core also contributes significantly to the capacity through its compressive strength and confinement effect. The optimal steel ratio must balance the cost of steel with the structural performance requirements.

Loading Method Effect

The loading method (uniform vs. concentrated loading) affects the distribution of internal forces and the location of plastic hinges. Uniform loading typically results in a more uniform stress distribution and a higher ultimate capacity, while concentrated loading creates localized stress concentrations and may lead to premature failure. The parametric analysis provides insight into the capacity reduction factors for different loading scenarios.

Engineering Practice Integration

Design Implications

The parametric analysis provides valuable data for the design of CFST ribbed arches:

• The rise-span ratio should be selected to optimize the capacity-to-weight ratio
• The slenderness ratio should be limited to ensure adequate stability
• The steel ratio should be selected based on the cost-performance trade-off
• The loading method must be accurately modeled to predict the ultimate capacity

Construction Considerations

The construction of CFST ribbed arches involves:

• Fabrication of the steel tube sections with precise dimensions
• Welding of the tube segments with full-penetration butt welds
• Erection of the arch ribs with temporary support structures
• Concrete pumping into the steel tubes with proper compaction
• Removal of the temporary supports after the concrete has cured

The construction sequence must be carefully planned to minimize residual stresses and to ensure the proper interaction between the steel tube and the concrete core.

Quality Control

The quality control of CFST ribbed arches involves:

• Inspection of the steel tube dimensions and tolerances
• Verification of the weld quality using non-destructive testing (RT, UT, MT)
• Monitoring of the concrete pumping process and compaction
• Testing of the concrete strength and the steel material properties
• Verification of the arch geometry and alignment

Safety Factors

The parametric analysis provides the ultimate bearing capacity, but the design must incorporate appropriate safety factors to account for:

• Material property variability
• Construction tolerances and workmanship
• Load uncertainties and combinations
• Long-term effects such as creep and shrinkage

The safety factors must be selected based on the applicable design codes and standards, such as GB 50017, JGJ 4, and the relevant railway bridge design codes.

Key Questions and Reflections

The research raises several important questions for further investigation:

1. How does the out-of-plane stability of the CFST ribbed arch compare to the in-plane ultimate capacity?
2. What is the effect of the concrete strength and the steel grade on the ultimate capacity?
3. How can the parametric analysis results be used to develop simplified design formulas for practical engineering?
4. What are the long-term performance implications of the CFST construction under fatigue and environmental loading?

The parametric analysis provides a comprehensive understanding of the factors influencing the ultimate capacity of CFST ribbed arches. However, the practical application of these results requires the development of simplified design methods that can be used by engineers in routine design practice. The research provides the foundation for such design methods, but further work is needed to translate the parametric results into practical design formulas.

Study Insights and Outlook

This research provides valuable parametric data on the in-plane ultimate bearing capacity of CFST single circular tube ribbed arches. The key insight is that the capacity is governed by a complex interaction between geometric parameters (rise-span ratio, slenderness ratio), material parameters (steel ratio), and loading conditions, and that the double nonlinearity finite element analysis is an effective tool for investigating these interactions.

The research contributes to the understanding of CFST arch structures and provides a basis for the development of design methods and code provisions. Future research should extend to out-of-plane stability, combined loading scenarios, and long-term performance to provide a more complete understanding of the structural behavior. The practical application of the research results requires the development of simplified design formulas and the integration of the findings into the relevant design codes and standards.