Optimal Inclination Angle of Diagonal Struts in CFST Arch Bridge Arch Hinges

Literature Overview

The paper by Wang Weimin, Gao Hejie, Zhou Shuixing, and Xie Yao (2021), published in the Journal of Chongqing Jiaotong University (Natural Science Edition), investigates the optimal inclination angle of diagonal struts in the arch hinge of steel tube concrete (CFST) arch bridges. Supported by the Guizhou Provincial Department of Transportation Science and Technology Project (No. 2015-122-052), this research addresses a practical design issue in the construction of CFST arch bridges, where temporary arch hinges are commonly used to facilitate cable-sling erection and arch rib alignment adjustment.

Core Technical Approach

The authors develop two simplified structural models for the arch hinge:

Model	Description	Assumptions
Ideal truss model	Arch hinge treated as a pin-jointed truss	All joints are frictionless pins, members are axially loaded
Triangular rigid frame model	Arch hinge treated as a fixed-end frame	Joints are rigid, bending moments are transmitted

From the ideal truss model, the axial force in the diagonal strut is derived using force equilibrium:

N_diagonal = P / sin(α)

where P is the vertical component of the arch thrust and α is the inclination angle of the diagonal strut. The stability coefficient is defined as the ratio of Euler critical load to the actual axial force:

η = N_cr / N_diagonal = π²EI / (L² · N_diagonal)

This coefficient is maximized when the diagonal strut angle is optimized, providing the best stability margin.

Analytical Results and Optimal Angle Determination

The study establishes the relationship between the stability coefficient and the diagonal strut inclination angle:

Inclination Angle (α)	Stability Coefficient (η)	Axial Force (N)	Bending Moment (M)
30°	0.52	High	Moderate
45°	0.71	Moderate	Moderate
60°	0.85	Moderate	Low
75°	0.92	Low	Low
90°	0.95	Low	Low
100°	0.93	Low	Low
115°	0.88	Low	Moderate

The analysis reveals that the optimal inclination angle for the diagonal strut is in the range of 60° to 100°, with the maximum stability coefficient occurring near 90°. This range satisfies both the stability requirement (high η) and the constructability requirement (angle not too steep).

Connection to JTG/T-D65-06-2015 Code Requirements

The Chinese highway design code JTG/T-D65-06-2015 specifies that the angle between the main tube and branch tube in CFST arch bridge nodes should not be less than 30°. The study's finding that the optimal diagonal strut angle is 60°-100° is consistent with this requirement, as the complementary angle (90° - α) ranges from -10° to 30°, ensuring that the main-branch tube angle remains within acceptable limits.

Design Parameter	JTG/T