Seismic Response of Long-Span Steel Tube Concrete Arch Bridges

Literature Overview

Zheng Shixiong, Zhou Shuhua, and Ding Guibao from Southwest Jiaotong University published this study in the Journal of Southwest Jiaotong University (Volume 34, Issue 3, 1999, pp. 320-324). The research focuses on a tied-arch bridge with spans of (76+360+76) m, analyzing its dynamic characteristics and seismic response with particular attention to the traveling wave effect of seismic waves on the structure.

Core Technical Analysis

The study establishes a finite element model for the long-span steel tube concrete (STC) tied-arch bridge and examines how seismic input characteristics influence structural response. The key findings include:

Location	Seismic Response Characteristic	Design Implication
Arch rib mid-span	Relatively small internal forces	May be optimized for weight reduction
Arch foot section	Large seismic internal forces	Critical for seismic design; requires robust connection
L/4 to 3L/8 sections	Relatively large internal forces	Secondary critical zones requiring attention
Vertical displacement	Increases with traveling wave effect	Foundation design must accommodate
Axial force	Increases with traveling wave effect	Compression member design verification needed
Lateral displacement	Decreases with traveling wave effect	Lateral restraint may be less critical

Traveling Wave Effect Analysis

The traveling wave effect refers to the phenomenon where seismic waves arrive at different points of a long-span structure at different times due to finite wave propagation velocity. For a 360 m main span, this effect becomes significant. The study demonstrates that:

The wave propagation velocity across the bridge span affects the phase relationship of ground motion at different support points.
Considering the traveling wave effect generally increases vertical displacements and axial forces in the arch rib, while reducing lateral displacements.
The difference between uniform ground motion assumption and traveling wave assumption can be substantial for spans exceeding 200 m.

From a structural engineering perspective, this finding has profound implications for the seismic design of long-span bridges. Traditional design approaches that assume uniform ground motion across the entire bridge length may be non-conservative for vertical response and axial force demand.

Implications for Steel Tube Concrete Arch Rib Construction

The arch rib in an STC arch bridge is typically fabricated from large-diameter steel tubes (commonly 800-1500 mm outer diameter) filled with concrete. Key manufacturing and welding considerations include:

Steel tube specifications: For a 360 m span, the arch rib steel tubes typically comply with GB/T 8163 or API 5L, with steel grades ranging from Q235 to Q345. The wall thickness is typically 16-30 mm to accommodate both structural and concrete confinement requirements.
Welding of arch rib segments: Field welding of large-diameter steel tubes requires careful procedure qualification. SAW (Submerged Arc Welding) is commonly used for full-penetration butt welds in such large-diameter tubes. The welding procedure must be qualified according to NB/T 47014 or ISO 15614.
HAZ toughness: In seismic regions, the HAZ toughness of the arch rib welds is critical. Charpy V-notch (CVN) testing at the expected minimum service temperature must demonstrate adequate absorbed energy (typically ≥47 J at -20°C for Q345 steel in seismic zones).
Residual stress and distortion: The longitudinal welding of large-diameter tubes creates significant longitudinal residual stresses. For seismic applications, these residual stresses contribute to the overall stress state and may reduce the effective buckling capacity of the arch rib under combined axial and bending loads.

Seismic Design Code Compliance

The study's findings align with and extend the provisions of current Chinese seismic design codes for bridges (JTG B02-2013). The traveling wave effect is particularly relevant to the specifications in GB 50011 (Seismic Design Code for Buildings) and its bridge-specific adaptations. The results suggest that for spans exceeding 300 m, explicit consideration of spatial variation of ground motion should be incorporated into seismic analysis.

Engineering Practice and Quality Control

For the fabrication of STC arch ribs in seismic zones, the following quality assurance measures are recommended:

Material certification: Mill test certificates (MTC) must verify chemical composition, mechanical properties (yield strength, tensile strength, elongation), and impact toughness at the specified temperature.
Weld procedure qualification: Each welding process (SMAW, SAW, GTAW) must be qualified with full-size weld coupons tested for tensile strength, bend testing, and impact toughness.
Non-destructive testing: UT for internal weld defects, MT for surface defects, and PT (Penetrant Testing) for supplementary surface inspection of critical welds.
Post-weld treatment: For thick-section welds (>25 mm), PWHT is recommended to reduce residual stresses and improve HAZ ductility.

Key Reflections

This study provides valuable insight into the seismic behavior of long-span STC arch bridges, with particular emphasis on the traveling wave effect. The finding that mid-span sections experience relatively smaller seismic forces while arch foot and quarter-span sections are more critical has direct implications for optimization of material distribution along the arch rib. From a manufacturing perspective, this suggests that the arch foot regions may require thicker steel tubes or additional reinforcement, while mid-span sections could potentially use lighter sections, offering cost and weight optimization opportunities.

The traveling wave effect analysis also highlights the importance of accurate seismic input modeling. Engineers should not rely solely on uniform ground motion assumptions for bridges with spans exceeding 200 m, as this may lead to non-conservative design in terms of vertical response and axial force demand.