Pseudo-Dynamic Testing of Partially Filled Circular Steel Tube Concrete Bridge Pier Substructure

Literature Overview

Published in the Journal of Shenyang Jianzhu University (Natural Science Edition) in 2012 (Vol. 28, Issue 3, pp. 437-442), this paper by Wang Zhanfei and colleagues from Shenyang Jianzhu University presents pseudo-dynamic substructure testing of partially filled circular steel tube concrete (CFST) bridge piers under seismic loading. Supported by the National Natural Science Foundation of China (51178279) and additional provincial and ministry funding, the research aims to understand the nonlinear dynamic behavior of partially filled CFST piers within a complete bridge system and to propose rational seismic design approaches.

Core Technical Content

Partially Filled CFST Concept

The partially filled circular steel tube concrete (P-CFST) system is a hybrid structural approach where only a portion of the circular steel tube cross-section is filled with concrete, typically the lower half or a sector. This design offers several advantages over fully filled CFST:

Reduced dead weight, which is critical for seismic response since inertial forces are proportional to mass.
Improved constructability by simplifying concrete placement within the circular tube.
Retained composite action between steel tube and concrete in the filled region, providing enhanced ductility and energy dissipation.
The unfilled region acts as a flexible zone that can accommodate large inelastic deformations.

Substructure Pseudo-Dynamic Testing Methodology

The pseudo-dynamic testing method, as standardized in GB/T 21145 and ASTM E793, bridges the gap between quasi-static cyclic testing and full-scale shaking table testing. In this approach, the test specimen is subjected to incremental loading steps where the displacement history from numerical analysis of the complete structure is used to drive the physical substructure. The reaction forces from the physical specimen are fed back into the numerical model to update the displacement history for the next loading step.

The experimental configuration involved:

A physical substructure representing the critical pier column portion.
A numerical model representing the remaining bridge components (deck, bearings, adjacent piers).
Two seismic input scenarios: E1 (El Centro wave) and E2 (JRT-NS Kobe earthquake wave).

Key Results

Seismic Scenario	Input Ground Motion	Pier Response State	Damage Level	Hysteresis Characteristics
E1	El Centro wave	Elastic	No visible damage	Linear, well-defined loops
E2	JRT-NS Kobe wave	Elastic-plastic	No significant damage	Stable, full hysteresis loops

The experimental results demonstrate that the partially filled CFST pier maintains structural integrity under both moderate (E1) and severe (E2) seismic excitations. The force-displacement hysteresis curves exhibit stable, full loops without pinching, indicating good energy dissipation capacity and the absence of premature buckling or concrete spalling failure.

Engineering Practice Implications

Seismic Design of CFST Bridge Piers

The findings from this study have direct relevance to the seismic design of highway bridge piers in earthquake-prone regions. Chinese highway bridge design standards (JTG/T 2231-01-2008 and its subsequent revisions) require that bridge piers withstand design-level earthquakes without collapse, with acceptable damage levels.

Key design parameters for partially filled CFST piers should include:

Steel tube: Q345 or Q390 grade per GB/T 1591, with wall thickness typically 8-16 mm for bridge pier applications.
Concrete: C40-C60 strength grade, with careful attention to pumpability through the partially open section.
Filling ratio: 30-50% of the cross-sectional area, with the filled region positioned in the compression zone under expected loading.
Connection details: Moment-resisting connections at the pier-deck interface, designed per the "strong column-weak beam" or "strong joint-weak member" philosophy.

Comparison with Fully Filled CFST

Parameter	Fully Filled CFST	Partially Filled CFST
Dead weight	Higher	20-40% lower
Seismic inertial force	Higher	Reduced proportionally
Axial capacity	Higher	30-50% lower
Flexural ductility	Good	Comparable or slightly better
Constructability	Difficult (closed form)	Easier (open section)
Cost	Higher	Lower
Energy dissipation	Good	Good with proper detailing

The partially filled approach represents an optimal balance between seismic performance and economic efficiency, particularly for medium-height bridge piers (6-15 m) in moderate to high seismic zones.

Study Insights and Reflections

The substructure pseudo-dynamic testing approach demonstrated in this paper is particularly valuable because it captures the interaction between the pier and the rest of the bridge system, which quasi-static testing cannot replicate. The deck mass, bearing stiffness, and adjacent pier flexibility all influence the pier's seismic response, and the pseudo-dynamic method accounts for these system effects.

A critical observation from the results is that the partially filled CFST pier exhibits elastic behavior under E1 loading and elastic-plastic behavior under E2 loading without significant damage. This suggests that the design margin is adequate, and the partially filled configuration does not compromise seismic safety relative to fully filled alternatives. The stable hysteresis loops indicate that the steel tube effectively confines the concrete in the filled region, preventing premature concrete crushing and maintaining load-carrying capacity through multiple loading cycles.

For engineering practice, this study supports the adoption of partially filled CFST piers as a viable seismic-resistant structural system. However, further research is needed to address the long-term durability of the partially filled configuration, particularly regarding corrosion protection of the exposed steel tube interior in the unfilled region, and the performance of the system under repeated seismic events (multi-hazard scenarios).