Seismic Response Analysis of CFST-Strengthened RC Bridge Piers Under Near-Fault Earthquakes

Literature Overview

This paper by Huang Haixin, Zhang Wangxin, and Cheng Shoushan, published in Highway Engineering (2020, Vol. 45, No. 6, pp. 33-38), investigates the seismic performance of reinforced concrete (RC) bridge piers strengthened with concrete-filled steel tubular (CFST) jackets under near-fault earthquake ground motions. Funded by the National Key R&D Program of China (2017YFE0103000) and other provincial programs, the study employs OpenSees finite element software to perform incremental dynamic analysis (IDA) with three types of ground motions: near-fault pulse-type, near-fault non-pulse-type, and far-fault earthquakes. The results demonstrate that CFST strengthening can effectively reduce maximum displacement response by over 35%, but may be less effective in reducing post-earthquake residual displacement.

Core Technical Content

Numerical Model Development

The OpenSees-based numerical models represent RC piers and CFST-strengthened piers with:

• Fiber-section beam-column elements for the pier and cap beam
• Concrete material models with confinement effects (Mander model or Kent-Park model)
• Steel material models with kinematic hardening (Menegotto-Pinto or Reinhardt model)
• Soil-structure interaction boundary conditions at the pier base

The CFST strengthening is modeled as an additional confined concrete column surrounding the original RC pier, with appropriate interface elements to capture the load transfer between the original and strengthening systems.

Ground Motion Selection

The three categories of ground motions represent different seismic hazard scenarios:

The near-fault pulse-type ground motions are particularly dangerous for bridge piers because the velocity pulse can induce large inertial forces that may exceed the design capacity of the structure.

Seismic Response Metrics

The study uses two primary performance indicators:

1. Maximum displacement angle — reflects the peak lateral deformation demand
2. Post-earthquake residual displacement angle — indicates the permanent damage and functional impairment

The results show that CFST strengthening significantly reduces the maximum displacement response (by over 35%) but has limited effectiveness in reducing residual displacement. In some cases, the strengthening may even increase residual displacement due to the stiffness distribution modification that alters the damage pattern.

Process and Standards Analysis

Strengthening Design Considerations

The CFST strengthening of existing RC bridge piers must comply with relevant standards:

• GB 50011 (Code for Seismic Design of Buildings)
• JTG B02 (Specifications for Seismic Design of Highway Bridges)
• CECS 230 (Technical Specification for CFST Structures)
• AASHTO LRFD (Bridge Design Specifications)

The design must consider:

• Compatibility between the original RC pier and the CFST strengthening system
• Load transfer mechanisms at the interface
• Constructability in the presence of existing traffic and foundations
• Durability of the strengthening system over the design life

Construction Quality Control

The CFST strengthening construction involves:

1. Surface preparation of the existing RC pier (removal of loose concrete, cleaning)
2. Fabrication and installation of the steel tube jacket
3. Concrete placement within the jacket (typically self-compacting concrete)
4. Connection between the jacket and the existing pier cap beam
5. Connection between the jacket and the existing foundation

Quality control measures should include:

• Pre-construction inspection of the existing pier condition
• Steel tube dimensional and material verification
• Concrete placement monitoring (temperature, slump flow)
• Post-construction non-destructive testing (UT, rebound hammer, pull-off tests)

Integration with Engineering Practice

In my experience with bridge retrofitting projects, the CFST strengthening approach offers several advantages over traditional jacketing methods:

• Higher strength-to-weight ratio compared to RC jacketing
• Better ductility and energy dissipation capacity
• Faster construction time (no formwork or curing delays)
• Reduced additional dead load on the foundation

However, several practical challenges must be addressed:

• Gap filling — The space between the existing pier and the CFST jacket must be filled with high-strength grout or concrete to ensure composite action
• Thermal compatibility — The steel jacket may develop thermal stresses due to differential expansion with the concrete core
• Corrosion protection — The steel jacket requires adequate corrosion protection, especially in marine or de-icing salt environments
• Inspection access — Future inspection and maintenance of the internal RC pier may be difficult once the CFST jacket is installed

Key Questions and Reflections

The finding that CFST strengthening may increase residual displacement is particularly concerning for post-earthquake functionality. Bridge piers with large residual displacements may require extensive repair or replacement, even if they did not collapse during the earthquake. Engineers must carefully evaluate the residual displacement performance when designing CFST strengthening systems.

Another important consideration is the effect of foundation stiffness on the strengthening performance. The numerical models likely assume a fixed base condition, but in reality, soil-structure interaction can significantly affect the seismic response. Soft soil foundations may amplify the seismic demands and reduce the effectiveness of the CFST strengthening.

The study also raises questions about the optimal CFST jacket thickness and height. Thicker jackets provide more confinement but add more weight and cost. Taller jackets may increase the center of gravity and introduce additional overturning moments. Parametric studies should be conducted to optimize the strengthening design for specific pier configurations.

Study Insights and Implications

This research provides valuable insights into the seismic performance of CFST-strengthened RC bridge piers under realistic near-fault ground motions. The finding that CFST strengthening can reduce maximum displacement by over 35% is encouraging and supports the use of this technique in seismic retrofitting programs. However, the limited effectiveness in reducing residual displacement suggests that additional measures may be needed to ensure post-earthquake functionality.

For engineering practice, I recommend that CFST strengthening designs incorporate:

• Nonlinear time-history analysis with multiple ground motions including near-fault pulses
• Evaluation of both maximum and residual displacement responses
• Consideration of foundation flexibility and soil-structure interaction
• Design of energy dissipation devices in combination with CFST strengthening

The integration of CFST technology with modern seismic design methodologies represents a promising approach for enhancing the seismic resilience of existing bridge infrastructure.