In a recent article published in the journal Buildings, Italian researchers presented a case study of a concrete building severely degraded by corrosion, requiring seismic retrofitting. They assessed the structural performance of the building both before and after intervention.
Study: Seismic Retrofitting of Corroded Concrete Buildings: A Case Study. Image Credit: Francesco Scatena/Shutterstock.com
Additionally, they suggested employing retrofit intervention techniques to improve the strength, stiffness, and ductility of the existing elements.
The findings of the research demonstrated how a comprehensive strategy involving mass and stiffness redistribution could achieve seismic adaptation for a corroded concrete structure through cost-effective interventions.
Background
Concrete structures are widely used in civil engineering for their durability, versatility, and affordability. However, they are vulnerable to degradation from environmental factors such as carbonation, chloride penetration, and moisture, leading to steel reinforcement corrosion. This corrosion reduces the cross-sectional area and bond strength of the steel, resulting in cracking, spalling, and loss of strength and stiffness in the concrete elements. Moreover, corrosion compromises the seismic performance of concrete structures by reducing ductility and energy dissipation capacity, increasing the risk of brittle failure under cyclic loading.
Seismic retrofitting of corroded concrete structures is challenging, requiring restoration of original capacity and enhancing seismic resistance in existing elements. Various techniques, including concrete jacketing, steel jacketing, fiber-reinforced polymer (FRP) wrapping, and base isolation, have been developed and applied for this purpose.
However, selecting the most suitable technique depends on factors such as degradation level, structural configuration, seismic demand, functional requirements, and economic feasibility.
Thus, each case of a corroded concrete structure requires a detailed assessment of the current conditions and potential solutions, carefully weighing the pros and cons of each method and their interactions.
About the Research
In this paper, the authors conducted a case study of a concrete building located in Sicily, southern Italy, that was built in the early 1960s as a three-dimensional (3D) frame supporting a belvedere square at the top, without intermediate floors. Decades of exposure to atmospheric agents led to significant corrosion and degradation of the external elements. The owner requested seismic retrofitting and functional transformation, including the addition of three new intermediate floors and structural layout changes.
Structural assessment was performed using both linear dynamic and nonlinear static analyses to evaluate the seismic performance of both the original and retrofitted structures. The analyses were conducted using finite element software, considering the geometry, the material properties, and the level of corrosion of the existing elements.
Moreover, the seismic load was defined according to the Italian codes, using elastic and design spectra for different limit states and behavior factors. The structural deficiencies and the failure mechanisms were also identified, and the ultimate displacement and the base shear were calculated.
Based on the assessment results, the study proposed a global strategy of seismic retrofitting, which included the following interventions:
- Constructing new steel-concrete composite floors to increase the mass and stiffness of the structure while reducing the eccentricity and torsional effects of the seismic load.
- Implementing concrete and steel jacketing for selected columns to enhance their cross-sectional area and confinement, thereby redistributing the base shear among the columns.
- Applying FRP plates and fabrics to certain beams to enhance their bending and shear strength as well as to prevent debonding of the concrete cover.
- Installing new steel elements, such as braces and ties, to enhance the stability and connection of the structure.
Furthermore, the researchers conducted nonlinear static analyses using the retrofitted model to verify the effectiveness of the intervention. The analyses demonstrated improved structural performance, evidenced by increased ultimate displacement and base shear as well as reduced stress and deformation of the elements.
Research Findings
The outcomes highlighted the following key points:
- Original structure analysis: The original structure demonstrated low seismic performance and high vulnerability due to several factors, including the absence of seismic design, irregular configuration, heavy mass at the top, lack of intermediate floors, and severe corrosion of elements. Linear dynamic analysis identified deficiencies in bending and shear capacity, particularly in outermost columns and beams heavily degraded by corrosion. Similarly, nonlinear static analysis revealed low displacement capacity and a disadvantageous collapse mechanism, primarily involving failure of the most stressed column at the northwest angle.
- Retrofit intervention impact: The retrofit intervention led to significant improvements in seismic performance and functionality by reducing mass and eccentricity, increasing stiffness and strength, and enhancing ductility and energy dissipation. It successfully restored degraded elements and adapted them to new functional requirements.
- Structural analysis post-retrofit: Following the retrofit, linear dynamic analysis demonstrated improved safety checks for both static and seismic actions, with reduced deficiencies and increased safety factors. Nonlinear static analysis revealed enhanced displacement capacity and a shift in the collapse mechanism towards more distributed and ductile beam failure.
- Effectiveness of retrofit strategies: The global strategy of mass and stiffness redistribution, coupled with local strengthening, proved effective and efficient. Various techniques such as concrete and steel jacketing, FRP wrapping, and composite floors provided adequate confinement of beam-column joints and increased axial and bending capacity of columns, with minimal impact on beam thickness and demolition.
Conclusion
The paper summarized that the seismic retrofitting and functional transformation of the concrete building in Sicily were successfully achieved by applying a global strategy of mass and stiffness redistribution and by using different techniques of strengthening and passive seismic protection.
Moving forward, the researchers suggested several directions for future work. They advised validating the retrofitting techniques by testing retrofitted elements under cyclic loading and comparing results with numerical analyses.
Additionally, they recommended optimizing intervention designs by balancing cost and performance and applying sustainability principles. They also proposed extending the study to analyze other cases of corroded concrete structures, considering degradation level, structural configuration, seismic demand, and functional requirements.
Journal Reference
Granata, M.F. Seismic Retrofit of Concrete Buildings Damaged by Corrosion: A Case Study in Southern Italy. Buildings 2024, 14, 1064. https://doi.org/10.3390/buildings14041064, https://www.mdpi.com/2075-5309/14/4/1064
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