■ Columns with Smart Plastic Hinges of Martensitic NiTi SMA bars 

The objective of this study was to examine the self-centering capacity of reinforced concrete (RC) columns with martensitic shape memory alloy (SMA) bars in the plastic hinge region. To achieve this, three RC columns were prepared: one was conventional and the other two had SMA bars in the plastic hinge zone. The columns had a diameter of 400 mm, height of 1400 mm, and aspect ratio of 3.5. The 400 mm long SMA bars were connected to the steel rebars using specially devised couplers. A cyclic lateral load was applied at the top of the columns, and the lateral displacements along the height of the columns and the corresponding forces were measured during cyclic lateral loading. The strain in the SMA bars was also measured. The RC column with SMA bars exhibited a plastic hinge around the couplers, thus concrete damage from cracking was concentrated at this site. This was completely different from the failure mode of the conventional RC column. The SMA bars in the column were in an elastic state, whereas the steel rebars yielded inside the couplers. The RC column with SMA bars exhibited excellent self-centering capacity, and its displacement recovery ratio was approximately 95% at a drift of 5%. However, its energy-dissipation capacity was lower than that of the conventional RC column. The equivalent damping ratio of the RC column with SMA bars was approximately 3% after 1.75% drift. It is conjectured that the excellent self-centering of the RC column with SMA bars was caused by the elastic behavior of the martensitic SMA bars.

Selected Publications

• Bui Thanh Nhan, Eunsoo Choi*, and Jong-Su Jeon* (2024). Investigation of flexural behavior of crimped SMA fiber-reinforced mortar beams through experimental study and mesoscale finite element modeling. Construction and Building Materials; 411: 134618, January 12. 

• Eunsoo Choi, Jong-Su Jeon*, and Jong-Han Lee (2023). Self-centering capacity of RC columns with smart plastic hinges of martensitic NiTi SMA bars. Smart Materials and Structures; 32(11): 115015, November 1.

■ Risk-Based Seismic Design of Diagonal SMA Wire-Based Bracing System

In this reseach, a risk-based seismic design method for diagonal self-centering braces (SCB) with shape memory alloy (SMA) wires implemented in multi-column bents is proposed to enhance the performance of older concrete bridge classes. An existing SMA-SCB system that improves the performance of older reinforced concrete building frames was adopted to numerically examine further applications to nonductile bridges. To this end, risk-based seismic design of the SMA-SCB to minimize the probability of collapse and demolition of bridge classes was developed by adopting a sophisticated finite element model that reflects the recentering behavior of an actual SMA-SCB; probabilistic bridge models considering uncertainties associated with the material and geometrical properties of bridges; refined failure condition; and the total probability theorem. A comparison of the results of the traditional seismic fragility-based and proposed designs indicated that the amount of SMA-SCB required by the former is nearly half of that required by the proposed approach because the traditional approach overemphasizes the demand at larger ground motion intensities.

Selected Publications

• Chang Seok Lee and Jong-Su Jeon* (2023). Seismic risk-based optimization of tension-only shape-memory alloy device for steel moment-resisting frames. Engineering Structures; 296: 116976, December 1. 

• Chang Seok Lee and Jong-Su Jeon* (2023). Risk-based seismic design of diagonal self-centering shape-memory alloy wire-based bracing system in multi-column bent bridges. Engineering Structures; 289: 116295, August 15. 

■ OpenSees Numerical Model of Superelastic SMAs with Functional Degradation

This study presents a simple hysteretic model to reproduce the stress–strain relationship of superelastic NiTi SMAs. The proposed model explicitly includes the functional degradation of SMAs, which has been ignored in earthquake engineering applications. This effect causes a reduction in the transformation stress and accumulation of residual strain. Because SMA devices are mainly used for seismic retrofit and account for a small portion of the structural system, their numerical model should not increase the computational time needed to perform nonlinear dynamic analyses. Computational efficiency can be achieved by representing their stress–strain response in a phenomenological way. Additionally, practitioners who may not have a professional background in materials science can easily manipulate the proposed model for the appropriate reproduction of model parameters such as transformation stress and residual strain. The ability to properly reproduce the experimental stress–strain response is validated for the test results of 65 NiTi SMA specimens. The amount of forward and reverse transformation stress degradation and the amount of residual strain accumulation per cycle, which are observed in the experimental results, are captured with reasonable accuracy in the proposed model. 

Selected Publications

• Chang Seok Lee and Jong-Su Jeon* (2022). Phenomenological hysteretic model for superelastic NiTi shape memory alloys accounting for functional degradation. Earthquake Engineering & Structural Dynamics; 51(2): 277-309, February 1.

■ Superelastic SMA reinforcement in concrete columns

Earthquake-induced damage in a reinforced concrete (RC) column is often concentrated in a region where the largest moment and deformation are expected. The ength of this region is widely known as plastic hinge length. To contribute to minimizing the operational and repair costs of damaged plastic hinge region in concrete columns under earthquake loading, this study focuses on deriving an analytical expression for the plastic hinge length of rectangular concrete columns reinforced with nickel-titanium shape memory alloy (SMA) bars based on the results from well-calibrated nonlinear finite element models. A parametric study was conducted to investigate the effect of various parameters on the plastic hinge length, such as the axial load ratio, section information, and material properties of concrete and SMA. A multiple stepwise regression analysis was performed to derive an expression to estimate the plastic hinge length of SMA-RC columns, which can help designers determine the optimal length for SMA reinforcement. Two SMA-RC building frames with their plastic lengths determined using the proposed and existing plastic hinge length expressions were analyzed to compare their nonlinear response under earthquake loading. When the proposed plastic hinge expression is used, the SMA usage can be nearly halved, while the maximum and residual story drift remained similar to the result obtained using the existing expression.

Selected Publications

• Chang Seok Lee, Sujith Mangalathu, and Jong-Su Jeon* (2024). Machine-learning-assisted drift capacity prediction models for reinforced concrete columns with shape memory alloy bars. Computer-Aided Civil and Infrastructure Engineering; 39(4): 595–616, February 15. 

• Chang Seok Lee and Jong-Su Jeon* (2022). Drift limit state predictions of rectangular reinforced concrete columns with superelastic shape memory alloy rebars. Journal of Building Engineering; 54: 104545, August 15.

• Chang Seok Lee, Eunsoo Choi, and Jong-Su Jeon* (2022). Estimating the plastic hinge length of rectangular concrete columns reinforced with NiTi superelastic shape memory alloys. Engineering Structures; 252: 113641, February 1.

■ Experimental and Analytical Studies of Shape Memory Alloy Rings

Shape memory alloys (SMAs) are a unique metallic alloy that has the ability to undergo large displacements while returning to their original shape. Bar and wire form have been widely studied in the mechanical and aerospace fields, however, little has been done on ring-shaped alloys, particularly at large scale. The research team consisting of structural engineers and a materials scientist developed a ring-shaped SMA bracing system since the ring-shape SMA has larger dimensions and higher capacities, and is easier to fabricate than existing SMA components. The feasibility for this application was examined by experimentally evaluating the performance of the new bracing system. The research team used the experimental results to determine the constitutive models in ABAQUS. Additionally, the research team is developing the simplified material model of SMA rings, which can be easily implemented into frame models for efficient design, analysis, and risk assessment of retrofitted steel and concrete frames.

Selected Publications

• Eunsoo Choi*, Hoan D Nguyen, Jong-Su Jeon and Joo-Won Kang (2019). Self-centering and damping devices using SMA dual rings. Smart Materials and Structures; 28(8): 085005, August 1.

• Eunsoo Choi, Jong-Su Jeon*, Woo Jin Kim, and Joo-Won Kang (2018). Investigation of MRS and SMA dampers effects on bridge seismic resistance employing analytical models. International Journal of Steel Structures; 18(4): 1325–1335, November 30. 

• Eunsoo Choi, Jong-Su Jeon, and Junwon Seo* (2017). Cyclic compressive behavior of polyurethane rubber springs for smart dampers. Smart Structures and Systems; 20(6): 739–757, December 1.

• Eunsoo Choi, Heejung Youn, Kyoungsoo Park, and Jong-Su Jeon* (2017). Vibration tests of precompressed rubber springs and a flag-shaped smart damper. Engineering Structures; 132: 372–382, February 1. 

• Nan Gao, Jong-Su Jeon*, Darel E Hodgson, and Reginald DesRoches (2016). An innovative seismic bracing system based on a superelastic shape memory alloy ring. Smart Materials and Structures; 25(5): 055030, May 1.