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Seismic Resistant Bridge Columns with NiTi Shape Memory Alloy and Ultra-High-Performance Concrete

Seismic Resistant Bridge Columns with NiTi Shape Memory Alloy and Ultra-High-Performance Concrete infrastructures Article Seismic Resistant Bridge Columns with NiTi Shape Memory Alloy and Ultra-High-Performance Concrete Hadi Aryan Viterbi School of Engineering, University of Southern California, Los Angeles, CA 90089, USA; haryan@usc.edu Received: 27 October 2020; Accepted: 19 November 2020; Published: 30 November 2020 Abstract: Reinforced concrete bridge columns often endure significant damages during earthquakes due to the inherent deficiencies of conventional materials. Superior properties of the new materials such as shape memory alloy (SMA) and ultra-high-performance concrete (UHPC), compared to the reinforcing steel and the normal concrete, respectively, are needed to build a new generation of seismic resistant columns. Application of SMA or UHPC in columns has been separately studied, but this paper aims to combine the superelastic behavior of NiTi SMA and the high strength of UHPC, in order to produce a column design with minimum permanent deformation and high load tolerance subjected to strong ground motions. Additionally, the excellent corrosion resistance of NiTi SMA and the dense and impermeable microstructure of UHPC ensure the long-term durability of the proposed earthquake resistant column design. The seismic performance of four columns, defined as steel reinforced concrete (S-C), SMA reinforced concrete (SMA-C), SMA reinforced UHPC (SMA-UHPC), and reduced SMA reinforced UHPC (R-SMA-UHPC) is analyzed through a loading protocol with up to 4% drift cycles. The use of NiTi SMA bars for the SMA reinforced columns is limited to the plastic hinge region where permanent deformations happen. All the columns have 2.0% reinforcement ratio, except the R-SMA-UHPC column that has a 1.33% reinforcement ratio to optimize the use of SMA bars. Unlike the S-C column that showed up to 68% residual deformation compared to peak displacement during the last loading cycle the SMA reinforced columns did not experience permanent deformation. The SMA-C and R-SMA-UHPC columns showed similar strengths to the S-C column, but with about 5.0- and 6.5-times larger ductility, respectively. The SMA-UHPC column showed 30% higher strength and 7.5 times larger ductility compared to the S-C column. Keywords: NiTi shape memory alloy; ultra-high-performance concrete; bridge column; earthquake 1. Introduction Besides durability issues involved with normal concrete and reinforcing steel these conventional materials provide insucient seismic capacity for the bridge columns. Development of new materials, such as shape memory alloy (SMA) and ultra-high-performance concrete (UHPC), with excellent durability and mechanical properties provides the opportunity to improve the performance of bridge columns against strong earthquakes. Accordingly, SMA and UHPC have been evaluated and implemented through various designs for improving the seismic performance of structures [1–3]. Di erent types of SMA such as NiTi SMA [4], Cu-based SMA [5], and Fe-based SMA [6] have been investigated for their mechanical and durability properties. One of the unique properties of SMAs, especially NiTi and Cu-based types, is the superelastic behavior that allows the material to retain its original shape after unloading, and dissipate energy through cycles of flag-shaped hysteretic loops [7,8]. SMAs have been used through di erent techniques to mitigate the earthquake e ects on structures. For example, SMA dampers are utilized in bridges [9] and buildings [10] to improve the damping and frequency response of these structure; SMA braces are proposed to retrofit bridges [11,12] and buildings [13] against seismic excitations; and SMA restrainers are used in bridges to control the relative Infrastructures 2020, 5, 105; doi:10.3390/infrastructures5120105 www.mdpi.com/journal/infrastructures Infrastructures 2020, 5, 105 2 of 11 movements of superstructure and the response of piers [14–17]. Moreover, base isolation systems are equipped with the SMAs to mitigate the seismic actions on bridges and buildings [5,18]. Applications of SMAs also include reinforced concrete elements [19], steel beam-column connections [20], and even marine structures [21–23]. UHPC is distinguished among cement-based materials including normal concrete due to its excellent compressive strength and dense microstructure. While normal structural concrete has a 28 days compressive strength of about 35 MPa the compressive strength of UHPC is at least 145 MPa at this age [24]. UHPC has been used in construction of several bridges worldwide such as the Mars Hill bridge in the U.S., the Cat Point Creek bridge in the U.S., the Jakway Park bridge in the U.S., the Sherbrooke overpass in Canada, the Peace bridge in South Korea, the Wild bridge in Austria, the GSE bridge in Japan, the Kampung Linsum bridge in Malaysia, the Celakovice Pedestrian bridge in Czech Republic, the Luan Bai Dried-Canal Railway bridge in China, and the Yuan Jiahe bridge in China [25]. Outstanding workability and long-term durability of UHPC also make it an ideal material for prefabrication of bridge elements and accelerated construction industry [26]. The unique compressive strength of UHPC along with its proper integrity under tensile loads prevent column failure subjected to major earthquakes with vertical acceleration component, during which significant axial load variations and large moment demands a ect the column [27]. Additionally, the dense microstructure of UHPC [28] makes it impermeable to moisture and adverse chemicals, and prevents aging reactions that often a ect normal concrete [29]. Several studies have implemented NiTi SMA or UHPC to advance the seismic design of columns. Varela and Saiidi [30] evaluated a plastic hinge rubber element with NiTi SMA bars in a quarter-scale column subjected to strong earthquake motions on a shake table. The use of this new concept limited the column residual deformation to less than 0.5% after experiencing up to 7.0% drifts. Billah and Alam [31] performed an analytical study to address the lack of corrosion resistance and significant permanent deformation of regular steel reinforcements in reinforced concrete columns. They presented three concrete columns in which NiTi SMA or stainless steel bars were used in the plastic hinge region. Two columns with NiTi SMA bars in the plastic hinge region had either stainless steel or fiber reinforced polymer bars above the hinge region and one column with stainless steel bars in the plastic hinge region had fiber reinforced polymer bars above the hinge region. The bars of these hybrid reinforced concrete columns were connected with couplers above the plastic hinge region. Accordingly, they ensured the entire height of columns was reinforced with corrosion resistant reinforcing materials. They analyzed the columns under seismic loading and found the residual deformation of the columns with NiTi SMA bars to be 87% less compared to the column with stainless steel bars in the plastic hinge region. Mohebbi et al. [32] presented a posttensioned precast bridge column with plastic hinge region made of UHPC material, which was connected to the foundation with a pocket connection. They used unbonded carbon fiber reinforced polymer posttensioning tendons inside the column to minimize the permanent drifts and tested this technique on shake table through the Northridge-Rinaldi earthquake record. Results showed that the posttensioning approach for the column with UHPC in the plastic hinge region was e ective in eliminating permanent deformations and increasing displacement ductility to 13.8 at maximum drift ratio of 6.9%. Farzad et al. [33] proposed a retrofitting technique for reinforced concrete columns using UHPC. They built eleven quarter-scale columns and intentionally damaged them with spalling concrete to implement their strengthening technique. They sandblasted the damaged part of columns and repaired seven columns with UHPC containing 2% and 4% steel fibers, one column with normal concrete, and left the rest unrepaired to be considered as reference. According to the column tests under constant axial load and cyclic lateral load repairing the damaged columns with UHPC shell increased the strength of columns without changing their size. Moreover, di erent fiber contents of UHPC resulted in similar strength gain in the columns. Unlike previous studies on the application of NiTi SMA or UHPC in bridge columns, this paper presents a column design with a combination of NiTi SMA reinforcements and UHPC at the same time to take advantage of both materials’ excellent properties against earthquake. The reduced ratio Infrastructures 2020, 5, 105 3 of 11 Infrastructures 2020, 5, x FOR PEER REVIEW 3 of 11 time to take advantage of both materials’ excellent properties against earthquake. The reduced ratio of SMA reinforcement is also evaluated in the column design with UHPC and SMA. Results of the of SMA reinforcement is also evaluated in the column design with UHPC and SMA. Results of the two UHPC columns, with full and reduced ratios of NiTi SMA reinforcement, are compared with two two UHPC columns, with full and reduced ratios of NiTi SMA reinforcement, are compared with concrete columns with SMA and steel reinforcements. In this study, the NiTi type of SMA is used, since two concrete columns with SMA and steel reinforcements. In this study, the NiTi type of SMA is it benefits from larger and more stable flag shaped hysteretic loops of superelastic behavior compared used, since it benefits from larger and more stable flag shaped hysteretic loops of superelastic to the other SMA types, in order to minimize residual deformation and raise energy dissipation [34]. behavior compared to the other SMA types, in order to minimize residual deformation and raise The NiTi SMA has a high corrosion resistance as well [35]. energy dissipation [34]. The NiTi SMA has a high corrosion resistance as well [35]. 2. Finite Element Models 2. Finite Element Models Four column sections are considered in this study, as shown in Figure 1. The columns are modeled Four column sections are considered in this study, as shown in Figure 1. The columns are in OpenSees program [36]. All the columns are 3.6 m high and have a diameter of 0.8 m. The steel modeled in OpenSees program [36]. All the columns are 3.6 meters high and have a diameter of 0.8 reinforced concrete (S-C), SMA reinforced concrete (SMA-C), and SMA reinforced UHPC (SMA-UHPC) meter. The steel reinforced concrete (S-C), SMA reinforced concrete (SMA-C), and SMA reinforced column sections are reinforced with 2.0% of steel or NiTi SMA bars, but the reduced SMA reinforced UHPC (SMA-UHPC) column sections are reinforced with 2.0% of steel or NiTi SMA bars, but the UHPC (R-SMA-UHPC) section has a reinforcement ratio of 1.33% to optimize the use of SMA bars. reduced SMA reinforced UHPC (R-SMA-UHPC) section has a reinforcement ratio of 1.33% to The SMA reinforcements are only provided at the bottom quarter of the column height, as the plastic optimize the use of SMA bars. The SMA reinforcements are only provided at the bottom quarter of hinge region and the top three quarters of the height is reinforced with steel bars. The SMA and steel the column height, as the plastic hinge region and the top three quarters of the height is reinforced reinforcements are connected with mechanical couplers above the plastic hinge region. Geometry and with steel bars. The SMA and steel reinforcements are connected with mechanical couplers above the material configurations of the columns are presented in Table 1. plastic hinge region. Geometry and material configurations of the columns are presented in Table 1. Figure 1. Column sections. Figure 1. Column sections. Table 1. Geometry and material configurations of columns. Table 1. Geometry and material configurations of columns. Height Diameter Aspect Reinforcement Height Reinforcement Column ID Material Section Height (m) (m) Ratio Ratio Range (m) Material Height Diameter Aspect Reinforcement Reinforcement Column ID Steel Material Section Range Plastic 0–0.8 Steel S-C 3.6 0.8 4.5 Concrete 2.0% (m) (m) Ratio Ratio Material Reinforced (m) Elastic 0.8–3.6 Steel Concrete SMA Steel PlasticPlastic 0 0–0.8 −0.8 Steel NiTi SMA SMA-C 3.6 0.8 4.5 Concrete 2.0% Reinforced Reinforced S-C 3.6 0.8 4.5 Concrete 2.0% Elastic 0.8–3.6 Steel Concrete Elastic 0.8−3.6 Steel SMA Concrete Plastic 0–0.8 NiTi SMA SMA-UHPC 3.6 0.8 4.5 UHPC 2.0% Reinforced SMA ElasticPlastic 0 0.8–3.6−0.8 NiTi Steel SMA UHPC SMA- Reduced SMA Reinforced 3.6 0.8 4.5 Concrete 2.0% Plastic 0–0.8 NiTi SMA R-SMA-UHPC 3.6 0.8 4.5 UHPC 1.33% Reinforced Elastic 0.8−3.6 Steel Concrete Elastic 0.8–3.6 Steel UHPC SMA Plastic 0−0.8 NiTi SMA SMA- Reinforced 3.6 0.8 4.5 UHPC 2.0% UHPC Elastic 0.8−3.6 Steel UHPC Infrastructures 2020, 5, x FOR PEER REVIEW 4 of 11 Reduced Plastic 0−0.8 NiTi SMA R- SMA SMA- 3.6 0.8 4.5 UHPC 1.33% Infrastructures 2020, 5, 105 4 of 11 Reinforced Elastic 0.8−3.6 Steel UHPC UHPC As shown in Figure 2, each column model consists of four equal-length elements along the height As shown in Figure 2, each column model consists of four equal-length elements along the height from which the bottom one is a distributed plasticity element, with a fiber section representing the from which the bottom one is a distributed plasticity element, with a fiber section representing the plastic hinge region, and the top three elements are elastic. Seven equally distanced integration points plastic hinge region, and the top three elements are elastic. Seven equally distanced integration points are used along the distributed plasticity element with fiber section, which are not shown in Figure 2 for are used along the distributed plasticity element with fiber section, which are not shown in Figure 2 simplicity. According to Figure 2, the column mass is concentrated at five points along the height from for simplicity. According to Figure 2, the column mass is concentrated at five points along the height which the three middle points hold the summation of mass from half of two elements below and above from which the three middle points hold the summation of mass from half of two elements below them, while the top and bottom points only hold the mass from half of their adjacent elements at the and above them, while the top and bottom points only hold the mass from half of their adjacent top and bottom, respectively. Therefore, the concentrated mass at the top and bottom points is half of elements at the top and bottom, respectively. Therefore, the concentrated mass at the top and bottom the concentrated mass at the middle points. points is half of the concentrated mass at the middle points. Figure 2. Column model in OpenSees. Figure 2. Column model in OpenSees. The Concrete02, ReinforcingSteel, and SelfCentering models are used for concrete, steel, and SMA The Concrete02, ReinforcingSteel, and SelfCentering models are used for concrete, steel, and materials, respectively. The Concrete02 model properly captures the post-peak behavior of concrete SMA materials, respectively. The Concrete02 model properly captures the post-peak behavior of during loading and unloading cycles and the ReinforcingSteel model accurately follows the linear concrete during loading and unloading cycles and the ReinforcingSteel model accurately follows the elastic, yield plateau, and strain hardening portions of reinforcing steel behavior in concrete in linear elastic, yield plateau, and strain hardening portions of reinforcing steel behavior in concrete in opposition to the common bilinear steel models. The SelfCentering model constructs the flag-shaped opposition to the common bilinear steel models. The SelfCentering model constructs the flag-shaped and energy dissipative SMA material hysteresis behavior in uniaxial direction under tension and and energy dissipative SMA material hysteresis behavior in uniaxial direction under tension and compression cycles [36]. The unconfined concrete has a compressive strength of 34.5 MPa, and peak compression cycles [36]. The unconfined concrete has a compressive strength of 34.5 MPa, and peak and ultimate strains of 0.0022 and 0.005, respectively. The Mander model [37] is used to obtain the and ultimate strains of 0.0022 and 0.005, respectively. The Mander model [37] is used to obtain the properties of the confined concrete. Accordingly, the confined concrete has a compressive strength properties of the confined concrete. Accordingly, the confined concrete has a compressive strength of 44.1 MPa, and peak and ultimate strains of 0.0054 and 0.0128, respectively. The properties of of 44.1 MPa, and peak and ultimate strains of 0.0054 and 0.0128, respectively. The properties of unconfined UHPC are obtained from [24], for the case of using 6 mm straight steel fibers with 2% unconfined UHPC are obtained from [24], for the case of using 6 mm straight steel fibers with 2% volumetric content in mixture. Accordingly, the unconfined UHPC has a compressive strength of volumetric content in mixture. Accordingly, the unconfined UHPC has a compressive strength of 145.8 MPa, and peak and ultimate strains of 0.0044 and 0.0146, respectively. The confined UHPC has a 145.8 MPa, and peak and ultimate strains of 0.0044 and 0.0146, respectively. The confined UHPC has compressive strength of 154.8 MPa, and peak and ultimate strains of 0.0137 and 0.0289, respectively. a com The tensile pressiv str e st engths rengtof h o concr f 154ete .8 MP and a,UHPC and pe ar ae k conservatively and ultimate st overlooked. rains of 0.01 The 37 and stress 0.02 strain 89, resp behavior ectively. The tensi of confined le strengths of concrete and concrete a UHPC ar nd UHPC a e plotted in re cons Figur er e vatively ove 3. The steel r rlooked. The st einforcement is ress str Grade ain be 60 with havior of confined concrete and UHPC are plotted in Figure 3. The steel reinforcement is Grade 60 with Infrastructures 2020, 5, 105 5 of 11 Infrastructures 2020, 5, x FOR PEER REVIEW 5 of 11 Infrastructures 2020, 5, x FOR PEER REVIEW 5 of 11 ultimate strain of 0.09, and yield and ultimate tensile strengths of 414 MPa and 621 MPa, respectively. ultimate strain of 0.09, and yield and ultimate tensile strengths of 414 MPa and 621 MPa, respectively. ultimate strain of 0.09, and yield and ultimate tensile strengths of 414 MPa and 621 MPa, respectively. The superelastic behavior of NiTi SMA is implemented based on [12,34], as shown in Figure 4. The superelastic behavior of NiTi SMA is implemented based on [12,34], as shown in Figure 4. The superelastic behavior of NiTi SMA is implemented based on [12,34], as shown in Figure 4. Confined Concrete 20 Confined Concrete Confined UHPC Confined UHPC 0 0.005 0.01 0.015 0.02 0.025 0.03 0 0.005 0.01 0.015 0.02 0.025 0.03 Strain Strain Figure 3. Stress-strain behavior of confined concrete and UHPC. Figure 3. Figure 3. Stress-strain behavior of co Stress-strain behavior of confined nfined concrete concrete and UHPC. and UHPC. NiTi SMA NiTi SMA 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 Strain Strain Figure 4. Superelastic behavior of NiTi SMA. Figure 4. Superelastic behavior of NiTi SMA. Figure 4. Superelastic behavior of NiTi SMA. An eigenvalue analysis is performed to obtain and compare the natural periods of the columns. An eigenvalue analysis is performed to obtain and compare the natural periods of the columns. An eigenvalue analysis is performed to obtain and compare the natural periods of the columns. Then, a loading protocol consisting of a constant axial load and lateral cycles of displacement control Then, a loading protocol consisting of a constant axial load and lateral cycles of displacement control Then, a loading protocol consisting of a constant axial load and lateral cycles of displacement control drifts is applied to the columns as shown in Figure 5. A constant vertical load of 1735 kN is applied to drifts is applied to the columns as shown in Figure 5. A constant vertical load of 1735 kN is applied drifts is applied to the columns as shown in Figure 5. A constant vertical load of 1735 kN is applied the columns as the service load, which is equivalent to 10% of the compression capacity of the S-C to the columns as the service load, which is equivalent to 10% of the compression capacity of the S-C to the columns as the service load, which is equivalent to 10% of the compression capacity of the S-C column. The columns are also subjected to eight displacement control lateral drift cycles at the top, column. The columns are also subjected to eight displacement control lateral drift cycles at the top, column. The columns are also subjected to eight displacement control lateral drift cycles at the top, including two cycles of 0.5%, two cycles of 1.0%, two cycles of 2.0%, and two cycles of 4.0% drifts, including two cycles of 0.5%, two cycles of 1.0%, two cycles of 2.0%, and two cycles of 4.0% drifts, including two cycles of 0.5%, two cycles of 1.0%, two cycles of 2.0%, and two cycles of 4.0% drifts, while their base is assumed as fixed support. Accordingly, the capacity of columns to respond a seismic while their base is assumed as fixed support. Accordingly, the capacity of columns to respond a while their base is assumed as fixed support. Accordingly, the capacity of columns to respond a action is analyzed and compared through this loading protocol. seismic action is analyzed and compared through this loading protocol. seismic action is analyzed and compared through this loading protocol. Stress (MPa) Stress (MPa) Stress (MPa) Stress (MPa) Infrastructures 2020, 5, 105 6 of 11 Infrastructures 2020, 5, x FOR PEER REVIEW 6 of 11 Figure 5. (a) Loading and boundary conditions; (b) lateral loading protocol. Figure 5. (a) Loading and boundary conditions; (b) lateral loading protocol. 3. Results of Analysis 3. Results of Analysis According to the eigenvalue analysis the natural periods of the S-C, SMA-C, SMA-UHPC, According to the eigenvalue analysis the natural periods of the S-C, SMA-C, SMA-UHPC, and and R-SMA-UHPC columns are 0.038, 0.040, 0.034, and 0.035 second, respectively. Based on Equation (1) R-SMA-UHPC columns are 0.038, 0.040, 0.034, and 0.035 second, respectively. Based on Equation (1) and assuming mass, m, as a constant the columns with lower natural period, T, have higher initial and assuming mass, m, as a constant the columns with lower natural period, T, have higher initial sti ness, K. Most importantly, the columns are analyzed subjected to the lateral cyclic loading protocol stiffness, K. Most importantly, the columns are analyzed subjected to the lateral cyclic loading and constant axial load, as explained in the previous section, to compare their seismic capacity. protocol and constant axial load, as explained in the previous section, to compare their seismic The base shear versus drift diagrams are shown in Figure 6 for all the columns. The essential seismic capacity. The base shear versus drift diagrams are shown in Figure 6 for all the columns. The essential parameters to compare between di erent columns are the strength, residual deformation, drift ductility, seismic parameters to compare between different columns are the strength, residual deformation, and energy dissipation. The obvious di erence between the behavior of the S-C column and the drift ductility, and energy dissipation. The obvious difference between the behavior of the S-C rest of columns with SMA reinforcement is the amount of residual deformation. During the 2.0% column and the rest of columns with SMA reinforcement is the amount of residual deformation. and 4.0% drift cycles the residual deformation of S-C column at zero load is equivalent to 0.9% and During the 2.0% and 4.0% drift cycles the residual deformation of S-C column at zero load is 2.7% drifts, respectively. This means that during 2.0% and 4.0% drift cycles, there is 45% and 68% equivalent to 0.9% and 2.7% drifts, respectively. This means that during 2.0% and 4.0% drift cycles, residual deformation, respectively, in the S-C column, after unloading to zero load compared to the there is 45% and 68% residual deformation, respectively, in the S-C column, after unloading to zero peak displacement. On the other hand, replacing the steel reinforcement with the SMA reinforcement load compared to the peak displacement. On the other hand, replacing the steel reinforcement with in the plastic hinge region leads to zero residual deformation in all the SMA reinforced columns during the SMA reinforcement in the plastic hinge region leads to zero residual deformation in all the SMA di erent drift cycles. This means that the S-C column cannot be serviceable after the earthquake, but the reinforced columns during different drift cycles. This means that the S-C column cannot be SMA reinforced columns retain their serviceability. The S-C column shows more energy dissipation serviceable after the earthquake, but the SMA reinforced columns retain their serviceability. The S-C through di erent loading cycles compared to the other columns but, since the energy is dissipated after column shows more energy dissipation through different loading cycles compared to the other large permanent deformations, it would not benefit the column. In other words, the larger inside area columns but, since the energy is dissipated after large permanent deformations, it would not benefit of base shear-drift cycles diagram of the S-C column is due to the significant residual deformations the column. In other words, the larger inside area of base shear-drift cycles diagram of the S-C column and damages after returning to zero load during di erent drift cycles. Between the SMA reinforced is due to the significant residual deformations and damages after returning to zero load during columns, the SMA-C column showed slightly more energy dissipation compared to the SMA-UHPC different drift cycles. Between the SMA reinforced columns, the SMA-C column showed slightly and R-SMA-UHPC columns, especially during the initial drift cycles up to 2%. more energy dissipation compared to the SMA-UHPC and R-SMA-UHPC columns, especially during the initial drift cycles up to 2%. T = 2 (1) (1) = 2 Infrastructures 2020, 5, 105 7 of 11 Infrastructures 2020, 5, x FOR PEER REVIEW 7 of 11 Infrastructures 2020, 5, x FOR PEER REVIEW 7 of 11 700 700 S-C SMA-C 700 700 600 600 S-C SMA-C 600 600 500 500 500 500 400 400 400 400 300 300 300 300 200 200 200 200 100 100 100 100 0 0 -100 -100 -100 -100 -200 -200 -200 -200 -300 -300 -300 -300 -400 -400 -400 -400 -500 -500 -500 -500 -600 -600 -600 -600 -700 -700 -4 -3 -2 -1 0 1 2 3 4 -4 -3 -2 -1 0 1 2 3 4 -700 -700 a) -4 -3 -2 -1 0 1 2 3 4 -4 -3 -2 -1 0 1 2 3 4 Drift (%) b) Drift (%) a) b) Drift (%) Drift (%) 700 700 SMA-UHPC R-SMA-UHPC 700 700 600 600 SMA-UHPC R-SMA-UHPC 600 600 500 500 500 500 400 400 300 300 300 300 200 200 200 200 100 100 0 0 0 0 -100 -100 -100 -100 -200 -200 -200 -200 -300 -300 -300 -300 -400 -400 -400 -400 -500 -500 -500 -500 -600 -600 -600 -600 -700 -700 -4 -3 -2 -1 0 1 2 3 4 -700 -4 -3 -2 -1 0 1 2 3 4 -700 c) -4 -3 -2 -1 0 1 2 3 4 d) -4 -3 -2 -1 0 1 2 3 4 Drift (%) Drift (%) c) d) Drift (%) Drift (%) Figure 6. Base shear versus drift diagrams for di erent columns: (a) S-C, (b) SMA-C, (c) SMA-UHPC, Figure 6. Base shear versus drift diagrams for different columns: (a) S-C, (b) SMA-C, (c) SMA-UHPC, Figure 6. Base shear versus drift diagrams for different columns: (a) S-C, (b) SMA-C, (c) SMA-UHPC, (d) R-SMA-UHPC. (d) R-SMA-UHPC. (d) R-SMA-UHPC. In order to compare the performance of di erent columns in terms of strength all the base shear In order to compare the performance of different columns in terms of strength all the base shear In order to compare the performance of different columns in terms of strength all the base shear versus drift diagrams are plotted in Figure 7 at the same time. The SMA-UHPC, R-SMA-UHPC, S-C, versus drift diagrams are plotted in Figure 7 at the same time. The SMA-UHPC, R-SMA-UHPC, S-C, versus drift diagrams are plotted in Figure 7 at the same time. The SMA-UHPC, R-SMA-UHPC, S-C, and SMA-C columns have shown the highest to the lowest strengths of 619 kN, 495 kN, 476 kN, and SMA-C columns have shown the highest to the lowest strengths of 619 kN, 495 kN, 476 kN, and and SMA-C columns have shown the highest to the lowest strengths of 619 kN, 495 kN, 476 kN, and and 441 kN, respectively. Therefore, the SMA-UHPC column design provides 30% higher strength 441 kN, respectively. Therefore, the SMA-UHPC column design provides 30% higher strength 441 kN, respectively. Therefore, the SMA-UHPC column design provides 30% higher strength compared to the S-C column, and removes the residual deformation. The R-SMA-UHPC column compared to the S-C column, and removes the residual deformation. The R-SMA-UHPC column compared to the S-C column, and removes the residual deformation. The R-SMA-UHPC column provides just about 4% higher strength compared to the S-C column. Accordingly, when UHPC is used provides just about 4% higher strength compared to the S-C column. Accordingly, when UHPC is provides just about 4% higher strength compared to the S-C column. Accordingly, when UHPC is along with the SMA reinforcement, reducing the reinforcement ratio by one third results in getting a used along with the SMA reinforcement, reducing the reinforcement ratio by one third results in used along with the SMA reinforcement, reducing the reinforcement ratio by one third results in similar strength to the S-C column, and still being able to remove the residual deformation. Moreover, getting a similar strength to the S-C column, and still being able to remove the residual deformation. getting a similar strength to the S-C column, and still being able to remove the residual deformation. results show that using SMA reinforcement in the plastic hinge region of the SMA-C column does Moreover, results show that using SMA reinforcement in the plastic hinge region of the SMA-C Moreover, results show that using SMA reinforcement in the plastic hinge region of the SMA-C not compromise the strength by more than 7% compared to the S-C column, while it prevents the column does not compromise the strength by more than 7% compared to the S-C column, while it column does not compromise the strength by more than 7% compared to the S-C column, while it permanent deformation. prevents the permanent deformation. prevents the permanent deformation. S-C S-C 500 SMA-C 500 SMA-C SMA-UHPC 400 SMA-UHPC R-SMA-UHPC R-SMA-UHPC -100 -100 -200 -200 -300 -300 -400 -400 -500 -500 -600 -600 -700 -700-4 -3 -2 -1 0 1 2 3 4 -4 -3 -2 -1 0 1 2 3 4 Drift (%) Drift (%) Figure 7. Base shear versus drift diagrams of the columns. Figure 7. Base shear versus drift diagrams of the columns. Figure 7. Base shear versus drift diagrams of the columns. Base Shear (kN) Base Shear (kN) Base Shear (kN) Base Shear (kN) Base Shear (kN) Base Shear (kN) Base Shear (kN) Base Shear (kN) Base Shear (kN) Base Shear (kN) Infrastructures 2020, 5, 105 8 of 11 It is important to note that the UHPC columns with full and reduced ratios of SMA reinforcement reach their maximum strength at about 4.0% drift, while the concrete columns with SMA and steel reinforcements reach their peak strength at about 3.0% and 1.2% drifts, respectively. The column drift ductility,  , is defined in Equation (2) as the drift at peak strength, D , divided by the drift D peak Infrastructures 2020, 5, x FOR PEER REVIEW 8 of 11 at yield point, D . Accordingly, the SMA-UHPC, R-SMA-UHPC, and SMA-C columns have a yeild It is important to note that the UHPC columns with full and reduced ratios of SMA ductility of about 22, 20, and 15, respectively, while the ductility of S-C column is 3. Therefore, the SMA reinforcement reach their maximum strength at about 4.0% drift, while the concrete columns with reinforced columns, with UHPC or concrete, provide much larger ductility than the S-C column. SMA and steel reinforcements reach their peak strength at about 3.0% and 1.2% drifts, respectively. The R-SMA-UHPC and SMA-C columns provide similar strength to the S-C column, but their ductility The column drift ductility, µD, is defined in Equation (2) as the drift at peak strength, Dpeak, divided is about seven and five times that of the S-C column, respectively. As summarized in Table 2, results by the drift at yield point, Dyeild. Accordingly, the SMA-UHPC, R-SMA-UHPC, and SMA-C columns show that the SMA-UHPC, R-SMA-UHPC, and SMA-C columns have superior seismic performance have a ductility of about 22, 20, and 15, respectively, while the ductility of S-C column is 3. Therefore, compared to the S-C column in terms of ductility and residual deformation. The SMA-UHPC column the SMA reinforced columns, with UHPC or concrete, provide much larger ductility than the S-C shows the best seismic performance among all the columns, given its highest strength and ductility. column. The R-SMA-UHPC and SMA-C columns provide similar strength to the S-C column, but Due to the high corrosion resistant of NiTi SMA bars used in the plastic hinge region, and the dense their ductility is about seven and five times that of the S-C column, respectively. As summarized in Taand ble 2impermeable , results show tha micrt the ostr S uctur MA- eUH of P UHPC C, R-SM over A-Uthe HPC entir , aned column SMA-C columns height, the have pr oposed superior SMA-UHPC seismic performance compared to the S-C column in terms of ductility and residual deformation. The column has excellent long-term durability in addition to its extraordinary seismic performance. SMA-UHPC column shows the best seismic performance among all the columns, given its highest strength and ductility. Due to the high corrosion resistant of D NiTi SMA bars used in the plastic hinge peak = (2) region, and the dense and impermeable microstructure of UHPC over the entire column height, the yeild proposed SMA-UHPC column has excellent long-term durability in addition to its extraordinary seismic performance. Table 2. Summary of the results. = (2) Column Strength (kN)  Residual Deformation S-C 476 3 68% Table 2. Summary of the results. SMA-C 441 15 0% Column Strength (kN) Residual Deformation SMA-UHPC 619 22 0% S-C 476 3 68% SMA-C 441 15 0% R-SMA-UHPC 495 20 0% SMA-UHPC 619 22 0% R-SMA-UHPC 495 20 0% As presented in Table 2, the S-C column su ered from 68% residual deformation after unloading to zero load during the last drift cycle. Figure 8 shows the distribution of corresponding residual As presented in Table 2, the S-C column suffered from 68% residual deformation after unloading to z curvatur ero loade, duri rotation, ng the last drif and deflection t cycle. Figure at the 8 sho plastic ws the d hinge istrirbution o egion of f cor S-C respondin columng r in esid the ual positive and curvature, rotation, and deflection at the plastic hinge region of S-C column in the positive and negative directions. negative directions. 0.9 0.9 0.9 0.75 0.75 0.75 0.6 0.6 0.6 0.45 0.45 0.45 0.3 0.3 0.3 0.15 0.15 0.15 0 0 -0.06 -0.04 -0.02 0 0.02 0.04 0.06 -0.045 -0.03 -0.015 0 0.015 0.03 0.045 -0.02 -0.01 0 0.01 0.02 a) b) c) Curvature (1/m) Rotation (rad) Deflection (m) Figure 8. Residual deformation at the plastic hinge region of S-C column: (a) curvature, (b) rotation, Figure 8. Residual deformation at the plastic hinge region of S-C column: (a) curvature, (b) rotation, (c) deflection. (c) deflection. Assuming linear behavior above the plastic hinge region, the residual deflection along the height Assuming linear behavior above the plastic hinge region, the residual deflection along the height of S-C column is shown in Figure 9, in comparison with its peak deflection. Elimination of this of S-C column is shown in Figure 9, in comparison with its peak deflection. Elimination of this significant residual deflection as accomplished in the other columns by using the NiTi SMA bars in significant residual deflection as accomplished in the other columns by using the NiTi SMA bars in the the plastic hinge region is crucial for immediate serviceability of the bridge after earthquake. plastic hinge region is crucial for immediate serviceability of the bridge after earthquake. Height (m) Height (m) Height (m) Infrastructures 2020, 5, 105 9 of 11 Infrastructures 2020, 5, x FOR PEER REVIEW 9 of 11 3.6 2.7 1.8 0.9 Peak Residual -0.15 -0.1 -0.05 0 0.05 0.1 0.15 Deflection (m) Figure 9. Residual versus peak deflections of the S-C column along the height. Figure 9. Residual versus peak deflections of the S-C column along the height. 4. Summary and Conclusions 4. Summary and Conclusions NiTi SMA bars are suitable alternatives for steel reinforcement of concrete elements in NiTi SMA bars are suitable alternatives for steel reinforcement of concrete elements in seismic seismic regions, due to their self-centering and energy dissipative properties. On the other hand, regions, due to their self-centering and energy dissipative properties. On the other hand, UHPC UHPC properties in terms of strength and integrity significantly outweigh the concrete properties. properties in terms of strength and integrity significantly outweigh the concrete properties. Among Among previous studies on the application of new materials in bridge columns, some have replaced previous studies on the application of new materials in bridge columns, some have replaced the steel the steel reinforcement with NiTi SMA bars and others utilized UHPC instead of normal concrete. reinforcement with NiTi SMA bars and others utilized UHPC instead of normal concrete. Replacing Replacing steel reinforcement with NiTi SMA bars resulted in minimum permanent deformation for steel reinforcement with NiTi SMA bars resulted in minimum permanent deformation for columns columns but with no strength advantage over conventional columns. On the other hand, replacing but with no strength advantage over conventional columns. On the other hand, replacing concrete concrete with UHPC only increased the column strength, and did not reduce the residual deformations with UHPC only increased the column strength, and did not reduce the residual deformations in the in the absence of a secondary measure. In order to take advantage of the excellent properties of both absence of a secondary measure. In order to take advantage of the excellent properties of both NiTi NiTi SMA and UHPC materials in the column, this study proposed and evaluated a column design SMA and UHPC materials in the column, this study proposed and evaluated a column design made made of UHPC with full and reduced ratio of NiTi SMA bars in the plastic hinge region, and compared of UHPC with full and reduced ratio of NiTi SMA bars in the plastic hinge region, and compared its its performance with existing designs. Four columns with 3.6 m height and 0.8 m diameter were performance with existing designs. Four columns with 3.6 m height and 0.8 m diameter were modeled in the OpenSees finite element program. Each column model consisted of four equal-height modeled in the OpenSees finite element program. Each column model consisted of four equal-height elements from which the bottom element was nonlinearly modeled using fiber section and the top elements from which the bottom element was nonlinearly modeled using fiber section and the top three elements were elastic. The first model was the conventional S-C column. The second model was three elements were elastic. The first model was the conventional S-C column. The second model was the SMA-C column, which was similar to the first model but with NiTi SMA bars in the plastic hinge the SMA-C column, which was similar to the first model but with NiTi SMA bars in the plastic hinge region, instead of longitudinal steel reinforcement. The third model was the SMA-UHPC column, region, instead of longitudinal steel reinforcement. The third model was the SMA-UHPC column, in in which the column was made of UHPC, and NiTi SMA bars were used in the plastic hinge region. which the column was made of UHPC, and NiTi SMA bars were used in the plastic hinge region. The The fourth model was the R-SMA-UHPC column, which was similar to the third model, but with fourth model was the R-SMA-UHPC column, which was similar to the third model, but with reduced reduced reinforcement ratio to optimize the use of SMA bars. Except for the R-SMA-UHPC column, reinforcement ratio to optimize the use of SMA bars. Except for the R-SMA-UHPC column, which which had 1.33% reinforcement ratio, the rest of the columns had 2.0% reinforcement. The columns had 1.33% reinforcement ratio, the rest of the columns had 2.0% reinforcement. The columns were were analyzed under constant axial load and lateral cyclic loading up to 4.0% drift. The main findings analyzed under constant axial load and lateral cyclic loading up to 4.0% drift. The main findings of of the study are summarized here: the study are summarized here: • Unlike Unlikethe the S-C S-C column, w column, which hich experienced experienced 68% 68% residual residual de deformation formation the the columns columns w with NiT ith i SMA NiTi rSMA einfor rein cement forcement did no did not su er t suffer from permanent from permanent d deformation. eformation. • The strength of SMA-UHPC column, 619 kN, was about 30% higher compared to the S-C The strength of SMA-UHPC column, 619 kN, was about 30% higher compared to the S-C column, column, 476 kN. 476 kN. • The strengths of SMA-C and R-SMA-UHPC columns were similar to the S-C column. The strengths of SMA-C and R-SMA-UHPC columns were similar to the S-C column. • The SMA-UHPC, R-SMA-UHPC, and SMA-C columns showed 7.5, 6.5, and 5 times larger The SMA-UHPC, R-SMA-UHPC, and SMA-C columns showed 7.5, 6.5, and 5 times larger ductility, ductility, compared to the S-C column. compared to the S-C column. Accordingly, the SMA-UHPC column showed the best seismic performance compared to the Accordingly, the SMA-UHPC column showed the best seismic performance compared to the other columns in terms of strength, ductility, and residual deformation. It is also important to note other columns in terms of strength, ductility, and residual deformation. It is also important to note that the columns made of UHPC benefit from its dense microstructure and impermeability. that the columns made of UHPC benefit from its dense microstructure and impermeability. Moreover, Moreover, the NiTi SMA bars are proven to have excellent corrosion resistance. Therefore, the SMA- the NiTi SMA bars are proven to have excellent corrosion resistance. Therefore, the SMA-UHPC and UHPC and R-SMA-UHPC designs ensure long-term durability for columns, in addition to providing excellent resilience against earthquakes. 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This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Infrastructures Multidisciplinary Digital Publishing Institute

Seismic Resistant Bridge Columns with NiTi Shape Memory Alloy and Ultra-High-Performance Concrete

Aryan, Hadi
Infrastructures , Volume 5 (12) – Nov 30, 2020
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infrastructures Article Seismic Resistant Bridge Columns with NiTi Shape Memory Alloy and Ultra-High-Performance Concrete Hadi Aryan Viterbi School of Engineering, University of Southern California, Los Angeles, CA 90089, USA; haryan@usc.edu Received: 27 October 2020; Accepted: 19 November 2020; Published: 30 November 2020 Abstract: Reinforced concrete bridge columns often endure significant damages during earthquakes due to the inherent deficiencies of conventional materials. Superior properties of the new materials such as shape memory alloy (SMA) and ultra-high-performance concrete (UHPC), compared to the reinforcing steel and the normal concrete, respectively, are needed to build a new generation of seismic resistant columns. Application of SMA or UHPC in columns has been separately studied, but this paper aims to combine the superelastic behavior of NiTi SMA and the high strength of UHPC, in order to produce a column design with minimum permanent deformation and high load tolerance subjected to strong ground motions. Additionally, the excellent corrosion resistance of NiTi SMA and the dense and impermeable microstructure of UHPC ensure the long-term durability of the proposed earthquake resistant column design. The seismic performance of four columns, defined as steel reinforced concrete (S-C), SMA reinforced concrete (SMA-C), SMA reinforced UHPC (SMA-UHPC), and reduced SMA reinforced UHPC (R-SMA-UHPC) is analyzed through a loading protocol with up to 4% drift cycles. The use of NiTi SMA bars for the SMA reinforced columns is limited to the plastic hinge region where permanent deformations happen. All the columns have 2.0% reinforcement ratio, except the R-SMA-UHPC column that has a 1.33% reinforcement ratio to optimize the use of SMA bars. Unlike the S-C column that showed up to 68% residual deformation compared to peak displacement during the last loading cycle the SMA reinforced columns did not experience permanent deformation. The SMA-C and R-SMA-UHPC columns showed similar strengths to the S-C column, but with about 5.0- and 6.5-times larger ductility, respectively. The SMA-UHPC column showed 30% higher strength and 7.5 times larger ductility compared to the S-C column. Keywords: NiTi shape memory alloy; ultra-high-performance concrete; bridge column; earthquake 1. Introduction Besides durability issues involved with normal concrete and reinforcing steel these conventional materials provide insucient seismic capacity for the bridge columns. Development of new materials, such as shape memory alloy (SMA) and ultra-high-performance concrete (UHPC), with excellent durability and mechanical properties provides the opportunity to improve the performance of bridge columns against strong earthquakes. Accordingly, SMA and UHPC have been evaluated and implemented through various designs for improving the seismic performance of structures [1–3]. Di erent types of SMA such as NiTi SMA [4], Cu-based SMA [5], and Fe-based SMA [6] have been investigated for their mechanical and durability properties. One of the unique properties of SMAs, especially NiTi and Cu-based types, is the superelastic behavior that allows the material to retain its original shape after unloading, and dissipate energy through cycles of flag-shaped hysteretic loops [7,8]. SMAs have been used through di erent techniques to mitigate the earthquake e ects on structures. For example, SMA dampers are utilized in bridges [9] and buildings [10] to improve the damping and frequency response of these structure; SMA braces are proposed to retrofit bridges [11,12] and buildings [13] against seismic excitations; and SMA restrainers are used in bridges to control the relative Infrastructures 2020, 5, 105; doi:10.3390/infrastructures5120105 www.mdpi.com/journal/infrastructures Infrastructures 2020, 5, 105 2 of 11 movements of superstructure and the response of piers [14–17]. Moreover, base isolation systems are equipped with the SMAs to mitigate the seismic actions on bridges and buildings [5,18]. Applications of SMAs also include reinforced concrete elements [19], steel beam-column connections [20], and even marine structures [21–23]. UHPC is distinguished among cement-based materials including normal concrete due to its excellent compressive strength and dense microstructure. While normal structural concrete has a 28 days compressive strength of about 35 MPa the compressive strength of UHPC is at least 145 MPa at this age [24]. UHPC has been used in construction of several bridges worldwide such as the Mars Hill bridge in the U.S., the Cat Point Creek bridge in the U.S., the Jakway Park bridge in the U.S., the Sherbrooke overpass in Canada, the Peace bridge in South Korea, the Wild bridge in Austria, the GSE bridge in Japan, the Kampung Linsum bridge in Malaysia, the Celakovice Pedestrian bridge in Czech Republic, the Luan Bai Dried-Canal Railway bridge in China, and the Yuan Jiahe bridge in China [25]. Outstanding workability and long-term durability of UHPC also make it an ideal material for prefabrication of bridge elements and accelerated construction industry [26]. The unique compressive strength of UHPC along with its proper integrity under tensile loads prevent column failure subjected to major earthquakes with vertical acceleration component, during which significant axial load variations and large moment demands a ect the column [27]. Additionally, the dense microstructure of UHPC [28] makes it impermeable to moisture and adverse chemicals, and prevents aging reactions that often a ect normal concrete [29]. Several studies have implemented NiTi SMA or UHPC to advance the seismic design of columns. Varela and Saiidi [30] evaluated a plastic hinge rubber element with NiTi SMA bars in a quarter-scale column subjected to strong earthquake motions on a shake table. The use of this new concept limited the column residual deformation to less than 0.5% after experiencing up to 7.0% drifts. Billah and Alam [31] performed an analytical study to address the lack of corrosion resistance and significant permanent deformation of regular steel reinforcements in reinforced concrete columns. They presented three concrete columns in which NiTi SMA or stainless steel bars were used in the plastic hinge region. Two columns with NiTi SMA bars in the plastic hinge region had either stainless steel or fiber reinforced polymer bars above the hinge region and one column with stainless steel bars in the plastic hinge region had fiber reinforced polymer bars above the hinge region. The bars of these hybrid reinforced concrete columns were connected with couplers above the plastic hinge region. Accordingly, they ensured the entire height of columns was reinforced with corrosion resistant reinforcing materials. They analyzed the columns under seismic loading and found the residual deformation of the columns with NiTi SMA bars to be 87% less compared to the column with stainless steel bars in the plastic hinge region. Mohebbi et al. [32] presented a posttensioned precast bridge column with plastic hinge region made of UHPC material, which was connected to the foundation with a pocket connection. They used unbonded carbon fiber reinforced polymer posttensioning tendons inside the column to minimize the permanent drifts and tested this technique on shake table through the Northridge-Rinaldi earthquake record. Results showed that the posttensioning approach for the column with UHPC in the plastic hinge region was e ective in eliminating permanent deformations and increasing displacement ductility to 13.8 at maximum drift ratio of 6.9%. Farzad et al. [33] proposed a retrofitting technique for reinforced concrete columns using UHPC. They built eleven quarter-scale columns and intentionally damaged them with spalling concrete to implement their strengthening technique. They sandblasted the damaged part of columns and repaired seven columns with UHPC containing 2% and 4% steel fibers, one column with normal concrete, and left the rest unrepaired to be considered as reference. According to the column tests under constant axial load and cyclic lateral load repairing the damaged columns with UHPC shell increased the strength of columns without changing their size. Moreover, di erent fiber contents of UHPC resulted in similar strength gain in the columns. Unlike previous studies on the application of NiTi SMA or UHPC in bridge columns, this paper presents a column design with a combination of NiTi SMA reinforcements and UHPC at the same time to take advantage of both materials’ excellent properties against earthquake. The reduced ratio Infrastructures 2020, 5, 105 3 of 11 Infrastructures 2020, 5, x FOR PEER REVIEW 3 of 11 time to take advantage of both materials’ excellent properties against earthquake. The reduced ratio of SMA reinforcement is also evaluated in the column design with UHPC and SMA. Results of the of SMA reinforcement is also evaluated in the column design with UHPC and SMA. Results of the two UHPC columns, with full and reduced ratios of NiTi SMA reinforcement, are compared with two two UHPC columns, with full and reduced ratios of NiTi SMA reinforcement, are compared with concrete columns with SMA and steel reinforcements. In this study, the NiTi type of SMA is used, since two concrete columns with SMA and steel reinforcements. In this study, the NiTi type of SMA is it benefits from larger and more stable flag shaped hysteretic loops of superelastic behavior compared used, since it benefits from larger and more stable flag shaped hysteretic loops of superelastic to the other SMA types, in order to minimize residual deformation and raise energy dissipation [34]. behavior compared to the other SMA types, in order to minimize residual deformation and raise The NiTi SMA has a high corrosion resistance as well [35]. energy dissipation [34]. The NiTi SMA has a high corrosion resistance as well [35]. 2. Finite Element Models 2. Finite Element Models Four column sections are considered in this study, as shown in Figure 1. The columns are modeled Four column sections are considered in this study, as shown in Figure 1. The columns are in OpenSees program [36]. All the columns are 3.6 m high and have a diameter of 0.8 m. The steel modeled in OpenSees program [36]. All the columns are 3.6 meters high and have a diameter of 0.8 reinforced concrete (S-C), SMA reinforced concrete (SMA-C), and SMA reinforced UHPC (SMA-UHPC) meter. The steel reinforced concrete (S-C), SMA reinforced concrete (SMA-C), and SMA reinforced column sections are reinforced with 2.0% of steel or NiTi SMA bars, but the reduced SMA reinforced UHPC (SMA-UHPC) column sections are reinforced with 2.0% of steel or NiTi SMA bars, but the UHPC (R-SMA-UHPC) section has a reinforcement ratio of 1.33% to optimize the use of SMA bars. reduced SMA reinforced UHPC (R-SMA-UHPC) section has a reinforcement ratio of 1.33% to The SMA reinforcements are only provided at the bottom quarter of the column height, as the plastic optimize the use of SMA bars. The SMA reinforcements are only provided at the bottom quarter of hinge region and the top three quarters of the height is reinforced with steel bars. The SMA and steel the column height, as the plastic hinge region and the top three quarters of the height is reinforced reinforcements are connected with mechanical couplers above the plastic hinge region. Geometry and with steel bars. The SMA and steel reinforcements are connected with mechanical couplers above the material configurations of the columns are presented in Table 1. plastic hinge region. Geometry and material configurations of the columns are presented in Table 1. Figure 1. Column sections. Figure 1. Column sections. Table 1. Geometry and material configurations of columns. Table 1. Geometry and material configurations of columns. Height Diameter Aspect Reinforcement Height Reinforcement Column ID Material Section Height (m) (m) Ratio Ratio Range (m) Material Height Diameter Aspect Reinforcement Reinforcement Column ID Steel Material Section Range Plastic 0–0.8 Steel S-C 3.6 0.8 4.5 Concrete 2.0% (m) (m) Ratio Ratio Material Reinforced (m) Elastic 0.8–3.6 Steel Concrete SMA Steel PlasticPlastic 0 0–0.8 −0.8 Steel NiTi SMA SMA-C 3.6 0.8 4.5 Concrete 2.0% Reinforced Reinforced S-C 3.6 0.8 4.5 Concrete 2.0% Elastic 0.8–3.6 Steel Concrete Elastic 0.8−3.6 Steel SMA Concrete Plastic 0–0.8 NiTi SMA SMA-UHPC 3.6 0.8 4.5 UHPC 2.0% Reinforced SMA ElasticPlastic 0 0.8–3.6−0.8 NiTi Steel SMA UHPC SMA- Reduced SMA Reinforced 3.6 0.8 4.5 Concrete 2.0% Plastic 0–0.8 NiTi SMA R-SMA-UHPC 3.6 0.8 4.5 UHPC 1.33% Reinforced Elastic 0.8−3.6 Steel Concrete Elastic 0.8–3.6 Steel UHPC SMA Plastic 0−0.8 NiTi SMA SMA- Reinforced 3.6 0.8 4.5 UHPC 2.0% UHPC Elastic 0.8−3.6 Steel UHPC Infrastructures 2020, 5, x FOR PEER REVIEW 4 of 11 Reduced Plastic 0−0.8 NiTi SMA R- SMA SMA- 3.6 0.8 4.5 UHPC 1.33% Infrastructures 2020, 5, 105 4 of 11 Reinforced Elastic 0.8−3.6 Steel UHPC UHPC As shown in Figure 2, each column model consists of four equal-length elements along the height As shown in Figure 2, each column model consists of four equal-length elements along the height from which the bottom one is a distributed plasticity element, with a fiber section representing the from which the bottom one is a distributed plasticity element, with a fiber section representing the plastic hinge region, and the top three elements are elastic. Seven equally distanced integration points plastic hinge region, and the top three elements are elastic. Seven equally distanced integration points are used along the distributed plasticity element with fiber section, which are not shown in Figure 2 for are used along the distributed plasticity element with fiber section, which are not shown in Figure 2 simplicity. According to Figure 2, the column mass is concentrated at five points along the height from for simplicity. According to Figure 2, the column mass is concentrated at five points along the height which the three middle points hold the summation of mass from half of two elements below and above from which the three middle points hold the summation of mass from half of two elements below them, while the top and bottom points only hold the mass from half of their adjacent elements at the and above them, while the top and bottom points only hold the mass from half of their adjacent top and bottom, respectively. Therefore, the concentrated mass at the top and bottom points is half of elements at the top and bottom, respectively. Therefore, the concentrated mass at the top and bottom the concentrated mass at the middle points. points is half of the concentrated mass at the middle points. Figure 2. Column model in OpenSees. Figure 2. Column model in OpenSees. The Concrete02, ReinforcingSteel, and SelfCentering models are used for concrete, steel, and SMA The Concrete02, ReinforcingSteel, and SelfCentering models are used for concrete, steel, and materials, respectively. The Concrete02 model properly captures the post-peak behavior of concrete SMA materials, respectively. The Concrete02 model properly captures the post-peak behavior of during loading and unloading cycles and the ReinforcingSteel model accurately follows the linear concrete during loading and unloading cycles and the ReinforcingSteel model accurately follows the elastic, yield plateau, and strain hardening portions of reinforcing steel behavior in concrete in linear elastic, yield plateau, and strain hardening portions of reinforcing steel behavior in concrete in opposition to the common bilinear steel models. The SelfCentering model constructs the flag-shaped opposition to the common bilinear steel models. The SelfCentering model constructs the flag-shaped and energy dissipative SMA material hysteresis behavior in uniaxial direction under tension and and energy dissipative SMA material hysteresis behavior in uniaxial direction under tension and compression cycles [36]. The unconfined concrete has a compressive strength of 34.5 MPa, and peak compression cycles [36]. The unconfined concrete has a compressive strength of 34.5 MPa, and peak and ultimate strains of 0.0022 and 0.005, respectively. The Mander model [37] is used to obtain the and ultimate strains of 0.0022 and 0.005, respectively. The Mander model [37] is used to obtain the properties of the confined concrete. Accordingly, the confined concrete has a compressive strength properties of the confined concrete. Accordingly, the confined concrete has a compressive strength of 44.1 MPa, and peak and ultimate strains of 0.0054 and 0.0128, respectively. The properties of of 44.1 MPa, and peak and ultimate strains of 0.0054 and 0.0128, respectively. The properties of unconfined UHPC are obtained from [24], for the case of using 6 mm straight steel fibers with 2% unconfined UHPC are obtained from [24], for the case of using 6 mm straight steel fibers with 2% volumetric content in mixture. Accordingly, the unconfined UHPC has a compressive strength of volumetric content in mixture. Accordingly, the unconfined UHPC has a compressive strength of 145.8 MPa, and peak and ultimate strains of 0.0044 and 0.0146, respectively. The confined UHPC has a 145.8 MPa, and peak and ultimate strains of 0.0044 and 0.0146, respectively. The confined UHPC has compressive strength of 154.8 MPa, and peak and ultimate strains of 0.0137 and 0.0289, respectively. a com The tensile pressiv str e st engths rengtof h o concr f 154ete .8 MP and a,UHPC and pe ar ae k conservatively and ultimate st overlooked. rains of 0.01 The 37 and stress 0.02 strain 89, resp behavior ectively. The tensi of confined le strengths of concrete and concrete a UHPC ar nd UHPC a e plotted in re cons Figur er e vatively ove 3. The steel r rlooked. The st einforcement is ress str Grade ain be 60 with havior of confined concrete and UHPC are plotted in Figure 3. The steel reinforcement is Grade 60 with Infrastructures 2020, 5, 105 5 of 11 Infrastructures 2020, 5, x FOR PEER REVIEW 5 of 11 Infrastructures 2020, 5, x FOR PEER REVIEW 5 of 11 ultimate strain of 0.09, and yield and ultimate tensile strengths of 414 MPa and 621 MPa, respectively. ultimate strain of 0.09, and yield and ultimate tensile strengths of 414 MPa and 621 MPa, respectively. ultimate strain of 0.09, and yield and ultimate tensile strengths of 414 MPa and 621 MPa, respectively. The superelastic behavior of NiTi SMA is implemented based on [12,34], as shown in Figure 4. The superelastic behavior of NiTi SMA is implemented based on [12,34], as shown in Figure 4. The superelastic behavior of NiTi SMA is implemented based on [12,34], as shown in Figure 4. Confined Concrete 20 Confined Concrete Confined UHPC Confined UHPC 0 0.005 0.01 0.015 0.02 0.025 0.03 0 0.005 0.01 0.015 0.02 0.025 0.03 Strain Strain Figure 3. Stress-strain behavior of confined concrete and UHPC. Figure 3. Figure 3. Stress-strain behavior of co Stress-strain behavior of confined nfined concrete concrete and UHPC. and UHPC. NiTi SMA NiTi SMA 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 Strain Strain Figure 4. Superelastic behavior of NiTi SMA. Figure 4. Superelastic behavior of NiTi SMA. Figure 4. Superelastic behavior of NiTi SMA. An eigenvalue analysis is performed to obtain and compare the natural periods of the columns. An eigenvalue analysis is performed to obtain and compare the natural periods of the columns. An eigenvalue analysis is performed to obtain and compare the natural periods of the columns. Then, a loading protocol consisting of a constant axial load and lateral cycles of displacement control Then, a loading protocol consisting of a constant axial load and lateral cycles of displacement control Then, a loading protocol consisting of a constant axial load and lateral cycles of displacement control drifts is applied to the columns as shown in Figure 5. A constant vertical load of 1735 kN is applied to drifts is applied to the columns as shown in Figure 5. A constant vertical load of 1735 kN is applied drifts is applied to the columns as shown in Figure 5. A constant vertical load of 1735 kN is applied the columns as the service load, which is equivalent to 10% of the compression capacity of the S-C to the columns as the service load, which is equivalent to 10% of the compression capacity of the S-C to the columns as the service load, which is equivalent to 10% of the compression capacity of the S-C column. The columns are also subjected to eight displacement control lateral drift cycles at the top, column. The columns are also subjected to eight displacement control lateral drift cycles at the top, column. The columns are also subjected to eight displacement control lateral drift cycles at the top, including two cycles of 0.5%, two cycles of 1.0%, two cycles of 2.0%, and two cycles of 4.0% drifts, including two cycles of 0.5%, two cycles of 1.0%, two cycles of 2.0%, and two cycles of 4.0% drifts, including two cycles of 0.5%, two cycles of 1.0%, two cycles of 2.0%, and two cycles of 4.0% drifts, while their base is assumed as fixed support. Accordingly, the capacity of columns to respond a seismic while their base is assumed as fixed support. Accordingly, the capacity of columns to respond a while their base is assumed as fixed support. Accordingly, the capacity of columns to respond a action is analyzed and compared through this loading protocol. seismic action is analyzed and compared through this loading protocol. seismic action is analyzed and compared through this loading protocol. Stress (MPa) Stress (MPa) Stress (MPa) Stress (MPa) Infrastructures 2020, 5, 105 6 of 11 Infrastructures 2020, 5, x FOR PEER REVIEW 6 of 11 Figure 5. (a) Loading and boundary conditions; (b) lateral loading protocol. Figure 5. (a) Loading and boundary conditions; (b) lateral loading protocol. 3. Results of Analysis 3. Results of Analysis According to the eigenvalue analysis the natural periods of the S-C, SMA-C, SMA-UHPC, According to the eigenvalue analysis the natural periods of the S-C, SMA-C, SMA-UHPC, and and R-SMA-UHPC columns are 0.038, 0.040, 0.034, and 0.035 second, respectively. Based on Equation (1) R-SMA-UHPC columns are 0.038, 0.040, 0.034, and 0.035 second, respectively. Based on Equation (1) and assuming mass, m, as a constant the columns with lower natural period, T, have higher initial and assuming mass, m, as a constant the columns with lower natural period, T, have higher initial sti ness, K. Most importantly, the columns are analyzed subjected to the lateral cyclic loading protocol stiffness, K. Most importantly, the columns are analyzed subjected to the lateral cyclic loading and constant axial load, as explained in the previous section, to compare their seismic capacity. protocol and constant axial load, as explained in the previous section, to compare their seismic The base shear versus drift diagrams are shown in Figure 6 for all the columns. The essential seismic capacity. The base shear versus drift diagrams are shown in Figure 6 for all the columns. The essential parameters to compare between di erent columns are the strength, residual deformation, drift ductility, seismic parameters to compare between different columns are the strength, residual deformation, and energy dissipation. The obvious di erence between the behavior of the S-C column and the drift ductility, and energy dissipation. The obvious difference between the behavior of the S-C rest of columns with SMA reinforcement is the amount of residual deformation. During the 2.0% column and the rest of columns with SMA reinforcement is the amount of residual deformation. and 4.0% drift cycles the residual deformation of S-C column at zero load is equivalent to 0.9% and During the 2.0% and 4.0% drift cycles the residual deformation of S-C column at zero load is 2.7% drifts, respectively. This means that during 2.0% and 4.0% drift cycles, there is 45% and 68% equivalent to 0.9% and 2.7% drifts, respectively. This means that during 2.0% and 4.0% drift cycles, residual deformation, respectively, in the S-C column, after unloading to zero load compared to the there is 45% and 68% residual deformation, respectively, in the S-C column, after unloading to zero peak displacement. On the other hand, replacing the steel reinforcement with the SMA reinforcement load compared to the peak displacement. On the other hand, replacing the steel reinforcement with in the plastic hinge region leads to zero residual deformation in all the SMA reinforced columns during the SMA reinforcement in the plastic hinge region leads to zero residual deformation in all the SMA di erent drift cycles. This means that the S-C column cannot be serviceable after the earthquake, but the reinforced columns during different drift cycles. This means that the S-C column cannot be SMA reinforced columns retain their serviceability. The S-C column shows more energy dissipation serviceable after the earthquake, but the SMA reinforced columns retain their serviceability. The S-C through di erent loading cycles compared to the other columns but, since the energy is dissipated after column shows more energy dissipation through different loading cycles compared to the other large permanent deformations, it would not benefit the column. In other words, the larger inside area columns but, since the energy is dissipated after large permanent deformations, it would not benefit of base shear-drift cycles diagram of the S-C column is due to the significant residual deformations the column. In other words, the larger inside area of base shear-drift cycles diagram of the S-C column and damages after returning to zero load during di erent drift cycles. Between the SMA reinforced is due to the significant residual deformations and damages after returning to zero load during columns, the SMA-C column showed slightly more energy dissipation compared to the SMA-UHPC different drift cycles. Between the SMA reinforced columns, the SMA-C column showed slightly and R-SMA-UHPC columns, especially during the initial drift cycles up to 2%. more energy dissipation compared to the SMA-UHPC and R-SMA-UHPC columns, especially during the initial drift cycles up to 2%. T = 2 (1) (1) = 2 Infrastructures 2020, 5, 105 7 of 11 Infrastructures 2020, 5, x FOR PEER REVIEW 7 of 11 Infrastructures 2020, 5, x FOR PEER REVIEW 7 of 11 700 700 S-C SMA-C 700 700 600 600 S-C SMA-C 600 600 500 500 500 500 400 400 400 400 300 300 300 300 200 200 200 200 100 100 100 100 0 0 -100 -100 -100 -100 -200 -200 -200 -200 -300 -300 -300 -300 -400 -400 -400 -400 -500 -500 -500 -500 -600 -600 -600 -600 -700 -700 -4 -3 -2 -1 0 1 2 3 4 -4 -3 -2 -1 0 1 2 3 4 -700 -700 a) -4 -3 -2 -1 0 1 2 3 4 -4 -3 -2 -1 0 1 2 3 4 Drift (%) b) Drift (%) a) b) Drift (%) Drift (%) 700 700 SMA-UHPC R-SMA-UHPC 700 700 600 600 SMA-UHPC R-SMA-UHPC 600 600 500 500 500 500 400 400 300 300 300 300 200 200 200 200 100 100 0 0 0 0 -100 -100 -100 -100 -200 -200 -200 -200 -300 -300 -300 -300 -400 -400 -400 -400 -500 -500 -500 -500 -600 -600 -600 -600 -700 -700 -4 -3 -2 -1 0 1 2 3 4 -700 -4 -3 -2 -1 0 1 2 3 4 -700 c) -4 -3 -2 -1 0 1 2 3 4 d) -4 -3 -2 -1 0 1 2 3 4 Drift (%) Drift (%) c) d) Drift (%) Drift (%) Figure 6. Base shear versus drift diagrams for di erent columns: (a) S-C, (b) SMA-C, (c) SMA-UHPC, Figure 6. Base shear versus drift diagrams for different columns: (a) S-C, (b) SMA-C, (c) SMA-UHPC, Figure 6. Base shear versus drift diagrams for different columns: (a) S-C, (b) SMA-C, (c) SMA-UHPC, (d) R-SMA-UHPC. (d) R-SMA-UHPC. (d) R-SMA-UHPC. In order to compare the performance of di erent columns in terms of strength all the base shear In order to compare the performance of different columns in terms of strength all the base shear In order to compare the performance of different columns in terms of strength all the base shear versus drift diagrams are plotted in Figure 7 at the same time. The SMA-UHPC, R-SMA-UHPC, S-C, versus drift diagrams are plotted in Figure 7 at the same time. The SMA-UHPC, R-SMA-UHPC, S-C, versus drift diagrams are plotted in Figure 7 at the same time. The SMA-UHPC, R-SMA-UHPC, S-C, and SMA-C columns have shown the highest to the lowest strengths of 619 kN, 495 kN, 476 kN, and SMA-C columns have shown the highest to the lowest strengths of 619 kN, 495 kN, 476 kN, and and SMA-C columns have shown the highest to the lowest strengths of 619 kN, 495 kN, 476 kN, and and 441 kN, respectively. Therefore, the SMA-UHPC column design provides 30% higher strength 441 kN, respectively. Therefore, the SMA-UHPC column design provides 30% higher strength 441 kN, respectively. Therefore, the SMA-UHPC column design provides 30% higher strength compared to the S-C column, and removes the residual deformation. The R-SMA-UHPC column compared to the S-C column, and removes the residual deformation. The R-SMA-UHPC column compared to the S-C column, and removes the residual deformation. The R-SMA-UHPC column provides just about 4% higher strength compared to the S-C column. Accordingly, when UHPC is used provides just about 4% higher strength compared to the S-C column. Accordingly, when UHPC is provides just about 4% higher strength compared to the S-C column. Accordingly, when UHPC is along with the SMA reinforcement, reducing the reinforcement ratio by one third results in getting a used along with the SMA reinforcement, reducing the reinforcement ratio by one third results in used along with the SMA reinforcement, reducing the reinforcement ratio by one third results in similar strength to the S-C column, and still being able to remove the residual deformation. Moreover, getting a similar strength to the S-C column, and still being able to remove the residual deformation. getting a similar strength to the S-C column, and still being able to remove the residual deformation. results show that using SMA reinforcement in the plastic hinge region of the SMA-C column does Moreover, results show that using SMA reinforcement in the plastic hinge region of the SMA-C Moreover, results show that using SMA reinforcement in the plastic hinge region of the SMA-C not compromise the strength by more than 7% compared to the S-C column, while it prevents the column does not compromise the strength by more than 7% compared to the S-C column, while it column does not compromise the strength by more than 7% compared to the S-C column, while it permanent deformation. prevents the permanent deformation. prevents the permanent deformation. S-C S-C 500 SMA-C 500 SMA-C SMA-UHPC 400 SMA-UHPC R-SMA-UHPC R-SMA-UHPC -100 -100 -200 -200 -300 -300 -400 -400 -500 -500 -600 -600 -700 -700-4 -3 -2 -1 0 1 2 3 4 -4 -3 -2 -1 0 1 2 3 4 Drift (%) Drift (%) Figure 7. Base shear versus drift diagrams of the columns. Figure 7. Base shear versus drift diagrams of the columns. Figure 7. Base shear versus drift diagrams of the columns. Base Shear (kN) Base Shear (kN) Base Shear (kN) Base Shear (kN) Base Shear (kN) Base Shear (kN) Base Shear (kN) Base Shear (kN) Base Shear (kN) Base Shear (kN) Infrastructures 2020, 5, 105 8 of 11 It is important to note that the UHPC columns with full and reduced ratios of SMA reinforcement reach their maximum strength at about 4.0% drift, while the concrete columns with SMA and steel reinforcements reach their peak strength at about 3.0% and 1.2% drifts, respectively. The column drift ductility,  , is defined in Equation (2) as the drift at peak strength, D , divided by the drift D peak Infrastructures 2020, 5, x FOR PEER REVIEW 8 of 11 at yield point, D . Accordingly, the SMA-UHPC, R-SMA-UHPC, and SMA-C columns have a yeild It is important to note that the UHPC columns with full and reduced ratios of SMA ductility of about 22, 20, and 15, respectively, while the ductility of S-C column is 3. Therefore, the SMA reinforcement reach their maximum strength at about 4.0% drift, while the concrete columns with reinforced columns, with UHPC or concrete, provide much larger ductility than the S-C column. SMA and steel reinforcements reach their peak strength at about 3.0% and 1.2% drifts, respectively. The R-SMA-UHPC and SMA-C columns provide similar strength to the S-C column, but their ductility The column drift ductility, µD, is defined in Equation (2) as the drift at peak strength, Dpeak, divided is about seven and five times that of the S-C column, respectively. As summarized in Table 2, results by the drift at yield point, Dyeild. Accordingly, the SMA-UHPC, R-SMA-UHPC, and SMA-C columns show that the SMA-UHPC, R-SMA-UHPC, and SMA-C columns have superior seismic performance have a ductility of about 22, 20, and 15, respectively, while the ductility of S-C column is 3. Therefore, compared to the S-C column in terms of ductility and residual deformation. The SMA-UHPC column the SMA reinforced columns, with UHPC or concrete, provide much larger ductility than the S-C shows the best seismic performance among all the columns, given its highest strength and ductility. column. The R-SMA-UHPC and SMA-C columns provide similar strength to the S-C column, but Due to the high corrosion resistant of NiTi SMA bars used in the plastic hinge region, and the dense their ductility is about seven and five times that of the S-C column, respectively. As summarized in Taand ble 2impermeable , results show tha micrt the ostr S uctur MA- eUH of P UHPC C, R-SM over A-Uthe HPC entir , aned column SMA-C columns height, the have pr oposed superior SMA-UHPC seismic performance compared to the S-C column in terms of ductility and residual deformation. The column has excellent long-term durability in addition to its extraordinary seismic performance. SMA-UHPC column shows the best seismic performance among all the columns, given its highest strength and ductility. Due to the high corrosion resistant of D NiTi SMA bars used in the plastic hinge peak = (2) region, and the dense and impermeable microstructure of UHPC over the entire column height, the yeild proposed SMA-UHPC column has excellent long-term durability in addition to its extraordinary seismic performance. Table 2. Summary of the results. = (2) Column Strength (kN)  Residual Deformation S-C 476 3 68% Table 2. Summary of the results. SMA-C 441 15 0% Column Strength (kN) Residual Deformation SMA-UHPC 619 22 0% S-C 476 3 68% SMA-C 441 15 0% R-SMA-UHPC 495 20 0% SMA-UHPC 619 22 0% R-SMA-UHPC 495 20 0% As presented in Table 2, the S-C column su ered from 68% residual deformation after unloading to zero load during the last drift cycle. Figure 8 shows the distribution of corresponding residual As presented in Table 2, the S-C column suffered from 68% residual deformation after unloading to z curvatur ero loade, duri rotation, ng the last drif and deflection t cycle. Figure at the 8 sho plastic ws the d hinge istrirbution o egion of f cor S-C respondin columng r in esid the ual positive and curvature, rotation, and deflection at the plastic hinge region of S-C column in the positive and negative directions. negative directions. 0.9 0.9 0.9 0.75 0.75 0.75 0.6 0.6 0.6 0.45 0.45 0.45 0.3 0.3 0.3 0.15 0.15 0.15 0 0 -0.06 -0.04 -0.02 0 0.02 0.04 0.06 -0.045 -0.03 -0.015 0 0.015 0.03 0.045 -0.02 -0.01 0 0.01 0.02 a) b) c) Curvature (1/m) Rotation (rad) Deflection (m) Figure 8. Residual deformation at the plastic hinge region of S-C column: (a) curvature, (b) rotation, Figure 8. Residual deformation at the plastic hinge region of S-C column: (a) curvature, (b) rotation, (c) deflection. (c) deflection. Assuming linear behavior above the plastic hinge region, the residual deflection along the height Assuming linear behavior above the plastic hinge region, the residual deflection along the height of S-C column is shown in Figure 9, in comparison with its peak deflection. Elimination of this of S-C column is shown in Figure 9, in comparison with its peak deflection. Elimination of this significant residual deflection as accomplished in the other columns by using the NiTi SMA bars in significant residual deflection as accomplished in the other columns by using the NiTi SMA bars in the the plastic hinge region is crucial for immediate serviceability of the bridge after earthquake. plastic hinge region is crucial for immediate serviceability of the bridge after earthquake. Height (m) Height (m) Height (m) Infrastructures 2020, 5, 105 9 of 11 Infrastructures 2020, 5, x FOR PEER REVIEW 9 of 11 3.6 2.7 1.8 0.9 Peak Residual -0.15 -0.1 -0.05 0 0.05 0.1 0.15 Deflection (m) Figure 9. Residual versus peak deflections of the S-C column along the height. Figure 9. Residual versus peak deflections of the S-C column along the height. 4. Summary and Conclusions 4. Summary and Conclusions NiTi SMA bars are suitable alternatives for steel reinforcement of concrete elements in NiTi SMA bars are suitable alternatives for steel reinforcement of concrete elements in seismic seismic regions, due to their self-centering and energy dissipative properties. On the other hand, regions, due to their self-centering and energy dissipative properties. On the other hand, UHPC UHPC properties in terms of strength and integrity significantly outweigh the concrete properties. properties in terms of strength and integrity significantly outweigh the concrete properties. Among Among previous studies on the application of new materials in bridge columns, some have replaced previous studies on the application of new materials in bridge columns, some have replaced the steel the steel reinforcement with NiTi SMA bars and others utilized UHPC instead of normal concrete. reinforcement with NiTi SMA bars and others utilized UHPC instead of normal concrete. Replacing Replacing steel reinforcement with NiTi SMA bars resulted in minimum permanent deformation for steel reinforcement with NiTi SMA bars resulted in minimum permanent deformation for columns columns but with no strength advantage over conventional columns. On the other hand, replacing but with no strength advantage over conventional columns. On the other hand, replacing concrete concrete with UHPC only increased the column strength, and did not reduce the residual deformations with UHPC only increased the column strength, and did not reduce the residual deformations in the in the absence of a secondary measure. In order to take advantage of the excellent properties of both absence of a secondary measure. In order to take advantage of the excellent properties of both NiTi NiTi SMA and UHPC materials in the column, this study proposed and evaluated a column design SMA and UHPC materials in the column, this study proposed and evaluated a column design made made of UHPC with full and reduced ratio of NiTi SMA bars in the plastic hinge region, and compared of UHPC with full and reduced ratio of NiTi SMA bars in the plastic hinge region, and compared its its performance with existing designs. Four columns with 3.6 m height and 0.8 m diameter were performance with existing designs. Four columns with 3.6 m height and 0.8 m diameter were modeled in the OpenSees finite element program. Each column model consisted of four equal-height modeled in the OpenSees finite element program. Each column model consisted of four equal-height elements from which the bottom element was nonlinearly modeled using fiber section and the top elements from which the bottom element was nonlinearly modeled using fiber section and the top three elements were elastic. The first model was the conventional S-C column. The second model was three elements were elastic. The first model was the conventional S-C column. The second model was the SMA-C column, which was similar to the first model but with NiTi SMA bars in the plastic hinge the SMA-C column, which was similar to the first model but with NiTi SMA bars in the plastic hinge region, instead of longitudinal steel reinforcement. The third model was the SMA-UHPC column, region, instead of longitudinal steel reinforcement. The third model was the SMA-UHPC column, in in which the column was made of UHPC, and NiTi SMA bars were used in the plastic hinge region. which the column was made of UHPC, and NiTi SMA bars were used in the plastic hinge region. The The fourth model was the R-SMA-UHPC column, which was similar to the third model, but with fourth model was the R-SMA-UHPC column, which was similar to the third model, but with reduced reduced reinforcement ratio to optimize the use of SMA bars. Except for the R-SMA-UHPC column, reinforcement ratio to optimize the use of SMA bars. Except for the R-SMA-UHPC column, which which had 1.33% reinforcement ratio, the rest of the columns had 2.0% reinforcement. The columns had 1.33% reinforcement ratio, the rest of the columns had 2.0% reinforcement. The columns were were analyzed under constant axial load and lateral cyclic loading up to 4.0% drift. The main findings analyzed under constant axial load and lateral cyclic loading up to 4.0% drift. The main findings of of the study are summarized here: the study are summarized here: • Unlike Unlikethe the S-C S-C column, w column, which hich experienced experienced 68% 68% residual residual de deformation formation the the columns columns w with NiT ith i SMA NiTi rSMA einfor rein cement forcement did no did not su er t suffer from permanent from permanent d deformation. eformation. • The strength of SMA-UHPC column, 619 kN, was about 30% higher compared to the S-C The strength of SMA-UHPC column, 619 kN, was about 30% higher compared to the S-C column, column, 476 kN. 476 kN. • The strengths of SMA-C and R-SMA-UHPC columns were similar to the S-C column. The strengths of SMA-C and R-SMA-UHPC columns were similar to the S-C column. • The SMA-UHPC, R-SMA-UHPC, and SMA-C columns showed 7.5, 6.5, and 5 times larger The SMA-UHPC, R-SMA-UHPC, and SMA-C columns showed 7.5, 6.5, and 5 times larger ductility, ductility, compared to the S-C column. compared to the S-C column. Accordingly, the SMA-UHPC column showed the best seismic performance compared to the Accordingly, the SMA-UHPC column showed the best seismic performance compared to the other columns in terms of strength, ductility, and residual deformation. It is also important to note other columns in terms of strength, ductility, and residual deformation. It is also important to note that the columns made of UHPC benefit from its dense microstructure and impermeability. that the columns made of UHPC benefit from its dense microstructure and impermeability. Moreover, Moreover, the NiTi SMA bars are proven to have excellent corrosion resistance. Therefore, the SMA- the NiTi SMA bars are proven to have excellent corrosion resistance. Therefore, the SMA-UHPC and UHPC and R-SMA-UHPC designs ensure long-term durability for columns, in addition to providing excellent resilience against earthquakes. 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Infrastructures – Multidisciplinary Digital Publishing Institute

Published: Nov 30, 2020

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