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Shape memory alloy (SMA)-based seismic isolation systems can successfully reduce the peak and residual displacements of bridges during strong earthquake, but they commonly lead to an increased force demands in substructure. This study explores the development of an SMA cable-based negative stiffness isolator to alleviate this problem. The proposed isolator is composed of superelastic SMA cables and a frictional sliding bearing with convex surfaces. The frictional sliding bearing limit the forces transferred to the superstructure and provides energy dissipation, while its built-in negative stiffness mechanism reduces the force demands in substructure. SMA cables provide critical restoring forces, additional energy dissipation, and displacement-limiting capacity. Based on the force balance, the negative stiffness and restoring requirements of the SMA cable-based negative stiffness isolator were analyzed first. Then, a prototype large-scale isolator was designed and fabricated. Next, the experimental testing of the developed isolator was performed under two different vertical load levels. Finally, finite element modeling of the proposed isolator was conducted, and the simulation results and experimental results were compared and discussed. The proposed isolator generates lower forces than the SMA-based zero and positive stiffness isolators and can exhibit stable energy dissipation capabilities with very good displacement-limiting and self-centering capabilities.

This study explores the flexural behavior of reinforced concrete (RC) columns strengthened with near surface mounted (NSM) shape memory alloy (SMA) bars or carbon fiber reinforced polymer (CFRP) bars. Seven RC column specimens were designed and fabricated to study the influence of different variables on the flexural response of the strengthened columns. These parameters include type of NSM reinforcement (SMA bars or CFRP bars), ratio of NSM reinforcement, and effect of CFRP jacketing. The columns were tested under cyclic lateral loading with constant axial force. The flexural behavior of each specimen was examined in terms of peak load, failure load, drift ratios, displacement ductility, stiffness degradation, energy dissipation, and seismic damage index. The experimental results indicate that strengthening of RC columns with NSM SMA or CFRP bars improves the flexural behavior of the columns through increasing the lateral load capacity, reducing the stiffness degradation and increasing the cumulative energy absorption up to failure. Further enhancement in the lateral response of RC columns was obtained by combining NSM bars and CFRP jacketing as the later provides an additional confinement to the critical sections of the test specimens.

This paper examines the buckling and post-buckling behavior of superelastic shape memory alloy (SMA) bars. A NiTi SMA bar with a diameter of 12 mm was used in all the experimental tests. First, the tensile and compression responses of NiTi bar were characterized under monotonic loading up to failure. A total of 15 specimens with slenderness ratios that range from 25 to 115 were tested to study the critical buckling load and post-buckling behavior of SMA bars. Digital image correlation (DIC) system was implemented to monitor full-field surface displacements. The interaction between material nonlinearity due to phase transformation and geometric nonlinearity was explored. Data obtained from the DIC measurement system were further analyzed to identify the onset of buckling and to extract experimental critical buckling loads. The effect of loading rate on buckling response of SMAs was investigated by conducting additional testing at higher loading rates on the specimens with three selected slenderness ratios. The temperature field on the surface of the specimens was recorded by an infrared camera. The analytical critical buckling loads were computed and compared with experimental results. All specimens exhibited a unique buckling behavior characterized with almost a complete shape recovery upon unloading.

The flexural behavior of reinforced concrete (RC) beams strengthened with near-surface mounted (NSM) 7075 aluminum alloy (AA) bars was investigated through experimental testing. In particul...

Self-sensing cementitious composites can enable structures that are capable of carrying the loads applied on them while monitoring their condition. Most of earlier research has focused on the incorporation of nanofillers or microfibers into cement paste or mortar composites. However, there have been very limited number of studies on the development of steel-fiber-reinforced cementitious composites with self-sensing capabilities. This study explores strain sensitivity of concrete mixtures that include coarse aggregates up to 15 mm diameter and steel fibers with a length of 13 mm and a diameter of 0.25 mm. Five different concrete mixtures with steel fibers at 0%, 0.2%, 0.35%, 0.5%, and 0.8% volume ratios were fabricated. Compression tests with simultaneous measurement of strain and electrical resistance were conducted on the cubic specimens. Gauge factor and percent linearity that is a measure of error in strain sensing were calculated. Concrete mixtures with 0.5% steel fibers possess a strong linear relationship between applied strain and electrical resistance change with a gauge factor over 20 times larger than that of traditional metal strain gauges. Phenomenological models for different resistivity and gauge factors of cement paste/mortar with respect to concrete with large aggregates and short–long fiber cement composites were presented.

This study investigates the tensile behavior of ductile and brittle epoxy polymers reinforced by graphene nanoplatelets (GNPs) dispersed by various methodologies. Dispersion of GNPs into the epoxy matrix using sonication or sonication in combination with high shear mixing was investigated. Also, the effectiveness of the dispersion of GNPs into epoxy resin or hardener was assessed. Then, the effect of GNP concentration on the tensile properties of GNP/epoxy composites fabricated by three selected dispersion techniques was explored. A large number of specimens were tested under uniaxial tensile loading and the results were analyzed in terms of tensile strength, fracture elongation, and tensile modulus. Finally, scanning electron microscopy imaging was used to assess the fractured surface of the selected specimens. Results indicate that the addition of GNPs into brittle epoxy does not provide significant improvements in tensile properties. On the other hand, up to 41% increase in tensile strength and 19% increase in tensile modulus were observed for ductile epoxy with 1 wt% GNPs. However, the addition of GNPs into either brittle or ductile epoxy reduces the fracture elongation of the developed composites.

This paper proposes an adaptive friction isolation system (AFIS) composed of shape memory alloy (SMA) cables and double friction pendulum bearings (FPBs). Double FPBs with different friction coefficients enable two-level response characteristics that are advantageous at frequent and design basis hazard levels, while SMA cables with initial slack provide adaptive response at maximum considered earthquake hazard level. SMA cables are arranged vertically and they limit excessive isolation displacement by providing hardening effects and additional energy dissipation capacity. They also improve the stability of FPBs under severe earthquakes. Based on the experimental response of FPB and SMA cables, a high-fidelity finite element model of AFIS is first established in ABAQUS. Then, to study the effectiveness of the AFIS in controlling seismic response of structures, a simplified model of the AFIS is developed in OpenSees and nonlinear time history analyses of a seven-story steel frame building isolated with AFIS are conducted. The results show that the AFIS can satisfactorily meet displacement demands under frequent and moderate earthquakes, while it effectively reduces the excessive displacement of the building under severe earthquakes. In particular, the AFIS considerably improves the performance of the isolated building under pulse-like near-fault earthquakes.

This paper investigates the rheological and printability characteristics of PVA fiber-reinforced cementitious composites. To fabricate 3D printable strain hardening cementitious mixtures, ordinary Portland cement, fly ash, silica fume, fine sand, water, and a polycarboxylate-based superplasticizer are used. The effects of a modified starch-based viscosity modifying agent and nano clay on the rheological properties of these mixtures are explored. A shear rheometer with a building materials cell and vane motor is used for rheological tests. First, stress-growth tests are conducted to determine the static yield stress evolution curves for the PVA fiber-reinforced cement composites. A constant low shear rate is applied to minimize the viscous contributions to yield stress. Then, structural recovery tests are conducted by applying three different shear rates that mimic initial rest, extrusion, and after deposition conditions of printable mixtures and the change in apparent viscosity is observed. Next, structural build-up of PVA fiber-reinforced cementitious composites is assessed through constant shear rate rheology tests at different rest intervals. Finally, the buildability of the PVA fiber-reinforced cementitious composites is evaluated using a 3D concrete printer equipped with a 15 mm diameter nozzle and screw pump.

Despite being most widely used construction materials, cement-based composites are brittle materials with low tensile strength and susceptible to cracking, especially under harsh environments. Over the past three decades, numerous studies have been conducted to enhance the mechanical properties and durability of cementitious composites through the use of various nanomaterials such as carbon nanotubes (CNTs) and carbon nanofibers (CNFs). More recently, graphene nanoplatelets (GNPs) has emerged as an ideal 2D nano-reinforcement for composite materials due to their favorable mechanical, thermal and electrical properties. However, the effects of different dispersing agents and particle size and surface area of GNPs on the mechanical properties of cement-based composites needs to be further investigated. This paper explores the influence of GNP addition on the mechanical properties and durability of cement-based composites. Two types of GNPs with different lateral size (

Superelastic shape memory alloys with ultra-high-strength, large mechanical hysteresis, and the ability to recover large deformations are promising materials for damping applications. In this study, the superelastic response and damping capacity of quaternary Ni45·3Ti39.7Hf10Pd5 polycrystalline alloys are investigated in terms of temperature and loading frequency dependency. It is shown that this alloy can show fully recoverable superelastic behavior with strain amplitude of 6–7% from −40 °C to 80 °C after heat treatments. The damping capacity of 550°C-3h aged alloy at 20 °C was 22.5 J/cm3 for quasi-static loading which decreased to 12.5 J/cm3 under 1 Hz loading frequency. At the high temperature of 80 °C the damping capacity was determined 9.2 J/cm3 under quasi-static loading and 4 J/cm3 under 1 Hz loading frequency. The decrease in damping capacity with loading frequency and temperature is linked to: the changes in local temperature of the specimen which significantly increases critical stress for reverse transformation while not affecting the critical stress for forward transformation, improved compatibility of transforming phases, and decrease in the contribution of superelastic strain to the total recovered strain.

This article presents a non-destructive approach for load rating of reinforced concrete bridges without structural plans. The approach is found on a hybrid method, which employs vibration and live ...

Superelastic shape memory alloys (SMAs) are smart materials that can recover 6–8% elastic strains due to their phase transformation. SMAs also possess unique characteristics such as good energy dissipation, excellent re-centering capabilities and corrosion resistance. Recent studies have incorporated the use of superelastic SMA fibers in cementitious composites to achieve re-centering and crack-closing capabilities. Consequently, it is important to investigate the performance of fiber reinforced concrete (FRC) members under cyclic loading. This study investigates the use of hybrid steel/SMA fibers as reinforcement in concrete members subjected to cyclic flexural loading. Digital image correlation (DIC) was used to monitor the full field displacements and strains of the concrete beam specimens. Fiber density and statistical spatial point pattern functions were used to assess the fiber distribution. Two acoustic emission sensors were attached to each side of the concrete specimens to characterize crack development. A correlation between the crack width propagation and cumulative energy captured by the acoustic emission sensors was established. Results showed that the hybrid specimen with equal fiber volume ratios for steel and SMA fibers exhibit a lower mid-span deflection and smaller crack width.

Superelastic viscous damper is a passive hybrid control device that combines shape memory alloy cables and a viscoelastic damper to mitigate dynamic response of structures subjected to multi-level seismic hazards. In the hybrid device, shape memory alloy cables that exhibit a nonlinear but elastic response are used mainly as re-centering unit, while the viscoelastic damper composed of high-damped butyl rubber compounds is employed to augment the equivalent viscous damping provided by the device. This study evaluates the effectiveness of superelastic viscous dampers in mitigating seismic response of steel frame structures through a probabilistic framework. First, a nine-story steel frame building is designed and modeled with and without superelastic viscous dampers, and extensive nonlinear response-history analyses are conducted. Then, probabilistic demand models are developed for selected engineering demand parameters. To quantitatively compare the performance of the designed buildings, seismic fragility curves and mean annual frequency of exceeding different performance levels are developed. In particular, the structural performance is evaluated using both peak inter-story drift and residual drift responses. Results indicate that superelastic viscous dampers can significantly improve structural performance; thus, it has the potential to lower the post-earthquake losses, as the better structural performance leads to less loss in relocation, rental, and economic loss.

The microstructure and shape memory properties of Ni40.3Ti39.7Hf15Pd5 and Ni40.3Ti44.7Hf10Pd5 alloys were investigated. It was revealed that transformation temperatures of these alloys can be tailored by thermal treatments and composition alteration. Heat treated Ni40.3Ti39.7Hf15Pd5 can show shape memory effect at above 100 °C. Both alloys showed shape memory effect, however, superelasticity was not observed due to the large thermal hysteresis and high rate of increase in transformation temperatures with stress. Both alloys have higher strength but considerably lower transformation strain than NiTi. Microstructure features also were investigated for grains, distribution and composition of secondary phase. It was revealed that secondary phases lead to decrease the recoverable strains of the NiTiHfPd alloys. As-received Ni40.3Ti44.7Hf10Pd5 and Ni40.3Ti39.7Hf15Pd5 have recoverable strains of 1.12% and 0.43% under 700 MPa, respectively.

Load rating describes the processing of quantifying the safe live load carrying capacity of an existing bridge. For most bridges, this load rating is derived as an analytical solution based on the structural design details and operational condition state. However, for structures without plans or insufficient structural details available it is difficult to formulate an analytical solution, especially for concrete structures where internal reinforcement details are needed to determine section capacity. For structures within this category, the potential solutions involve destructive evaluation to characterize the materials and components, proof load testing, or engineering judgement-based characterization. In this paper, a nondestructive method for load rating of reinforced concrete (RC) slab bridges without structural plans is proposed. To determine a bridge’s load rating factor, the capacity as well as dead load and live load effects need to be determined. In the proposed approach, a series of finite element analyses were conducted to describe the modal properties of a large population of bridges with different geometric characteristics. Results and geometric inputs were then used to develop an artificial neural network model that predicts the flexural rigidity of a bridge based on the measured modal frequencies derived from vibration testing. Due to the uncertainty in internal geometry of concrete, nondestructive approaches are presented to obtain the cross-section dimensions of bridge as well as the elastic modulus and compressive strength of concrete. Next, the cross-sectional area of the internal reinforcing steel is estimated through a quasi-static load test coupled with an optimization approach. These structural and material properties are then used to determine load effects and ultimately the bridge’s capacity. As a validation of the proposed approach, a skewed RC slab bridge with structural plans was tested and analyzed as if plans were not available. The bridge was instrumented with accelerometers and strain gages to record its response under ambient vibration, impact excitation, and quasi-static live load testing. Results show that the proposed nondestructive approach can be used to satisfactorily determine the load rating factor of the test bridge and can ultimately be used for load rating of concrete slab bridges without structural information.

This study explores the aftershock collapse performance of steel buildings designed with superelastic viscous dampers (SVDs) under seismic sequences. The SVD strategically combines shape memory alloy (SMA) cables and a viscoelastic compound to provide good self-centering and damping capabilities. A nine-story steel special moment resisting frame (SMRF) is first designed with or without SVDs to satisfy modern seismic design requirements. A mainshock incremental dynamic analysis (IDA) is conducted for the SMRF and SVD frames using a total of ten as-recorded seismic sequences. The specific levels of post-mainshock interstory drift ratios are then induced in both frames and an aftershock IDA analysis is conducted for the mainshock-damaged buildings. The maximum interstory drift and residual drift IDA curves are developed and compared for both frames at different mainshock damage levels. The results are analyzed in terms of aftershock collapse capacity, collapse fragility, and collapse capacity at demolition. The effect of aftershock ground motion polarity on the performance of both frames is also explored. The study reveals that the SMRF has increased vulnerability to aftershocks when higher damages are induced during mainshock, while the aftershock collapse performance of the SVD frame is not affected from the intensity of mainshock event. It is also shown that the SVD frame significantly improves the aftershock capacity associated to a residual story drift that leads to major alignment or demolition.

This study explores seismic performance of steel frame buildings with SMA-based self-centering bracing systems using a probabilistic approach. The self-centering bracing system described in this study relies on superelastic response of large-diameter cables. The bracing systems is designed such that the SMA cables are always stressed in tension. A four-story steel frame building characterized until collapse in previous research is selected as a case-study building. The selected steel frame building is designed with SMA bracing systems considering various design parameters for SMA braces. Numerical models of these buildings are developed by taking into account the ultimate state of structural components and SMA braces as well as the effect of gravity frames on lateral load resistance. Nonlinear static analyses are conducted to assess the seismic characteristics of each frame and to examine the effect of SMA brace failure on the seismic load carrying capacity of SMA-braced frames. Incremental dynamic analyses (IDA) are performed to compute seismic response of the designed frames at various seismic intensity levels. The results of IDA are used to develop probabilistic seismic demand models for peak inter-story and residual inter-story drifts. Seismic demand hazard curves of peak and residual inter-story drifts are generated by convolving the ground motion hazard with the probabilistic seismic demand models. Results show that steel frames designed with SMA bracing systems provide considerably lower probability of reaching at a damage state level associated with residual drifts compared to a similarly designed steel moment resisting frame, especially for seismic events with high return periods. This indicates reduced risks for the demolition and collapse due to excessive residual drifts for SMA braced steel frames.

This study investigates the response of a shape memory alloy (SMA)-based isolation system that combines multiple groups of SMA cables and a lead rubber bearing (LRB). The isolation device, named as multi-level SMA/lead rubber bearing (ML-SLRB), is designed such that it maintains its efficiency under frequent, design and extreme levels of seismic events. Two large-size ML-SLRB isolation devices were designed and fabricated. The response of the proposed isolation systems was evaluated together with a conventional LRB under increasing amplitudes of cyclic loads. The effects of loading rate and vertical pressure on the response of the ML-SLRB isolator were evaluated. Finite element models of the fabricated ML-SLRB isolators were developed and analyzed to assess the response of the different SMA cable groups at different stages of the loading. The test results, supported by the finite element analyses, revealed that the SMA cable groups used in a loop configuration in the ML-SLRB isolator are prone to stress concentrations and early damage. The ML-SLRB isolators that employed its main SMA cable groups in a straight configuration successfully achieved a multi-level performance where the stiffness of the isolator increased as the demands of the displacement increased. The developed isolator also exhibited lower residual drifts compared to the LRB isolator.

Multifunctional self-sensing smart concrete can provide a structural health monitoring (SHM) solution that is robust, reliable, and low-cost. Smart concrete, which includes coarse aggregates with 15 mm size, brass fibers, silica fume, superplasticizer and water, has been developed as a promising multifunctional material. The normal and cross compression and split tensile tests were conducted on 75 mm cube samples; large scale compression test was conducted on 15 × 15 × 30 cm prism; large scale bending tests were applied to prisms with sizes 15 × 15 × 75 cm and 15 × 30 × 150 cm. The normal and cross compressive strain gage factors were 54 and 59 while linearities were 6.6% and 7%, respectively. Normal and cross tensile strain gage factors were 3.0 and 2.9 while corresponding linearities were 6.6% and 13%. As the sample size increased, the electrical resistivity increased and strain sensitivity decreased due to obstruction of electrons by aggregates that revealed a “size effect” for piezoresistivity of smart concrete. Large scale bending test results verified the piezoresistivity of smart concrete while crack formation and propagation increased the electrical resistance dramatically. Smart concrete can be utilized to monitor both strain and damage.

In this paper, an experimental investigation is carried out to study the effects of graphene nanoplatelets (GNPs) on the mechanical properties of cementitious composites with coarse aggregates. The concrete mixtures with GNPs concentrations ranging from 0.025% to 0.10% by weight of the cement are prepared, where a wet dispersion technique that employs high shear mixing and polycarboxylate-based superplasticizer to disperse GNPs in water. The effects of various dispersion parameters including different high shear mixing durations as well as the use of ultrasonication together with high shear mixing on the dispersion of GNPs are studied. The dispersion quality of GNPs is assessed through optical microscopy, Raman spectroscopy, and Scanning Electron Microscopy tests. Compressive strength and flexural strength tests are conducted to assess the effects of GNPs on mechanical properties of the fabricated GNP-reinforced concrete specimens. Results show that all the dispersion procedures considered in this study can disperse GNPs in water without causing any basal or vacancy defects in graphene sheets and reduce the size of graphene flakes. The use of ultrasonication together with high shear mixing leads to the smallest size graphene sheets. When the GNPs are added to the concrete mixture at a dosage of 0.025%, a maximum increase of 17% in compressive strength is observed, while no significant effect of GNPs on the flexural strength is noticed.

This study investigates the flexural behavior of reinforced concrete beams strengthened with near surface mounted (NSM) basalt fiber reinforced polymer (BFRP) bars. A total of five concrete beams with a length of 2100 mm were prepared with different internal longitudinal reinforcement and strengthening reinforcement ratios. Four-point bending tests were conducted under monotonic loading. A digital image correlation (DIC) technique was used to monitor full-field displacement and strain contours as well as mid-span deflections. The strains in the BFRP bars were recorded using strain gauges. The force-displacement curves, longitudinal strains in NSM reinforcement, and crack widths were computed and used to evaluate the performance of the strengthened beams. Results indicate that the NSM BFRP bars can successfully restore the original capacity of a concrete beam with a corroded internal reinforcement while providing a more ductile behavior. In addition, the ultimate load capacity of the reinforced concrete beams can be augmented using NSM BFRP bars but in an expense of smaller deflection capacity.

Superelastic shape memory alloys (SMAs) are metallic alloys that can recover their nonlinear deformations upon unloading. The use of SMAs in cementitious composites as fibers can enable crack recovery and re-centering capabilities. In this study, the crack recovery characteristics of SMA fiber reinforced mortar specimens subjected to flexural cyclic loading are studied through acoustic emission analysis. SMA fiber reinforced mortar specimens with fiber volume ratios of 0.3%, 0.5% and 1.0% are tested under cyclic flexural loads. Acoustic emissions (AE) signals are captured and analyzed to characterize the crack formation and crack width development at each cycle of loading. In addition, the crack width propagation of the specimens is monitored by Digital Image Correlation measurement system and is used as a basis in AE analyses. Results show that the SMA fiber reinforced mortars exhibit considerable re-centering and crack recovery characteristics at large deformation amplitudes.

Over the past decade, shape memory alloy (SMA) in the form of wires and cables have been extensively studied for various structural engineering applications. There are numerous application areas where pure compression (or coupled with tension) is the primary load bearing scenario, which requires larger size SMA bars. However, the compression behavior of SMA bars is not well known, and little is reported in the literature. In that perspective, this paper presents an experimental study on large diameter superelastic Nickel-Titanium (NiTi) bars subjected to a cyclic compression load. A total of nine SMA bars having slenderness ratios ranging from 60 to 90 were tested. Hysteretic stress-strain responses are plotted and critical buckling load, energy dissipation and residual strain of SMA bars with different slenderness ratios are presented.

Shape memory alloys (SMAs) are a class of metallic alloys that possess remarkable characteristics such as superelasticity and shape memory effect. Superelastic SMAs have been considered as fiber in polymer composites due to their ability to recover their deformation upon removal of load, good energy dissipation capacity and impact resistance. Graphene nanoplatelets (GNPs) consists of small stacks of graphene sheets that are two-dimensional. Both sides of atomic lattice of GNPs contact matrix of a composite system and can generate more sites for potential chemical and physical bonding with the host material. Most importantly, graphene sheets and their derivatives can be produced at large-scale for industrial demand at low-cost. This study explores the fabrication of multi-scale reinforced epoxy matrix composites in which GNPs and SMA strands are employed as nano- and micro-scale reinforcements, respectively. First, GNPs are dispersed into a ductile and brittle epoxy matrix to produce GNP/epoxy nanocomposites. To study the effect of GNP content on the behavior of the developed nanocomposite, GNPs are added to the epoxy-hardener mixture at different weight percentages (neat, 0.1%, 0.25%, 0.5%, 1%, and 2%). Uniaxial tensile tests of the developed nanocomposites are conducted under monotonic load up to failure. The optimum GNP content for GNP-reinforced epoxy matrix is determined and used in the fabrication of SMA fiber/epoxy composite. The developed multiscale reinforced epoxy composites are tested under tensile loading and their full-field strain and temperature behavior are monitored and evaluated using a digital image correlation system and an infrared thermal camera.

Over the past decades, a number of structural health monitoring methods have been developed for condition assessment of concrete structures. Most of these methods require the installation of external sensors. Accelerometers are commonly used for vibration-based damage detection for the entire structure, while strain gauges are installed in order to detect cracking and damage at the component level. Conventional strain sensors, such as metal foil strain gauges, have been widely used to monitor local conditions in concrete structures. However, all of these sensors have certain shortcomings such as exhibiting limited durability and low gauge factor, and providing only pointwise strain measurements. Multifunctional cement-based composites that can determine their own strain and damage can overcome the limitations of these traditional sensors. This study explores the use of two different nanomaterials, namely graphene nanoplatelets (GNP) and carbon black (CB) for the development of self-sensing cementitious composites and the synergetic effects in their hybrid utilization. A simple fabrication method that does not require special treating procedures such as ultrasonication for dispersing nanomaterials is pursued. Twelve batches of mortar specimens reinforced with only GNP or CB at different concentrations, or with both GNP and CB fillers are prepared. A polycarboxylate-based superplasticizer is used to disperse nanomaterials and to increase the workability of the nano-reinforced mortar. Scanning electron microscope (SEM) is utilized to assess the distribution quality of nanomaterials. Standard cubic specimens are tested for compressive strength at 28 days. The bulk resistivity of the standard prismatic specimens is measured using the four-point probe method. The piezoresistive response of nano-reinforced cement composites is evaluated under the cyclic compressive loads.

Fiber reinforced polymer (FRP) composites have been increasingly used in engineering applications due to their lightweights, high strength, and high corrosion resistance. However, the conventional FRPs exhibit brittle failure at relatively low ultimate tensile strains, low toughness, and limited fatigue strength. Shape memory alloys (SMAs) are a class of metallic alloys that can recover large strains upon load removal with minimal residual deformations. Besides their ability to recover large deformations, SMAs possess excellent corrosion resistance, good energy dissipation capacity, and high fatigue properties. This study investigates the cyclic behavior of composite materials that consists of a thermoset polymer matrix reinforced with superelastic NiTi SMAs wires. SMA-FRP coupons with three different reinforcement ratios were fabricated using a special-made mold and following a modified hand lay-up technique. The uniaxial tensile tests were conducted under cyclic loading protocols at various stress levels to characterize the behavior of the composite. Low-cycle fatigue properties of SMA-FRPs were also investigated. Microstructural analysis using the scanning electron microscopy (SEM) technique was conducted on fractured surfaces to fully understand the failure mechanism. Results revealed that the SMA-FRP composites can recover relatively high strains upon unloading and exhibit very high failure strains. Keywords: Fiber-reinforced polymers, Shape memory alloys, Cyclic behavior, Superelastic SMA-FRP, Low-cycle fatigue

This paper discusses vibration serviceability assessment of a highly trafficked local footbridge based on the experimental tests and analytical studies. The selected bridge is an approximately 60 m (196 ft) long multi-span steel structure with a continuous reinforced concrete slab supported on two longitudinal steel girders. The experimental study consists of ambient vibration and pedestrian interaction tests to describe the dynamic characteristics of the selected bridge structure. The fundamental frequency of the bridge in the vertical direction obtained through ambient vibration tests was within the critical range described by available design guidelines. This required further analysis to assess the performance of the bridge relative to the maximum acceleration thresholds. In addition to the peak dynamic response obtained from the pedestrian interaction tests, peak acceleration values were calculated analytically based on current design guidelines and compared to the comfort limits. Results from both experimental and analytical studies suggest that the footbridge possesses satisfactory serviceability performance under low and dense traffic conditions, but the comfort level under very dense traffic loads was classified as minimum according to the results of the analytical calculations.

This study investigates the seismic collapse resistance of steel moment resisting frames equipped with superelastic viscous dampers (SVD) through incremental dynamic analysis (IDA). The SVD is a hybrid passive control device that strategically combines a viscoelastic device and shape memory alloy cables in parallel. The hybrid device exhibits improved re-centering and energy dissipating capabilities compared to only viscoelastic or only SMA-based devices. First, the design and mechanical behavior of SVDs are described. A nine-story steel frame building is selected for the numerical analyses. The building is first designed as a conventional special moment resisting frame (SMRF) to meet the strength and stiffness requirements of the design codes. Then, a reduced strength version of the fully code compliant frame is developed and upgraded with either SVDs or buckling restrained brace (BRB) system. Analytical models of the steel building for each configuration are developed to simulate global frame behavior by considering both geometric nonlinearities and cyclic strength and stiffness deterioration of structural steel components under dynamic loads. Incremental dynamic analysis is employed to assess the seismic resistance of steel frame structures up to collapse using 44 ground motion records. A sensitivity analysis is also performed to evaluate the influence of SVD design parameters on the seismic response of the frame. The results indicate that the steel frame designed with SVDs has the largest median collapse capacity and minimal residual drifts under various seismic hazard levels.

One of the main factors that lead to the deterioration of reinforced concrete structures is the corrosion of reinforcing steel. The aluminum alloy (AA) bars, which have favorable characteristics such as good ductility, low specific weight, good corrosion resistance, and recyclability, can be used as an alternative to steel reinforcement to increase service life of concrete structures. This study investigates the feasibility and performance of AA reinforced concrete beams. A total of nine specimens reinforced with AA bars and two specimens reinforced with plain steel bars, which serve as benchmark, were fabricated and tested under four-point bending up to failure. The longitudinal reinforcement ratio and the concrete strength were the main test variables for the specimens. The load-deflection curves, failure modes, crack patterns, crack width, and reinforcement strains were evaluated and discussed for each specimen. A modified section analysis and a strut-and-tie model were used to predict the load carrying capacities of AA reinforced beams for flexural and shear failure modes. The results indicate that the AA bars, if properly treated, can be utilized as reinforcement in concrete beam with satisfactory performance.