Steel-FRP composite bar (SFCB) is promising for marine infrastructure by combining high stiffness, strength, and ductility with superior durability. However, the construction of marine infrastructure is still hindered by the brittleness of concrete, the lack of freshwater and river sand, and harsh construction conditions. This study proposes a novel solution by integrating SFCB with seawater sea-sand engineered cementitious composite (ECC) permanent formwork and seawater sea-sand concrete. Experimental and analytical investigation was conducted on composite slabs, examining the effects of reinforcement type and ratio, ECC thickness, interface treatment, and fiber content. Results indicate that bottom ECC acted as permanent formwork, suppressed crack propagation, and prevented shear failure. While SFCB maintained high stiffness and strength, slabs with high reinforcement ratios were prone to shear failure, which was mitigated by ECC layer. Longitudinal grooves outperformed transverse ones at ECC-concrete interface, while low fiber content in ECC caused ineffective shear crack restraint and interfacial failure. Further adding ECC at the top of slabs improved deformability, increasing bearing capacity and ductility by up to 87.7 % and 107.9 %, respectively. Cross-sectional analysis identified three failure modes, and calculation methods for reinforcement limits and flexural capacity were proposed. These results support the safe design of high-performance composite slabs.

To evaluate the seismic performance of high-rise modular structures incorporating innovative grouted joints, seismic tests were carried out in this study on a substructure taken from an actual 20-story modular structure. Cyclic loading tests were performed on a full-scale, two-story, two-span substructure under realistic axial compression ratios to systematically evaluate the seismic performance of the structural system. The test specimen consisted of box-shaped welded steel columns, box-shaped cold-formed steel beams, and innovatively designed grouted joints. A novel grouting process enabled rapid assembly of the modules. Detailed descriptions of the specimen design, test apparatus, loading scheme, construction process, and measurement schedule were provided. The experimental behavior of the beams, columns, and joints were presented in detail. The hysteretic performance of the specimen, including hysteretic curves, skeleton curves, global deformation, and energy dissipation behavior, was thoroughly discussed. The results demonstrated that the structure exhibited excellent cyclic performance, lateral bearing capacity, and ductility, with an ultimate loading drift ratio up to 5% rad. The primary failure mode was characterized by local buckling and tearing at the ends of the module beams, with plastic hinges primarily forming at the beams of each story and the bases of the columns at the bottom story. This study provides important experimental evidence for the seismic design of modular steel structures and validates the engineering applicability of the novel grouted joint.

In this study, an icing model of a transmission line is established by using the secondary development of the Fluent user-defined scalar (UDS) transport equation. An Eulerian two-phase flow model is established by using the UDS transport equation framework to determine the impact of water droplets and the local water-droplet collection coefficient. As per the Messinger thermodynamic equilibrium principle, a user-defined function is developed to assess the icing amount on the microelements of the wire surface and convert it into the ice thickness. To verify the accuracy of the results, the calculation reference values, results, and procedures of the Lagrangian and Euler methods developed by using the discrete phase model are compared and analyzed. The two-dimensional three-dimensional results are compared to determine the relationship between the three-dimensional effect and particle size of the water droplets.

Steel corrosion risk has motivated the development of hybrid reinforcement systems (combining steel-FRP composite bars and FRP ties) in marine concrete infrastructure, especially for seawater sea-sand concrete. However, conventional pultruded FRP ties suffer from low confinement efficiency, adversely affecting the axial-bending behavior of concrete columns by reducing bearing capacity and ductility. Novel closed FRP ties offer significant potential to mitigate these limitations. To elucidate the confinement mechanism and optimize the design of closed FRP tie, this study experimentally investigates hybrid reinforcement reinforced concrete (hybrid-RC) columns with varied tie parameters under combined axial-bending loading. Results demonstrate that replacing conventional steel reinforcement with the novel hybrid system yields similar failure modes while maintaining comparable bearing capacity in compression-controlled failures and achieving a 12.5% increase in tension-controlled failures. Notably, increasing tie ratio has a marginal effect on bearing capacity but significantly improves ductility. The high strength and linear-elastic behavior of closed FRP ties enable continuously enhanced confinement. Furthermore, the strength limit of closed FRP ties is recommended. FRP ties with equivalent strength effectively substitute steel ties in columns under axial-bending load. These findings offer valuable insights for developing and designing durable and ductile hybrid reinforcement systems for marine applications.

This study presents a novel offshore expandable HMFS system that integrates the functions of a floating breakwater and wave energy converters (WECs), offering an alternative approach to marine space development. A scaled experimental model was developed, consisting of hybrid functional modules, inter-module connectors, a mooring system, and integrated WEC units. The design incorporates WECs driven by parallel-axis gears and hinge connectors with linear torsional stiffness. The research experimentally examines the effects of connector stiffness, longitudinal spatial expansion, and mooring line fracture on key dynamic responses. The findings demonstrate that enhanced connector stiffness reduces module motion responses while increasing connector loads, providing experimental guidance for connector selection. The hinge-connected system demonstrates stable dynamic performance, while the rigid-connected system exhibits significant increases in pitch bending moment during longitudinal spatial expansion. Analysis of single mooring line fracture suggests avoiding systems with minimal mooring lines to prevent excessive planar displacement. These findings provide valuable experimental insights for the engineering implementation of the HMFS system.

This study investigates the tension-tension fatigue performance of pultruded carbon fiber-reinforced polymer (CFRP) bars, employing expansion cement grout as a load transfer medium (LTM) for bonded-type anchorages. The transverse shear strength, apparent interlaminar shear strength, and ultimate tensile strength (UTS) of CFRP bars were first quantified. Subsequently, tension-tension fatigue tests under three stress ratios (R = 0.3, 0.5, and 0.65) were conducted at 25 degrees C, respectively, generating corresponding stress-life (S-N) and probabilistic stress-life (P-S-N) curves through Basquin's model and Weibull distribution fitting. Notably, the fatigue limit boundary was determined based on the Goodman relation, with safe stress amplitudes identified as 385 MPa (R = 0.3), 300 MPa (R = 0.5), and 245 MPa (R = 0.65). Throughout the fatigue testing, the bonded-type anchorage demonstrated exceptional performance, exhibiting cumulative residual displacements within 0.15 mm after 2 million load cycles. Furthermore, the fatigue damage accumulation in CFRP bars manifests a highly nonlinear trend: minimal damage occurs during the initial 90 % of fatigue life, followed by rapid degradation in the final 10 % of fatigue life prior to fracture. A model has been proposed which can effectively describe the nonlinear damage evolution. Additionally, the long-term sensing performance of carbon fiber reinforced polymer-optical fiber Bragg grating (CFRP-OFBG) bars was evaluated. No apparent Bragg wavelength drift was observed over 2 million fatigue cycles, confirming their sensing stability.

Aerodynamic characteristics of a square cylinder fitted with vertical axis wind turbines (VAWTs) at its corners are experimentally investigated. Detailed analyses are conducted on the effects of VAWT-to-cylinder spacing, wind turbine arrangement, and wind turbine solidity. The principle of optimizing the aerodynamic characteristics of the square cylinder is elucidated using proper orthogonal decomposition (POD). In addition, the flow control effect under various relationships between the rotational direction of VAWTs and the direction of incoming flow is examined. The results demonstrate that the integration of VAWTs can significantly suppress both the fluctuating lift and mean drag of the square cylinder. The flow control effect improves as the VAWT-to-cylinder spacing decreases. At the optimal spacing of 0.1d, the fluctuating lift and mean drag of the square cylinder are reduced by 25% and 10%, respectively. Installing four VAWTs provides better flow control than installing two VAWTs, with the upstream wind turbines playing a more significant role in flow control. A smaller wind turbine solidity is more favorable for flow control, whereas increased solidity enhances the rotation of VAWTs installed at the corners of the square cylinder. The presence of the VAWTs decreases both the energy proportion and POD coefficient amplitude of the principal mode of the surface pressure coefficient, leading to a notable suppression of the fluctuating lift of the square cylinder. The flow control is always effective in the wind direction critical to wind-induced vibration control. Furthermore, the flow field around the square cylinder can promote the rotation of the corner-mounted VAWTs, particularly the downstream ones.

The blade installation system for large offshore wind turbines comprises multiple components and presents considerable operational challenges. This system primarily consists of the installation vessel, the ship-mounted crane, and the double pendulum system, with the transmission of dynamic responses among these components exhibiting significant complexity. Jack-up and floating installation vessels are widely deployed for the installation; however, different types of vessels will produce varying dynamic responses. These distinct responses serve as the motion boundaries of the installation system, ultimately affecting the blade oscillations. Therefore, this investigation develops an analysis program for blade installation and evaluates the dynamic responses of the system when different vessels are employed. The results indicate that, during operation with the jack-up vessel, blade oscillations are mainly governed by the first order mode frequency of the double pendulum system and the turbulent wind frequency. In contrast, when the floating vessel is utilized, the spectral peak frequency of the wave and the natural frequencies of the vessel must also be considered. Moreover, if the spectral peak frequency closely aligns with the first order mode frequency of the double pendulum system and the natural frequencies of the vessel, the blade oscillations will be significantly amplified.

Submerged floating tunnels (SFTs) are primarily subjected to environmental loads from waves and currents. Based on a previously developed multibody dynamics model, this study introduces a wave–current interaction (WCI) formulation, establishing a numerical framework for coupled analysis. Using measured environmental data under WCI conditions, the dynamic responses of SFTs are investigated under following, opposing current, and pure wave conditions.The results show that WCI exhibits a clear period-dependent effect on wave forces. For T* > 3.6, the horizontal wave force is slightly larger under following current, while for T* < 3.6, it is maximized under opposing current conditions. The vertical wave force shows a similar trend, with a critical period of T* = 3.14. Under regular waves, both horizontal and vertical displacements at the middle location are largest under following current and smallest under opposing current conditions. With increasing wave height, amplification (following) and attenuation (opposing) effects increase linearly, with vertical responses being more sensitive. Under irregular waves, based on the standard deviation, responses follow following > pure > opposing for short peak periods (8.5 s), while differences are minor for long peak periods (14.3 s). The coupling effect is significant for short periods and remains within ±5% for long periods.

In the design of offshore wind turbine monopile foundations per ultimate limit states (ULS) criteria, the p-y curves specified in API code for sand significantly overestimates the lateral bearing capacity of monopiles, as it fails to accurately predict the initial subgrade reaction modulus (E*py) and the ultimate soil resistance (pu), while neglecting the effects of small-strain stiffness in sand. This study developed a refined finite element model for pile-soil interaction by integrating a conventional hypoplastic model and an intergranular strain-based model, the latter of which incorporates small-strain stiffness characteristics. The effects of pile diameter, embedment depth, relative density, and small-strain stiffness of sand on the p-y curves were systematically analysed. Building upon Broms’ (1964) formula, the improved pu expression integrates both the density correction factor (CR) and the diameter-depth coupling factor (CzD) to capture nonlinear influences of these parameters. The modified p-y curves method is scaled from a hyperbolic soil stress-strain correlation and incorporates the small-strain stiffness behaviours of sand (denoted as SS p-y curves). The results reveal that the small-strain stiffness reduction behaviour significantly influences the subgrade reaction modulus (Epy) at small pile displacements (where y < 0.02D), E*py increases exponentially with embedment depth and logarithmically with pile diameter. The SS p-y curves method achieves superior predictive accuracy for ultimate lateral bearing capacity, with discrepancies under 5% for flexible piles and within 13% for semi-rigid and rigid piles.

The appropriate application of model predictive control (MPC) for oscillating buoy wave energy converters (OBWEC) can significantly improve energy capture efficiency while ensuring safe operation. In this paper, a gradient-based MPC, demonstrated to have high computational efficiency, is proposed for OBWEC. The state space model of buoy is established based on the Cummins theory, and wave prediction is achieved through deterministic sea wave prediction method. The optimal control problem of MPC is formulated as a two-point boundary value problem and solved using the gradient projection algorithm with adaptive line search method. To handle the constraints associated with the physical limitations of the power-take-off system, the augmented Lagrangian approach is implemented. For benchmarking purposes, passive control is utilized for algorithm parameter tuning under regular wave conditions. The proposed controller performance under irregular wave conditions with prediction errors is subsequently evaluated against an MPC approach formulated as a nonlinear programming problem and solved via sequential quadratic programming. Comparative results demonstrate that the proposed gradient-based MPC method achieves a notable improvement in energy capture efficiency relative to these benchmark strategies.

In marine environment, reducing the alkalinity of seawater sea sand concrete (SWSSC) is an effective approach to prolonging the service life of basalt fiber reinforced polymer (BFRP) bars. This study analyzed the previous experimental data on the deterioration of FRP and identified the optimal environmental pH range for delaying the degradation of BFRP. A design method for low-alkalinity SWSSC was proposed using activated excavated sediment and silica fume as supplementary cementitious materials. Microscopic characterization techniques were employed to analyze the evolution of the microstructure and chemical composition of low-alkalinity SWSSC, revealing the underlying alkalinity regulation mechanism. Results indicated that in the dual-component admixture system, activated sediment and silica fume consumed Ca(OH)2 through pozzolanic reactions, generating secondary hydrated calcium silicate gel while inhibiting the formation of the AFm phase. This process reduced the pH value of pore solution from 12.8 to 11.6 while maintaining compressive strength. Furthermore, to evaluate the feasibility of applying low-alkalinity SWSSC in BFRP-reinforced structures, the long-term mechanical performance and bond-slip behavior of BFRP bars embedded in low-alkalinity SWSSC were examined. The low OH-concentration in low-alkalinity SWSSC decreased the activation energy for resin hydrolysis, significantly retarding the degradation of BFRP. This resulted in a 47.7 % increase in the long-term tensile strength of BFRP bar and a 57.5 % improvement in long-term bond strength between BFRP bar with low-alkalinity SWSSC. This study promotes the application of low-alkalinity SWSSC in marine infrastructure, providing novel insights for enhancing the durability of FRP-reinforced concrete structures.

Tuned liquid damper (TLD) mitigates excessive vibration responses of offshore platform structures by utilizing liquid oscillation within a tank. However, the conventional pure-water TLD exhibits limited energy dissipation capacity, which is often insufficient to meet structural vibration control requirements. To enhance the damping performance of TLD, the incorporation of internal obstruction devices is necessary. Moreover, when applied to offshore platform structures, the large dimensions of the TLD tank and the significant liquid sloshing forces require robust internal supporting components to ensure safe and stable operation. In this study, an innovative TLD configuration with built-in rectangular poles is proposed. First, based on computational fluid dynamics (CFD), the two-phase flow solver in OpenFOAM is improved by coupling the level set and volume of fluid (VOF) algorithms (CLS-VOF). This improvement effectively suppresses spurious flows and enhances the accuracy of free-surface capturing. The numerical simulation results show good agreement with experimental data, demonstrating the reliability and accuracy of the proposed numerical model. Subsequently, the effects of critical parameters, including the liquid filling level, number of poles, pole installation position, pole blockage ratio, and excitation amplitude, on the nonlinear sloshing behavior of the internal liquid are systematically investigated, establishing a fundamental database of the dynamic characteristics of the rectangular liquid tank with poles. Then, an equivalent mechanical model is developed using the particle swarm optimization (PSO) algorithm to estimate the sloshing frequency and damping performance of the rectangular liquid tank with poles. The theoretical predictions are compared with numerical simulation results to validate the accuracy of the proposed model. The equivalent mechanical model enables rapid and reliable determination of the dynamic parameters of the TLD, providing effective support for the preliminary design and optimization of TLD in engineering applications.

In this paper, the authors conducted full-scale seismic tests on a modular steel frame, monitoring strains in beams, columns, and grouted connections, and measuring deformations in the connections. The first accompanying paper analyzes and discusses the overall response of the frame. This paper further investigates the seismic performance of the components and connections tested in the frame. Compared to conventional T-shaped joint and cruciform joint tests, the use of realistic boundary conditions in the connections allows for a more accurate study of the cyclic response of grouted connections. The seismic responses of module beams and columns were analyzed. The hysteretic behavior, skeleton curves, and energy dissipation of the grouted connections were discussed. Based on this information, the plastic development sequence and failure mode of the frame were summarized. Results show that the components and connections exhibit good cyclic performance, deformation capacity, and energy dissipation. The plastic development sequence and failure mode of the frame are yielding of the module beams (drift ratio of 0.67% rad - 0.86% rad), yielding of the horizontal connection plates (0.8% rad), yielding of the column bases (0.81% rad - 0.99% rad), and yielding of the modular column bases (0.94% rad - 1.33% rad).

Durable fiber-reinforced polymer (FRP) bars are well-suited to marine infrastructure, particularly in applications involving seawater sea-sand concrete. However, their adoption is constrained by the low elastic modulus and brittle failure behavior of longitudinal FRP bars, as well as the inadequate confinement provided by conventional pultruded FRP ties. These limitations adversely affect serviceability (e.g., deflection and crack width), strength, and ductility of concrete columns subjected to axial–flexural loading. To address these challenges, this study proposes a novel hybrid reinforcement scheme for concrete columns, combining longitudinal steel–FRP composite bars (SFCBs) with closed-type FRP ties to enhance axial–flexural performance. An experimental program was conducted on hybrid longitudinal SFCB and FRP tie reinforced concrete (hybrid-RC) columns under various loading conditions to elucidate axial–flexural failure mechanisms, quantify the mechanical contribution of SFCBs, and evaluate the confinement behavior of closed-type FRP ties. Similar to conventional steel bars reinforced concrete columns, hybrid-RC columns exhibit compression-controlled and tension-controlled failure modes while achieving comparable serviceability, strength, and ductility. Notably, the post-yielding stiffness of SFCBs increases the load-carrying capacity by 11.3% under tension-controlled failure. In addition, closed-type FRP ties provide significantly improved confinement compared with pultruded FRP ties, resulting in substantial enhancement of axial–flexural capacity. An analytical model is further developed to predict axial–flexural failure modes and strength, incorporating the distinctive constitutive behavior of the hybrid reinforcement system and the experimentally observed failure mechanisms of hybrid-RC columns.

During long-term service, modular structures are subjected to various loads, and their key components, intermodule connections, experience complex multiaxial stress states. This paper investigates the performance of inter-module grouted connections under combined axial force and bending moment. Building upon previous experimental studies, the compressive-bending failure modes and force transfer mechanisms of the connection are revealed through numerical simulation and theoretical analysis, and corresponding calculation methods are proposed. The numerical model is first validated, followed by a comprehensive parametric study. The results show that increasing the axial compression ratio, grout height, and inner sleeve thickness can all significantly enhance the flexural resistance of the connection, among which the axial compression ratio has the most significant influence on flexural performance. When the axial compression ratio increases from 0 to 0.8, the flexural bearing capacity of the connection increases by 49.7%. The observed failure modes include failure of the grouted connection, simultaneous failure of both the grouted connection and the connected column, and failure of the connected column. In addition, the stiffness characteristics of the grouted connection are analyzed, and the required stiffness for equivalent-stiffness design is theoretically derived. Furthermore, a predictive equation for the compressive-bending resistance of the connection is proposed, and a conservative envelope curve for the compressive-bending resistance is provided. Finally, the recommended dimensions and design guidelines for module columns to achieve equivalent-strength connections using grouted connections are presented.

In recent years, global renewable energy has developed rapidly, with the offshore wind industry experiencing particularly notable growth. Approximately 80 % of the potential offshore wind energy resources globally are located in areas where water depths exceed 60 m, making floating offshore wind turbines (FOWTs) especially semi-submersible platforms, a focus of research. However, existing semi-submersible platforms are predominantly steel, and their diverse designs hinder standardised manufacture and drive up construction costs. To address the lack of studies on optimizing, characterising, and verifying the structural strength of semisubmersible FOWT's platforms with reinforced concrete, this paper proposes a modular-based conceptual design for a novel semi-submersible FOWT's platform that supports standardisation. This research carry out main dimension parameter design, select optimization variables, and establish an integrated multi-objective optimization framework driven by long-term dynamic responses. A reduced-order model (ROM) computes long-term dynamic response and economic indicators as objective functions, while verifying whether the design scheme satisfies the constraint conditions, and the NSGA-II genetic algorithm is used to trace the Pareto front. The results demonstrate good convergence and significant objective-function improvements, yielding multiple promising design solutions. Selecting the cost-optimal solution, a comprehensive performance evaluation was conducted and compared with the LIFES 50+ OO-Star scheme; numerical simulations show that the novel platform outperforms OO-Star in long-term service performance while dramatically reducing total construction cost. Finally, a finite element model of the optimal design is used to assess structural strength under Ultimate Limit State (ULS) conditions, confirming that the platform with added prestressed tendons meets strength requirements. Overall, this work has significant potential for engineering applications and provides a comprehensive reference for the structural design and optimization of semi-submersible FOWT's platforms with reinforced concrete.

Transmission lines are highly sensitive to wind, making their safety under tropical cyclones (TCs) critical. While reliability theory provides a framework for assessing structural performance, uncertainties in dynamic wind loads, which are commonly represented by gust response factors (GRFs), remain insufficiently characterized, limiting accurate risk assessment. This paper develops probabilistic models of GRFs to capture the randomness of dynamic loads on transmission tower-line systems (TTLSs). Three representative TTLSs with voltages of 110 kV, 220 kV, and 500 kV are employed, and the static finite element analysis (FEA) using equivalent static wind loads (ESWLs) is compared with the nonlinear time-history analysis (NLTHA) to quantify tower-line coupling effects. Random GRF models accounting for TC wind turbulence are developed, and sensitivity analysis is performed to identify the key factors driving GRF variability. Nonlinear static pushover analysis (NSPA) and incremental dynamic analysis (IDA) reveal differences in failure modes and structural capacity under TC winds. Fragility and reliability assessments indicate that GRF randomness increases tower failure probability at moderate wind speeds but reduces fragility under extreme winds, whereas neglecting GRF variability leads to overestimated tower reliability, underscoring the importance of incorporating dynamic load uncertainty in design and risk management of transmission lines.

The novel hybrid hydrophobic longitudinal steel-FRP composite bars (SFCBs) and FRP hoops reinforced lowalkalinity seawater sea sand concrete (SWSSC) column has been developed. It overcomes the issues of steel bar corrosion and the shortage of freshwater and river sand. However, its long-term performance in marine environment remains unclear. Hence, this study investigates the effects of marine exposure on the failure modes, long-term load capacity and ductility of hybrid-reinforced column. Results show that the hydrophobic coating on reinforcement and the low OH- concentration of concrete effectively inhibits the degradation of FRP and the FRP-concrete interfacial debonding, reducing localized strain concentration and significantly improving the longterm performance of hybrid-reinforced column. After immersion in seawater at 55 degrees C for 8 months, the load capacity and pre-peak ductility coefficient of hybrid-reinforced column are 34.7 % and 25.0 % higher, respectively, than those of conventional reinforced SWSSC column. Moreover, a prediction model for long-term load capacity, incorporating the time-dependent deterioration of materials, is proposed and validated, showing superior feasibility and accuracy. It predicts that the service life of hybrid-reinforced column is approximately 3time longer than that of conventional column. These findings provide reliable theoretical models and experimental evidence to support durable design of marine infrastructure.