Mohammad AlHamaydeh

Various control strategies have been developed to mitigate wind turbine (WT) vibrations. Among these, active vibration control offers superior performance, although its effectiveness can be compromised by time delays stemming from the relative complexity of the control system. This study investigates the feasibility of a novel long short-term memory (LSTM)–driven active cable-based damper (LSTM–ACD) for WT vibration control, in which the LSTM neural network functions as the active control algorithm addressing the time-delay issues. The LSTM network trained using tower-top response data with given time delays as inputs, while the corresponding ideal control forces generated by the Linear Quadratic Regulator (LQR) serve as outputs. Detailed theoretical mechanisms of cable-based control and LSTM–ACD are introduced first. Subsequently, a coupled excitation–WT–LSTM–ACD model is established using an NREL 5 MW WT as a case study. The control performance of an active cable-based damper with LQR control is then evaluated under seismic and wind load scenarios, and the LSTM–ACD’s time-delay compensation capability is validated under seismic excitations. Results demonstrate that: (1) ideal active control outperforms the passive counterpart in seismic and wind load scenarios; (2) the LSTM–ACD achieves comparable vibration mitigation performance compared to the ideal LQR-based controller; and (3) the LSTM–ACD exhibits robustness against variations of time delays. These findings demonstrate that the LSTM–ACD is an effective and robust active control method for WTs, showing a strong potential to address the time-delay challenges in active vibration control systems.

This study examines the influence of steel fiber reinforcement on the mechanical properties of geopolymer concrete incorporating different slag to fly ash binder ratios (75:25, 50:50, and 25:75). Three fiber contents (0%, 1%, and 2%) by volume were used to assess their impact on compressive strength, flexural strength, initial stiffness, and toughness. Compressive tests were conducted at 1, 7, and 28 days, while flexural behavior was evaluated through a four-point bending test at 28 days. The results showed that geopolymer concrete with 75% slag and 25% fly ash experienced the highest compressive strength and modulus of elasticity, regardless of the steel fiber content. The addition of 1% and 2% steel fiber content enhanced the compressive strength by 17.49% and 28.8%, respectively, compared to the control sample. The binder composition of geopolymer concrete plays a crucial role in determining its compressive strength. Reducing the slag content from 75% to 50% and then to 25% resulted in a 15.1% and 33% decrease in compressive strength, respectively. The load–displacement curves of the 2% fiber-reinforced beams display strain-hardening behavior. On the other hand, after the initial crack, a constant increase in load causes the specimen to experience progressive strain until it reaches its maximum load capacity. When the peak load is attained, the curve gradually drops due to a loss in load-carrying capacity known as post-peak softening. This behavior is attributed to steel’s ductility and is evident in specimens 75S25FA2 and 50S50FA2. Concrete with 75% slag and 25% fly ash demonstrated the highest peak load but the lowest ultimate displacement, indicating high strength but brittle behavior. In contrast, concrete with 75% fly ash and 25% slag showed the lowest peak load but the highest displacement. Across all binder ratios, the addition of steel fibers enhanced the flexural strength, initial stiffness, and toughness. This is attributed to the bridging action of steel fibers in concrete. Additionally, steel fiber-reinforced beams exhibited a ductile failure mode, characterized by multiple fine cracks throughout the midspan, whereas the control beams displayed a single vertical crack in the midspan, indicating a brittle failure mode.

This paper comprehensively reviews structural vibration control systems for earthquake mitigation in civil engineering structures. Structural vibration control is vital for enhancing the resilience and safety of infrastructure subjected to seismic activity. This study examines various control strategies, including passive, active, and hybrid methods, with a focus on the advantages of semi-active systems, which offer a balance of energy efficiency and adaptive capabilities. Semi-active devices, such as magnetorheological dampers, are highlighted for their ability to offer adaptive control without the high energy demands of fully active systems. The review discusses challenges like time delays, sensor placement, and model uncertainties that can impact the practical implementation of these systems. Experimental studies and real-world applications demonstrate the effectiveness of semi-active systems in reducing seismic responses. This paper emphasizes the need for further research into optimizing control algorithms and addressing practical challenges to enhance the reliability and robustness of these systems. It concludes that semi-active control systems are a promising solution for enhancing structural resilience in earthquake-prone areas, offering a practical alternative that strikes a balance between performance and energy requirements.

Reinforced concrete (RC) beams with openings in shear spans exhibited a significantly reduced structural performance due to disruptions in load transfer mechanisms. This numerical study investigated the influence of pre-stressed iron-based Shape Memory Alloy (Fe-SMA) rebars on the behavior of RC beams with web openings, focusing on the effect of shear-oriented design parameters, including the stirrup spacing, stirrup diameter, and horizontal reinforcement around the opening. A nonlinear finite element analysis (NLFEA) was conducted using ABAQUS/CAE software 2020 to simulate the response of RC beams under these conditions. The results showed that the presence of web openings in RC beams reduced the ultimate load capacity and stiffness. However, the pre-stressed Fe-SMA reinforcement effectively mitigated these adverse effects, restoring much of the solid beam’s performance. Among the studied parameters, reducing the stirrup spacing significantly improved the load-bearing capacity, with the smallest spacing (100 mm) restoring 86% of the solid beam’s ultimate load. Increasing the Fe-SMA stirrup diameter further enhanced performance, with T16 stirrups recovering 92% of the solid beam’s ultimate load capacity. The most substantial improvement occurred when horizontal reinforcement was introduced, particularly with T16 stirrups, achieving a 95% load recovery, nearly matching the solid RC beam structural performance. These findings demonstrated the promising potential of pre-stressed Fe-SMA reinforcement as a viable solution for restoring the structural strength of RC beams with web openings.

This comprehensive review paper delves into the utilization of Glass Fiber-Reinforced Polymer (GFRP) composites within the realm of concrete column reinforcement, spotlighting the surge in structural engineering applications that leverage GFRP instead of traditional steel to circumvent the latter’s corrosion issues. Despite a significant corpus of research on GFRP-reinforced structural members, questions about their compression behavior persist, making it a focal area of this review. This study evaluates the properties of GFRP bars and their impact on the structural behavior of concrete columns, addressing variables such as concrete type and strength, cross-sectional geometry, slenderness ratio, and reinforcement specifics under varied loading protocols. With a dataset spanning over 250 publications from 1988 to 2024, our findings reveal a marked increase in research interest, particularly in regions like China, Canada, and the United States, highlighting GFRP’s potential as a cost-effective and durable alternative to steel. However, gaps in current knowledge, especially concerning Ultra-High-Performance Concrete (UHPC) reinforced with GFRP, underscore the necessity for targeted research. Additionally, the contribution of GFRP rebars to compressive column capacity ranges from 5% to 40%, but current design codes and standards underestimate this, necessitating new models and design provisions that accurately reflect GFRP’s compressive behavior. Moreover, this review identifies other critical areas for future exploration, including the influence of cross-sectional geometry on structural behavior, the application of GFRP in seismic resistance, and the evaluation of the size effect on column strength. Furthermore, the paper calls for advanced studies on the long-term durability of GFRP-reinforced structures under various environmental conditions, environmental and economic impacts of GFRP usage, and the potential of Artificial Intelligence (AI) and Machine Learning (ML) in predicting the performance of GFRP-reinforced columns. Addressing these research gaps is crucial for developing more resilient and sustainable concrete structures, particularly in seismic zones and harsh environmental conditions, and fostering advancements in structural engineering through the adoption of innovative, efficient construction practices.