This study investigates the use of External Adaptive Tensioning (EAT) systems as a retrofit strategy to reduce the structural response and fatigue damage in steel bridges. A high-fidelity three-dimensional model, including detailed welded-joint sub-models, is combined with the Active System Utilisation (ASU) metric to account for actuator reliability and fallback configurations. Results show that EAT reduces the stress response by up to 66% relative to the unretrofitted condition and achieves near-zero fatigue damage at critical welded details under representative traffic loading. Even with partial downtime (ASU < 1), EAT provides substantial gains in remaining service life compared with passive external post-tensioning. These findings demonstrate the structural benefits of retrofitting with active components and establish a computational framework for evaluating fatigue and reliability effects in actively controlled bridge systems.

The durability of aging bridge structures has become a serious societal concern. It has been estimated that 40%-50% of the bridge stock in Europe and 36% in the US is approaching and exceeding the intended service life in some cases. Conventional retrofitting methods are generally effective under predetermined loading scenarios and can mitigate to some extent the effect of damage through strengthening and stiffening. However, typical retrofit measures involve the addition of components, which might cause unwanted stress accumulation, and in addition, they cannot perform adaptation after damage to recover functionality. Adaptive structural systems can modify the response under loading using sensors and mechanical actuators, instead of relying solely on the resistance offered through material and geometry. Previous work has shown that well-designed adaptive structures are effective in reducing peak responses under strong loading resulting in configurations that embody far fewer material and carbon resources than conventional passive systems. This work investigats retrofit strategies using active control through mechanical actuators integrated into the bridge's primary load path or as external systems. The objective is to extend the durability of most common bridge types including beam, tied-arch, and cable-stayed. Two active retrofit systems are considered: (1) an external adaptive tensioning (EAT) for beam bridges; (2) linear actuators placed in the hangers and stays of tied-arch and cable-stayed bridges. Depending on the failure mode (e.g., corrosion-, fatigue-induced), the effect of active control is simulated through a quasi-static controller based on least-squares minimization or through a linear quadratic regulator and explicit time-history analysis. Results shows that the stress reduction achieved by the EAT system retrofitted to a concrete bridge with corrosion-induced damage could extend service by approximately 12 years. In both cable-stayed and tied-arch bridges, the stress range amplitude caused by vehicular traffic is reduced below the constant amplitude fatigue limit, potentially extending service beyond 75 years.

The paper explores the potential applications of adaptive components based on shape memory polymer (SMP) composites in vibration control of plate/shell structures and rigidization of inflatable structures. These components achieve stiffness and damping variation by thermally actuating SMP between its glassy and rubbery states. In CASE A, steel-SMP sandwich plates of a truss bridge are actuated to glass transition temperature (Tg), where material damping reaches the peak to mitigate dynamic responses. CASE B proposes a simple and reversible rigidization method for inflatable structures, creating high compaction ratio and design flexibility. Converting the SMP layer between its glassy and rubbery states, inflatable structures achieve multiple functions during transportation, construction, and service life. SMP-based adaptive components enhance structural performance and mitigate dynamic effects in demanding environments for various structures.

This paper presents experimental testing of a new semi-active vibration control device comprising a shape memory polymer (SMP) core that is reinforced by an SMP-aramid composite skin. This control device works as a load-transfer component that can be integrated into truss and frame structures in the form of a joint. At the material level, thermal actuation from ambient (25 °C) to transition temperature (65 °C) causes a significant 40-fold increase in damping due to viscoelastic effects. At the component level, uniaxial tensile and four-point bending tests have shown that tensile strength depends primarily on the bond strength between the reinforcement skin and the structural element while flexural strength depends on the strength of the reinforcement skin fibers. Through cyclic testing, it has been observed that material viscoelasticity is beneficial to ductility and energy dissipation. When the joint core is actuated to the SMP transition temperature, axial and flexural stiffness decrease by up to 50% and 90%, respectively. The property change at material and component levels enable tuning the frequency and damping ratio at the structure level, which has been successfully employed to mitigate the dynamic response of a 1/10 scale three-story prototype frame under resonance and earthquake loadings.

This paper presents numerical and experimental studies on semi-active seismic response control of structures equipped with variable stiffness and damping structural joints. Such adaptive joints, which are comprised of a shape memory polymer (SMP) core reinforced by an SMP-aramid composite skin, function as load-transfer components as well as semi-active control devices. The SMP core material can transition from a glassy to a rubbery state through thermal actuation resulting in a shift of the structural natural frequencies and a parallel increase of damping ratio, which enables a new semi-active control strategy. Control performance has been evaluated on a three-story frame equipped with 12 adaptive joints and subjected to seismic excitations. Full-transient analysis has shown that when the joints are thermally actuated to the transition temperature (65°C), acceleration and base shear are reduced by up to 62% and 65%, respectively. Shake-table tests have been carried out on a 1/10-scale prototype, confirming that through thermal actuation of the adaptive joints the structural damping ratio increases from 2.6% to 11.3% and the first natural frequency shifts by up to 37%. As the structure becomes more flexible, an increase of displacements and interstory drift might occur. However, depending on the seismic excitation, top-story acceleration and base shear are significantly reduced in the range 43%–50% and 35%–51%, respectively. These results confirm that semi-active control through thermal actuation of variable stiffness and damping structural joints is effective to mitigate the structure response under seismic excitation.

This paper introduces a new semi-active strategy for vibration control of truss and frame structures equipped with variable stiffness and damping joints which consist of a shape memory polymer (SMP) core reinforced by an SMP-aramid composite skin. When the joints are actuated to the transition temperature through thermal actuation, the SMP core transitions from a glassy to a rubbery state through a viscoelastic region, which causes a stiffness reduction and an increase of damping. The mechanic behavior of the joint can be thought of as transitioning from a moment to a pin connection. This way, it is possible to cause a shift of the structure natural frequencies and to increase damping, which is employed to obtain a significant reduction of the dynamic response. This paper comprises two parts: (1) characterization of a variable stiffness and damping material model through experimental testing; (2) numerical simulations of a truss bridge and a four-story frame, which are equipped with variable stiffness and damping joints. The truss bridge (case A) is subjected to a resonance and a moving load while the four-story frame (case B) is subjected to El Centro earthquake loading. For case A under resonance loading, the dynamic response can be reduced exclusively through a frequency shift and ignoring viscoelastic effects. For case A under moving load and case B under earthquake loading, vibration suppression is mostly caused by the increase of damping due to viscoelastic effects. Control time delays due to joint heating have been included in the analysis. When the joints are actuated to the transition range 55°C–65°C, which is specific to the SMP adopted in this study, the acceleration peak amplitude reduces by up to 95% and 87%, for case A and case B, respectively. For both cases, damping increases by up to 2.2% from undamped conditions (25°C). This work has shown that the adoption of variable stiffness and damping structural joints has great potential to enable a new and effective semi-active control strategy to significantly reduce the structure response under a wide range of dynamic loading conditions.

This paper presents design and characterization of a new type of structural joint which can vary its stiffness through actuation. Stiffness variation is employed to control the dynamic response of frame structures equipped with such joints. The joint is made of a shape memory polymer (SMP) core which is reinforced by an SMP-aramid composite skin. A controlled stiffness reduction of the joint core material, induced by resistive heating, results in a shift of the structure natural frequencies. This work comprises two main parts: 1) characterization of material thermomechanical properties and viscoelastic behavior; 2) numerical simulations of the dynamic response of a one-story planar frame equipped with two such variable stiffness joints. The experimental material model obtained through Dynamic Mechanical Analysis has been used to carry out modal and non-linear transient analysis. However, control time delays due to heating and cooling as well as fatigue are not considered in the numerical simulations. Results have shown that through joint stiffness control, the fundamental frequency shifts up to 8.72% causing a drastic reduction of the dynamic response under resonance loading. The SMP-aramid skin is effective to restrain the joint deformation in the activated state while maintaining viscoelastic damping properties.

The environment around buildings keeps changing, while the static design solutions of buildings cannot perform well during the whole service life. In order to improve structural performances including strength (i.e. avoid collapse) and serviceability, adaptive structures are likely to establish as one of future trends in both research and application for the built environment. This project aims to synthesize a type of structural joints with variable stiffness capabilities. Stiffness variation is achieved by strategically arranged materials with transduction properties. Shape memory polymers (SMPs) feature large variation of stiffness between a glassy and a rubbery state, which makes them good candidates for application in shape control of adaptive structures. The structures will change themselves into optimal shapes corresponding to different load conditions. However, large shape changes require significant flexibility of the joints because their fixity can affect load-path and shape control. To address this problem, a variable stiffness joint is proposed. During shape/load-path control, the joint reduces its stiffness so that required deformation patterns can be achieved with low actuation energy. After shape control the joint recovers rigidity. Experimental studies showed the potential for application of joints with variable stiffness in adaptive structures.

This paper presents a new strategy to control the structure dynamic response via a shift of the natural frequencies obtained through using variable stiffness joints. The joints are made of shape memory polymers and are fabricated through 3D printing. Stiffness variation is activated through resistive heating which causes a phase change from a glassy to a rubbery state in order to switch the joint between a ‘locked’ (e.g. a moment connection) and a ‘released’ (e.g. pin) state. This work comprises an experimental and a numerical part: 1) dynamic mechanical thermal analysis (DMTA) has been carried out to characterize the viscoelastic behavior of a 3D printed SMP specimen; 2) the dynamic response of a 2-floor planar frame equipped with four variable stiffness SMP joints is simulated through transient analysis. Numerical results show that through joint stiffness control, the fundamental frequency shifts up to 19% causing a drastic reduction of the dynamic response under resonance loading. A comparison between variable stiffness joints with viscoelastic and only-elastic behavior shows that viscoelasticity increases mechanical damping up to 4.7 times during the transition phase.