María del Mar Corral, Leonardo Todisco, José M. Soria
Under-deck cable-stayed systems are commonly employed to enhance the structural performance of footbridges. However, their ability to withstand live loads is most effective only for a particular load distribution, thereby limiting opportunities for material savings. To overcome this challenge, the authors previously introduced the concept of smart cable-stayed footbridges, demonstrating its theoretical feasibility and potential for substantial material savings. This paper drives forward this idea and presents the first experimental proof-of-concept of a smart under-deck cable-stayed footbridge. The smart behavior is achieved through the use of a linear electric actuator that replaces the midspan strut of a conventional under-deck cable-stayed footbridge. The smart system is completed by a sensor and a control system that interprets displacement data and communicates with the actuator, which elongates and contracts depending on the acting live load. This paper aims to experimentally validate the concept of smart under-deck cable-stayed footbridges. To achieve this goal, a 6-meter-span prototype was designed, constructed, and subjected to a comprehensive testing campaign, including static, quasi-static, and dynamic load scenarios. The experimental response under static loads closely matches the analytical and numerical models, with differences ranging from 4%–8%. In addition, dynamic analysis revealed that the smart control significantly increases the stiffness related to the bending vibration mode, reducing vibrations by up to 20% compared to a conventional structure. In summary, this investigation provides the first successful physical validation of the theoretical concepts and offers insights into the structural response of smart under-deck cable-stayed footbridges, demonstrating their potential for a more efficient and sustainable footbridge design.
The final tensile fracture of any composite structure is primarily due to the failure of its constituents, namely fibers and matrix in the present case. To date, no experimental data exists, to the author’s knowledge, to define the behavior of constitutive fibers of a composite structure throughout its life span. The prime candidate for a fiber-based investigation is unidirectional zero-degree composite coupons. But unidirectional coupons do not demonstrate any significant loss of stiffness during fatigue cycling compared to other lay-ups. Even if stiffness degradation was significant, due to the nature of damage in this material system it would be impossible, practically, to monitor that change using conventional techniques (e.g. an extensometer) because the damage and failure process destroys the integrity of the contact between those devices and the material, under cyclic conditions. This investigation presents the findings of a fiber-based investigation of unidirectional composite material systems. In particular, a unidirectional graphite/epoxy system was studied, and the influence of applied load level on fiber fractures, and their influence on damage growth documented. A damage monitoring technique (patent pending) was developed to accurately record the state of damage in this material system without the usage of extensometers or strain gages. Following this method, two new damage norms were introduced, namely, “percent phase damage” and “percent gain damage”. Fiber fracture, strength degradation, and the life of unidirectional specimens were investigated and recorded as a function of various load levels. Fiber fracture, in general, showed no definitive growth pattern during fatigue cycling. It appears that the majority of the broken fibers that occur over nearly 90% of the life are due to the initial applied load cycle. This is one of the key findings of this investigation. “Proof testing” which is a common practice in industry for “verifying” the integrity of a structure, could very well be causing significant subsequent reductions in life. With these findings as a base, it is now possible to postulate the first well-founded mechanistic model of fiber-dominated fatigue degradation under tensile loading.