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Significance
The use of Fibre Reinforced Polymer (FRP) in structural engineering has become increasingly attractive and popular because they offer excellent material properties of high strength-to-weight ratio, corrosion resistance, and also can easily be installed. They are especially advantageous in harsh environments where other materials like steel and concrete are susceptible to degradation. However, design of FRP structures still has some limitations particularly in the development of reliable and efficient connection systems which compromise the structural integrity due brittle failures and stress concentrations. Traditionally, bolted and bonded joints have been used to connect FRP members and while bolted joints, though common, tend to introduce stress concentrations and weaken the FRP structure due to the anisotropic and brittle nature of the material. This often results in premature failure at the bolt holes which limits the joint’s load-bearing capacity. On the other hand, the use of adhesive-bonded joints offered better load distribution and reduced stress concentrations, however they suffer from brittle failure modes and lack sufficient ductility especially under dynamic or seismic loads. Moreover, adhesive joints are highly sensitive to environmental conditions such as temperature and humidity, which degrade their performance over time. Thus, there is a growing interest in developing hybrid connections that combine the advantages of both bolting and bonding techniques which will enhance the ductility of the connection with ductile elements incorporated, which can undergo plastic deformation, and by this brittle failure of the FRP components. However, the current hybrid designs still face are not optimal with complexity in assembly and difficulties in integrating transverse beams into real-world 3D structural systems. These issues are a true obstacle for the practical application of FRP in more complex structural frameworks especially in scenarios where ductility and repairability are critical, such as in seismic zones or modular construction. To this account, recent paper published in Composite Structures Journal and led by Prof. Francesco Ascione from the Department of Civil Engineering at University of Salerno, presents a new and novel ductile hybrid connection for FRP assemblies. The goal of their study was to create a connection system that can both improves the mechanical performance of the joints and also offers easy reparability and adaptability for complex structural systems.
In brief, the proposed joint is designed to bond FRP pultruded profiles to steel elements to maintain the continuity of the pultruded fibres. The steel elements are bolted to other steel components which are designed to be weaker than the bond strength in order to ensure a ductile response. This arrangement makes it possible to optimise the mechanical response of the materials used and to maintain the typical construction technology of FRP and steel structures. Figure 1 shows the geometrical features of the proposed connection, which consists of (1) a “nodal element” made of a steel section welded to a top and bottom end plate (see the red I-section, but a hollow section is also feasible); (2) a steel sleeve (see the blue element) arranged with an end plate to be bolted to the nodal element and bonded to the FRP column; (3) a steel channel (the green element) designed as the weaker component of the assembly; (4) a steel T-joint (the purple element) bolted to the steel channel and bonded to the FRP beam (i.e. the two yellow C-profiles). The geometry of the beam, column and nodal element can vary according to the design requirements and construction needs, while the concept of the other components of the connection can remain the same.
Authors tested various configurations of the proposed joint with key parameters such as the bonding width between the FRP beams and steel components, the pitch of the bolts, and the thickness of the steel web (fuse) being varied across different test specimens. To begin with, the researchers evaluated how the connection behaved under increasing loads and to identify the point at which failure occurred and observed that the damage was concentrated in the fuse. This finding confirmed their hypothesis that the steel elements would act as sacrificial components which allowed the FRP profiles and adhesive to remain intact. We believe this is significant because it demonstrated that the proposed connection could undergo controlled plastic deformation in the steel elements, thus preventing brittle failure in the FRP. As a result, the connection has high ductility which is vital for structural systems in seismic zones where energy absorption is necessary. Additionally, the researchers also evaluated the influence of bonding width on the joint’s performance. They found that if you increase the bonding width this significantly improve the overall stiffness and resistance of the connection. Indeed, the increased bonding area allowed for a more efficient distribution of stresses across the adhesive layer, reducing the risk of failure in the FRP components. However, they noted that this improvement came with a trade-off as the bonding width increased, the ductility slightly decreased due to the stiffer nature of the joint. It is interesting to note that the pitch of the bolts also played important role in the connection’s behavior. The Italian scientists tests revealed that larger bolt spacing resulted in increased resistance and stiffness while smaller bolt spacing promoted higher ductility. This balance between bolt pitch and the connection’s performance provided important insights for optimizing the joint design, depending on the specific structural requirements.
The team also evaluated different steel fuse thicknesses to assess their impact on the joint’s mechanical behavior and found that thicker fuses provided higher stiffness and strength but this came at the cost of reduced ductility. On the other hand, they reported that thinner fuses allowed for greater plastic deformation and ductility but showed lower resistance. The force-displacement and moment-rotation curves generated during the tests provided further insights into the joint’s behavior. The curves indicated that the connection could sustain large displacements and rotations before reaching failure, which is indicative of a ductile response. This behavior is highly desirable in structural engineering, as it ensures that the joint can absorb significant energy before failing, reducing the likelihood of catastrophic collapse during events such as earthquakes. In conclusion, Dr. Francesco Ascione and colleagues successfully developed a novel ductile hybrid connection for FRP pultruded beam-to-column assemblies, which managed to overcome long-standing challenges in the structural use of FRP materials. This innovation offers a significant advancement in the structural engineering field particularly in applications where energy absorption and ductility are essential such as in earthquake-prone areas. We believe the implications of this comprehensive and well-designed study are wide-reaching. First, the proposed connection system improves the safety and longevity of FRP structures and thus make them more viable for a broader range of applications including high-performance buildings and infrastructure projects. The repairability of the connection is particularly important as it allows for damaged steel components to be easily replaced without the need for extensive repairs to the FRP elements. This is expected to reduce maintenance costs and extends the lifespan of the structures and by this make them more sustainable and cost-effective over time. Moreover, the researchers’ experimental setup and methods can guide future design and construction practices and be the blueprint for optimizing hybrid connections based on specific structural requirements. Furthermore, the results of Prof. Francesco Ascione and his colleagues could influence future building codes and standards for FRP structures and potentially lead to wider adoption of these materials in construction.
Reference
Francesco Ascione, Mario D’Aniello, Luciano Feo, Luigi Granata, Raffaele Landolfo, A novel ductile connection for FRP pultruded beam-to-column assemblies, Composite Structures, Volume 337, 2024, 118091,
