Safeability of a beam-to-column adhesive connection for large scale pultruded profiles: experimental investigation and modeling

Fibre Reinforced Polymer (FRP) materials are appealing as alternative to traditional construction materials due to their high tensile strength, excellent resistance to aggressive environments, high strength to weight ratio, simple and rapid installation time. Also, due to their low maintenance requirements, these materials offer a promising alternative for the development of more durable and sustainable structures. The connections in GFRP structures are deemed to be essential in providing the required load-carrying capacities. The connection technology for pultruded GFRP profiles presents numerous challenges due to the brittle and anisotropic nature of the material. GFRP profiles are usually connected via bolting, adopting the design rules for similar steel connections, but in the last decade the adhesive technique has gained more and more interest. International Standards stipulates that bonded connections should not be allowed for primary load bearing components. Their use is permitted only in combination with or as a backup for bolted connections. The main reason for the prohibition of bonded connections is lack of knowledge about and experience with the performance of such connections. Hence, there is need for research on bonded connections in order to understand their behavior, in terms of strength and stiffness, and to assess their performance vis-a-vis similar bolted connections. The knowledge thus gained can be used by designers to safely design composite structures with bonded connections, provided that it can be demonstrated to be more advantageous than using bolted connections. Theoretically, there are reasons to believe that bonded connections can be superior to bolted connections in FRP composite structures. In fact, compared with the traditional mechanical assembly technologies (e.g., bolted, pinned or riveted methods), adhesive bonding has a lot of advantages. First of all, nearly all types of materials, including composite materials in particular, can be bonded by adhesives. Secondly, adhesive bonding technology makes bonded structures light in comparison to other assembly technologies (e.g. mechanical fasteners). Thirdly, due to the characteristic of making no holes in the surface prior to bonding, stress concentration can be decreased compared to other methods such as bolting and/or riveting. However, as confirmed by current literature, the mechanical response of structural adhesives in general and that of the bonded joints, in particular, is significantly dependent on several factors such as the temperature (both high and low values) and the moisture which may limit the applicability of structural adhesives. The environmental temperature may exceed the glass transition temperature (Tg) of the adhesive formulation entailing relevant changes in its properties, determining a transition from a hard to a rubbery behaviour, thus compromising its specific application. Due to different environmental parameters experienced by the assembled structures during the use, among which the temperature values, the adhesive can be naturally subjected to a delay or increase in the curing degree. This can lead to adverse or positive changes in strength and stiffness. The speed and extent of the changes depend on the magnitude and duration of the temperatures experienced by the adhesive. [edited by Author]