رفتار سازه از مفاصل GFRP چند منظوره برای سازه های بتنی

A multifunctional all-FRP joint has been developed for the transfer of bending moments and shear forces in thermal insulation sections of concrete slab structures used in building construction. Tensile forces from moments are transferred by horizontal GFRP bars, while a pultruded cellular GFRP element transfers the compression forces. The shear forces are transferred by inclined GFRP bars and the webs of the GFRP element. The new joint considerably increases energy savings for buildings due to the low thermal conductivity of GFRP materials. The quasi-static behavior of the joint at the fixed support of cantilever beams was investigated. Two parameters were studied: shear- or moment-dominated loading mode and concrete strength. Results show that the all-FRP joint does not play a critical role at the ultimate limit state. Ductile failure occurs through concrete crushing. The GFRP bars lead to a significant improvement in joint performance compared with similar joints comprising steel bars. Higher concrete strength does not, however, significantly improve the ultimate load.

In the context of the sustainable use of non-renewable raw materials and energy, a significant trend towards energy saving construction concepts and methods can be observed in building construction. There is an ongoing movement towards construction methods with high thermal insulation and new standards requiring very low energy consumption are being defined. In this respect, thermal bridges in insulating facades created by penetrations of structural components (e.g., cantilever balcony structures) and built from materials with high thermal conductivity (concrete or steel) are a major concern. New codes comprising stringent requirements regarding thermal insulation have, therefore, been implemented, e.g., the new Swiss Code SIA 380/1 [1], prescribing decreased values for linear thermal bridge allowance limits in facade constructions. As a result of the new requirements, efforts are being made by the construction industry to develop new structural components for facade penetrations offering improved thermal behavior. The use of new high strength GFRP materials (Glass Fiber-Reinforced Polymers) is being explored – GFRP composites present a thermal conductivity approximately 200 times lower than that of steel [2] and in addition provide load-bearing functions. Two examples of load-bearing and insulating GFRP components were presented by Keller et al.: a hybrid-FRP/steel joint [2] and [3] and an all-FRP joint [4] for thermal insulation and load transfer in concrete slabs. The hybrid FRP/steel joint, shown in Fig. 1, is inserted in cantilever slabs at the location of the facade penetration, providing a transfer of shear forces and bending moments as well as thermal insulation [5].The present paper describes the further development of the hybrid-FRP/steel joint presented in [2] to an all-FRP joint, shown in Fig. 2, through the replacement of the steel bars of the hybrid joint with GFRP bars. The all-FRP joint provides a reduction of approximately 50% of the linear thermal bridge allowance of the hybrid joint (value obtained from testing and FE simulation). To verify the load-bearing behavior of the new all-FRP joint in detail, quasi-static full-scale experiments on concrete cantilever beams with integrated insulating joints were performed. The experimental results for the beams with all-FRP joints are presented in the following and their load-bearing performance is compared with that of similar beams with hybrid-FRP joints.

A multifunctional all-FRP joint – combining load-bearing and building-physical functions (thermal insulation) – was developed to improve thermal insulation in concrete slab structures which penetrate insulating facades in buildings. The new joint represents a development of an existing hybrid FRP/steel joint through the replacing of the steel bars by GFRP bars. Four cantilever beam specimens integrating the all-FRP joint were designed, constructed and tested. The parameters investigated were concrete strength and loading mode. The results of the all-FRP beam experiments were then compared with similar experiments previously performed on hybrid FRP/steel joint beams to evaluate joint performance. The following conclusions can be drawn from this work: (1) The GFRP components of the new joint (CS-element and shear and tension bars) are not a critical concern at ultimate limit state, provided that a strain limit of 0.7–0.9% is respected for the tensile bars. Ductile failure occurs through concrete crushing. In design, therefore, standard reinforced concrete theory can be used to dimension the cross sections adjacent to the insulating joint. This represents an important advantage in practice, since most structural engineers are not familiar with FRP design theory. (2) The pultruded multi-cellular GFRP element transfers part of the shear forces in the element webs due to element tilting, independent of loading mode. The predominant joint deformation mode and degree of shear participation are determined by the upper tension bar elongation. Smaller elongations lead to tilting and shear transfer, while larger elongations lead to joint rotation without shear transfer. GFRP bars can prevent joint rotation due to the absence of yielding and therefore can lead to a significant improvement in joint performance. Although the materials in the all-FRP joint are not ductile, concrete crushing leads to a similar ductility as that observed for the hybrid FRP/steel joint beams. (3) Even though failure occurs through concrete crushing, higher concrete strength does not significantly increase ultimate loads. Due to the lower redistribution capacity of the higher strength beams in the failure zone, the ultimate load cannot be increased proportionally to concrete strength. (4) Further studies are in progress with regard to the elaboration of a detailed design method for the application of the new all-FRP joint in practice.