رفتار سازه ای اتصالات برشی T-Perfobond در تیرهای کامپوزیت: رویکرد تجربی

This paper presents the results from eighteen push-out tests made at the Civil Engineering Department of the University of Coimbra, Portugal, on T-Perfobond shear connectors. The investigated variables were: concrete slab thickness, concrete compressive strength, connector geometry, relative position of the connector to the direction of loading, shear connector hole number and disposition, among others. The results are presented and discussed, focusing on the T-Perfobond structural response in terms of shear transfer capacity, ductility and collapse modes. Finally, a comparison of the experimental results with existing analytical formulae was also made to develop guidelines for designing the T-Perfobond connectors.

The shear connector is the component that assures shear transfer between the steel profile and the reinforced concrete slab, enabling the development of the composite action in composite beams. Several different types of connectors have been studied, proposed, and used in the past. Reference is made to headed or Nelson studs (Fig. 1a), Perfobond (Fig. 1b) and Crestbond (Fig. 1c) shear connectors.Among these connectors, the most widely used, due to a high degree of automation in workshop or site, is the Nelson stud (Fig. 1a), designed to work as an arc welding electrode and, at the same time, after the welding, as the resisting shear connector. It has a shank and a head that contributes to the shear transfer and prevents the uplift. However, it has some limitations in structures submitted to fatigue, and its use requires specific welding equipment and a high power generator at the construction site. Additionally, in applications where a discrete distribution of the connectors is needed, for example in precast concrete decks or in strengthening, repairing or even retrofitting existing structures taking advantage of the steel and concrete composite action, the stud may be substituted with advantages by stronger shear connectors. The Perfobond type connector has some common properties with the specific connector studied in this paper. It is formed by a rectangular steel plate with holes welded to the beam flange (Fig. 1b). The Perfobond or Perfobond rib shear connector was developed in the eighties, as referred by Zellner [23], motivated by the need of a system that, under service loads, only involved elastic deformations, with specific bond behaviour and also was associated to higher fatigue strength. Several authors have recently studied the behaviour of the Perfobond connector, mostly from push-out tests. Among these, reference is made to the studies of Al-Darzi et al. [1], Iwasaki et al. [11], Machacek & Studnika [12], Medberry & Shahrooz [13], Neves & Lima [14], Oguejiofor & Hosain [15], [16], Ushijima et al. [17], and Valente & Cruz [18] and [19]. These authors concluded that their structural response was influenced by several geometrical properties such as the number of holes, the plate height, length and thickness, the concrete compressive strength, and the percentage of transverse reinforcement provided in the concrete slab. Ferreira [6] has adapted the Perfobond geometry for thinner slabs, usually used in residential buildings, and isolated the contributions to the overall shear connector strength from the reinforcement bars in shear and from the concrete cylinders formed through the shear connector holes. The motivation of developing new products for the shear transfer in composite structures is related to issues involving particular technological, economical or structural needs of specific projects. In this context, some other alternative shear connectors have been proposed for composite structures. Reference can be made to the studies of Fink and Petraschek [7], Gündel and Hauke [8], Hechler et al. [9], Hegger and Rauscher [10], Machacek and Studnika [12], Vellasco et al. [20], Veríssimo et al. [21], and Zellner [23]. Also, an alternative connector, named as T-Perfobond (Fig. 2), was presented by Vianna et al. [22], in the scope of a study on Perfobond connectors, where a comparison of the behaviour of these connectors and a limited number of T-Perfobond connectors was made. This connector derives from the Perfobond connector by adding a flange to the plate, acting as a block. The motivation for developing this T-Perfobond connector is to combine the large strength of a block type connector with some ductility and uplift resistance arising from the holes at the Perfobond connector web.The present work focuses on T-Perfobond connectors and involved eighteen push-out tests performed at the Civil Engineering Department of the University of Coimbra, Portugal. Specimens were fabricated from an IPN 340 section cut at the symmetry axis parallel to the flanges, and were produced without holes, and with, respectively, two or four holes, located in one or two rows in the load transfer direction, with slabs of 120 mm (Fig. 3a) and 200 mm thicknesses, (Fig. 3b). Six tests were made from the nominal C25/35 concrete compressive strength class, and twelve tests from the nominal C35/45 class according to EN-1992-1-1 (Eurocode 2 [2]).

The results from a set of experimental tests conducted at the Civil Engineering Department of the University of Coimbra, Portugal, were used to evaluate the behaviour of T-Perfobond connectors, focusing on their resistance and slip capacity. T-Perfobond connectors have shown to stand high shear loads resulting in a smaller number of connectors in a beam. In addition, they have the additional advantage of being produced with ordinary rolled I or H sections, and may be welded with easily available equipment. These factors contribute to a potential saving of material and workmanship, leading to a more economical design of composite girders. The onset of failure is related to a slip at the connector-concrete interface, followed by the formation of cracks in the concrete that open and propagate as the load increases, followed by the concrete crushing at the connector’s front face. This concrete failure was at later stages of loading accompanied by the connector yielding, and, in some cases by a failure of the connector welds. The slip observed in the tests was smaller than the minimum required slip capacity of 6 mm according to Eurocode 4 [4]. This value does not satisfy the requirements for a plastic distribution of shear force in the connectors along the structural element. This fact is not significant if an elastic distribution of shear along the beam length is to be adopted in design. Concrete block resistance was found to be of much greater importance than the resistances related to the holes and to the reinforcement bars. In fact, comparison of test resistances from tests without and with holes, and without and with reinforcement bars in the holes, showed limited gains in resistance for the investigated specimens range. The authors believe that if the tests were repeated at the inverse position, i.e the Perfobond flange located close to the jack, the connector ductility would be improved. Another improvement in the connector ductility could be achieved if connector positioned at the inverse position could be used with additional reinforcement bars used at the concrete slabs through the Perfobond holes and/or below the Perfobond web the specimens. Increasing the slab thickness led to a subsequent increase of the characteristic resistance and of the slip capacity δuδu, related to a larger concrete block resistance. When comparing these differences to the similar differences from the tests with a different concrete cylinder compressive strength, it may be concluded that for the higher concrete compressive strength the relative gain in resistance is slightly less significant but the relative gain in ductility is also quite important (about 100%). An increase of the concrete compressive strength also led to a higher shear connector capacity, but in a comparatively smaller proportion to the concrete resistance in itself. This resistance enhancement was observed to be less relevant in thicker slabs. Application to T-Perfobond connectors of an available model for predicting the shear resistance of T or block connectors [5] was found to be on the safe side for 120 mm thick slab connections (especially for stronger concrete), since it neglects the favourable effects of the steel reinforcement bars in the concrete slab and of the holes in the connector. However, for thicker slabs, this model considerably overestimated the connector’s resistance, suggesting that it depends on the connector height and slab thickness differently to the formulation expressed by Eq. (2). The next steps of the present investigation will consider the development of consistent and accurate formulae for the evaluation of the shear capacity of the investigated T-Perfobond connectors, taking into account the interaction between the block resistance and the contributions from the slab steel reinforcement and the connector’s holes. Full scale tests of composite beams using the investigated shear connector will also be the main focus of that study. These new tests will be centred on composite girders with shear connectors evenly spaced or spaced according to the shear force diagram. Their results will help to investigate questions related to the maximum and minimum spacing, its application in a partial interaction design and on hogging moment regions and on its influence over the composite beam effective width.