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AN ANALYTICAL MODEL FOR THE PREDICTION OF THE RESISTANCE OF T-STUBS WITH FOUR BOLTS IN A ROW

The behaviour of the moment resisting steel frames (MRFs) is strongly influenced by the rotational behaviour of joints which can affect both the internal actions arising in beams and columns and the overall deformability of the structure. The fast development of powerful computers and the growing use in professional practice of advanced structural software have allowed designers to accurately account for the non-linear behaviour of structural members and connections. Due to this reason, in order to grasp the MRFs behaviour, it is necessary to have accurate mechanical models able to predict the whole rotational response of steel joints. In particular, the analysis of the connections of a steel structure can be carried out according to the so-called Component Method reported in Eurocode 3 [1]. Such a method is a general approach that can be used to model every type of joint, provided that all the base components are properly identified and modelled. In case of bolted connections, the main joint components, such as the column flange in bending, the end-plate in bending and the web and flange angles in tension are modelled by means of the so-called equivalent T-stub in tension. The T-stub is a sub-assemblage realized by fastening two Tees through the flanges with one or more bolt rows. In technical literature, several works deal with the characterization of the monotonic and cyclic behaviour of Tstubs with two bolts in a row, but with reference to T-stubs with four bolts in two rows, which is a sub-assemblage applicable to many real structural situations, few experimental and modelling works are available and, therefore, more research efforts are still needed. Within such framework, in this paper the attention is focused on the experimental and theoretical analysis of the plastic resistance of T-stubs with four bolts in one row. To this scope, the work mainly focuses on three experimental tests carried out at the University of Coimbra (Fig.1a), on the set up of a finite element model (FEM) in ABAQUS software (Fig.1b) and on the development of an analytical model for predicting the plastic resistance of the T-stub. During the experimental activity at the laboratory of the University of Coimbra, tests were performed on three specimens designed in order to reproduce the typical T-stub failure modes. On the base of these tests a finite element model has been carried out in ABAQUS software. Successively, aiming to carry out a complete model for the prediction of the resistance of T-stubs with four bolts in a row, the analysis of the possible yield line patterns arising in the plate has been necessary. Therefore, by exploiting the developed FEM, several cases with different bolt/plate stiffness ratio and with different values of the T-stub width have been considered (Fig.1c). In this phase, particular attention has been devoted to the type-2 failure mechanism, which is the collapse mechanism most influenced by the presence of the second bolt row. Finally, the approach already suggested in [2,3] has been applied. In particular, by equating the external and internal work, the effective length of the equivalent T-stub in tension has been determined for all the possible cases depending on the failure mechanism and plate geometry. As a conclusion of the work, a general formulation able to provide the strength of a T-stub with four bolts in a row has been developed and