۱- Introduction

۲- Design criteria of joints

۳- Experimental campaign

۴- FEM

۵- Parametric Fe analysis

۶- Conclusive remarks

References

Abstract

Beam-to-column joints equipped with friction dampers are promising solutions to improve the performance of steel moment resisting frames due to the possibility to guarantee large dissipation capacity limiting the structural damage under severe seismic conditions. In this paper, the experimental tests and the numerical simulations of two types of joints are shown and discussed with the aim of developing pre-qualified configurations. The friction dampers are designed to be easily removable from both the lower beam flange and the column face by means of bolted connections. The devices are composed of a stack of steel plates conceived to assure symmetrical friction. The friction surface is set in vertical direction in first case and in horizontal direction in the second type. The experimental tests confirmed the effectiveness of both examined joints and the finite element analyses allowed characterizing their local response, thus providing additional insights to improve the design requirements.

Introduction

The design criteria currently implemented by seismic codes are based on the philosophy of hierarchy of resistances, which aims to guarantee overall ductile and dissipative behavior by enforcing the plastic deformation, namely the damage, into specific ductile zones of the structure. However, economic and social reasons have recently pushed researchers and designers towards systems that can resist severe ground motions with low or without structural damage [1]. Traditional seismic resisting systems widely adopted for steel buildings (e.g. moment resisting frames, concentrically and eccentrically braced frames) entail a dissipation mechanism based on plastic deformations of several structural elements, which may correspond large repairing costs in the aftermath of a seismic event. Therefore, the idea of low or free from damage structures has become very appealing in the last decades [2–۴]. The use of friction connections is a viable and promising strategy to achieve this objective for steel structures [5–۲۴]. In the framework of Eurocodes, this type of connections can be classified as partial strength according to EN1993:1-8 [25] because their design resistance should be lower than the strength of the connected members to prevent any damage into the primary structural members. EN1998-1 [26] allows the use of partial strength connections provided that their rotation capacity is properly demonstrated. In the case of conventional partial strength joints the ductility can be designed by imposing local hierarchy rules among the components constituting the joints [27–۳۱] and verified by means of pre-qualification tests [32–۳۴]. Moment resisting friction connections are conceived to develop the dissipation mechanism by means of the relative slip into ad-hoc devices located between the lower beam flange and the outer cap plate connected to the column flange, while the upper flange of the beam is connected to a plate either bolted or welded to the column. The cover plate connecting the upper beam flange may be subjected to some moderate plastic bending deformations to accommodate the joint rotation following the sliding of the device, thus enforcing the formation of an ideal center of rotation that prevents the damage of the slab. To increase the moment capacity, friction devices can be also adopted for the beam web. In addition, the resistance of the joint can be modulated keeping the same assembly but varying the friction resistance that changes with the clamping force used for the bolts. Indeed, the friction device is composed of a stack of steel plates that are clamped together by means of tightened high strength bolts, which are inserted in the slotted holes of the plates to allow the relative sliding.