Experimental analysis and design of friction joints equipped with sprayed aluminium dampers

It is well known that, in seismic zone, the adoption of the classical design philosophy for MR-frames provides to concentrate the energy dissipation at beam ends requiring, due to random variability and strain-hardening of the materials, the employment of heavy structural details for joints and a significant oversizing of columns due to the application of members’ hierarchy criterion. In order to overcome such limitations, in last decades mainly due to economy issues, partial strength connections have been proposed as an effective alternative to full-strength connections. In fact, with this approach the action transferred to the column is limited to the joint resistance and a more cost/effective design can be reached especially in case of frames of medium rise with long spans. Within this framework, in this paper, an intermediate approach between the classical and innovative one is presented through the application of friction dampers to steel joints. In particular, it is proposed to connect the beam to the column with a classical fixed T-stub fastening the upper flange and a friction damper located at the beam lower flange. The friction damper, composed by a stack of steel plates with one of the inner plates thermally sprayed with aluminium dust, is designed in order to slide in correspondence of a force equal to the nominal resistance of the connected beam divided by the lever arm (which is equal to the distance between the T-stub and the friction damper). In this way, provided that all the joint components are designed to be over-strength with respect to the actions corresponding to the friction damper sliding force, it is possible to obtain practically full-strength connections without providing any damage to the steel elements. In the paper, the approach is validated reporting the results of an experimental campaign developed at the Laboratory of Materials and Structures of the University of Salerno.