Design procedure and analysis of innovative steel frames with reinforced concrete infill walls

Steel frames with reinforced concrete infill walls (SRCWs) are earthquake-resistant hybrid systems classified as Type 1 in EN 1998-1 [1] where they are considered to behave essentially as reinforced concrete walls able of dissipating energy in the vertical steel sections and in the vertical reinforcements of the walls. In addition, detailing provisions are the same provided for reinforced concrete walls except for indications on the edge shear connections. In reality, their behaviour might be very different and characterised by a resisting system in which diagonal compression struts develop within the reinforced concrete infill walls, resulting in cracks in the tension diagonal and crushing in the compression diagonal. Such issues are strictly related to the lack of a specific capacity design procedure that allows controlling the formation of a proper dissipating mechanism. Refined analyses previously carried out on SRCW systems [2] designed according to Eurocodes demonstrated an unsatisfactory fragile behaviour due to the severe damage occuring to concrete long before yielding of the ductile elements. The failure mechanism is generally characterised by yielding of the steel frame concentrated mainly in the elements near the bottom of the wall (more specifically at the connections of the horizontal to the vertical parts). The plastic deformation on the concrete infill walls concentrates in a diagonal path clearly indicating the distribution of cracking. In addition, localized plastic deformations are also present near the corners of the infill walls due to the local action of the first studs of the horizontal and vertical elements (Figure 1a). The innovative system depicted in Figure 1b is proposed to overcome the previous critical aspects. The RC infill walls are not connected to the vertical columns where the energy dissipation is expected. The system is conceived to control the formation of diagonal struts in the infill walls and behaves as a latticed brace instead of a shear wall. The energy dissipation takes place only in the vertical elements of the steel frame subjected mainly to axial forces without involving the reinforcements of the infill walls. Detailing of the connection of the dissipating elements should allow their replacement and the possible use of buckling-restrained elements. The formation of the diagonal strut is ensured by joint stiffeners and bearing plates (Figure 2). The joint may be welded in shops allowing speeding up the erection phases. The stud connectors are not required to transfer shear forces but they are used to connect the infill and the frame together during the seismic shakings