Keywords: dual structures, equivalent diagonal model, push-over analysis, seismic design, serviceability limit state, shear walls, steel frames.

In the field of seismic design of steel structures, unstiffened steel plate shear walls are more and more used as an innovative and effective system for the protection of buildings against lateral forces [1]. The main characteristics of this system, which consists of thin steel plates inserted into frame bays and connected to both beams and columns, are the large initial stiffness, the strong resistance and the dissipative capacity arising from the tension-field behaviour developing in the steel plates under shear. When used also as cladding of the building, such a system allows a significant reduction of both gravity and seismic loads, leading to a considerable cost saving for the construction and fabrication process. In the last years, especially in Canada and Japan, steel plate shear walls have been employed for seismic protection of new constructions and also for seismic upgrading of existing steel and reinforced concrete structures [2,3].

An extensive survey of the recent literature concerning the structural application of slender steel shear walls is presented in the paper. Then, based on the assumption of the "equivalent diagonal" model, the behaviour of two typical MR steel frames (having 7 and 15 storeys), is analysed (Figure 1). For each frame typology, the primary structure has been designed according to Eurocode 8, starting form lighter structural configurations, i.e. designed according to vertical loads only, and arriving to heavier configuration, where also the serviceability limit state is fulfilled. Then, the frames designed according to vertical load only have been upgraded by using several configurations of unstiffened plate shear walls, giving rise to different frame typologies whose seismic behaviours has been evaluated on the basis of push-over analysis. On the other hand, it has been considered that the insertion of steel plates in CV resisting frames can determine local collapse mechanisms due to column failure. Therefore, in a second phase of the design process, the obtained configurations (in terms of both frame member sections and thickness of shear walls) have been modified in order to optimise the seismic behaviour of the whole structure. In particular, strength, stiffness, ductility, type of collapse mechanism and structural weight of the different structural configurations have been compared in relation to the SLD bare frame.

In the whole, obtained results show that the use of shear walls allows (i) a significant improvement of the global seismic performance, (ii) the exploitation of global plastic mechanism in the structure and (iii) a significant reduction of structural weight (up to 35% for the 7-storey frame and to 15% for the 15-storey frame) respect to the simple bare SLD frame configuration. Therefore, they prove the effectiveness of the structural systems based on the use of moment resisting frames designed according to vertical load only and then upgraded through the use of slender steel shear panels.

1

G. De Matteis, R. Landolfo, F.M. Mazzolani, "Seismic Response of MR Steel Frames with low-yield Steel Shear Panels", Engineering Structures, 25, 155-168, 2003. doi:10.1016/S0141-0296(02)00124-4

2

R.G. Driver, G.L. Kulak, D.J.L. Kennedy, A.E. Elwi, "Cyclic Test of Four-Story Steel Plate Shear Wall", Journal of Structural Engineering, 124 (2), 112-120, 1998. doi:10.1061/(ASCE)0733-9445(1998)124:2(112)

3

A.S. Lubell, H.G.L. Prion, C.E. Ventura, M. Rezai, "Unstiffened Steel Plate Shear Wall Performance under Cyclic Loading", Journal of Structural Engineering, 126 (4), 453-460, 2000. doi:10.1061/(ASCE)0733-9445(2000)126:4(453)

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