## 1 Introduction  
For decades, seismic-resistant structures have been designed using the conventional approach of ensuring life safety against earthquakes, the main objective being to prevent buildings from collapsing. However, this approach comes at the cost of assuming important damage to the structure after the earthquake, requiring in most cases its demolition and rebuilding or a high cost of repair. Strong ground motions such as the Northridge (Northridge, 1994) or Hyogo-ken Nambu (Kobe, 1995) earthquakes have long since proven that this approach is no longer valid, as economic losses are also to be considered in seismic-resistant design. Controlling damage is a key aspect of the so-called Performance-Based Seismic Design: the new paradigm that has guided the development of codes, strategies and new technologies in the earthquake engineering field since the beginning of this century. Substantial research efforts have been devoted to the development of innovative systems to control structures' vibration. These are categorized into four groups: passive systems, active systems, semi-active systems and hybrid systems [1; 2; 3; 4]. Among them, passive control systems based on the use of displacement-dependent energy dissipation devices (EDDs)--also referred to herein as dampers--have been proven to be a cost-effective solution. When installed in the main structure, they can enhance the overall performance of the building, reducing the seismic response in terms of displacements and,more importantly, attracting most of the energy input by the earthquake. The damage concentrates in the EDDs, which are specifically engineered elements easy to inspect, repair and/or replace after an earthquake. This prevents damage to the main structure and minimizes disruption on the building use. In displacement-dependent dampers, energy is dissipated through different mechanisms: metal yielding, metal phase transformation, friction, sliding, etc. Dampers based on metal yielding (metallic dampers) are made of materials with a large inherent plastic deformation capacity and shaped to avoid any source of stress concentration that could jeopardise the inherent high energy dissipation capacity of the material. Many metallic dampers have been developed in the last decades, and a state-of-the-art review of metallic dampers can be found in [5]. The original designs were based on simple torsion or bending of steel plates and uniaxial deformation of U-shape strips. Some widely used examples are: Buckling Restrained Braces, Added Damping and Stiffness, and its triangular version. Other energy dissipation devices that have drawn attention lately are the slit type (Figure 1a) and the honeycomb damper (Figure 1b). They are built from steel plates with openings of constant width referred to as slits (Figure 1a), or a variable honeycomb shape (Figure 1b), leaving between them steel strips of constant (Figure 1a) or variable (Figure 1b) width. These strips dissipate energy through plastic flexural/shear deformations. Several recent studies have investigated slit-type dampers made of mild steel. [PERSON] [6] studied a slit damper fabricated from a standard structural wide-flange section with some slits cut in the web. [PERSON] [7] investigated several steel dampers with a honeycomb geometry with the aim of yielding all the length of the strip simultaneously. [PERSON] [8] proposed several optimized nonuniform shapes for the openings of the damper and compared their overall performance through quasi-static cyclic tests. [PERSON] [9] developed a series of hourglass-shaped strip dampers and tested them under quasi-static and dynamic monotonic and cyclic loading. [PERSON] [10] studied a symmetric slit damper cut from a steel plate having a low height-to-thickness ratio. The shape and dimensions of several specimens were optimized and tested experimentally under cyclic loading in quasi-static conditions.