A shake table study of single-bay two-storey model of conventional ordinary concentric braced frame (OCBF) and aluminum shear-link enabled braced frame (SLBF) was conducted to evaluate the performance of shear-link as energy dissipation device. The 1:12 reduced scale models were subjected to Taft ground motion of increasing peak ground accelerations, representing seismic loads of increasing severity. Similitude laws for adequate modeling of dynamic behavior of the reduced models were satisfied and time, frequency and acceleration values were scaled. The test indicated that SLBF frame attracted about 41–64% less base shear compared to OCBF for varying PGA levels of the ground motions. Similar trend was noticed for overturning moments and floor acceleration as well. However, the first storey floor drifts for the SLBF were always greater than the OCBF. Significant amount of energy was absorbed by aluminum shear-links leading to satisfactory response up to the scaled PGA of 1.7g of the Taft motion, while the OCBF frame could not survive the scaled PGA of 0.8g. These proof-of-concept tests also helped validate the design methodology developed for proportioning aluminum shear-link enabled steel frames.
Under seismic action, reliance for survival of fixed-base structure is placed on its ability to dissipate seismic energy, which occurs while undergoing large inelastic deformations in specially detailed regions of beams and column bases of the gravity load system. With the use of energy dissipation devices (EDDs), which can be easily replaced, it is possible to prevent accumulation of inelastic deformation in the main gravity load resisting members and localization of the damage induced. The basic function of EDDs is to reduce and/or absorb a portion of the input energy, and thereby reducing the energy dissipation demand on primary structural members and minimizing possible structural damage.
A widely considered strategy for the dissipation of energy in the structure during an earthquake is through the inelastic deformation of metallic devices [1–3]. Flexure yielding of steel dampers such as TADAS, ADAS were developed using steel plates of triangular shapes to maximize energy dissipation potential [4–6]. Solid and slit webs of steel section has also shown to yield in shear and act as a damper under lateral loads [7–9]. A few studies have also been carried out on low yielding steel shear panels utilizing its shear deformation as a means to dissipate the energy .
The shear yielding of low yielding alloy metals, such as aluminum, has been found to be very ductile and large inelastic deformations are possible without tearing or buckling. The yielding in shear mode maximizes the material participating in plastic deformation without excessive localized strains. In this regard I-shaped shear-links of low yield ductile alloys of Aluminum have been found to be excellent energy dissipative devices limiting the energy dissipation demand on structural members of the primary structure [11–13]. Further, the addition of aluminum shear-links to an ordinary chevron braced frame (OCBF) as shown in Fig. 1 has shown to improve its seismic performance remarkably. The analytical study indicated that addition of shear-links leads to considerable reductions in the base shear which acted as dampers by dissipating significant amount of seismic energy induced in the structure. A number of element tests on the reduced and full-scale shear-links showed satisfactory performance over a wide range of frequencies [11,14]. However, no system test by means of fixing the shear-link within a steel frame has been conducted to verify the effectiveness of shear-link braced frames (SLBFs).
Earthquake simulation tests are an invaluable source of information for understanding the behavior of the structural systems in the nonlinear range. Shaking table tests were conducted to evaluate the load resistance mechanism, failure/damage pattern and the hysteretic behavior of shear-link systems and to provide the data for developing suitable design procedures for proportioning various elements of the overall system.
The primary objective of this research effort is to study the performance of the SLBF, designed as per the simplified method developed by Rai and Wallace , using shake table experiments. A 1:12 reduced scale model was fabricated with due care of dynamic similitude relations and earthquake simulation tests were conducted. The performance of the SLBF was evaluated in terms of floor accelerations, base shears, overturning moments, and hysteretic response of shear-links. Similarly, OCBF model having same details as that of SLBF model was tested in order to compare the performance with the SLBF.
2. Aluminum shear link as seismic damper
Typical shear-link with two panels is shown in Fig. 1, which is fabricated from thin plates forming its flanges, web and stiffeners. The aluminum shear-link is designed to yield in shear mode to limit the maximum lateral force, which is transmitted to the primary structure, and to provide significant energy dissipation potential during the earthquake ground shaking by means of inelastic deformation in the damping device. In addition, significant amount of strain hardening of aluminum alloys allows the shear-links to resist additional lateral loads after the first yield and thus forcing shear-links of other stories to share the load in a multi-storied structure. Consequently, the inelastic activities are spread out across various bays and stories of the building structure. These properties make the aluminum shear-link attractive for both new buildings and improvement to existing structures. Aluminum links should be placed strategically in the structure to yield in shear; for example, in ordinary chevron braced frames it is placed in between the diagonal braces and the floor beam as shown in Fig. 1 .
3. Earthquake simulator testing
3.1. Prototype building
A two-storey community building assumed to be located in seismic zone V (PGA = 0.36g) on the soil profile Type I (Rock, or hard soil) of IS 1893(Part 1)  was considered for analysis. In the plan, the building is 36 m long in the E–W direction (six bays @ 6 m) and 18 m (three bays @ 6 m) wide in the N–S direction as shown in Fig. 2. In the elevation, floor to floor heights are 4.5 m and the building is assumed to possess no irregularity of any kind. In the N–S direction, six bracing frame system were designed to provide the code level lateral resistance. The building was assumed to have a dead load and live load of 3.8 kPa and 3 kPa, respectively, on roof and floor. The six bracing frame systems at the middle bay in N–S direction are designed as SLBF systems and all the other interior frames were designed to resist only gravity loads associated with their tributary areas.
The capacity design approach is followed in proportioning various components of the SLBF system. They are designed for the capacity of the dampers such that the frame does not yield before the dampers reach their failure shear stress. Similar design philosophy has been used for such yielding energy dissipation devices [16,17]. The shear-links are designed based on two limit states of strength and ductility demands of the design level and maximum credible earthquakes. Size of shear link is calculated by determining the horizontal web area required to resist the design storey shear taken same as that of OCBF. The design shear strain, cd = d/ d corresponds to the allowable storey drift, d at design level earthquake (typically 0.4% of storey height) and the depth d, of shear-link is chosen between 1/10th and 1/12th of storey height. The peak shear stress and strain values in the shear-links follow a power law relation given by smax ¼ 2:6 r0:2c0:2 where r0.2 is the tensile yield strength of web material .