A novel double slip loads friction damper to control the seismic response of structures

Highlights

• Controlling structural response under two seismic intensity levels input.

• Providing extra stiffness under intense excitations by a 2^nd stiffening phase.

• Reducing the base shear force by adjusting the lateral stiffness.

• Producing a more uniform drifts and accelerations along the building height.

• Developing the hysteresis behavior of a novel double slip load friction damper.

Abstract

In this study, the performance of a proposed friction damper with two slip loads in controlling the seismic response of steel moment resisting buildings structures under moderate and intense earthquake excitations is investigated. The proposed friction damper initially operates with a smaller slip load and could shift to the higher slip load after a certain amount of slippage when subjected to intense ground motions. In this regard, the hysteresis behavior of the proposed double slip loads (DSL) friction damper is modeled and after necessary verifications is imported to the material library of OpenSees software to be used for parametric study. To evaluate the performance of the proposed DSL damper, three steel moment resisting frame (SMRF) models with 4, 8 and 14 stories are considered that are designed according to the AISC-LRFD and ASCE7-16 codes. Using a suite of earthquake records, nonlinear time history analyses (N-THA) are carried out to determine the optimal slip loads as well as the optimal amount of the slippage in between to minimize the seismic response of the considered models. The smaller slip load is optimally determined by performing N-THA on the structural models equipped with a single slip load friction damper using the earthquake records that are scaled to the design basis earthquake (DBE) level. Then, utilizing the obtained first slip load, N-THA are repeated for the models equipped with DSL friction dampers using the same earthquake records that are now scaled to the maximum credible earthquake (MCE) level to optimally determine the second slip load as well as the amount of the slippage between two slip loads. The genetic algorithm is employed to find the optimal slip loads and the amount of slippage in between. Two scenarios for height-wise variation of the DSL parameters, i.e, the slip loads and the extent of slippage, of the SMRF models are considered. In the first scenario as a practical approach, an upward decreasing pattern of slip loads along the height of the buildings is considered. In second scenario a simple search is performed to optimally identify those parameters for all floors. Furthermore, the performance of the structural models with DSL dampers are compared with those equipped with conventional single slip load systems. The results clearly demonstrate the efficiency of the proposed DSL dampers in effectively reducing the seismic response of the structures in terms of story drifts, residual drifts, absolute floor accelerations, and base shear forces in comparison to the conventional single slip load friction dampers.

Introduction

During severe earthquakes, the seismically induced inertial energy is expected to inflict heavy damages to building structures. Obviously, dissipation of this input energy could reduce the dynamic response of the structural systems. In the last couple of decades, various innovative energy dissipating systems (EDS) have been introduced to safeguard the structures against earthquake loadings and improve their seismic performance. These devices are supposed to minimize the structural damages by dissipating a considerable part of the earthquake or wind induced input energy and generally are categorized into velocity-dependent and displacement-dependent energy dissipation mechanisms [1]. Friction dampers as displacement-dependent devices are among the most widely used EDSs for seismic damage mitigation of building structures in recent years. These dampers significantly enhance the structural seismic performances through formation of stable hysteresis behavior with acceptable energy dissipation capacity. Other advantages of friction dampers include simple energy dissipation mechanism, inexpensive manufacturing and facile utilization [2].

Different kinds of friction dampers have been proposed by researchers in recent years. As one of the pioneers, Pall et al. [3] introduced the limited slip bolted (LSB) joint to control the large panel structures subjected to dynamic loading. The input energy is dissipated by friction joints when the concrete panels slide relative to each other under lateral loading. In another effort, Pall and Marsh [2] developed a friction damper to be used in the x-braced steel frames which benefits from the friction pads. Their proposed damper dissipates energy through slippage of the braces at the friction joints. FitzGerald et al. [4] proposed the slotted bolted connection (SBC) implemented in concentrically braced frames that dissipates energy through friction generated by the steel sliding plates. In the meantime, more sophisticated friction devices were introduced to maintain a consistent coefficient of friction as well as ensuring stable hysteresis behavior. Sumitomo Metal Industries Ltd, introduced a friction damper which consists of a cylindrical steel casing with copper alloy friction pads, pre-compressed internal spring and inner and outer wedges [5]. The energy dissipating restraint (EDR) damper that was first proposed by Fluor Daniel Corporation and further developed by Nims et al. [6], is capable of producing different types of hysteresis behaviors. Mualla and Belev [7] proposed a rotating slotted bolted friction damper that is comprised of circular steel plates with friction pads. Cho and Kwon [8] presented a numerical model of a wall-type friction damper which was implemented in reinforced concrete (RC) frame structures. This damper consists of a Teflon slider and an RC wall and was modeled as a wall rather than a brace-type member.

Differently, Samani et al. [9] employed the idea of semi-active friction dampers by introducing adjustable frictional damper (AFD) which controls the seismic response of the structure by adjusting the clamping force of the dampers using hydraulic pressure. Pardo-Varela et al. [10] studied a semi-active friction damper to control seismic responses of large-scale structures. Martínez and Curadelli [11] presented the multiple friction damper (MFD) which is installed on columns, and a preloading device provides the normal force required to generate the friction. Wang et al. [12] proposed the arc-surfaced frictional damper (AFD) in which the friction force is secured by pre-compression of the polyurethane elastomer (PUE) that varies with displacement due to its curved frictional surfaces.

The performance based application of friction dampers has also attracted the attention of researchers in recent years in which multi-level control systems consisting of friction dampers are used to dissipate energy for different levels of earthquake intensity. Balendra et al. [13] presented a new device consisting of a slotted connection and a knee brace. The energy exerted by low to moderate ground motions is dissipated through friction damping of the slit connection while the input energy of severe earthquake is dissipated through plastic behavior of the knee member. Recently, Lee et al. [14] proposed a new type of hybrid damper for multi-level seismic protection of structures. Seismic energy is dissipated exploiting plasticity and friction through combined behavior of shear-type friction damper and non-uniform strip damper. More information could be found regarding various introduced friction dampers in Refs. [15], [16], [17], [18], [19], [20], [21].

The energy dissipation capacity of a structure equipped with friction dampers is depended on the damper’s slip loads. So, developing a practical approach for determining the optimal slip load is extremely important. Aiken et al. [22] used a nine-story friction damped braced steel frame to experimentally and analytically determine the optimal slip loads. Filiatrault and Cherry [23] proposed a simplified approach to determine the optimal slip loads by introducing a design slip-load spectrum. Moreschi and Singh [24] employed a genetic algorithm to optimize the slip loads of structures equipped with friction dampers. Lee et al. [25] considered the story shear forces of an elastic building structure as a criterion to determine the optimal slip loads and brace stiffness distribution. Krack et al. [26] presented a design procedure for friction-damped structures through reliability based optimization under considered uncertainties such as the excitation level, the friction coefficient, and the linear damping. Miguel et al. [27] proposed a robust design optimization of friction dampers by employing the non-dominated sorting genetic algorithm (NSGA-II) for optimization procedure.

On the other hand, the concept of performance-based seismic design emphasizes on the seismic performance of structures for different levels of ground motion intensities. However, since the conventional friction dampers developed earlier mostly operate with a single slip load, they would not have a desirable performance under both moderate and intense ground motions. If the slip loads are determined for severe excitations, no considerable slippage could be expected during moderate earthquakes. On the other hand, by designing the damper slip loads for moderate earthquakes, the dissipated energy will not be significant, although the amount of slippage might be considerable. In this regard, variable friction dampers or active/semi-active friction dampers were proposed to improve the efficiency of conventional friction dampers by controlling the applied normal force to the contact area in real time. However, drawbacks are reported on the application of active/semi-active friction dampers such as their potential instability due to performance dependency on control algorithm, high energy consumption, and demand for system monitoring [28].

The friction dampers can be easily repaired after a severe earthquake, mainly by replacing the friction pads [29]. Also, they are less sensitive to variation of external parameters in comparison to other dissipating devices [30]. One of the main drawbacks of previously discussed friction dampers is their relatively low post‐sliding stiffness characteristic. During an intense earthquake there remains no capacity to transmit the earthquake loads and energy to neighboring stories after the dampers start to slip. Recent investigations clarified that the EDSs with considerable post‐sliding stiffness prevent excessive plastic deformation and abrupt stiffness loss in a weak story [31], [32], [33].

In this paper, a novel double slip load (DSL) friction damper is proposed which consists of two slippage levels. The first slippage level with lower slip load is activated under moderate earthquakes while the higher slip load is activated when the building is subjected to intense part of the earthquakes after some slippage with lower slip load. Therefore, the building is protected against earthquake excitations at different seismic intensity levels. Also, the so-called hardening phase, i.e., the phase of arriving at the 2nd slip load is provided after the first slippage level to prevent abrupt stiffness loss in the structure. The genetic algorithm is employed to determine the optimal design parameters of the proposed damper located at different floors to have a practical and economical design. Furthermore, a parametric study is carried out to demonstrate the functionality of the proposed DSL friction damper in controlling the seismic response of the structural models under earthquake excitations with different intensities.