۱- Introduction

۲- Friction damped braced frames

۳- Fragility function

۴- A method of estimating design parameters of friction damper-brace assembly

۵- Illustrative examples and parametric study

۶- Results and discussion

۷- Comparison with an existing design method

۸- Conclusions

References

Abstract

In this paper, a probability-based design methodology of the friction dampers in multi-story steel frames is proposed. Both the slip force of the device and the stiffness ratio of the system are analyzed as two important parameters, which affect the behavior of the structures equipped with friction devices. The seismic fragility of friction damped braced frames is evaluated and used to identify the optimal ranges of the above-mentioned design parameters so as to minimize the overall damage probability of the structure under the action of strong ground motions. For this purpose, fragility functions of the structural models are derived using nonlinear incremental dynamic analyses. To demonstrate the efficiency of the proposed method, three structural models of steel moment resisting frames with friction damper systems (including chevron braces and damper devices) are considered for the purpose of the seismic performance analysis. The results of the analyses show that the largest damage probability in each structural model corresponds to the case with the higher slip force and the lower stiffness ratio, where the undesirable buckling failure will govern before full activation of friction damper. For the three considered building frames, the optimal range of slip force lies between 40% and 55% of the total weight of structures and the recommended value for stiffness ratio is 2.

Introduction

Since several decades the seismic response control techniques have been used as complementary solutions to the existing seismic force resisting systems and several types of passive energy dissipation devices have been developed. Friction dampers are considered one of the most effective passive control devices for building structures against earthquake actions. Compared to the velocity-dependent devices, such as viscous and viscoelastic dampers, friction dampers provide energydissipation capacity with a sufficient lateral stiffness. The action of these dampers is based on the mechanism of dry friction which develops between two solid bodies, sliding relatively one to another for providing a specified energy dissipation capacity [1]. The friction devices exhibit hysteretic behaviors similar to those achieved by the metallic devices. Recently, considerable progress has been made in the development of this type of devices. Pall friction damper is the most commonly and widely used type of friction dampers, which was originally introduced by Pall et al. [2,3], based upon the automotive brake. It can be located in structures either as a part of braces [4] or, more recently, as beam-tocolumn joints [5,6]. The friction devices basically consist of steel plates tightened together by means of high strength steel bolts, whose either axial or rotational deformation mechanisms lead to a transformation of kinetic energy into thermal energy; so, the maximum amount of friction force can be controlled by adjusting the friction coefficient of the sliding surfaces, as well as the compression force of the tightened bolts, but it is independent of the sliding velocity and the contact area of the sliding surface. Since the 90s of the last Century, Sumitomo Metal Industry in Japan developed a different type of friction dampers [7]. This device consists of copper pads impregnated with graphite in contact with the wedges inside of the steel cylinder. The load on the contact surface is induced by a series of wedges that act under the compressive force of the Belleville washer springs (cup springs). The graphite serves as a lubricant between the contact surfaces and ensures a stable coefficient of friction and silent operation. The experimental study of Aiken and Kelly [7] allows the Sumitomo damper to be used as a structural damper system. They compared the response of moment resisting frames without and with this damper.