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Abstract
Within the course of structure engineering, several mechanisms have been used to limit the force applied to structures during high seismic events. These mechanisms, known as dampers, dissipate the seismic energy and hence reduce the actual loads in lateral load resisting elements of the system and allow for a more efficient and economical design. This leads to design and fabrication of different types of energy dissipating systems within the industry for various applications. This research focuses on one type of damping system known as the friction damper. Applied to a standard braced frame lateral system, the friction damper is made up of a friction pad placed between two plates. These plates are attached by means of a series of bolts within predetermined length slotted holes. By post tensioning the bolts, a specific friction force [i.e., slip force] between the plates and friction pads can be achieved and seismic energy can be dissipated at the interface of the friction pads and the plates. This process dissipates the seismic energy and reduces the force resisted by the braced frame.
Currently, friction dampers come in a single stage, allowing structural engineers to design for a specific slip force and limit state [service or strength limit state]. This research focuses on the development and applications of a two-stage friction dampers that can be designed and tuned to dissipate seismic energy at both the service and strength limit states. This device, functioning at each stage like a one-stage friction damper, will consist of two plate and friction pad mechanisms with separate bolts and slotted holes. The first stage, slipping at a service level loading, will be activated at a low intensity and more frequent earthquake events. The second stage, will be activated at a seismic event with higher intensity [DBE or MCE]. This system will allow the structural engineers to take advantage of the two-stage friction dampers at low and high intensity seismic events and meet both the serviceability and strength criteria defined.
The finite element simulations and numerical analyses in this study, conducted using CSI’s structural building analysis and design software ETABS©, consisted of a series of preliminary analyses to validate the behavior of the system and accuracy of the model. Using a series of nonlinear links, a four-story, four-bay building was modeled with two-stage friction dampers. This structure was designed to meet predetermined inter-story drift limits and then subjected to a suite of 44 ground motions. The ground motions were scaled to both the service level earthquake [SLE] and maximum considered earthquake [MCE] spectrum for San Diego, California. The story shear and inter-story drift ratios were recorded and compared to those of a system without friction dampers [i.e. elastic system] to check the effectiveness of the two-stage friction dampers.
The results showed that the structure with the dampers produced an average base shear of 160 kips versus nearly 1000 kips in the elastic system and 50 kips versus nearly 250 kips subjected to MCE and SLE ground motions, respectively. This reduction factors of 6.25 for MCE level and 5.0 for SLE level proved the effectiveness and damping capabilities of the two-stage friction dampers to control the performance of the structure.
The study was limited to a preliminary analysis and development of the two-stage friction damper. Future research and studies can be conducted on this topic to further develop this type of damping system in terms of fabrication, design, optimization, and efficiency in building applications.