مقاله انگلیسی رایگان در مورد خواص مودال و رفتار لرزه ای ساختمان های مجهز – الزویر ۲۰۱۸

This paper deals with the seismic protection of building frames by means of external dissipative systems. Dampers and external framing system can be arranged in several configurations, involving different kinematic behaviours and seismic performances. This study analyses a recently-developed solution called “dissipative tower”, which exploits the rocking motion of a steel braced frame, hinged at the foundation level, for activating the dampers. This system aims at controlling both the global response and the local storey deformation of the frame, by using a reduced number of viscous dampers. A state space formulation of the dynamic problem is presented in general terms, together with the solution of the seismic problem via the modal decomposition method. A parametric study is carried out to evaluate the influence of the added damping and of the braced frame stiffness on the modal properties and seismic response of a benchmark reinforced concrete frame retrofitted with the external dissipative towers. It is shown that the addition of the towers yields a regularization and reduction of the drift demand along the building height, but it may induce significant changes, not always beneficial, in the distribution of internal actions of the frame and in the absolute storey accelerations.

Introduction

Passive damping systems have proven to be very efficient solutions for the seismic protection of new constructions and retrofitting of existing structures [1–۴]. Dampers are traditionally installed within a building frame in either diagonal or chevron brace configurations connecting adjacent storeys. This type of damping system, whose effectiveness has been investigated in numerous studies (e.g. [5–۱۱]), presents some disadvantages, particularly when employed for retrofitting existing buildings. Usually, the addition of dissipative diagonal in existing frames provides an increment of axial forces in the columns and this may lead to premature local failures, as observed numerically in the case of moment resisting frames equipped with nonlinear hysteretic dampers [12,13] as well as with linear viscous dampers [14]. In order to avoid this, column strengthening may be required, in the case of existing frames [12], or application of specific capacity design rules, in the case of newly designed moment resisting frames [14]. Furthermore, there may be some feasibility limits on the strengthening of the existing foundations at the base of the bracing system. Also, the indirect costs related to the interruption of the building utilization during the installation of the retrofit system can be very high, particularly for strategic buildings, hospitals or schools. These problems could be solved efficiently by using external damper configurations, where the dissipative bracings and the relevant foundations are placed outside the construction [15]. External dampers and bracing components can be arranged in very different configurations and the possible solutions can be grouped into three main categories, characterized by substantially different kinematic behaviours, but all permitting the control of both the total amount of the dissipated energy and the frame deformation at the various storeys. In the first arrangement (Fig. 1a), the dampers are placed horizontally at floor level, between the frame and an external stiff structure [16–۱۸]. This way, the links are activated by the relative displacements between the frame and the external structure. A similar configuration can be obtained by placing the dampers between adjacent buildings, though this solution is efficient if the two buildings have strongly different dynamic properties [19–۲۲]. An alternative arrangement consists in coupling the frame with an external shear deformable bracing structure (Fig. 1b). The new and existing structures are connected at the storey level and the dissipative devices, incorporated in diagonal braces within the new structure, are activated by the relative displacements between adjacent floors, as in the more traditional case of dissipative braces placed within the existing structure [3]. A third arrangement, denoted as “dissipative tower” consists in external stiff bracings linked to the frame at the storey level and connected at the foundations by a hinge (Fig. 1c). The energy dissipation is provided by dampers placed at the external frame base and activated by its rocking motion. The high stiffness of the braced frame promotes a uniform distribution of the inter-storey drift of the protected frame. Recently, this system has been employed for the seismic design of new constructions and for retrofitting existing buildings [23,24] and a patent covering a technological solution was registered [25]. A theoretical study [26], in the field of the stochastic dynamic, has compared the performance of this system with that of the system of Fig. 1a).