Earthquake performance assessment of long span reinforced concrete structures using nonlinear dynamic time history analysis and seismic retrofitting with viscous dampers

It's always crucial to select assessment methods for the determination of seismic performance of existing structural systems located on an active fault zones. Selected in compliance with their dynamic characteristics under the effect of an earthquake on structural systems, these assessment methods can be classified into two groups. The methods are based on elastic and elasto-plastic structural material behavior. Calculation methods which are selected by considering that the material is linear, i.e. elastic, are the studies conducted for determination of capacities of load-bearing system elements under the effect of seismic loads. On the other hand, calculation methods based on elasto-plastic material behavior – i.e. material is nonlinear- consist of the studies conducted for determination of demands for elongation/shortening which will take place in structural materials under the effect of a seismic design. Another key point regarding the existing structural systems located on seismic zones is compensation of structural inadequacy identified as a result of performance assessment, which means determination of retrofitting methods eligible for existing structural systems with resistance inadequacy. In the last two decades, with the development of practices in structural technology, use of passive control systems for both designing structural systems and studies for seismic retrofit of existing structures has started to be common. As one of the passive control systems, viscous dampers involve stainless steel cylindrical pipes and pistons on the head of these pipes. Velocity responses to which structural systems exposed under the effect of an earthquake create two different pressures on the pistons located on both ends of viscous dampers. The pressure difference created on both ends of dampers occurs due to silicon-based oil and can be transformed into thermal energy. Thus, a part of the seismic energy to which the system is exposed can be damped. Within the scope of this study, seismic performance of existing reinforced concrete blocks separated by expansion joints and whose load-bearing system is made of shear wall systems, and approximately 60 m long reinforced concrete shell system as a whole extension of these blocks is determined. For determination of the seismic performance of the structural system, nonlinear time history analysis method is used regarding the dynamic characteristics of the system. For the assessment of seismic performance of the structural system, at first, definition of plastic hinges are specified and these hinges are defined at analysis models in terms of the section characteristics of load-bearing system elements and by using the method in which structural materials present elasto-plastic behavior, i.e. these materials are nonlinear, and on the basis of section damage limits for load-bearing elements estimated by regulations. In the second process, ground motion records selected in accordance with the dynamic characteristics of the structural system and seismological and geotechnical characteristics of the region are identified. With 5% damping ratio, acceleration spectrum of selected ground motion records are registered and these spectral curves are scaled according to defined 5% elastic design acceleration spectrum by the regulations within the predefined matching range with respect to the first natural period of the structural system, regarding the seismicity of the region. The selected ground motion records are scaled in accordance with the acceleration spectrum design with regards to the requirements defined by regulations. The results of the seismic performance assessment conducted on the structural system are summarized under two main sub-headings: • Load-bearing systems of structural blocks generally have inadequate lateral resistance. Particularly the shear wall systems formed in the short direction in the plan are exposed to 95% of the base shear force since the blocks are constructed with shear wall systems. Such a high rate creates excessive tension-compression responses at these shear wall system headings over their own capacity. These responses lead to the formation of plastic hinges on shear wall headings mentioned above at collapse performance level. The plastic hinges formed in the structural systems of Block 4 and 5 reduce the performance of the system to the collapse level. • Since the connection between nearly 60 m long reinforced concrete shell system and structural blocks is designed by hinges, the system doesn't have any lateral stiffness. This results in the shell end to move in high levels under the estimated effects of an earthquake. Additionally, shell elements swing independently at high levels since they don't have connection among each other. The last chapter of this study focuses on the retrofitting method to reduce response on displacement of reinforced concrete shell system under analyzed effects of an earthquake. In this method, shear wall systems are placed on both end points of shell elements working as a cantilever shear wall systems. These shear wall systems and edge shell elements are connected with four viscous dampers to reduce the velocity response which will be placed on the cantilever shear wall systems. The main objective of placing viscous dampers is to reduce velocity response of the shell end system under the effect of an earthquake. Thus, the cantilever shear wall systems will be exposed less lateral force and the elements will be designed with a lower cost. Variables defining dynamic characteristics of the four viscous dampers used are determined via trial and error method with the estimation of approximately 50% damping on the shell system, regarding the first natural lateral period of the system. In the last chapter, energy demands and capacities under the estimated earthquake motion of the system within the scope of retrofitting the shell system with viscous dampers are evaluated. According to the results of these energy assessments, energy capacity of the shell systems is above the demands on earthquake motion energy in all ground motion records.