Structural design and evaluation of seismic performance of a tower

In recent years, with increasing economic power, technological opportunities and demands a tendency has been emerged toward tall buildings. As a result of this tendency, competition for both engineering and architectural design has come to fruition, and a monumental identity of high structures has also became clear. Together with the instinct of ascending to higher levels of human beings, high buildings have come to the forefront as a symbolic structure for some important events. Existing design and application practice, reinforced concrete shear walls are accepted as the main element of rigidity lateral stiffness. However, some secondary structural elements are also used according to the geometry of the structures and the service requirements. As a result of this role sharing, the behavior and safety of reinforced concrete shear walls under lateral forces determine the behavior and safety of whole structure. As a result of the reinforced conrete application which is used for about a century, we have serious knowledge and experience accumulation on the section behavior. However, as the interactions of the structural elements increase, the behavior of the building becomes more complex. For this reason, structural design in practical engineering is carried out not to seek real behavior exactly, but to provide the necessary structural safety. Mechanisms that form the bending and shear resistances of the sections must be well understood in order to ensure the safety of the reinforced concrete wall. In addition, when detailing shear walls according to these mechanisms, all the behavioral phases of the structural elements must be considered in addition to the initial conditions. In this study, the effects of both cross-sectional dimensions and designs in which sections can safely transfer shear loads under bending and shear interaction were investigated. This master thesis work consists of six parts. Section contents can be described as follows: In the first part, the purpose, scope and method of study are given. The aim of this study is to investigate shear design approach for shear wall at tubular plan type and shear behavior at damaged sections like cracked sections in tension. The scope of this study is nonlinear behavior and performance evaluation of 115.9 m high tubular section tower with 10 m diameter. In the second part, general shear behaviors in reinforced concrete sections, shear behavior in reinforced concrete walls and shear failure modes have been examined and summarized. In the third chapter, the structure that studied is introduced. The sample structure is 115 m high and designed by the author of the thesis and is currently built in Antalya. The structure has a tubular core wall plan and have 10 m diameter. Thickness of shear wall is 80 cm for first XX m and 60 cm for remaining part. This study includes xxii a sensitivity analysis for shear wall thickness to achieve optimum structural behavior. Construction site classified as second earthquake region and Z3-C soil class. In order to execute nonlinear time-history analysis of structure, original earthquake records with scale factors which are presented in seismic hazard report of the constructions site used. Also analysis are executed at directions at 0° and 45° to cover most inconvenient cases. In the fourth chapter, general structure behavior, bending and shear behavior of reinforced concrete shear walls are observed over four different reinforced concrete wall thicknesses. In this section, using only linear analyzes, all parameters are evaluated and the most effective working reinforced concrete wall thickness is determined. All linear analysis go through with 60 cm, 70 cm, 80 cm and 90 cm shear wall thickness in order to pick most efficient section dimensions. Natural periods, axial compressive stress, tension reinforcement, shear reinforcement and lateral displacements vary nearly linearly with shear wall thickness. But increase at base shear force ratio to mass of structure with 80 cm thickness is optimum with respect to shear wall thickness. In the fifth section, nonlinear analyzes of the structure with determined section dimensions and reinforcement detailing are performed in the time domain. Taking into account the interaction of bending and shear behavior, shear safety in reinforced concrete shear walls has been investigated for different behavioral steps. To execute nonlinear time-history analysis, all shear walls are modelled as fiber section with expected material properties. Shear behavior of shear wall are considered as elastic with an effective shear modulus. To assess performans of shear walls in bending, strains are monitored with respect to specified thresholds in specifications. To evaluate the performance of tower structure lateral displacement, tension and compression strains at shear wall, average compressive stress at shear wall sections, average shear stresses at shear wall sections considered with respect to limits at specifications. Also in order to evaluate shear safety of shear walls, sliding shear failure mode of shear wall is investigated. As a result of geometry of structure, always nearly half of shear walls are exposed to tension in bending behavior. Shear wall sections which are exposed to tension are considered as vulnerable to sliding shear failure in absense of axial compression. In this very section, no tension capacity of concrete, interlocking of aggregates etc. is not considered as a mechanism to create shear resistance. Only dowel action of longitudinal reinforcements is liable for shear resistance of shear wall. Combined action of shear and tension on rebars are monitor at every time step in every shear wall section. Except only a fractional part of rebars, all longitudinal reinforcements are adequate for combined tension and shear action. No nonlinear behavior is considered at steel structure elements of building and all steel frame structures which placed on top of structure is modelled as elastic frame element. In the sixth chapter, the results of the studies are compiled and some recommendations are given. Sensivity analysis that investigates optimum shear wall thickness, exhibits that larger sections does not guarantees safe conditions, neither it does not offer an efficient engineering solutions. All the lateral deformations values are within the thresholds of specification with considered shear wall thickness. xxiii The sample structure achieves "Life Safety" performance criteria. All shear wall sections are achieves safety for axial compressive stress level and deformations levels. Also detailed sections by linear analysis does contain adequate longitudinal reinforcement for axial tension stress due to earthquake loads. All shear wall sections has enough section area for principle compressive capacity for shear demand and enough tranverse reinforcement for principle tension capacity for shear demand. Only longitudinal reinforcements does provide adequate shear resistance againt sliding shear failure without any concrete contribution.