Effect of outrigger beam location to the structural design of high-rise buildings under seismic loads

Outrigger braced systems contain core wall and exterior columns which connected by rigid girders to core. These rigid elements depth size can be one or two-storey height. When outrigger braced systems exposed lateral loads pheriperal columns which restrained by outrigger beams resist core rotation. This restistance causes tension and compression forces on exterior columns. There are two main results of this resistance; decreasing overturning moment of the core wall and decreasing top deflection of the system. In theoretical studies, outrigger systems are idealized by a single-redundant cantilever which restrained by rotational spring. In this system cantilever beam represents core wall and rotational spring represents sum of outrigger beams and exterior columns. Furthermore, in the same studies it is obtained that rigidity of rotational spring changes in direct proportion to the axial-rigidty (EA) of exterior columns which under the outrigger beam and it changes in the inverse proportion to the height of exterior columns. By using this idealized system, in order to reduce overturning moment of core wall, outrigger beam must be located at bottom of structure. On the other hand, in order to reduce top deflection of bulding outrigger beam must be located at midheight of structure. In this thesis, behaviours of structural outrigger braced systems under earthquake loads are studied with using 40 storeys building that has two outrigger beams. In consideration of the lack of studies in the literature, outrigger beams are selected reinforced concrete structural members and reinforced concrete design of outrigger beam is included in the research topic. There are no rules for design and sizing of the outrigger systems in actual codes. However there are some recommendation clauses which point to the necessity of limiting of overstrengths of outrigger beams in TBDY code that being come into force at 2019 and PEER-TBI guideline. In this thesis, behaviours of the outrigger braced structural systems are studied in two stages, linear and non-linear analysis due to lack of design rules in codes. The study is classified in six main section. In the first section, general informations about high rise buildings are given and the process of determining the structure load-bearing systems is emphasized. At the end of the first section, the aim of the studies are briefly mentioned. Second section is the literature review part. It includes summary of studies that analyzed effects of outrigger beams location and systems behaviour under earthquake effects. In the third section, effects of outrigger beams location are studied on idealized systems which includes in literature review parts in second section. In the fourth section, linear and non-linear behaviour of a reinforced concrete building which height is H and has two outrigger beams at 0.33H-0.66H location are investigated. Strut-and-tie method is used for reinforced concrete calculation of outrigger beam. Outrigger beam is not suitable to the Bernoulli-Navier hypothesis cause of the load and the size of the beam. Stress orbits are the result of idealization of the outrigger beam with frequent finite elements which are used for generating the strut-and-tie model. Incremental equivalent earthquake load method that one of the pushover method which is the capacity of the structure can be obtained independently of earthquake ground motion is used for non-linear behavior of the system. In the fifth section, analyzes of four different systems (0.25H-0.75H, H-0.25H, H-0.50H, H-0.75H) obtained by changing the positions of the outrigger beams in the structural system used in the fourth section were performed. The results obtained from the systems are compared in two parts as linear and nonlinear analysis. In the last part, all the results obtained from this dissertation are summarized.