The following paper presents the design and verification steps of tall steel frame structures with braced core, belt and outrigger trusses. The two case study structures refer to sky scrapers of 180m and 300m respectively, located in Istanbul, Turkey Keywords tall steel buildings, concentrically braced frames, response spectrum analysis, nonlinear dynamic analysis IntroductionThe design of a tall building, characterized by definition by its significant slenderness and a long vibration period [1-8] must pay particular attention not only to vertical loads, but also to the horizontal ones, such as wind and earthquakes, which require greater control of displacement and structural strength, since they may induce significant oscillations and effects of the second order are no longer negligible. The structural systems designed to withstand these actions, are the bracing structures which by their nature, affect the choice of the entire structural scheme. It should be highlighted that, since this paper focuses on buildings that are 180m tall, at least, to calculate the wind action, it was not possible to refer to European codes, because they do not specify how to study the effects of such an action when building is taller than 100m. Therefore, it was necessary to refer to American guidelines and standards [9,10] that were assumed as an alternative approach, as shown in the following sections. Instead, the effect of the seismic action was assessed comparing two models: the first one is based on a linear elastic constitutive approach and subjected to modal dynamic analysis with a design spectrum, carried out through SAP2000 software [11]; the second one, having more sophisticated constitutive laws, was developed adopting a distributed plasticity approach and then subjected to a series of nonlinear dynamic analyses, carried out within the open platform OpenSees [12]. Although high-fidelity finite element (FE) models commonly used for seismic analysis and/or topology optimization of steel and reinforced concrete structural systems and components [13][14][15][16][17][18][19][20][21] are able to reproduce local stress/strain concentrations, the computational effort increases tremendously. Therefore, such an approach is currently unfeasible for design office use, especially whether the response of entire super-tall mega-frame buildings with their key structural components has to be assessed in a nonlinear dynamic fashion. In light of this, mechanical idealizations and fiber-based representations were proven to be a promising and viable approach when used to predict the nonlinear behavior of such systems at a global level, as shown by several comparisons against experimental tests [22][23][24][25][26][27].