THE EFFECT OF ANGLE CHANGE OF DIAGONAL MEMBERS ON R-FACTOR AND COLLAPSE FRAGILITY CURVES OF MID-RISE STEEL DIAGRID STRUCTURES

A new structural system of diagonally gridded elements, the so called ``diagrid'', has been introduced in recent years in which the frames that are made up of intersected members surround the main structure. This type of system provides excellent shear rigidity and stiffness, in contrast to the similar tubular structures. The triangular configuration offers structural stability for gravity and lateral loads, and the removal of external columns gives the building a unique architectural beauty. Swiss Re (London), Hearst Tower (New York), Cyclone Tower (Asan, South Korea), Capital Gate Tower (Abu Dhabi) and Jinling Tower (Nanjing, China) are prominent cases with diagrid system worldwide. The response modification factor (R-factor) for diagrid systems is not yet explicitly introduced in the existing building codes and few studies have been conducted on the assessment of this parameter. On the other hand, previous researches show that the angle of diagonals play an important role in the structural behavior. In this paper, the effect of the angle change of diagonal elements on R-factor, collapse capacity and collapse fragility curves of steel diagrid structures are investigated. To this end, 6, 8, 10, and 12 story diagrid buildings with different angles for diagonals varying from 580 to 780 were selected and modelled using OpenSees software. The post-buckling behaviour of diagonal members was taken into account in modelling process. The models were then analyzed using nonlinear static analysis and the R-factor of the system was calculated according to the proposed method by ATC-19 guielines. Nonlinear incremental dynamic analysis (IDA) were performed and collapse fragility curves were extracted using 44 far-field records in order to evaluate their seismic performance and to validate the calculated R-factor. Next, the reliability of the applied seismic performance coefficients were controlled through the methodology proposed by FEMA P-695 taking into consideration the uncertainties in modeling, designing, earthquakes and experimental data. Results show that the effect of diagonal angles on the response modification factors depend on the building height. However, it can be stated that, in general, changing the angles of diagonals in steel diagrid systems do not have a significant effect on their R-factor. It is also observed that an increase in the angle of diagonal members will reduce the uncertainty in collapse data.