The effects of structural geometry on the stiffness of diagrid high-rise structures

Abstract

This dissertation presents further analysis towards the subject of diagrid high-rise structures, with specific reference to the variations in structural geometry and their effects on the overall stiffness of these structures. A historical account of the development of this structural typology through the ages is provided, along with literature relating to the current studies being carried out on diagrid structures. The aims and objectives of this study are to investigate the behaviour of diagrid high-rise structures with respect to variations in height, base aspect ratio as well as the inclusion and exclusion of corner columns. This was accomplished by creating a control model of the diagrid high-rise structure and then altering its geometric variable parameters. The methodology of the parametric structural analysis is presented in this dissertation. Firstly, a list of geometric variables and fixed parameters that were implemented in an earlier study carried out by Moon et al. (2007) on the "optimal diagrid angle for high-rise structures" were used as the starting point for the analyses carried out in this dissertation. These variables and parameters were used to construct a 30 model using commercial parametric software, Karamba3D, which was then analyzed to compare the results obtained with those of Moon et al. (2007). This initial analysis served to validate the control model, whose specific variables and parameters were then altered to deal with other situations not considered by Moon et al. (2007). The variables and parameters used in this dissertation include the building height, the base aspect ratio and, the inclusion and exclusion of corner columns. The results obtained indicated that the structural behaviour of the control model compared well with that of Moon et al. (2007). With regards to the increase in height, the optimal angle of the diagrid to the horizontal continued to increase until 76° with regards to a structural configuration that included corner columns. On the other hand, the corresponding optimal angle for diagrid configurations that excluded corner columns plateaued at 69° as the height was increased up to 400m. The effects of corner columns were found to be significant, so much so that an average decrease of 30% in the total roof top horizontal deflection was noted in the structural configuration that corresponded to the optimal diagrid angle. The increase in the base aspect ratio iv resulted in the structures being designed according to their shear rather than their flexural requirement. This trend persisted at increasingly lower building heights, proportional to the base aspect ratio. Hence, it can be said that, for an increase in building height, the optimal diagrid angle also increases. The removal of the corner columns has more of an effect on the overall structural stiffness than that reported by Moon (2007). The increase in base aspect ratio had an effect on the overall design considerations of the structures in terms of their shear and bending area requirements.