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The raising level of migration of people to urban cities and unavailability of land to accommodate this migration have given birth to multi-storey buildings or tall buildings. Currently, numerous systems comprising beams with columns, shear walls or bracings either individually or in combinations are used for designing structures of tall buildings. From them diagrid system is one of new trends. Diagrid system is adopted due to architectural limitations of providing vertical elements, as in this system only diagonal members in form of grids are provided to support both gravity as well as lateral loads. In present study, behavioural investigation of different diagrid buildings is assessed using ETABS software. Three RCC buildings 6, 12 and 18 storied are considered for this study. The objective of the study is to compare the conventional frame (CF) system with diagrid (DG) system with respect to varying number of storeys. To elaborate the comparison, diagrid system is further divided into three types as per diagrid angles, i.e. 33.42°, 52.85° and 63.2°. From comparison of results, it is concluded that, for 6-storey buildings adopting CF system reduces construction cost and time, for 12-storey buildings any system except 33.42°DG system can be adopted as per construction cost, time and aesthetical appearance and for 18-storey buildings 63.2°DG system is preferred. Hence, diagrid structural system can be adopted for buildings above 40 m height approximately.

Setback buildings are those types of structures which have geometric vertical irregularities. The historical tremor records show that, the setback buildings attracts greater damage as compared to the regular buildings. This poor seismic response may be ascribed to the insufficiency of current codes, with which these buildings were designed. So, it becomes obligatory to investigate the seismic response of these structures with more advanced seismic iterative assessments and practical adjustments in order to reduce the potential seismic damages. This research focuses mainly on seismic behaviour of vertical geometrical irregular structure and the method to avoid or minimize such problems during the design of structures. To carry out the study, a regular symmetric frame (M1) and seven irregular (M2, M3, M4, M5, M6, M7 and M8) frame G + 10 buildings with different vertical setbacks each were modelled and considered for the analysis by using ETABS 2018 software. The result proves that buildings with vertical setbacks are more vulnerable to torsion than the regular buildings. To reduce the torsional irregularity effects on vertical irregular structure, shear wall system was recommended as practical solution which is an effective method of seismic retrofitting of an existing structure and for other future projects. It was concluded that fundamental time period of regular building is less than the similar vertical geometric irregular building, and it depends on upon the setback ratio. Regular buildings give higher participation of first mode as compared to buildings with vertical setback because as the irregularity increases the contribution of first mode decreases.

Due to the client demand on architectural drawings and land structure of Nepal, it is very difficult to make the structure with regular plan shapes, and thus, horizontal or vertical irregularity may be developed. These irregularities of the structure are more responsible for the collapse of building under the dynamic loads action during strong ground motions of earthquake and cause a significant torsional reaction in the structure. The main objectives of this paper are to study the torsional behaviour of reinforced concrete building with plan asymmetry which is located in zone V subjected to earthquake load and its parameters reflect the torsion effect and recommending the practical solution in order to reduce torsional effect. To carry out the study, both symmetric and asymmetric (L-SHAPE, + shape and T shape) plan buildings of eleven storey were modelled and ETABS software is used for analysis. The result proves that asymmetrical plan buildings are more vulnerable than symmetrical plan buildings. To reduce the torsional irregularity effect of asymmetrical plan structure curtailed shear wall system is recommended as practical solution with considering the stiffness. It was concluded that curtailed shear wall system improves the seismic performance of the structure reducing the cost criteria. After additional of shear wall, the torsional irregularity ratio of asymmetrical plan structure comes with in permissible limit as per IS 1893. Torsion creates damage or complete structures collapse, to control torsion, irregular structures should be carefully analysed.