ANALYSIS OF STRUCTURES WITH SOME UNCOMMON IRREGULARITIES- TYPICAL FINDINGS

Architects are focusing on planning high-rise structures with various irregularities, there is a great deviation from the expected simple and symmetric configuration to irregular configurations. The IS 1893:2016 has detailed many vertical as well as horizontal irregularities in structures. Many technical papers are available on stiffness irregularity, mass irregularity, diaphragm irregularity, torsional irregularity, however no much research work is available in literature on some uncommon irregularities such as vertical geometric irregularity, floating or stub columns, non-parallel lateral force system. Thus there is a great need to investigate development of forces, displacements and storey drift in the structures with these irregularities and understand the limitations and scope for seismic safety of these structures. The analyses of structures with vertical geometric irregularities have clearly demonstrated that the column location and its position from the plane of major stiffness variation play important role in development of forces and displacements. The outcome of this research paper shall prove to be beneficial while designing structures with vertical geometric irregularity.


Introduction
Structural engineers are before different new developments in planning safe structures because of an expansion in the extreme activities surrendered on structures which has the imminent to cause incredible annihilation, harm and hazard to individuals. A vast part of India is susceptible to have an adverse effect on structures due to the seismic hazards. Hence it is important to take a note of seismic load for the design of high rise structures [2]. The conduct of a building during seismic tremors depends on its general shape, size and geometry, not withstanding how the seismic tremor forces are conveyed to the ground [1]. The earthquake forces created at various floor levels in a building should be transferred down along the height to the ground by the shortest path; any deviation or discontinuity in this load transfer path results in the poor execution of the building [3]. Proper design considerations along with appropriate safety measures can result in the seismic safety of structure with vertical geometric irregularity [4]. We have to examine the critical column locations and the increment in the forces and displacement due to the irregularities in structures. By finding these critical locations the plan of the building can be advanced. In this paper, a G+17 regular ideal structure is analyzed and designed by using STAAD-PRO Software and also three different models having vertical geometric irregularity are analyzed and designed by using same Software. In the models having vertical geometric irregularity, the irregularities are introduced following the Guidelines as specified in IS 1893:2016. Three different limits are adopted for three different models.

Study of Codal Provisions
For the design of structures with vertical geometric irregularity, Indian Standard Code IS 1893:2016 has specified some guidelines. The guideline is as follows:-1. Vertical irregularity a) Vertical Geometric Irregularity: "Vertical geometric irregularity shall be considered to exist, when the horizontal dimension of the lateral force resisting system in any storey is more than 125 percent of the storey below."

Methodology
Initially a G+17 regular ideal structure is analyzed and designed by using Staad-Pro Software. Then three models having vertical geometric irregularity are analyzed and designed. For considering the vertical geometric irregularity IS 1893:2016 has specified a limit of 125 percent. While creating the models with vertical geometric irregularity we have adopted three limits, one limit equal to 125 percent, other limit lesser than 125 percent i.e, a limit of 100 percent and the third limit of 150 percent which is greater than 125 percent. For the analysis of structures the grade of concrete used is M30 and the grade of steel used is Fe-415. Seismic analysis of ideal structure and the set of structure with vertical geometric irregularity were performed by using Equivalent Static Analysis approach. The plan and elevation of Regular ideal structure is shown in figure 1 and 2. The structures with vertical geometric irregularity are as shown in figure 3.

B) Structural Details
The ideal structure and the structures with irregularity are having same specifications.   1. Lateral shear force may be much more in case of 100% irregularity than the other cases. With the increase in irregularity this may irratically deacrease. (Fig. 4) 2. Torsion in the selected column shows irratic increase, torsion at middle floors is much higher for the 150% geometric irregularity while the ideal structure shows lowest torsion. (Fig. 5) 3. Similar observations are seen in case of lateral moments. (Fig. 6) 4. Displacement may be much more in case of 100% irregularity than the other cases. With the increase in irregularity this may irratically increase or deacrease. (Fig. 7)

Conclusion
Vertical geometric irregularities may result in erratic increase or decrease in torsions, shears and displacements. Position of column from the plane of major stiffness variation plays important role in these. 50% of geometric irregularity does not play a direct role in development of forces and displacements in all columns. The results may significantly vary with the configuration and column positions.