Seismic Analysis and Design of Irregular Buildings

Abstract

Earthquakes are known to produce one of the most destructive forces on the earth. The January 26th,2001 earthquake in Gujarat once again brought into focus on prepa- ration for such natural disasters. The situation was worst because of the huge loss of life. Buildings can be made quake-resistant, if no one quake- proof, by following building codes defined by the seismic history of a particular region. As a result, even engineered structures in urban areas like Ahmedabad crashed literally like a pack of cards. It has been observed in earthquake that the presence of irregularity in the plan of a structure makes it vulnerable to seismic damages. Buildings with plan irregular- ities (e.g., those with re-entrant corners such as L-shape plans on corner plots) and those with elevation irregularities (e.g., large vertical setbacks in elevation such as a plaza-type configuration in commercial structures) are common in the affected area. So it is very important to analyze the structure for Earthquake forces. Many studies have been done for different type of irregular structures. In Re- search papers different irregularity like soft storey , Floor diaphragm irregularity , Geometrical irregularity , Set back irregularity etc have been studied. Strengthening of existing structure is also mentioned in some of the research papers2. But very few studies have been done for improvement of structure by different parameters like shear wall, Regular grid , Stub column during earthquake forces. Eccentricity is the major concern for highly irregular structure. It needs to be considered while analyzing any of the structure. Torsional moment is generated due to eccentricity and due to that shear in column has to be calculated. The main objective of this study is to understand different irregularity and tor- sional response due to plan and vertical irregularity and also to analyze different shapes like \Ten Storey Building",\U", \Triangle" buildings while earthquake forces acts and to calculate additional shear due to torsion in the columns. Additional shear

due to torsional moment needs to considered. Due to Torsional Moment, shear force increases in the column so that in design procedure that shear must be considered. Design steps have been shown to understand the design procedure by IS 13920:1993. Design of beam and column have been done and ductile detailing has also been done and drawing is made to show detailing. In addition to this effects of soft storey, Shear wall, Stub Column, Regular grid is considered. Physical models have been pre- pared to check the behavior of the structure and to check torsional moment at corner columns. Additional shear is considered while designing the beams and columns. Co- dal Comparison of Design eccentricity is also presented to check amplification factors of different codes and also to check accidental eccentricity that needs to be considered. Ten storey building is analyzed by using STAAD.Pro. Different Irregularities like Soft storey and Mass Irregularity have been studied by different parameters like Storey shear, Time period, SF, BM , Displacement etc. It has been observed that Mass irregularity is having higher values of time period and also higher lateral forces. Soft storey is having larger values of shear force and bending moment. Physical mod- els have been prepared of different shapes like \L", \U", \Triangle",` `T" and \Soft Storey". Behavior has been checked by testing them on shake table and it has been observed that torsional behavior is the highest in corner columns. U shaped building has been analyzed by ETABS. It is 81m long and 46m in width. Shear wall model and Stub Column model have been prepared and comparison has been done for Storey shear, Time period, Displacement, SF and BM. Results shows that shear wall resists much lateral forces compare to frame structure. Stub column is very highly irregular thing in which beams can be failed due to higher forces transfer from column to beam. Transverse girder must be provided. Seismic analysis of Triangle shaped building has been done and effects due to ec- centricity has been measured. Torsional Moment has been calculated and shear has

been calculated in columns. It has been observed that inspite of Irregularity in the plan or in the elevation ,if Column grid is provided regular and if shear wall is pro- vided for lateral forces, it is much more efficient and effects of vulnerability can be minimized. The work has been carried out to show that if study of irregularity can be done properly than failures can be minimized. The content of major project is divided in various chapters. Chapter 1 includes in- troduction of irregularity and need of seismic analysis of structure. Chapter 2 includes literature review of various topics like irregularity, shear center, Eccentricity, Torsion etc. Chapter 3 includes understanding of torsion and codal provisions of eccentricity by different codes. Chapter 4 includes Seismic analysis of Ten Storey buildings which includes soft storey and Mass irregularity. Chapter 5 includes Seismic analysis of \U" shaped building analysis and stub column and shear wall model has been prepared to see the effects. Chapter 6 includes seismic analysis of \Triangle" shaped building. Different models have been prepared like \Regular Grid" , \L" shaped ,Shear wall model and results have been obtained for additional shear due to torsional moment. Chapter 7 includes design of Column and beam by following IS 13920:1993. Chapter 8 includes Summary and conclusions and also future scope of work.