ABSTRACT
Humans has started rising higher due to lace in horizontal space, Building taller structures has become a common trend. But as this vertical limits is pushed the risk factor increase. Skyscraper has to be analyzed for various loads to provide safety. Behavior of a structure is most critical during earthquake thus it is important to analyze a structure for seismic resistance. Earthquake is an unpredictable hazard having power to bring a high raised structure to ground. Thus it is important to analysis a structure for seismic force. Seismic coefficient method can be used to analyze small and medium height structure up to 40m. It is a simple method of analysis and required less calculation. The behavior of a structure depends critically on overall shape, size and geometry. As an asymmetrical structure behave more critically during earthquake as compared to a symmetrical structure. A study on the behavior of symmetrical, L-shape and T-shape RC building during earthquake is done to understand different in seismic loading and difference in behavior due to different shape. A 10 store building with height of each floor 3m having different shape as symmetrical, L-shape and T-shape in plan are analyses by seismic coefficient method with the help of ETABS software. As manual analysis of as structure that is 10 store high is a time consuming job and probability of errors is also increases. ETABS may provide an easy, efficient and accurate way to analyze a structure. The analysis is done as per IS 1893:2002 (Part-1). Estimation of response such as; lateral forces, base shear, storey drift, storey shear etc. is carried out for all the different shaped RC buildings and Compared based on response given by different shaped building.
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CHAPTER- 1
Introduction
An earthquake is the perceptible shaking of the Earth, which can be violent enough to destroy major buildings and kill thousands of people. The severity of the shaking can range from barely felt to violent enough to toss people around. Thus to reduce the loss of life due to earthquake it is important to study earthquake and behaviour of building during earthquake.
1.1) Earthquake and its effect on buildings
An earthquake is the resulted due to sudden release of energy in the earth's crust that creates seismic waves. Earthquakes are caused mostly by rupture of geological faults, but also by other events such as volcanic activity, landslides, mine blasts, and nuclear tests.
Fig No. 1.1- Number of Earthquake per year 1|Page
1.1.1) Behavior of Structure during Earthquake
In recent times, reinforced concrete buildings have become common in India, particularly in towns and cities. Ground shaking is experienced during earthquake, building resting on the on it will experience motion at its base. From Newton's First law of Motion, even though the base of the building moves with the ground, the roof has a tendency to stay in its original position. But since the walls and columns are connected to it, they drag the roof along with them. This is much like the situation that you are faced with when the bus you are standing in suddenly starts; your feet move with the bus, but your upper body tends to stay back making you fall backwards!! This tendency to continue to remain in the previous position is known as Inertia. In the building, since the walls or columns are flexible, the motion of the roof is different from that of the ground (Figure 1.2). The force experienced by the roof due to drag of base structure is knows as Inertia Force. [13]
Fig. No. 1.2- Effect on Inertia in building during earthquake
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Inertia force in brief can be explained by Newton's Second Law of Motion, the inertia force F is mass M times acceleration A (F=M x A), and its direction is opposite to that of the acceleration. Clearly, more mass means higher inertia force. Therefore, lighter buildings sustain the earthquake shaking better. As the load transferred take place the inertia force experienced by the roof is transferred to the ground via the columns, causing forces in columns. The forces are also generated due to earthquake shaking, the columns undergo relative movement between their ends. (Figure 1.3). As columns try to come back to the straight vertical position, i.e., columns resist deformations. In the straight vertical position, the columns carry no horizontal earthquake force through them. But, when forced to bend, they develop internal forces. The larger is the relative horizontal displacement u between the top and bottom of the column, the larger this internal force in columns.
Fig. No. 1.3- Inertia force in columns 3|Page
1.1.2) Importance of Size and Shape of Building during Earthquake
The behaviour of a building during earthquakes depends critically on its overall shape, size and geometry, in addition to how the earthquake forces are carried to the ground. A desire to create an aesthetic and functionally efficient structure drives architects to conceive wonderful and imaginative structures. Sometimes the shape of the building catches the eye of the visitor, sometimes the structural system appeals, and in other occasions both shape and structural system work together to make the structure a marvel. In general, buildings with simple geometry in plan (Fig. No. 1.4) have performed well during strong earthquakes. Buildings with re-entrant corners, like those U, V, and + shaped in plan (Fig. No. 1.4), have sustained significant damage. Many times, the bad effects of these interior corners in the plan of buildings are avoided by making the buildings in two parts. For example, an L-shaped plan can be broken up into two rectangular plan shapes using a separation joint at the junction (Fig. No. 1.4). Often, the plan is simple, but the columns/walls are not equally distributed in plan. Buildings with such features tend to twist during earthquake shaking. [13]
Fig No. 1.4- Shapes of building 4|Page
Twisting or torsional movement in a building is created due to irregularity in structure. As earthquake force act on the center of mass of building and its resultant acts at the center of stiffness of building. Due to difference in center of mass and center of stiffness of building torsional movement is generated. The twisting can be easily understood by the classic swing example Swing - a wooden cradle tied with coir ropes to the sturdy branch of an old tree. Consider swing tied by two identically equal rope. It swings equally, when you sit in the middle of the cradle. Buildings too are like these rope swings; just that they are inverted swings (Fig. No. 1.5). The vertical walls and columns are like the ropes, and the floor is like the cradle. Buildings vibrate back and forth during earthquakes. Buildings with more than one storey are like rope swings with more than one cradle.
Fig. No. 1.5- Behavior of building 5|Page
Now consider same cradle and if you sit at one end of the cradle, it hoists (i.e., moves more on the side you are sitting). This also happens sometimes when more of your friends bunch together and sit on one side of the swing. Likewise, if the mass on the floor of a building is more on one side (for instance, one side of a building may have a storage or a library), then that side of the building moves more underground movement (Fig. No. 1.6). This building moves such that its floors displace horizontally as well as rotate. Twisting is due to unequal distribution of load.
Fig. No. 1.6- Twisting due to unequal distribution of mass
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Now considering the same cradle but with two ropes which the cradle is tied to the branch of the tree be different in length. Such a swing also twists even if you sit in the middle (Fig. No. 1.7). Similarly, in buildings with unequal structural members (i.e., frames and/or walls) also the floors twist about a vertical axis (Fig. No. 1.7) and displace horizontally. Likewise, buildings, which have walls only on two sides (or one side) and flexible frames along the other, twist when shaken at the ground level (Fig. No. 1.7). Twisting is due to Unequal distribution of Stiffness.
Fig. No. 1.7- Twisting due to irregularity in stiffness 7|Page
1.1.3) Necessity of Seismic Analysis
The engineers do not attempt to make earthquake proof buildings that will not get damaged even during the rare but strong earthquake; such buildings will be too robust and also too expensive. Instead, the engineering intention is to make buildings earthquake resistant; such buildings resist the effects of ground shaking, although they may get damaged severely but would not collapse during the strong earthquake. Thus, safety of people and contents is assured in earthquake-resistant buildings, and thereby a disaster is avoided. This is a major objective of seismic design codes throughout the world. [13] The earthquake design philosophy may be summarized as follows (Figure 1.8):
Fig. No. 1.8- Performance of building under different intensity of earthquake shake 8|Page
(a) Under minor but frequent shaking, the main members of the building that carry vertical and horizontal forces should not be damaged; however building parts that do not carry load may sustain repairable damage. (b) Under moderate but occasional shaking, the main members may sustain repairable damage, while the other parts of the building may be damaged such that they may even have to be replaced after the earthquake (c) Under strong but rare shaking, the main members may sustain severe (even irreparable) damage, but the building should not collapse. Thus, after minor shaking, the building will be fully operational within a short time and the repair costs will be small. And, after moderate shaking, the building will be operational once the repair and strengthening of the damaged main members is completed. But, after a strong earthquake, the building may become dysfunctional for further use, but will stand so that people can be evacuated and property recovered.
Damage in Buildings: Unavoidable
Design of buildings to resist earthquakes involves controlling the damage to acceptable levels at a reasonable cost. Contrary to the common thinking that any crack in the building after an earthquake means the building is unsafe for habitation, engineers designing earthquakeresistant buildings recognize that some damage is unavoidable. Different types of damage occur in buildings during earthquakes. Some of these cracks are acceptable (in terms of both their size and location), while others are not. For instance, in a reinforced concrete frame building with masonry filler walls between columns, the cracks between vertical columns and masonry filler walls are acceptable, but diagonal cracks running through the columns are not (Fig. No. 1.9). Earthquake-resistant design is therefore concerned about ensuring that the damages in buildings during earthquakes are of the acceptable variety, and also that they occur at the right places and in right amounts.
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Fig. No. 1.9- Diagonal crack in column
This approach of earthquake-resistant design is much like the use of electrical fuses in houses: to protect the entire electrical wiring and appliances in the house, you sacrifice some small parts of the electrical circuit, called fuses; these fuses are easily replaced after the electrical overcurrent. Likewise, to save the building from collapsing, you need to allow some predetermined parts to undergo the acceptable type and level of damage.
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1.2) Necessity of High Rise Building Scarcity of land in fast growing city like Mumbai, Ahmedabad, Surat, etc. has raised the need of vertical growth. With increasing population and restriction in horizontal development due to scarcity in land, construction of number of skyscrapers or high raised building has increased. Increase in high raised building is due to: Rapid growth of population in urban communities, and therefore the constant pressure of the limited land area affected the evolution of building. Table No. 1.1- Population growth [15] Current Population of Gujarat in 2015
65,105,237
Population in 2014
63,891,303
Population of Gujarat in 2013
62,700,003
Population of Gujarat in 2012
61,530,916
Population of Gujarat in 2011
60,383,628
Horizontal Development gets restricted due to less space & then vertical growth is the ultimate option available. Expensive land prices. The high cost of setting up infrastructure for new cities. Improving the esthetic view of the city.
With increase in height of building, human & economic losses due to various natural & human hazards also increases. Thus high raised building should be designed in such a way that they can resist against various forces. Behaviour of a structure is vulnerable during an earthquake. Thus it is necessary to study the forces and behaviour of structure during earthquake and designing structure in such a way that it can withstand earthquake without collapsing.
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1.3) Introduction to ETABS
In this study, behaviour of building during earthquake were deliberate by the help of ETABS. ETABS stands for Extended Three-dimensional Analysis of Building System. ETABS is an analysis and design software for analyzing and designing a building.
1.3.1) Introduction to ETABS
ETABS is an engineering software for analysis and design of multistory structure. ETABS is in use for 30 years and is developed by Computers and Structures, Inc. (CSI). CSI was founded in 1975, is recognized globally as the pioneering leader in software tools for structural and earthquake engineering. Software from CSI is used by thousands of engineering firms in over 160 countries for the design of major projects. CSI also provides various different products for analyzing and designing of different structures as SAP2000, CSiBridge, ETABS, SAFE, PERFORM-3D and CSiCOL. [16]
Fig No. 1.10- ETABS 12 | P a g e
ETABS is a 3D object based modeling and visualization software, blazingly fast linear and nonlinear analytical power, sophisticated and comprehensive design capabilities for a widerange of materials, and provides reports, and schematic drawings that allow quick and easy understanding of analysis and design results.
ETABS integrates every aspect of the engineering design process. Creation of models in ETABS is easier - intuitive drawing commands allow for the rapid generation of floor and elevation framing. CAD drawings can be converted directly into ETABS models or used as templates onto which ETABS objects may be overlaid. ETABS also helps in Design of steel and concrete frames (with automated optimization), composite beams, composite columns, steel joists, and concrete and masonry shear walls. Models can be rendered, and all results can be seen directly on the structure. Comprehensive and customizable report is available for all analysis and design output, and schematic construction drawings of framing plans, schedules, details, and cross-sections may be generated for concrete and steel structures.
ETABS has a wide selection of templates for quickly starting a new model. At this model template stage, the user has the ability to define grid and grid spacing, the number of stories. It follow various international codes which helps user to analyze and design the structure as per their code. Various kind of different load can be applied in etabs such as Super dead load, Live load, Seismic load, Wind load, etc. ETABS provides the support of IS 1893:2002 for seismic analysis of a building & provides the analysis results for various load combinations.
1.3.2) Features of ETABS
One Window, Many Views: - ETABS provides a single user interface to perform: Modeling, Analysis, Design, Detailing, and Reporting.
Templates: - ETABS has a wide selection of templates for quickly starting a new model. The user has the ability to define grid and grid spacing, the number of stories, the default structural system sections. 13 | P a g e
Automated Code Based loading: - ETABS will automatically generate and apply seismic and wind loads based on various domestic and international codes.
Load Cases and Combinations: - ETABS allows for an unlimited number of load cases and combinations. Load combination types.
Mixed Units: - ETABS gives users full control of the units used with all model data and displays results in the units desired. Whether architectural units or analysis results units, you can have any combination of units throughout your model.
Deformed Geometry: - Users can display deformed geometry based on any load or combination of loads, as well as animations of modes.
Reaction Diagrams: - Support reactions can be displayed graphically on the model either as vectors or as tabular plots for selected reaction components.
Report Generation: - The report generator features include an indexed table of contents, model definition information, and analysis and design results in tabulated format. Reports are viewable within ETABS 2013 with live document navigation connected to the Model Explorer and directly exportable to Microsoft Word.
1.3.3) History of ETABS
Etabs is originally development of TABS over 30 year by CSI.
The software is used by millions of engineers over 160 countries and is leading tool for structural and earthquake engineering
ETABS was used to create the mathematical model of the Burj Khalifa, currently world's tallest building designed by Skidmore, Owings and Merrill (SOM).
ETABS was used to analyze Taipei 101 Tower in Taiwan, One World Trade Center in New York, the 2008 Olympics Birds Nest Stadium in Beijing, etc.
ETABS is commonly used to analyze: Skyscrapers, parking garages, steel & concrete structures, low rise buildings, portal frame structures and high rise buildings.
Other CSI software also helped in designing the cable-stayed Centenario Bridge over the Panama Canal. 14 | P a g e
1.4) Objectives 1. To study behavior of high-rise RC building during earthquake. 2. Effect of seismic load on structure due to plan irregularity. 3. To compute lateral force on each levels due to seismic force. 4. To check the maximum allowable response of structure. 5. To study behavior of different shaped buildings in plan during earthquake. 6. Compute the torsional movement in structure due to irregularity in mass and stiffness. 7. To study the software and provide accurate and quicker analysis results by the use of ETABS.
Learn software
Save time
objectives
Safety/ economy
Achieve accuracy
Fig. No. 1.11- Objectives
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CHAPTER- 2
Literature Survey
For the brief understanding of earthquake, forces generated in building due to earthquake and behaviour of building during earthquake Various research papers were studied.
2.1) Reviews of Research Papers
Mohammed yousuf ET.AL [1] carried out dynamic analysis of RC building with plan irregularity. Four models of G+5 building with one symmetry plan and remaining irregular plan. The analysis was carried out by ETABS 9.5 as per IS 1893:2002(Part-1). After analyzing they concluded that Irregularity in plan can result in complex dynamics and irregular response. Base shear is higher in rectangular building comparing to L-shaped building. Asymmetrical building requires more accurate analysis therefore Response Spectrum Method should be used.
Rucha S. Banginwar, ET.AL [2] attempted to evaluate the effect of plan configurations on the response of structure by response spectrum method. They studied the effect of difference geometrical configurations on the behavior of structure of the already constructed building located in the same area during earthquake. After analyzing as per IS 1893:2002 (Part-1) they concluded the plan configurations of structure has significant impact on the seismic response of structure in terms of displacement, story drift, story shear. Building with severe irregularity shows maximum displacement and storey drift. Maximum torsion observed in building with high irregularity plan.
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DEEPAK SUTHAR, ET.AL [3] studied the evolution of tall building’s structural systems and analyzed the residential high rise building for seismic forces by both static and dynamic method considering two type of structural system. Model I -regular frame system i.e. normal column-shear wall beam slab system and Model II - modular wall slab system. Carried out the analysis by using ETABS. After analyzing they concluded the story drift, lateral displacement and time period is more in model I as compared to model 2. The results were more conservative in Static analysis as compared to the dynamic method resulting uneconomical structure. As building was in irregular “L-shape” the behavior in both direction is not similar.
Mr. Sandesh N. Suryawanshi, ET.AL [4] studied the complex behavior of structure under asymmetric form and influence of torsional moment on the behavior of structure by using Response spectrum method. A simplified nonlinear pushover analysis was used to find structural descriptors required in seismic vulnerability assessment by using software SAP2000. After analysis they concluded that the time period and base shear calculation by using equivalent static method is approximately equal with response spectrum method in SAP. Comparing the torsional moment (TM) in beam shows that TM in asymmetrical building is more than symmetrical. Equivalent static method and response spectrum method in SAP showed that, base shear and roof displacement for asymmetrical building is more than symmetrical building. Formation of hinges in asymmetrical building is more and early than symmetrical building.
Prof. Wakchaure M. R, ET.AL [5] studied the influence of the torsion effects on the behavior of structure. 2 cases are considered in study, case one is without considering torsion and case two is considering torsion. The Indian standard code of practice IS-1893 (Part I: 2002) is used for guidelines and methodology. After analyzing they concluded that asymmetric building case 2, that is without considering torsion, the area of steel in the beams at critical stage are much smaller than those in case 1, that is with considering torsion. Therefore torsional behavior of asymmetric building is one of the most frequent source of structural damage and failure during strong ground motions. 17 | P a g e
C. Justine Jose, ET.AL [6] studied response of tall symmetric buildings during earthquake. Design moments of columns and roof displacements were taken as the parameters of the study to find the torsional effect on building during earthquake. After analysis they concluded that the torsional mode has significant effect from low raised buildings onwards even for symmetric structures. The symmetric buildings exhibited torsional effects only when the torsional time period T has a value between 0.55s and 4s.
Neha P. Modakwar, ET.AL [7] study different irregularity and torsional response due to plan and vertical irregularity and to analyze cross shape and L shape building while earthquake forces acts and calculated additional shear due to torsion in the columns. The study initiated to quantify the effect of different degrees of irregularity on Structures designed for earthquake using simplified analysis. The types of irregularity Considered was (a) Horizontal Irregularity- Reentrant corner (b) Vertical Irregularity -Mass Irregularity. The analysis showed the Re-entrant corner columns are needed to be stiffened for shear force in the horizontal direction perpendicular to it as significant variation is seen in these forces. For torsional effect the re-entrant corner columns must be strengthen at lower floor levels and top two floor levels and from the analysis they observed that behavior of torsion is same for all zones.
Dr. S.K. Dubey, ET.AL [8] studied different irregularity and torsional response due to plan and vertical irregularity, and to analyze “T”-shaped building while earthquake forces acts and calculated additional shear due to torsion in the columns. After analyzing they concluded that buildings with irregularities are prone to earthquake damage. Soft storeyFor all new RC frame buildings, the best option is to avoid such sudden and large decrease in stiffness and/or strength in any storey.
Shantanu S. Magdum, ET.AL [9] studied inelastic seismic behaviour of multistoried building with vertical setbacks were analyzed by IS code approach. Analyzed the torsion effect on building and studied the Designs of asymmetric multistoried building. Study 18 | P a g e
shows that there is increase in shear force due to torsion in column and increase in area of steel reinforcement in column particularly at the edge member of the building. There was 4.53 % increase in the displacement due to torsion.
Sachin G. Maske, ET.AL [10] studied influence of the torsion effects on the behaviour of structure. In study two cases are considered, case one is without considering torsion and case two is considering torsion. Results were compared in terms of % Ast in columns. The analysis showed the asymmetric building case 2, the forces in the columns located in the stiff side of the plan are much smaller than those in the elements of the flexible side of the plan. There is no significant change in column forces around center of rigidity.
P. P. Chandurkar, ET.AL [11] studied the effect of shear wall on building. 2 modal with and without shear wall were analyzed with Parameters like Lateral displacement, story drift and total cost required for ground floor were calculated in both the cases replacing column with shear wall. After seismic analysis they concluded that changing the position of shear wall will affect the attraction of forces, so that wall must be in proper position. Major amount of horizontal forces are taken by shear wall. Providing shear walls at adequate locations substantially reduces the displacements due to earthquake.
M. L. Saldivar, ET.AL [12] studied seismic vulnerability of existing mid-rise buildings. A nonlinear incremental static analysis of building was done. Study evaluated two apartment complexes built with obsolete regulations. After analyzing they show the damages were mainly found where there in discontinuity of rigidity.
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2.2) Conclusion base on Review of Research Papers 1. Seismic coefficient method is easy and convenient method of seismic analysis of building up to the height of 40m (10-12 storey). 2. Building height more than 40 meter should be analyzed by response spectra method. The method is highly accurate compared to seismic coefficient method. 3. An asymmetrical building will behave different in both direction during earthquake. 4. Torsional effect is observed in a building having vertical and horizontal irregularity. 5. Displacement in building increase by 4-5% due to torsional effect. 6. Earthquake effect on building can be minimized by providing shear wall. 7. Damage in a building due to earthquake is mainly found where there is discontinuity of rigidity.
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CHAPTER- 3
Methodology and Input Data Seismic analysis of buildings can be done by various different methods. For the study Seismic coefficient method of analysis is used as the method is easy and convenient. Method analysis is generally used for the building up to the height of 40m (10-12 storey).
3.1) Seismic Coefficient Method
The seismic coefficient is easiest method for calculation of seismic forces on building. In this method, first, the design base shear is calculated for the whole building, and it is then distributed along the height of the building, as explained below: [17] a) Determination of base shear : The total design lateral force or design seismic base shear (VB) along any principal direction shall be determined by the following expression: (IS: 1893 (Part-1) – 2002, Cl. 7.5.3, Pg. 24)
VB = Ah.w Where, VB = total design lateral force at the base of a structure. Ah = design horizontal acceleration spectrum value W = seismic weight of the building
Seismic weight (w):
Seismic weight of floor system = dead weight of slab + weight of beam + half of the weight of columns above and below the floor + half of the weight of wall above and below the flow. 21 | P a g e
Imposed load (live load):
As per IS 1893 (1) – 2002, cl. 7.3.1 the percentage of imposed load as given in table 1.3 should be used. For calculating the design seismic forces of the structure the imposed load on the roof need not to be considered.
Table No. 3.1- Percentage of imposed load to be considered in seismic weight calculation (IS 1893 (Part-1):2002, Cl. 7.3.1) Imposed uniformly distributed floor loads
Percentage of imposed
(KN/M2)
load
Up to and including 3.0
25
Above 3.0
50
Design horizontal seismic coefficient (Ah):
Seismic coefficient is calculated as per IS: 1893 (1) -2002, cl. 6.4.2, pg. 14.
Ah = (Z I Sa / 2 R g) For any structure with T < 0.1 S, the value of Ah will not be taken less than Z/2 whatever be the value of I/R.
Where, Z = zone factor I = importance factor R = response reduction factor Sa/g = average response acceleration coefficient
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Zone factor (Z):
The value of zone factor (Z) given in table is for the maximum considered earthquake. The factor 2 in the denominator of Z is used so as to reduce the MCE zone factor to the factor for design basis earthquake (DBE). Table No. 3.2- Zone factor (Z) (IS 1893 (Part-1):2002, cl. 6.4.2) Seismic zone Seismic Intensity Z
II
III
IV
low
Moderate
severe
0.10
0.16
0.24
V Very severe 0.36
Fig. No. 3.1- Seismic zone in INDIA 23 | P a g e
Importance factor (I):
Importance factor depends on the functional use of the structures, characterized by hazardous consequences of its failure, post- earthquake function need, historical value, or economic importance. The values of importance factor (I) are given table. Table No. 3.3- Importance factor, I (IS 1893 (Part-1):2002, cl. 6.4.2) SI. NO.
STRUCTURE
Importance Factor
(1)
(2)
(3)
Importance service and community building, such as hospitals; schools; monumental structures; emergency (I)
building like telephone exchange, television stations,
1.5
radio community halls like cinemas, assembly halls and subway station, power station. (ii)
All other building
1.0
Response reduction factor (R):
It depends on the perceived seismic damage performance of the structure, characterized by ductile or brittle deformations. However, the ratio I/R shall not be greater than I. Table No. 3.4- Response reduction factor, R for building systems (IS 1893 (Part-1):2002, cl 6.4.2) Sr. no.
Lateral load resisting system
R
(1)
(2)
(3)
(I)
Ordinary RC moment – resisting frame(OMRF)2
3.0
(ii)
Special RC moment – resisting frame (SMRF)3
5.0
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Average response acceleration coefficient (Sa/g):
It is a factor which represent the acceleration response of the structure under earthquake ground vibrations. It depends on the natural period and damping of the structure. For rock or different types of soil the values of Sa/g are given in fig.
Fig. No. 3.2- Average response acceleration coefficient- Chart
Multiplying factor for obtaining values for other damping is given in table below. Table No. 3.5- Damping factor (IS 1893 (Part-1):2002, cl. 6.4.2)
Damping Percent Factor
0
2
5
7
10
15
20
25
3.20
1.40
1.00
0.90
0.80
0.70
0.60
0.55
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Fundamental natural period (Ta): Ta = 0.075 h0.75 …… for moment resisting frame building without brick infill panel (RC frame). Ta = 0.085 h0.75 …… for moment resisting frame building without brick infill panel (Steel Panels). Ta = 0.09 h / √d …… for all other building including moment resisting frame with brick infill panels.
Where, h = height of building (m) d = base dimension of building at the plinth level in m, along the considered direction of the lateral force.
b) Distribution of base shear : The design base shear shall be distributed along the height of the building as per the following expression:
Q = Vn (Wi hi 2 / ∑ Wi hi 2)
Where, Q = design lateral force at floor I, Wi = seismic weight of floor I, hi = height of floor I measured from base, n = number of storey
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3.2) Input Data
Shape:
Rectangular
Fig. No. 3.3- Rectangular building
L- Shaped
Fig. No. 3.3- L- shape building 27 | P a g e
T- Shaped
Fig. No. 3.3- T- shape building
Grid Data:Table No. 3.6- Grid data Grid Data No. of grid Span Size of Grid
1
2
3
4
5
6
X (m)
4
4
4
4
4
4
Total length of grid 24
Y (m)
4
4
4
4
4
-
20
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Height of building:Table No. 3.7- Height of building Height of Building Height
Elevation
m
m
STORY10
3
30
STORY9
3
27
STORY8
3
24
STORY7
3
21
STORY6
3
18
STORY5
3
15
STORY4
3
12
STORY3
3
9
STORY2
3
6
STORY1
3
3
BASE
0
0
Name
Material detail:Concrete- M 25 Steel- Fe 415
Structural Detail:Table No. 3.8- Structural Detail Structural Detail Stru.\Size Beam Column Slab
B
D
300 mm 500 mm 300 mm 300 mm -150 mm
Clear Cover 40 mm 40 mm --
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Load applied and mass sources for seismic analysis:-
Table No. 3.9- Loading & Mass sources
Load applied & Mass source for seismic analysis
Name
Load Detail
Multiplier for seismic analysis
DEAD
Self-Weight
1
LIVE
5kN/m2 on slab
0.5 (IS 1893 (Part-1):2002)
FF
1.2 Kn/m2 on slab
1
WALL
13.8 Kn/m- on beam
1
Seismic Details:Data for seismic analysis are taken from IS 1893 (Part-1):2002 as per criteria and selected location of building. In this case building is situated at Ahmedabad and is constructed over a medium soil strata. Table No. 3.10- Seismic Data Seismic Details (As per IS:18920) Z Damping (Zone Factor) 5%
0.16
Soil Type II
T(time) 𝑡=
0.09ℎ √𝑑
X (Sec)
Y (Sec)
0.551
0.603
I (Impor. Factor) 1.5
R (Response reduction) 5
Scale Factor for Response spectrum 𝑆𝐹 =
𝐼𝐺 2𝑅
0.981
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CHAPTER- 4
Modeling and Analysis in ETABS ETABS 2013 is used for the seismic analysis of buildings by seismic coefficient method. Building response for Base shear, Story drift, Displacement, torsional movement were studied using ETABS.
4.1) Modeling in ETABS
Procedure of modeling in ETABS is as under:1) Selection of Standards:
Unit: - Metric IS
Code: - IS 800:2007 & IS 456:2000
Fig. No. 4.1 – Selection of Standards in ETABS 31 | P a g e
2) Defining Grid:
Number and size of bay in X & Y direction.
Number of storey and height of typical store.
Modeling can be started with a predefined templates or grid can be imported from a CAD file.
Grid of various beam can prepared by centerline command in AutoCAD & with the help of drawing given by architect columns and beam can be provided in ETABS.
Fig. No. 4.2 – Defining Grid
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3) Define material:
Concrete (M20, M25, M30...)
Rebar’s (Mild steel, HYSD...)
Properties such as ultimate strength, characteristic strength, yield strength etc. can be redefined.
New material can be added with its different properties such as elasticity, density, Poisson ratio, shear modulus etc.
Fig. No. 4.3 – Defining Material
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4) Define Frame Section:
Beam, Column and Slab.
Reinforcement with different material can be provided.
Various details can be defined such as type of tie, dimensions of cover, minimum & maximum diameter of bar.
Fig. No. 4.4- Defining Column
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5) Define Loads:
Dead Load, Floor finish, Wall load
Live Load
Seismic load (Detail for defining seismic load is describe in Cl-3.4)
Fig. No. 4.5- Defining Load
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6) Mass Source Participation:
Participation of mass during earthquake has to be defined.
Seismic weight of building depends on mass participation during earthquake.
Dead load provide 100% mass participation during earthquake.
Where live load participation depends on, LL > 3; multiplied by 0.5
Fig. No. 4.6- Mass Participation during earthquake 36 | P a g e
7) Modeling:
Column, Beam, Slab, Defining Support
8) Assigning Load:
Floor Finish & LL on Slab
Wall load on Beam
Fig. No. 4.7- Loading on slab and beam
9) Analyzing Frame:
Frame is analyzed for Base Shear, CM & CR, Story Drift, etc.
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4.2) Calculation of seismic load in ETABS
Before modeling in etabs, various loads on the structure are defined as shown above. While defining seismic load various other data inputs are require for the analysis. 1) Code to use for seismic analysis 2) Direction of earthquake (X & Y) 3) Accidental eccentricity to be considered or not (5% as per code) 4) Response reduction factor (R) as per IS 1893 (Part-1):2002 5) Zone factor (Z) as per IS 1893 (Part-1):2002 6) Type of soil as per IS 1893 (Part-1):2002 7) Importance Factor as per IS 1893 (Part-1):2002 8) Time period as per IS 1893 (Part-1):2002 T= 0.09h/Sqrt (d) 9) Defining mass sources as per IS 1893 (Part-1):2002
Fig. No. 4.8- Seismic Load data 38 | P a g e
4.3) Check list for ETABS
Model in ETABS is prepared with the help of ETABS check list. Check list helps user during modeling and data input. Check list shows various contents that are to be taken care of during modeling a structure. Check list shows a step wise modeling process with instructions for user for data input and analysis required for different type of structures. Check list helps a structural engineer to model a structure properly and an engineer to check the different type of analysis carried out for designing the structure.
Table No. 4.1 - Check list for ETABS Project No. : -
Model by : -
Date : -
Project Name : -
Checked by : -
Date : -
Sr. Content / Description No. A
Geometry Check
1
Basic layout of model & compare with Arch drawings & DBR
2
Check that distance between columns is from beam center to beam center
3
Check the code specifications i)
Concrete Frame Design
ii)
Shear Wall Design
Engineer
Engineer
(Design)
( Check )
iii) Live Load Reduction 4
5
Check story data i)
Check no. of floors & compare with DBR & Arch drawings
ii)
Check story height & compare with DBR & Arch drawings
Check all material properties in detail i)
Check concrete grade used & compare with DBR
ii)
Check steel grade used & compare with DBR
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iii) Check the modulus of elasticity 6
Check all defined frame sections i)
Check c/s dimensions
ii)
Check property modifiers
iii) Check reinforcement & cover provided iv) Check material used for section 7
Check all defined wall/ slab sections i)
Check section thickness
ii)
Check material used for section
iii) Check type of section ( Shell or Membrane ) i f shell (Mesh & Edge Constrain) 8 9
Check foundations restraints applied Check all defined frame sections in geometry and modeled landing beams of stairs to consider short column effect. ( Enable landing Beam for Torsion )
10 Check releases (Secondary Beam) 11 Check rigid diaphragms applied (check with senior engineer) 12 Check pier label for all wall sections and Check Overwrites for Wall Sections B
Loa ding Check
13 Check all defined static load cases i)
Check number of static load cases & its type
ii)
Check the seismic code selected
iii)
Check the seismic factors & compare with DBR (Consider Minimum eccentricity for Eq.Forces) Check time period and compare with DBR (From Basement slab; if NO
iv)
14
basement than from Foundation)
Check load combinations defined & compare with DBR (For Regular geometry structure NO 30% COMBO; for irregular geometry consider 30% combos.
15 Check mass source defined 16 Loading i)
Check unit of all load applied
ii)
Check all loads to see if applied in proper category
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iii) Check Big LL & Small LL iv) Check temperature load if any C
Analysis Check
17 Check model for warnings or errors ( under "Analyze" tab ) 18 Check Overwrites for frame and 1.2 DL + 0.5 LL, If P-Delta analysis is considered 19 Check the live load reduction factor as No live load reduction and also do not consider special seismic design data 20 Run Analysis 21 Check deflection for beams and check all deformed shape of structure 22 Check LAST RUN LOG ANALYSIS for analysis warning and Error 23
24
If P-DELTA analysis than Check LAST RUN LOG ANALYSIS to See the ITERATION CONVERGED; if not than INCREASE the number of ITERATION. Check Mode-1 and Mode-2. It should be in pure translation, either X or Y direction.(RZ should be less than 10% for Mode-1 and Mode-2) Calculate design eccentricity. If it is less than permissible value than ok, otherwise
25 applied additional torsional moment at each diaphragm level. Consider the load combination with additional torsional moment Check the building structure for SOFT STOREY. The ratio of two successive story 26 should not be less than 0.7 if it is not than design the structure for twice the magnitude of EQ. 27 Check limiting drift as per IS: 1893-2002 for service load combo. 28 Check the combinations that have been selected for design 29 Check applied EQ. Force, correctly as per IS CODE 1893:2002 30 Check for "load per unit area" ( for DL + LL ) D
Design Check
31 Run Design for frame section and shear wall 32 Check for any frame section failures and shear wall failures
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4.4) Analyzed results and their evaluation
After analyzing the structure the analysis results can be viewed and analysis report is generated. Required analysis results are collected from the report: Center of mass (CM) center of stiffness (CS) Store shear Store stiffness Store displacement Store drift (should not be more than 0.004*h. (IS 1893(part-1):2002, Cl. 7.11.1)) Lateral store force By the help of this results, torsional movement in a building is found. Torsional movement = Base shear X Design eccentricity (edi) Where, Base shear is obtained by ETABS analysis. Design eccentricity (edi) = 1.5 esi + 0.05bi or esi - 0.05bi Where, esi= Static eccentricity = CM – CS bi= Floor dimensions perpendicular to direction of force. Here, Factor 1.5 represents dynamic amplification and 0.05 represents accidental eccentricity.
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CHAPTER- 5
Validation
For validation of results ETABS analysis results were cross checked by manual analysis results of a problem given below:Plan and elevation of a three storey RCC school building is shown below (Fig. No. 5.1). The intensity of dead load is 12 Kn/m2 (including column, beam and wall) and floor caries an imposed load of 4 Kn/m2. The building is situated in zone IV. The type of soil encounter is medium soil and it is proposed to design the building with special movement resistant frame. Determining the seismic force using static coefficient method.
Fig. No. - 5.1- Plan and Elevation of building 43 | P a g e
5.1) Analysis by ETABS
IS1893 2002 Auto Seismic Load Calculation This calculation presents the automatically generated lateral seismic loads for load pattern seismic according to IS1893 2002, as calculated by ETABS. Direction and Eccentricity Direction = X-direction Eccentricity Ratio = 5% for all diaphragms Structural Period Period Calculation Method = User Specified User Period
T = 0.286 sec
Factors and Coefficients Z = 0.24 R=5 I = 1.5
Seismic Zone Factor, Z [IS Table 2] Response Reduction Factor, R [IS Table 7] Importance Factor, I [IS Table 6] Site Type [IS Table 1] = II Seismic Response Spectral Acceleration Coefficient, Sa 1.36 = Sa /g [IS 6.4.5] g T
Sa = 2.5 g
Equivalent Lateral Forces Seismic Coefficient, Ah [IS 6.4.2]
S ZI ga Ah = 2R
Calculated Base Shear Table No. 5.1- Base shear in building Direction
Period Used (sec)
W (Kn)
Vb (Kn)
X
0.286
3200.00
288.00 44 | P a g e
5.2) Manual Analysis
Step: - 1- Lump mass of building
Dead load = (10 x 8) x 12 = 960 KN (Dead load of each floor and terrace). Live load= 4 x0.5= 2 (50% as per IS 1893 (P-1):2002, page-24, Table- 8). So L.L= 2 x (10 x 8) = 160 KN Total load of building = DL of roof + DL of typical floor + LL of typical floor Lump mass of building = 3200 KN Step: - 2 – Fundamental natural period (Ta)
By IS 1893 (P.1):2002, Cl. 7.6.2 𝑡=
0.09ℎ = 0.286 Sec √𝑑
Step: - 3 – Design seismic Base shear (VB)
VB= Ah X w All the value of Ah are taken from IS 1893 (P.1):2002 Ah = 0.09 So. VB = 0.09 X 3200 = 288 KN.
Analysis results of ETABS and manual analysis matches, thus it can be said that ETABS provide genuine results as per Indian Standards. As manual analysis take more time and possibility of errors are high it can be recommended to use ETABS for analysis of a structure is very helpful.
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CHAPTER- 6
Results and Evaluation
After modeling of building or structure in ETABS, it is analyzed. On the base of the analysis result a report is generate by ETABS containing detailed analysis results. Following results were obtained after analysis:-
6.1) Structural Analysis Result by ETABS
6.1.1) Center of mass & Stiffness:-
Table No. 6.1- Center of Mass & Center of Rigidity Shape (m) Rectangular
T- Shape
L- Shape
Center of
Center of
Center of
Center of
Center of
Center of
mass
Rigidity
mass
Rigidity
mass
Rigidity
X
Y
X
Y
X
Y
X
Y
X
Y
X
Y
12
10
12
10
12
7.406
12
7.532
13.46
8.547
13.54
8.637
6.1.2) Seismic weight of building:-
Table No. 6.2- Seismic weight of building Shapes
Rectangular
T- shape
L- Shape
Weight (Kn)
118022.96
75078.63
96550.76
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6.1.3) Story Shear:-
Table No. 6.3- Story Shear
Story- Shear (Kn) Shape Store
Rectangular
T- Shape
L- Shape
X
Y
X
Y
X
Y
Store 10
1783.109
1627.748
1131.757
1033.148
1457.433
1330.449
Store 9
3262.877
2978.586
2073.813
1893.123
2668.345
2435.855
Store 8
4432.076
4045.914
2818.153
2572.61
3625.115
3309.262
Store 7
5327.244
4863.087
3388.039
3092.842
4357.642
3977.965
Store 6
5984.919
5463.459
3806.731
3475.054
4895.825
4469.256
Store 5
6441.638
5880.384
4097.489
3740.478
5269.563
4810.431
Store 4
6733.937
6147.216
4283.574
3910.35
5508.756
5028.783
Store 3
6898.356
6297.309
4388.247
4005.903
5643.301
5151.606
Store 2
6971.431
6364.017
4434.768
4048.371
5703.1
5206.194
Store 1
6989.7
6380.694
4446.398
4058.988
5718.049
5219.841
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6.1.4) Story Drift:Table No. 6.4- Story Drift Story- Drift (Story shear/Story stiffness) (mm) Shape Store
Rectangular
T- Shape
L- Shape
X
Y
X
Y
X
Y
Store 10
9.84
9.42
9.91
9.53
9.83
9.49
Store 9
16.39
15.45
15.89
15.06
16.14
15.32
Store 8
21.62
20.27
20.66
19.47
21.18
19.97
Store 7
25.59
23.92
24.25
22.79
25
23.49
Store 6
28.45
26.54
26.81
25.14
27.73
25.99
Store 5
30.35
28.27
28.47
26.64
29.54
27.63
Store 4
31.46
29.25
29.37
27.44
30.57
28.53
Store 3
31.92
29.63
29.66
27.65
30.96
28.83
Store 2
31.82
29.46
29.38
27.33
30.79
28.59
Store 1
27.23
25.04
24.67
22.8
26.16
24.1
As value of store drift falls under 0.004*h = 0.004*30 = 120mm as per Indian Standard requirement thus building is safe against drift.
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6.1.5) Story Displacement:-
Table No. 6.5- Story Displacement Story- Displacement (mm) Shape Store
Rectangular
T- Shape
L- Shape
X
Y
X
Y
X
Y
Store 10
254.67
237.25
239.07
223.85
247.9
231.94
Store 9
244.83
227.83
229.16
214.32
238.07
222.45
Store 8
228.44
212.38
213.27
199.26
221.93
207.13
Store 7
206.82
192.11
192.61
179.79
200.75
187.16
Store 6
181.23
168.19
168.36
157
175.75
163.67
Store 5
152.78
141.65
141.55
131.86
148.02
137.68
Store 4
122.43
113.38
113.08
105.22
118.48
110.05
Store 3
90.97
84.13
83.71
77.78
87.91
81.52
Store 2
59.05
54.5
54.05
50.13
56.95
52.69
Store 1
27.23
25.04
24.67
22.8
26.16
24.1
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6.2) Evaluation of Analysis Result’s
6.2.1) Design Eccentricity:-
Eccentricity due to asymmetrical shape of building is calculated as per IS 1893 (Part-1): 2002, Cl. 7.9.1 Table No. 6.6- Design Eccentricity Shape (m) Eccentricity
Rectangular
T- Shape
L- Shape
X
Y
X
Y
X
Y
Static Eccentricity (esi)
0
0
0
0.1251
0.0879
0.0903
1) Design Eccentricity (edi)
1
1.2
1
1.38765
0.86815
1.33545
2) Design Eccentricity (edi)
-1
-1.2
-1
-1.0749
-0.9121
-1.1097
Whichever of both design eccentricity gives more severe effect in shear of frame.
6.2.2) Torsion movement:-
Due to asymmetrical plan of building torsional movement or rotation in plan is generated. Torsional movement is calculated by multiplying base shear to design eccentricity.
Table No. 6.7- Torsional Movement Shape (Kn*m) Directions Rect.
T-Shape
L- Shape
X
6989.70
4446.39
- 5215.43
Y
7656.83
5632.45
6970.84
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CHAPTER- 7
Discussions and Conclusion’s
On the base of analysis results a discussion is be done on the behaviour of building and seismic forces acting on building. On the base of this discussion conclusions are provides.
7.1) Discussion
1) Eccentricity in building
Eccentricity in building may be generated due to many reasons like in proper distribution of mass, direction of rectangular columns, shape of building etc. this eccentricity is known as static eccentricity.
Design eccentricity calculated as per codal provision provides multiplication (1.5) and addition (0.05*bi) factor for dynamic amplification and accidental eccentricity for safety of a structure.
Due to this multiplication and addition factor a symmetrical building with equal mass distribution may also show some eccentricity. Which helps in providing higher safety and overcome accidental eccentricity due to change in purpose of building, soft story effect etc.
Eccentricity is mainly effected by shape of building. Irregular shaped building has his higher eccentricity.
From the graph (Fig. No. 7.1) it is clear that eccentricity is high in T- Shaped building than in Rectangular or L- Shaped building.
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Eccentricity due to shapes 1.45 1.4 1.35 1.3 1.25 1.2 1.15 1.1 Rectangular
L- Shape
T- Shape
Fig No. 7.1- Eccentricity due to different shapes (Graph)
2) Relation of Torsional movement-Mass of building During the study it is clearly seen that the torsional movement depends on the mass of the structure. As the base shear is directly related to mass of building, thus higher the mass higher is base shear and higher the torsional movement. The torsional movement in rectangular building is high due to mass. By the graph it is clear that mass plays important role in torsional movement of building with eccentricity.
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Torsional Movement (Kn m)
Mass vs Torsional movement 8000.00 7500.00
Rectangular
7000.00
L- Shape
6500.00 6000.00 5500.00
T- Shape
5000.00 70000.00
80000.00
90000.00
100000.00
110000.00
120000.00
130000.00
Mass (Kn)
Fig No. 7.2- Mass v/s Torsional Movement Graph
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7.2) Conclusion’s
1) Base shear in building depends on the mass of building. 2) Irregularity in plan results in eccentricity of Center of mass and center of stiffness. 3) Behavior of all three shaped building are different in both direction due to difference in size of building in both direction. 4) T- Shaped building in plan has high eccentricity compared to Rectangular and L-shaped building. 5) It is observed that 3 floor of all the three shaped building shows the higher drift value than any other floor in both the direction. 6) It is observed that displacement of building is higher when direction of earthquake is perpendicular to longer face of building. In this case displacement is high in x-direction. 7) It is observed that torsion movement generated due to eccentricity is directly related to mass. As mass of building increases torsional movement increases. 8) Base shear is high in Rectangular building. 9) Due to high base shear, torsional movement in rectangular building is also high. 10) Torsional movement is comparatively more when direction of earthquake is perpendicular to the shorter face of building. In This case torsional movement is high in Y-direction. 11) Clear relation between mass & torsion and Eccentricity & Shape is obtained.
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REFERENCES
1. MOHAMMED YOUSUF, P.M. SHIMPALE; Dynamic Analysis of Reinforced Concrete Building with Plan Irregularities. International Journal of Emerging Technology and Advanced Engineering, ISSN 2250-2459, Volume 3, Issue 9, September 2013. 2. RUCHA S. BANGINWAR, M. R. VYAWAHARE, P. O. MODANI; Effect of Plans Configurations on the Seismic Behavior of the Structure by Response Spectrum Method. International Journal of Engineering Research and Applications, ISSN: 2248-9622, Vol. 2, Issue 3, May-Jun 2012, pp.1439-1443. 3. DEEPAK SUTHAR, H.S.CHORE, P.A. DODE; High rise structure subjected to seismic forces and its behavior. 12th IRF International Conference, ISBN: 978-93-84209-31 -5. 4. Mr. Sandesh N. Suryawanshi, Prof. S. B. Kadam, Dr. S. N. Tande; Torsional Behavior of Asymmetrical Buildings in Plan under Seismic Forces. International Journal of Emerging Engineering Research and Technology, Volume 2, Issue 4, July 2014, PP 170-176, ISSN 23494395 (Print) & ISSN 2349-4409 (Online). 5. Prof. Wakchaure M. R, Nagare Y U; Effect of Torsion Consideration in Analysis of Multi Storey frame. International Journal of Engineering Research and Applications (IJERA), ISSN: 2248-9622, Vol. 3, Issue 4, Jul-Aug 2013, pp.1828-1832. 6. C. Justine Jose, T. P. Somasundaran, V. Mustafa; prediction of seismic torsional effects in tall symmetric buildings. IJRRAS 5 (2), November 2010. 7. Neha P. Modakwar, Sangita S. Meshram, Dinesh W. Gawatre; Seismic Analysis of Structures with Irregularities. IOSR Journal of Mechanical and Civil Engineering (IOSR-JMCE), e-ISSN: 2278-1684, p-ISSN: 2320-334X, PP 63-66. 8. Dr. S.K. Dubey, P.D. Sangamnerkar; Seismic behavior of asymmetric RC buildings. International Journal of Advanced Engineering Technology, E-ISSN 0976-3945. 9. Shantanu S. Magdum, Dr.P.S.Patil; Study of seismic behaviour of vertical asymmetric multistoried building. International Journal of Advanced Technology in Engineering and Science, Volume No.02, Issue No. 06, June 2014, ISSN (online): 2348 – 7550. 10. Sachin G. Maske, Dr. P. S. Pajgade; Torsional Behaviour of Asymmetrical Buildings. International Journal of Modern Engineering Research (IJMER), Vol.3, Issue.2, March-April. 2013, pp-1146-1149 ISSN: 2249-6645. 11. P. P. Chandurkar, Dr. P. S. Pajgade; Seismic Analysis of RCC Building with and Without Shear Wall. International Journal of Modern Engineering Research (IJMER), Vol. 3, Issue. 3, May - June 2013, pp-1805-1810, ISSN: 2249-6645. 12. M. L. Saldivar, L.M. Garino Libardi, G. Navarta Navarro; Seismic Performance of Existing Residential Buildings. 15 WCEE, LISBOA 2012. 13. Earthquake tips- IITK 14. IS 1893 (Part-1):2002 15. http://www.indiaonlinepages.com/population/gujarat-population.html 16. www.csiamerica.com/products/etabs 17. Book : Earthquake Engineering by R.P.Rethaliya
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