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International Journal of Research and Innovation (IJRI)

International Journal of Research and Innovation (IJRI)PERFORMANCE BASED ANALYSIS OF VERTICALLY IRREGULAR STRUCTURE

UNDER VARIOUS SEISMIC ZONES.

Mohammed Azemuddin1, Venkata Ratnam 2, Mohammed Abdul Hafeez 3

1 Research Scholar, Department of Civil Engineering, Aurora Scientific Technological and Research Academy, Hyderabad, India.2 Associate Professor, Department of Civil Engineering, Aurora Scientific Technological and Research Academy, Hyderabad, India.3 Associate professor , Department of Civil Engineering, Maulana Azad National Urdu University, Hyderabad, India.

*Corresponding Author:

Mohammed Azemuddin, Research Scholar, Department of Civil Engineering, Aurora Scientific Technological and Research Academy,Hyderabad India.

Published: July 25, 2015Review Type: peer reviewedVolume: II, Issue : II

Citation: Mohammed Azemuddin, Research Scholar (2015) "PERFORMANCE BASED ANALYSIS OF VERTI-CALLY IRREGULAR STRUCTURE UNDER VARIOUS SEISMIC ZONES."

INTRODUCTION

General

Earthquake is a natural phenomenon, which is generated in the earth’s crust. Duration of the earthquake developed is generally very short, which might not last more than few seconds or a minute or so but, it is the intensity of the earthquake that’s makes the big difference from the mod-erate damage to the mass destruction. However, it is the fact that thousands of people lose their lives due to the earthquake in different parts of the world and this gets to its peak when this occurs with larger intensity. The ef-fects of an earthquake are strongest in a broad zone sur-rounding the epicenter. Surface ground cracking associ-ated with faults that reach the surface often occurs, with horizontal and vertical displacements of several yards common. Such movement does not have to occur dur-

ing a major earthquake; slight periodic movements called fault creep can be accompanied by micro earthquakes too small to be felt. The worst damage occurs in densely populated urban areas where structures are not built to withstand intense shaking.Seismic hazard in the context of engineering design is generally defined as the predicted level of ground accel-eration which would be exceeded with 10% probability at the site under consideration due to the occurrence of an earthquake anywhere in the region, in the next 50 years. A lot of complex scientific perception and analytical mod-eling is involved in seismic hazard estimation. A compu-tational scheme involves the following steps: delineation of seismic source zones and their characterization, selec-tion of an appropriate ground motion attenuation relation and a predictive model of seismic hazard. Although these steps are region specific, certain standardization of the approaches is highly essential so that reasonably compa-rable estimates of seismic hazard can be made worldwide, which are consistent across the regional boundaries.Damage and loss of life sustained during an earthquake result from falling structures and flying glass and objects. Flexible structures built on bedrock are generally more re-sistant to earthquake damage than rigid structures built on loose soil. In certain areas, an earthquake can trig-ger mudslides, which slip down mountain slopes and can bury habitations below. So as to avoid these damages the structure has be built considering the earthquake that it may experience during its life span. Many methods are available which can be used to analyze the performance of the earthquake but the most commonly or the method which is more popular among structural designers is the pushover analysis. The pushover analysis predicts the re-

Abstract

In the recent years a lot of attention has been given to the earthquake analysis of structure it is one of the most dev-astating natural calamity and which causes severe damage not only to the properties but also to the lives. This is the reason there has been a lot of focus on the structures to be earthquake resistant. Buildings get damaged mostly due to the earthquake ground motions. In an earthquake, the building base experiences high frequency movements, which results in the inertial force on the building and its components and this problem gets worse when a structure is irregu-lar in shape, size etc,. Therefore, there is a lot to work on the seismic behavior of the irregular building which might not respond the way regular building does. It makes the irregular building quite more complex and unpredictable during the course of an earthquake.

Pushover analysis is one of the effective tool by which the response of a structure can be understood. The procedure basically consists of application of lateral loads in increasing order from top story to the bottom story which might follow a particular pattern and the results obtained from this are compared basically in terms of base shear and roof displace-ment and further these are used to obtain the performance point of the structure.

The study aims to predict the response of a structure in different zones according to the IS 1893:2002 (part 1) for a G+15 storied building with and without steel cross bracing. The structure is irregular in geometry vertically. The analysis of he structure has been performed on the SAP 2000 finite element software.

It has been observed that the structure can resist more loads with the inclusion of steel bracings, base shear capacity of the building is observed to be doubled and the roof displacement of the building has reduced considerably. The base shear capacity of the building increases with increase in zone factor i.e. from zone ii to zone v. The story drift changes suddenly at the level of setback due to the large concentration of forces at that level.Keywords: G+15 storied building, vertical irregularity, pushover analysis, base shear, story drift.

1401-1402

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International Journal of Research and Innovation (IJRI) sponse of the structure to quite a reliable extent and even is not that complex in process most of the engineers opt for pushover analysis due to this.

The study attains even more importance when the build-ing under consideration has vertical geometric irregular-ity. As, the behavior of such type of structure is not that predictable or if even predicted that might be very ap-proximate. So, an effective analysis is required such as pushover analysis so as to study their response to the earthquake event minutely. Pushover analysis is applica-tion of gradually increasing lateral loads at every level of the structure from bottom to the top story of the struc-ture. The structure is subjected to the lateral loads until the collapse and from there a pushover curve is obtained which is then converted into capacity curve. This capac-ity curve is merged with the demand curve which final-ly gives the performance point of the structure. This is an important insight to the buildings condition after an earthquake that to what extent is the building damaged or if it can be strengthened by retrofitting. The primary objective of the pushover analysis was to identify the need of retrofitting to the structure which has now been ex-tended to the analysis of the existing building and it has become a boon for the structural engineering to design earthquake resistant buildings. Pushover analysis has been the preferred method for seismic performance evalu-ation of structures by the major rehabilitation guidelines and codes because it is conceptually and computationally simple. Pushover analysis allows tracing the sequence of yielding and failure on member and structural level as well as the progress of overall capacity curve of the struc-ture.

Generally, global modifications to the structural system are conceived such that the design demands, often denot-ed by target displacement, on the existing structural com-ponents, are less than their capacities. Lower demands may reduce the risk of brittle failures in the structure and avoid the interruption of its functionality. The present work aims at assessment of seismic performance of the RC framed structure with vertical geometric irregularities. The structure is analyzed with and without inclusion of cross steel bracing. The inelastic seismic response has been quantified in terms of global performance param-eters derived by means of non linear static analysis. The steel bracing increases the lateral resisting capacity of the structure and even in the base shear capacity of the structure. Further, the steel bracings decrease the bend-ing moments and shear forces in columns, they increase the axial compression in the column to which they are connected.

METHODS OF ANALYSIS

For seismic performance evaluation, a structural analysis of the mathematical model of the structure is required to determine force and displacement demands in various components of the structure. Several analysis methods, both elastic and inelastic, are available to predict the seis-mic performance of the structures.

EQUIVALENT STATIC ANALYSIS

This approach defines a series of forces acting on a build-ing to represent the effect of earthquake ground motion, typically defined by a seismic design response spectrum. It assumes that the building responds in its fundamental mode. For this to be true, the building must be low-rise and must not twist significantly when the ground moves. The response is read from a design response spectrum,

given the natural frequency of the building (either calcu-lated or defined by the building code). The applicability of this method is extended in many building codes by ap-plying factors to account for higher buildings with some higher modes, and for low levels of twisting. To account for effects due to "yielding" of the structure, many codes apply modification factors that reduce the design forces (e.g. force reduction factors).

RESPONSE SPECTRUM METHOD

Static procedures are appropriate when higher mode ef-fects are not significant. This is generally true for short, regular buildings. Therefore, for tall buildings, buildings with torsional irregularities, or non-orthogonal systems, a dynamic procedure is required. In the linear dynamic procedure, the building is modelled as a multi-degree-of-freedom (MDOF) system with a linear elastic stiffness ma-trix and an equivalent viscous damping matrix.

NON LINEAR STATIC ANALYSIS

In general, linear procedures are applicable when the structure is expected to remain nearly elastic for the level of ground motion or when the design results in nearly uniform distribution of nonlinear response throughout the structure. As the performance objective of the struc-ture implies greater inelastic demands, the uncertainty with linear procedures increases to a point that requires a high level of conservatism in demand assumptions and acceptability criteria to avoid unintended performance. Therefore, procedures incorporating inelastic analysis can reduce the uncertainty and conservatism.

CAPACITY CURVE

The overall capacity of a structure depends on the strength and deformation capacity of the individual components of the structure . In order to determine capacity beyond the elastic limits , some form of nonlinear analysis of the structure is required.

A capacity curve is converted into capacity spectrum by using a set of equation from ATC 40 which is known as ADRS format. Initially the curve is obtained between base shear and roof displacement which is converted into a curve between Spectral acceleration and spectral dis-placement, an example of capacity curve is shown in fig.

DEMAND CURVE

Ground motion during an earthquake produces complex horizontal displacement patterns which may vary with time. Tracking this motion at every time step to deter-mine structural design requirement is judge impractical . Demand curve is a representation of earthquake ground motion .It is given by spectral acceleration vs time period

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International Journal of Research and Innovation (IJRI)

ADVANTAGES OF PUSHOVER ANALYSIS

The pushover analysis is an effective tool for the perfor-mance evaluation of a structural system, by estimating its strength and deformation demand induced during a seismic event , by means of a static nonlinear analysis the demands are then compared to available capacities at the performance levels of interest. The evaluation is based on assessment of important performance parameters such as global drift, inter storey drift and inelastic element de-formations .

NON LINEAR DYNAMIC ANALYSIS

Nonlinear dynamic analysis utilizes the combination of ground motion records with a detailed structural model, therefore is capable of producing results with relatively low uncertainty. In nonlinear dynamic analyses, the de-tailed structural model subjected to a ground-motion re-cord produces estimates of component deformations for each degree of freedom in the model and the modal re-sponses are combined using schemes such as the square-root-sum-of-squares.

TOOLS FOR PUSHOVER ANALYSIS

Many softwares are available on which pushover analysis can be carried out, they are• STAAD PRO• ETABS• SAP2000• ADINA • SC- PUSH3D

In this project the analysis is carried out using SAP2000 as it can provide most productive solution from a 2D frame to a complex 3D model for nonlinear analysis. Advanced analytical techniques provide step by step deformation; Eigen and Ritz analyses based stiffness of nonlinear cas-es. It is finite element software which works with complex geometry. It also has by default all material properties and codes like ATC 40, FEMA 356, FEMA 440, IS 1893 (part 1) : 2002 so as to facilitate easy and quick solution for a set of boundary conditions.

PROCEDURE FOR PUSHOVER ANALYSIS

Pushover analysis can be performed as either force con-trolled or displacement controlled depending on the phys-ical nature of the load and the behavior expected from the structure .Force controlled option is useful when the load

is known (such as gravity loading) and the structure is expected to be used when specified drifts ar sought (such as in seismic loading), where the magnitude of the ap-plied load is not known in advance or where the structure can be expected to lose strength or become unsuitable. A displacement controlled pushover analysis is basically composed of the following steps

VERTICAL IRREGULARITY

Due to the growing demands of aesthetic appearance of the buildings engineers are bound to construct structures with irregularities. Sometimes, due to the functionality of the building the irregularities might have to be provided i.e. for buildings which may have unusual purposes. How-ever, it is undeniable that such type of irregularities in-creases the vulnerability of the structures to earthquake or any dynamic event. Torsion is one of the concerns that might affect the building heavily in addition to that mass and stiffness have considerable effect on the response of the building. Geometrically as the structures reduces due to the provision of setbacks, the stiffness of the structure decreases and makes the structure more susceptible to the large displacement which might turn quite cataclys-mic. And even the mass of the building makes significant contribution in the response of the building, if the mass of the building is concentrated at certain portion then large torsional moment will be developed which is again not recommended for a structure. So as to overcome these de-fects buildings of irregular configuration effective method of analysis must be applied which is capable of detecting the weak zones in the structures one of such analysis is pushover analysis which is being carried out in the pro-ject.

TYPES OF IRREGULARITIES

Structural irregularities are basically demarcated into two categories:

i) Plan irregularity ii) Vertical irregularity

Plan Irregularity (IS 1893 (Part 1): 2002)

a)Torsion Irregularity

To be considered when floor diaphragms are rigid in their own plan in relation to the vertical structural elements that resist lateral forces. Torsional irregularity is to be considered to exist when the maximum storey drift, com-puted with design eccentricity, at one end of the struc-tures transverse to a axis is more than 1.2 times the aver-age of the storey drifts at the two ends of the structure.

b)Re – entrant corners

Plan configuration of a structure and its lateral force re-sisting system contain re-entrant corners, where both projections of the structure beyond the re-entrant corner are greater than 15 percent of its plan dimension in the given direction.

c)Diaphragm Discontinuity

Diaphragm with abrupt discontinuities or variations in stiffness, including those having cut-out or open areas greater than 50 percent of the gross enclosed diaphragm area, or changes in effective diaphragm stiffness of more than 50 percent from one storey to the next.

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International Journal of Research and Innovation (IJRI) d)Out of plane Offsets

Discontinuities in a lateral force resistance path, such as out-of-plane offsets of vertical elements.

e)Non parallel Systems

The vertical elements resisting the lateral force are not parallel to or symmetric about the major orthogonal axes or the lateral force resisting elements.

Vertical Irregularities (IS 1893 (Part 1): 2002) a)Stiffness Irregularity

A soft storey is one in which the lateral stiffness is less than 70 percent of that in the storey above or less than 80 percent of the average lateral stiffness of the three storeys above.

A extreme soft storey is one in which the lateral stiffness is less than 60 percent of that in the storey above or less than 70 percent of the average stiffness of the three sto-reys above. For example buildings with STILTS will fall under this category.

b)Mass Irregularity

Mass irregularity shall be considered to exist where the seismic weight of any storey is more than 200 percent of that of its adjacent storeys. The irregularity need not be considered in case of roofs.

c)Vertical Geometrical Irregularity

Vertical geometrical irregularity shall be considered to ex-ist where the horizontal dimension of the lateral force re-sisting system in any storey is more than 150 percent of that in its adjacent storey.

d)In-Plane Discontinuity in Vertical Elements resisting Lateral ForceA in-plane offset of the lateral force resisting elements greater than the length of those elements.

e)Discontinuity in Capacity – Weak Storey

A weak storey is one in which the storey lateral strength is less than 80 percent of that in the storey above. The storey lateral strength is the total strength of all seismic force resisting elements sharing the storey shear in the considered direction.

Following are the figures by which the irregularities in structure are depicted such as mass irregularity, vertical geometrical irregularity etc,.

Stiffness Irregularity

Mass Irregularity

Vertical Geometrical Irregularity

(A) (B)

(A) In-plane discontinuity in vertical lateral force-resisting ele-ment

(B) Discontinuity in capacity (Weak storey)

DIFFERENT SEISMIC ZONES OF INDIA

The Indian subcontinent has a history of devastating earthquakes. The major reason for the high frequency and intensity of the earthquakes is that the Indian plate. Geographical statistics of India show that almost 54% of the land is vulnerable to earthquakes. A World Bank & United Nations report shows estimates that around 200 million city dwellers in India will be exposed to storms and earthquakes by 2050. The latest version of seismic zoning map of India given in the earthquake resistant de-sign code of India [IS 1893 (Part 1) 2002] assigns four

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International Journal of Research and Innovation (IJRI) levels of seismicity for India in terms of zone factors. In other words, the earthquake zoning map of India divides India into 4 seismic zones (Zone 2, 3, 4 and 5) unlike its previous version which consisted of five or six zones for the country. According to the present zoning map, Zone 5 expects the highest level of seismicity whereas Zone 2 is associated with the lowest level of seismicity.

Center for Seismology, IMD under Ministry of Earth Sci-ences is nodal agency of Government of India dealing with various activities in the field of seismology and al-lied disciplines. The major activities currently being pur-sued by the Center for Seismology include, a) Earthquake monitoring on 24X7 basis, including real time seismic monitoring for early warning of tsunamis, b) Operation and maintenance of national seismological network and local networks c) Seismological data centre and informa-tion services, d) Seismic hazard and risk related studies e) Field studies for aftershock / swarm monitoring, site response studies f) Earthquake processes and modeling, etc. The IS code follows a dual design philosophy: (a) under low probability or extreme earthquake events (MCE) the structure damage should not result in total collapse, and (b) under more frequently occurring earthquake events, the structure should suffer only minor or moderate struc-tural damage. The specifications given in the design code (IS 1893: 2002) are not based on detailed assessment of maximum ground acceleration in each zone using a de-terministic or probabilistic approach. Instead, each zone factor represents the effective period peak ground accel-erations that may be generated during the maximum con-sidered earthquake ground motion in that zone.

Zone 5

Zone 5 covers the areas with the highest risks zone that suffers earthquakes of greater Intensity. The IS code as-signs zone factor of 0.36 for Zone 5. Structural designers use this factor for earthquake resistant design of struc-tures in Zone 5. The zone factor of 0.36 is indicative of effective (zero period) level earthquake in this zone. It is referred to as the Very High Damage Risk Zone. The region of Kashmir, the western and central Himalayas, North Bihar, the North-East Indian region and the Rann of Kutch fall in this zone.Generally, the areas having trap rock or basaltic rock are prone to earthquakes.

Zone 4

This zone is called the High Damage Risk Zone. The IS code assigns zone factor of 0.24 for Zone 4. The Indo-Gangetic basin and the capital of the country (Delhi), Jammu and Kashmir fall in Zone 4. In Maharashtra, the Faltan area (Koyananager) is also in zone no-4. In Bihar the northern part of the state like- Raksaul, Near the bor-der of India and Nepal, is also in zone no-4.

Zone 3

The Andaman and Nicobar Islands, parts of Kashmir, Western Himalayas fall under this zone. This zone is clas-sified as Moderate Damage Risk Zone and also 7.8 The IS code assigns zone factor of 0.16 for Zone 3.

Zone 2

This region is liable to have less intensity and is classified as the Low Damage Risk Zone. The IS code assigns zone factor of 0.10 (maximum horizontal acceleration that can be experienced by a structure in this zone is 10% of gravi-

tational acceleration) for Zone 2

PROCEDURE FOR PUSHOVER ANALYSIS IN SAP2000

SAP 2000 is a finite element software which is capable of performing analysis for any type of structures in less time . It gives a better result for non linear analysis . The pro-cedure for the analysis consist of following steps :i) Modelingii) Static Analysisiii) Designingiv) Pushover Analysis

Steps involved in SAP2000 to perform non linear static analysis are,• Creating the model in usual manner• Defining the material properties for the analysis which includes concrete, rebar and others, if necessary. All the properties corresponding to a particular are by default present in the software, the user has to just select those details.• Defining the frame properties like beam, column, slab etc. in the similar way by making appropriate selections.• Define properties and acceptance criteria for the pusho-ver hinges. The program includes several built-in default hinge properties that are based on average values from FEMA-356 for concrete members. These built in proper-ties can be useful for preliminary analyses, but generally user defined properties are recommended.• Assigning loads for respective member and hinges to beams and columns of bending axial type respectively on both ends of members.• Define the pushover case, more than one pushover anal-ysis can also be defined in one analysis.• Run the basic analysis till the gravity loads and the members of the structure must be safe under these loads and then the pushover case.• Display pushover curve and table which is the structure response plotted between spectral acceleration and spec-tral displacement.

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International Journal of Research and Innovation (IJRI) BARE FRAME RESULTS AND DISCUSSION

GENERAL:

All models under the study has be analyzed using pusho-ver analysis which is often referred as non linear static analysis in various zones of India. The models have been subjected to the loads as per codal provisions and the results as discussed earlier are analyzed in terms of base shear, roof displacement etc,. Shear capacity of the build-ing has been enhanced by using steel bracings. perfor-mance of each building are obtained through this proce-dure.

RESULTS:

Pushover curves for Zone II and Zone III (Bare Frame)

Push over curves for Zone III and Zone IV(Bare Frame)

Pushover curves for Zone II and Zone III (X steel Bracing)

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International Journal of Research and Innovation (IJRI)

Pushover curve for Zone IV and Zone V (Steel bracing)

COMPARISION OF PUSHOVER CURVES

Pushover curves for different zones (Bare Frame)

Pushover curves for different zone (X Steel Bracing)

From the above plot we conclude that base shear devel-oped in the structure for zone V is more and it decreases from Zone V to Zone II as the intensity of earthquake is more in Zone V and it decreases from Zone V to Zone II.

Comparison of Pushover curve for Bare Frame and X Steel Bracing

Comparison of Pushover curve for Bare Frame and X Steel Bracing(Zone II)

Comparison of Pushover curve for Bare Frame and X Steel Bracing(Zone III)

Comparison of Pushover curve for Bare Frame and X Steel Bracing(Zone IV)

Comparison of Pushover curve for Bare Frame and X Steel Bracing(Zone IV)

From the above figures it is evident that the performance of the structure is enhanced when lateral systems (x steel bracings) are included for a structure. As from the figure the structure with steel bracing is able to withstand more lateral loads thereby increasing the base shear capacity of the structure and increment of the base shear is quite considerable in all the zones

Drift:

As discussed earlier that the drift of the structure is the difference of the displacement of successive stories. . Ac-cording to IS 1893 (part1): 2002 the story drift for any level should not be greater than 0.004H.

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International Journal of Research and Innovation (IJRI) BARE FRAME STRUCTURE PERFORMANCE POINT

Performance point can be obtained by capacity spectrum and demand spectrum and the intersection point of these two curve is performance point.

Performance point for G+15 storied Bare Frame building(Zone II)

Table: Resultant Base Shear vs Roof Displacement

Above tabulated format is for the G+15 storied Bare Frame building in Zone 2 which lies in IO to LS performance level

Performance point for G+15 storied Bare Frame building(Zone III)

Table: Resultant Base Shear vs Roof Displacement

Above tabulated format is for the G+15 storied Bare Frame building in Zone 3 which lies in IO to LS performance level

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International Journal of Research and Innovation (IJRI) CONCLUSIONS

• we conclude that base shear developed in the structure for zone V is more and it decreases from Zone V to Zone II as the intensity of earthquake is more in Zone V and it decreases from Zone V to Zone II.

• performance of the structure is enhanced when lateral systems (x steel bracings) are included for a structure.

• Due to the provision of setback there is significant change at the level of setback which causes uneven dis-tribution of forces as the structure is Geometrically verti-cal irregular.

• considerable decrease in the value of Drift in various zones by the provision of X Steel bracing (lateral support) in the structure.

• It is also notable that the drift ratio % for all the struc-tures under consideration changes abruptly at the level of setback.

• Base shear of the Bare Frame is less then that of struc-ture with X Steel Bracing (lateral support) as the struc-ture is capable of observing more lateral forces when X Steel Bracings are provided.

• In Zone V it is observed that structure can be collapsed even after the provision of X Steel Bracing due to high intensity of Earthquake

• Performance level of the structure is observed to in-crease considerably when Lateral support is provider in the structure.

REFERENCES

IS 1893-2002(Part 1), “Criteria for Earthquake Resistant Design of Structures” , Bureau of Indian Standards.

• ATC 40, “Seismic Evaluation and Retrofit of Concrete Buildings” , California Seismic Safety Commission.

• FEMA 356, “NEHRP Guidelines for the Seismic Rehabili-tation of Buildings ” ,American society of civil engineers, Washington, D.C.

• Federal Emergency Management Agency (FEMA 273) NEHRP GUIDLINES (1997) developed a set of technically sound, nationally applicable guidelines (with commen-tary) for the seismic rehabilitation of buildings, Washing-ton DC, U.S.A.

• IS 456 : 2000 “Plain and Reinforced Concrete Code of Practice” , Bureau of Indian Standards

Author

Mohammed AzemuddinResearch Scholar,Department of Civil Engineering, Aurora S Scientific and Technological and Research Academy, Bandlaguda,Hyderbad India.

Mythili Rao, Assistant Professor, Department of Civil Engineering, Aurora's Scientific Technological and Research Academy, Hyderabad India.

Mohammed Abdul HafeezAssociate professor,Department Of Civil Engineering,Maulana Azad National Urdu University,Hyderabad India.