EFFECT OF IRREGULARITY SHAPE ON FLAT SLAB ...

EFFECT OF IRREGULARITY SHAPE ON FLAT SLAB MULTISTOREY

BUILDING UNDER LATERAL LOADING USING ETABS

Khaja Ateequddin1, waseemsohail

2

1M.Tech, Lords institute of engineering & technology, Himayathsagar, Hyderabad, Telangana, India.

Mail Id: [email protected]

2 M. Tech. Asst pro.. Lords institute of engineering & technology, Himayathsagar, Hyderabad, Telangana, India.

Mail Id: [email protected]

I. ABSTRACT

In order to compete in the ever growing competent

market it is very important for a structural engineer to

save time, as a sequel to this an attempt is made to

analyze and design multistoried building by using a

software package Etabs. In the construction industry the

need of flat slabs supported on columns in an irregular

layout is raising design methods valuated by

experimental evidence for flat slabs supported on non

rectangular grid or however scarce. To fill thus vacuum

different irregular shaped flat slab models are being

tested. The building analyzed for the following models:

Model 1 : 10 Storey Building with Regular plan

Model 2 : 10 Storey Building with Re-entrant corners

Model 3 : 10 Storey Building with Re-entrant corner L-

Shape

Model 4: 10 Storey Building with Vertical Irregularity

on one side


on both side

Model 6: 10 Storey Building with Rectangle shape as

Diaphragm discontinuity

Model 7: 10 Storey Building with Inverse-T as

Diaphragm Irregularity

For analyzing a multi storied building one has a consider

all possible loading and see that the structure is safe

against all possible loading conditions. There are several

methods for analysis of different frames like kani’s

method, cantilever method, portal method, and Matrix

method.

The present project deals with the analysis of irregular

flat slab multistory building under lateral loads like

seismic ,wind loads. and to evaluate seismic conditions

of a building like lateral displacement, storey drift, base

shear, time period. From structural engineering point of

view, that challenges to built such taller and taller

structures are wind and seismic forces. On taller

structure

ETAB Pro with its new features surpassed its

predecessors , and compotators with its data sharing

capabilities with other major software like AutoCAD

and MS Excel. We conclude that ETAB is a very

powerful tool which can save much time and is very

accurate in designs. Thus it is concluded that ETAB

package is suitable for the design of a multistoried

building.

INTRODUCTION

The rapid growth of the urban population needs to

construct big prestigious buildings for companies and

industrial purpose. The construction of multi-storey

buildings and large span structures is becoming a

necessary part of our living style. A flat slab is a one-

way or two-way system with thickenings in the slab at

the columns and load bearing walls called „drop panels‟.

A JOURNAL OF COMPOSITION THEORY

Volume XII Issue VII JULY 2019

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Drop panels act as T-beams over the supports. This form

of construction has become popular in recent years

because of the large spans of about 10 m for reinforced

slabs and about 12 m for pre-stressed slabs. Reinforced

flat slabs may need to be sensibly pre-cambered (not

overdone) to control deflection.

EFFECT OF STRUCTURAL IRREGULARITIES

Earthquake resistant design of reinforced concrete

buildings is a continuing area of research. In spite of all

the weaknesses in the structure, either code

imperfections or error in analysis and design, the

structural configuration system has played a vital role in

catastrophe. The IS 1893(Part 1):2002 has recommended

building configuration system in Section 7 for the better

performance of RC buildings during earthquakes.

Vertical discontinuities in load path

One of the major contributors to structural damage

in structures during strong earthquake is the

discontinuities/ irregularities in the load path or

load transfer. The diaphragms distribute these

forces to vertical resisting components such as

columns, shear walls, frames and other vertical

elements in the structural system which transfer the

forces into the foundation. The diaphragms must

have adequate stiffness to transmit these forces.

Irregularities in strength and stiffness

A “weak” story is defined as one in which the

story’s lateral strength is less than 80 percent of

that in the storey above. A “soft storey” is one in

which the lateral stiffness is less than 70% of that

in the story immediately above, or less than 80%

of the combined stiffness of the three stories

above. The essential characteristic of a “weak” or

“soft” storey consists of a discontinuity of strength

or stiffness which occurs at the second storey

connections.

Mass irregularities

Mass irregularities are considered to exist where the

effective mass of any storey is more than 200% of the

effective mass of an adjacent storey. The effective mass

is the real mass consisting of the dead weight of the floor

plus the actual weight of partition and equipment.

Vertical geometric irregularity

A vertical setback is a geometric irregularity in a

vertical plane. It is considered, when the horizontal

dimension of the lateral force resisting system in

any storey is more than 15% of that in an adjacent

storey.

Figure 1.1 Vertical Geometric Irregularities



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Re-entrant corners

The re-entrant, lack of continuity or “inside”

corner is the common characteristic of overall building

configurations that, in plan, assume the shape of an L, T,

H, +, or combination of these shapes occurs due to lack

of tensile capacity and force concentration.


The diaphragm is a horizontal resistance element that

transfers forces between vertical resistance elements.

The diaphragm discontinuity may occur with abrupt

variations in stiffness, including those having cut-out or

open areas greater than 50% of the gross enclosed

diaphragm area, or change in effective diaphragm

stiffness of more than 50% from one storey to the next

storey.

II. EARTHQUAKE RELATED

TERMINOLOGY

Earthquake: An earthquake is a spasm of ground shaking

caused by a sudden release of energy in the earth’s

lithosphere (i.e. the crest plus of the upper mantle).

Focus: It is also termed as hypocenter. The point

on the fault where the slip starts is focus. The

depth of focus from the ground is called focal

depth.

SEISMICITY AND SEISMIC ZONING MAPS

The geographic distribution of past earthquakes is not

uniform over the globe. Also, in one region only small

earthquakes may occur, whereas in another region bigger

ones may take place. It is this aspect of earthquake

occurrence that is covered by the term seismicity.

OBJECTIVES OF THESTUDY

To evaluate the response of irregular flat slabs

structure subjected to lateral loads.

To identify the values of lateral displacement,

storey drift, time period and base shears subjected to

seismic and wind loads.

To compare the variation in maximum lateral

displacement for different types of irregular flat slabs.

To compare the values of all the above

parameters among irregular flat slabs under lateral

loading.

SCOPE OF THEPROJECT

The present study is limited to reinforced concrete

(RC) multi-storied building with different irregular

flat slabs.

The RC building used in this study is eleven

storied (G+10) building have different floor plans

with 5 bays having 7 m distance along longitudinal

direction and 7 bays having3.57 m distance along

transverse direction as shown in figure. The floor

slabs are modeled as rigid diaphragms.

ORGANIZATION OF THE REPORT

Chapter 1: This introductory chapter has presented

the earthquake related terminology, wind

performance on irregular flat slabs, seismicity and

seismic zoning of maps, objective and scope of the

present study.

Chapter 2: This chapter deals with literature

review. It discusses the previous work done in the

area of study.

Chapter 3: This chapter deals with the methodology

adopted for the present study.

Chapter 4: This deals with the modeling and analysis of

the structure considerations.

Chapter 5: Deals with analysis of results.

Chapter 6: Conclusions are arrived based on the results.

Chapter 7: References

Chapter 8: Appendix



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III.METHODOLOGY

When a structure is subjected to earthquake, it responds

by vibrating. An earthquake can be resolved in any three

mutually perpendicular directions-the two horizontal

directions (x andy) and the vertical direction (z).

A number of methods are available for the earthquake

analysis of buildings; two of them are presented here:

• Equivalent Static Lateral Force Method (pseudo

static method)

• Dynamic analysis

i) Response spectrum method.

ii) Time history method.

IMPORTANCE OF SEISMIC DESIGNCODES

Ground vibrations during earthquakes cause forces and

deformations in structures. Structures need to be

designed to withstand such forces and deformations.

Countries around the world have procedures

outlined in seismic codes to help design engineers

in the planning, designing, detailing and

constructing of structures. An earthquake-resistant

building has four virtues in it, namely:

1. Good Structural Configuration: Its size, shape

and structural system carrying loads are such

that they ensure a direct and smooth flow of inertia forces to theground.

2. Lateral Strength: The maximum lateral

(horizontal) force that it can resist is such that

the damage induced in it does not result in

collapse.

3. Adequate Stiffness: Its lateral load resisting

system is such that the earthquake induced

deformations in it do not damage its contents

under low to moderate shaking.

4. Good Ductility: Its capacity to undergo large

deformations under severe earthquake shaking even

after yielding is improved by favorable design and

detailing strategies.

IV. MODELLING AND ANLYSIS

Modeling of column ends at foundation

The column end at foundation can be modeled by

considering the degree of fixity provided by the

foundation. Depending on the type of footing the

end condition is modeled as follows:

i) Isolated footing: A hinge can be provided at

the column end at the bottom of the foundation.

When it is founded on hard rock, column end may

be modeled as fixed.

ii) Raft foundation: The column end can be modeled as

fixed.

iii) Combined footing: Engineering judgment

must be exercised in modeling the fixity provided

by the combined footings. If the footings are

adequately restrained by tie beams, the column end

can be modeled as fixed.

iv) Single pile: Fixity of column at a depth of five

times to ten times the diameter of pile depending

upon the type of soil, from the top of pile cap.

v) Multiple piles: Assume fixity of column at top of pile

cap.

The column-end support for buildings under

consideration the following considerations are

made in the analysis:

1. 1. The supports are assumed to be fixed. Wherever slab

is modeled as plate element, element is connected at four

nodes only.

2. Soil structure interaction is not taken into

consideration.

3. Accidental eccentricity is not considered



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ABOUT ETABS

The modeling and the analysis of the building frames

were carried out using commercial software ETABS.

The important features of this software are as follows:

ETABS is widely used software package from

Computers and Structures, Inc for building

structures.

ETABS is fully graphical user interface: it is

used to generate the model, which can then be

analyzed.

The ETABS engine: It is a general –purpose

calculation engine for structural analysis &

integrated steel, concrete, timber & aluminum.

Using ETABS a ten storey RC structure with various

types of steel bracing is modeled analyzed.

DESCRIPTION OFBUILDING

Model 1 : 10 Storey Building with Regular plan

Model 2 : 10 Storey Building with Re-entrant corners

Model 3 : 10 Storey Building with Re-entrant corner L-

Shape


on one side


on both side

Model 6: 10 Storey Building with Rectangle shape as


Model 7: 10 Storey Building with Inverse-T as

Diaphragm Irregularity

MODEL DATA OF THEBUILDINGS

BASIC DATA OF THE STRUCTURE

Basic Data Of The Structure

MATERIAL PROPERTIES

Table 4.2: Material Properties

MEMBER PROPERTIES

Table 4.3: Member properties

Table 4.4: Loads Considered

The load cases considered in the seismic analysis are as

per IS 1893– 2002

Figure 4.3: Plan of the Building



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Figure 4.4: Elevation of ten storey building

ETABS MODELINGPICTURES

Figure 4.5: ETABS Model with regular plan (Model

1)

Figure 4.6: ETABS Model with Re-entrant corners

(Model 2)

Figure 4.7: ETABS Model with Re-entrant corner L-

Shape (Model 3)

Figure 4.8: ETABS Model with vertical irregularity

on one side (Model 4)

Figure 4.9: ETABS Model with vertical irregularity

on both side (Model 5)

Figure 4.10: ETABS Model with Rectangle shape as

Diaphragm discontinuity (Model 6)



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Figure 4.11: ETABS Model with Inverse-T as

Diaphragm (Model 7)

V. RESULTS AND DISCUSSIONS

RESULTS FOR EARTHQUAKELOADING

LATERALDISPLACEMENT:

Table 5.1: Lateral Displacement

Figure 5.1:Graph showing maximum lateral

displacement in X and Y directions

STOREYDRIFT:

Table 5.2: Storey Drift

Figure 5.2:Graph showing Storey Drift in X and Y

directions

BASE SHEAR:

Table Base shear

Figure 5.3:Graph showing Base Shear in X and Y

directions



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TIME PERIOD:

Table 5.4: Time Period

Figure 5.4: Graph showing Time Period in X and Y

directions

RESULTS FOR WINDLOADING

LATERALDISPLACEMENT:

Table 5.5: Lateral Displacement

Figure 5.5: Graph showing maximum lateral

displacement in X and Y directions

STOREY DRIFT:

Table 5.6: Storey Drift

Figure 5.6: Graph showing Storey Drift in X and Y

directions

BASE SHEAR:

Table 5.7: Base Shear

Figure 5.7:Graph showing Base Shear in X and Y

directions



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VI. CONCLUSIONS

1. From the above graph5.1 it is observed that the

lateral displacement is reduced in X direction

compared to Y direction by 58% as the width of

the building in X direction is more than the

width of the building in Y-direction.

2. From the above graph 5.2 it is observed

that the storey drift in X direction for is less

compared to Y direction. And the values

obtained from the table above shows us that the

model 5 among all the other irregular models has

least amount of storey drift in Y-direction.

3. According to the cl.7.8.2 IS 1893-2002

part1, says that the minimum base shear should

be that of response spectrum when compared to

static analysis. From above table we can

conclude that the base shear for response

spectrum is less than the base shear for static

analysis which is found true.

From the above graph 5.3 it is observed that base

shear is minimum for model 3 among all the

models for both the analysis results.

4. From the above graph 5.4it is observed

that the least amount of time period occurs at

mode 3 for model with vertical irregularity on

both side. Maximum time period occurs at mode

1 for model with Re-entrant corner L-Shape.

5. From the above graph 5.5 it can be

observed that the maximum displacement for

tenth floor is found to be in X direction for

model III with Re-entrant corners L-Shape. And

also the minimum displacement for ground floor

is in Y direction for model V.

6. From the above graph5.6 it is observed

that the Storey drift due to wind loading for

G+10 building is hardly considered because of

very low values obtained.

7. From the above graph 5.7 it is observed

that the base shear for models without vertical

irregularity is same. For model IV and V with

vertical irregularity on one side and two side

respectively are relatively low compared to other

models.

SCOPE FOR FUTUREWORK

1. A non rectangular building has to be studied with

G+25floors.

2. The present study is based on linear static analysis

and dynamic analysis. The results need to be verified

with the push-over analysis results.

3. The study can be extended to find out a method to

control irregularity in such buildings.

REFERENCES 1. IS 456-2000 “Code of Practice for Plain and

Reinforced Concrete”, Bureau of Indian Standards,

NewDelhi

2. IS 1893-2002 “Criteria for earthquake resistant

design of structures”, Bureau of Indian Standards,

NewDelhi

3. IS: 875-part-3 (1987), “Indian standard code of

practice for design loads (other than earthquake) for

buildings and structures”, Bureau of Indian

Standards, New Delhi

4. DubeyS.K., P.D. Sangamnerkar. “Seismic behaviour

of asymmetric RC buildings”, International Journal

of Advanced Engineering Technology E-ISSN

0976- 3945, Professor & Head Dept of Civil

Engineering, MANIT, Bhopal.

5. CassiJ. H. s and Cornejo- E, “Influence of Vertical

Irregularities in the Response of Earthquake

Resistant Structures”.



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6. Monish.S, Karuna .S, “A Study On Seismic

Performance Of High Rise Irregular Rc Framed

Buildings”, International Journal of Research in

Engineering and Technology, Volume: 0, Issue:

05,May-2015

7. Muralidhar G.B and Mrs. Swathi Rani K.S,”

Comparison of Performance of Lateral Load

Resisting Systems in Multi Storey Flat Slab

Building”, International Journal of Research in

Engineering and Technology, Vol.5, Issue No.3

March2016

8. Nonika. N, Gargi DandaDe,“Seismic Analysis Of

Vertical Irregular Multistoried Building”,

International Journal of Research in Engineering

and Technology, Volume:04, Issue: 09,September-

2015.

9. Ravikumar C M, Babu Narayan K S, Sujith B V,

Venkat Reddy D, “Effect of irregular configurations

on seismic vulnerability of RC buildings”. National

Institute of Technology, Surathkal, Architecture

Research 2012, 2(3): 20-26.

10. Sharon L. Wood, (1992), “Seismic Response of R/c

Frames With Irregular Profiles”, Journal of

Structural Engineering (JOSE), Vol.5, Issue 5,

May2015.



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