analysis of effects of openings on rcc beam

CHAPTER 1

INTRODUCTION

1.1 GENERAL INTODUCTION

In the construction of modern buildings, many pipes and ducts are necessary to accommodate

essential services like water supply, sewage, air-conditioning, electricity, telephone, and

computer network. Fig1.1 shows a view of the typical layout of pipes for a high-rise building.

Usually, these pipes and ducts are placed underneath the soffit of the beam and, for aesthetic

reasons, are covered by a suspended ceiling, thus creating a “dead space”. In each floor, the

height of this dead space that adds to the overall building height depends on the number and

depth of ducts to be accommodated. The depth of ducts or pipes may range from a couple of

centimeters to as much as half a meter.

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An alternative arrangement is to pass these ducts through transverse openings in the floor beams.

As shown in fig.1.2, this arrangement of building services leads to a significant reduction in the

head room and result in a more compact design. For small building, the savings thus achieved

may not be significant compared to the overall cost. But for multistory buildings, any saving in

story height multiplies by the number of stories can represent a substantial saving in total height,

length of air-conditioning and electrical ducts, plumbing risers, walls and partition surfaces, and

overall load on the foundation.

It is due to economy and a growing trend toward the use of systems approach to building design

that structural engineers are often required to keep provisions for transverse opening in beams.

Most engineers permit the embedment of small pipes, provided some additional reinforcement is

used around the periphery of the opening. But when large opening are encountered, particularly

in reinforced or prestressed concrete members, they show a general relctance to deal with them

because adequate technical information is not readily available. There is also a lack of specific

guidelines in building codes of practice(ACI,1995;BS 8110-97), although they contain detailed

treatment of openings in floor slabs. As a result, designs are frequently based on intuition, which

may lead to disastrous consequences. There is at least one case on record describe by

merchant(1967), in which the failure of a large building was averted when severe sestress at a

large opening in the stem of a beam was discovered and mitigated in time.

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It is obvious that inclusion of openings in beams alters the simple beam behavior to a more

complex one. Due to an abrupt change in the cross-sectional dimensions of the beam, the

opening corners are subject to high stress concentration that may lead to wide cracking that is

unacceptable from aesthetic and durability viewpoints. The reduced stiffness of the beam may

also give rise to excessive deflection under service load and result in a considerable

redistribution of internal forces and moments in a continuous beam. Unless special reinforcement

is provided in sufficient quantity, the strength of such a beam may be reduced to a critical

degree.

Web openings in the beams occur quite often in practice to provide convenient passage of

environmental services. As a result, story heights in buildings can be reduced and slight

reduction in concrete beams weight would improve the demand on the supporting frame both

under gravity loading and seismic excitation which resulting in major cost savings. These

Openings may be of different shapes and sizes, and are generally located close to the supports

where shear is predominant. In fact, openings should be positioned on the concrete beams to

provide chords with sufficient concrete area to develop the ultimate compression block in flexure

and adequate depth to provide effective shear reinforcement. Although numerous shapes are

possible, circular and rectangular openings are the most common ones.

Passing utility services through opening in the floor beam webs minimizes the required story

height (see Figure 1.1). For small buildings, the savings thus achieved may not be significant, but

for multistory buildings, any saving in story height multiplied by the number of stories can

represent a substantial saving in total height, length of air-conditioning and electrical ducts,

plumbing risers, walls and partition surfaces, and overall load on the foundation. Story heights in

buildings can be reduced and a slight reduction in concrete beams weight would improve the

demand on the supporting frame both under gravity loading and seismic excitation resulting in

major cost savings. As a result, the ceiling may be attached directly to the underside of the floor.

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Figure 1-1photograph of circular openings in reinforced concrete beam.

1.2 CLASSIFICATION OF OPENING

Transverse opening in beams may be of different shapes and sizes. Prentzas(1968),in his

extensive experimental study, considered openings of circular, rectangular, diamond,

triangular, trapezoidal and even irregular shapes as shown in fig.1.3. although numerous

shapes of openings are possible, circular and rectangular openings are the most common

ones. Circular openings are required to accommodate service pipes, such as for plumbing

and electrical supply. On the other hand, air-conditioning ducts are generally rectangular in

shape, and they are accommodated in rectangular openings through beams. Sometimes the

corners of a rectangular opening are rounded off with the intention of reducing possible

stress concentrated at sharp corners, thereby improving the cracking behavior of the beam in

service.

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With regard to the size of openings, many researchers use the terms small and large opening

without any definition or clear-cut demarcation line. Mansur and Hasnat(1979) have defined

openings circular, square, or nearly square in shape as small openings, whereas, according to

somes and Corley(1974), a circular opening may be considered as large when its diameter

exceeds 0.25 times the depth of the beam web. However, the authors consider that the

essence of classifying an opening as either small or large lies in the structural response of the

beam. When the opening is small enough to maintain the beam-type behavior or, in other

words, if the usual beam theory applies, then the opening may be termed a small opening.

When beam-type behavior ceased to exist due to the provision of opening, then the opening

may be classified as a large opening.

According to the above criterion, the definition of an opening being small or large depends

on the type of loading. For example, if the opening segment is subject to pure bending, then

beam theory may be assumed to be applicable up to a length of the compression chord

beyond which instability failure takes place. Similarly, for a beam subject to combined

bending and shear, test data reported in the literature have shown that the beam type

behavior transform into a vierendeel truss action as the size of opening is increased.

1.3 DESIGN CONSIDERATION

In the design of a concrete beam with web openings. The usual acceptance criteria for structurel

members should apply. These are basically the strength and serviceability requirements. An

accurete assessment of ultimate strength is necessary to provide adequate safety against possible

collapse. Serviceability, in general, requires that the deflection produced under working loads be

sufficiently small and cracking, if any, be controlled with maximum crack width not exceeding

some tolerable limits. However, due to the provision of transverse openings, the usual design

procedure for solid beams is not applicable to beams with openings. For example, the indirect

way of satisfying the serviceability requirement of maximum deflection by ensuring a minimum

span-to-effective depth ratio is not valid for beams with openings.

In a statically determinate structure, bending moment and shear force distribution are uniquely

defined by statics alone. In statically indeterminate structures, however, the distribution of

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internal forces and moments depends on the relative stiffness of each individual member.

Provision of openings through a member reduces its stiffness relative to others and causes a

redistribution of internal actions from what is obtained by analyzing the structure, assuming

prismatic members. This possible redistribution due to the provision of openings should be taken

into account in the analysis of a statically indeterminate structure.

In prestressed concrete beams, it is also necessary to evaluate the effects of prestress on cracking

around the opening to fulfill the crack control requirements both at transfer of presress and at full

service load. All tha above aspects need to be addressed properly before a satisfactory design

method for beams with transverse openings can evolve.

1.4 OBJECTIVES OF STUDY:

1. Finite elementaAnalysis will be carried out by ANSYS 14.

2. A reference of papers by soroush amiri and reza masoudnia published in Austalian journal of

basic and applied science in the year of 2011 is taken for analysis and its further scope is taken

for the thesis.

3. Rectangle Beams with different shape of openings like rectangle and circular and at different

location will be analysed using finite element analysis software. The load versus deflection

diagram and stress concentration due to opening is analysed for all the beam with different

openings and at different locations horizontal as well as vertical.

4 I will model beams of 150mm x 200mm cross section and length of 2000 mm simply

supported at its ends with following circular openings:

Solid beam,120mm,100mm, 80mm

5 I will locate these openings at various places in the span of the beam in horizontal as well as in

vertical direction along the span of the beam.

6. All these beams will be modelled and the finite element analysis for these beams shall be

carried out.

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7. The result obtained from finite element analysis will be interpreted and the best position for

the opening and the size of the opening shall be determined.

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CHAPTER 2

LITERATURE STUDY

2.1 GENERAL

This chapter includes various technical papers, which are related to the analysis of the reinforced

concrete beam with web openings. Literature survey is carried out to be familiar with the amount

of work done in the area throughout the world. The survey gives ideas about the extent of work

to be carried out during project. Various research papers published till date on different aspects

of behavior of beam with various size of web openings. Some papers gives us the reinforcement

configuration around the periphery of the opening.

2.2 REVIEW OF RESEARCH PAPERS

2.2.1. Paper1: Investigation of the Opening Effects on the Behavior of

Concrete Beams without Additional Reinforcement in Opening Region Using

Fem Method

Author: Soroush Amiri, Reza Masoudnia

Year of publishing: 2011

Abstract: In this study a three-dimensional nonlinear finite element method using ANSYS

10.0, finite element analysis software, has been employed to simulate the simply supported

concrete beams consisting of circular openings with varying diameters. The effects of circular

opening size on the behavior of such beams were investigated in this research. Two cases were

carried out for verification study. Subsequently, numerous models of simply supported

reinforced concrete rectangular section beams with circular and square opening were loaded

monotonically with two incremental concentrated loads. The beams were simulated to obtain the

load-deflection behavior and compared with the solid concrete beam. All beams had an identical

cross section of 100 mm × 250 mm and 2000 mm in length with the circular opening in seven

diameters: 150 mm, 130 mm, 120 mm, 110 mm, 100 mm, 80 mm and 60 mm and an equivalent

square opening with 133 mm in width. The results obtained from this study showed that the

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performance of the beams with circular openings with diameter less than 0.48D (D is depth of

the beam web) has no effect on the ultimate load capacity of the RC rectangular section beams.

On the other hand, introducing the circular opening with diameter more than 0.48D reduces the

ultimate load capacity of the RC rectangular section beams at least 26%.

Conclusions: Following conclusions can be drawn with respect to the results obtained from

the analysis of the solid beam without opening, beams with circular opening with 150 mm, 130

mm, 120 mm, 110 mm, 100 mm, 80mm and 60 mm in diameter and beam with equivalent square

opening with 133 mm in width. The suggested recommendations for the future works are also

presented. This research mainly comprises the load-deflection curves behavior for solid beam

and the effect of beams with circular opening and square opening on the ultimate load capacity

of the beams. Furthermore, the effect of circular opening compared to the equivalent square

opening has been discussed. The following conclusions can be stated based on the evaluation of

the analyses of the calibration model and the RC beams with circular and square openings:

1. The ultimate load obtained by ANSYS for the RC beam without opening is very close to the

ultimate load measured during experimental testing.

2. Introducing the circular opening with diameter less than 48% of the depth of the beam

(without special reinforcement in opening zone) has no effect on the ultimate load capacity of the

RC rectangular beams, meaning that these beams behave similar to the beams without opening.

This result is clearly observed and shown in Figure 14. Moreover, according to the crack pattern,

mode of failure is flexure at mid span in these beams.

3. Introducing the circular opening with diameter more than 48% of the depth of the beam

(without special reinforcement in opening zone) reduces the ultimate load capacity of the RC

rectangular beams at least 26%. Mode of the failure is shear at the opening at a low load level.

4. The circular opening has more strength than equivalent square opening with difference of 9%

in ultimate load capacity.

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The following recommendations are suggested for future researches which are not covered in the

present study:

Based on the finite element method, future studies can include different types of loading to

further test the design method. This comprises studies with continuous beams, multiple load

points, and different opening shapes and also includes examining the vertical location of the

transverse openings in the RC beams. Moreover, strengthening of the opening in the RC beams

with composite sheets and additional steel reinforcement around the opening can be investigated

and simulated by finite element computer software.

2.2.2. Paper2: The Study of the Effects of Web Openings on the Concrete

beams.

Authors: Soroush Amiri, Reza Masoudnia and Ali Akbar Pabarja

Year of publishing:2011

Abstract: This paper aims to review the results of previous research studies on precast and

prestressed beams, T-beams, deep beams, and rectangular concrete beams with web openings.

The behavior of both reinforced concrete beams with rectangular and circular openings and

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precast beams with rectangular and circular openings was investigated. The effects of the size

and location of the openings on the behavior of such beams are examined and the strengths of

these openings are explored as well. This paper accordingly reviews and describes the previous

researches which are related to the openings in the concrete beams.

Conclusion:

• Reinforced and prestressed concrete beams can accommodate large web openings without

sacrificing strength or serviceability;

• Web openings must be outside the strand development length for prestressed concrete

members;

• An inflection point in the chord exists approximately at the midpoint of the opening; and

• Cracking in the tension chord causes the shear force to redistribute from the bottom chord to

the top chord. A critical issue for the researchers in the field is how to divide the shear force

between the chords at an opening. Thompson and Pessiki (2006) made some design

recommendations including several important elements which were collectively called the initial

design model (IDM). The following are the recommendations:

• Restrictions on the applicability of the obtained results;

• Prescriptive requirements for both placement and size of the opening, and the distance between

the openings as well;

• Prescriptive requirements for transverse reinforcement at the abutments;

• An analytical procedure to determine the chord forces; and

• Recommendations on the applicable ACI 318 sections to design the chords for combined axial

force, shear force, and moment.

A considerable progress has been made during the last three decades in the investigation of

concrete beams with opening in the web. Hence, this paper reviewed and described the previous

researches which were related to the openings in the concrete beams. The details of some works

for precast and RC beams with different opening size and type are available in the present study.

The effects of the size and location of the openings on the behavior of such beams were

examined and the strengths of these openings were explored as well. Intensive research is

required on the different types of loading to further test the design method. This includes studies

with continuous beams, multiple load points, and different opening shapes.

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2.2.3. Paper3: DESIGN OF REINFORCED CONCRETE BEAMS WITH

WEB OPENINGS

Author: M.A. Mansur

Year of publishing: 2006

Abstract: The provision of transverse openings in floor beams to facilitate the

Passage of utility pipes and service ducts results not only in a more systematic layout of pipes

and ducts, it also translates into substantial economic savings in the construction of a multi-story

building. To investigate the problem of openings in beams, the author initiated a research

program in the early 1980s. Since then extensive research has been carried out giving a

comprehensive coverage on both circular and large rectangular openings under various

combinations of bending, shear and torsion. In this paper, major findings relevant to the analysis

and design of such beams under the most commonly encountered loading case of bending and

shear are extracted and summarized. An attempt has been made to answer the frequently asked

questions related to creating an opening in an already constructed beam and how to deal with

multiple openings. It has been shown that the design method for beams with large openings can

be further simplified without sacrificing rationality and having unreasonable additional cost.

Conclusion: This paper gives a brief but comprehensive treatment of the analysis and design

of reinforced concrete beams that contain transverse openings through the web and are subjected

to combined bending and shear. Recognizing the differences in beam behavior, circular and large

rectangular openings are treated separately. Practical situations of drilling an opening in existing

beams and special design considerations for beams with multiple openings are also briefly

discussed. It has been shown that the design method for large rectangular openings may be

considerably simplified if it is decided to use symmetrical arrangement of reinforcement in the

chord members. Further details are available in the only book (Mansur and Tan, 1999) available

to date on openings through concrete beams.

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2.3 BOOKS REFERRED

“concrete beams with openings: Analysis and Design”

M.A.Mansur

Kiang-Hwee Tan

This book compiles information in the behavior, analysis, and design of concrete beam that

contain transverse openings through the web. The behavior of such beams under bending, shear

and torsion is treated in this book. Design of beams with openings is given in this book. Suitable

guidelines on the detailing of beams with web opening are also included.

2.4 SUMMARY

All the different papers gives an idea about the amount of various research works carried out on

this topic so far, current trends of research works and further scope of detailed studies required in

this topic, many research work done in this topic but still now no perfect procedure given in any

code for designing of the beam with web openings. There is not any codal provision for design of

beam with opening in India.

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CHAPTER-3

METHODOLOGY

3.1 METHODS USED FOR ANALYSIS:

There are two methods used for analysis of reinforced concrete beam with web openings.

(1) Finite element analysis using ANSYS14.

(2) Experimental Testing of the beam.

I will model reinforced concrete beam with different opening size at different location and for

validation of my analysis also cast the beam and test it in the laboratory.

Detail of the beam:

I have design the beam using IS-456. I have design limiting section for 150mm x 200mm size

and having a length of 2000mm.

3-12mm bar used as main bar & 2-8mm bar used as anchor bar.

Fe415 steel and M20 concrete used.

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3.2 Modeling of beam with different size of circular opening in ANSYS14.

Different models of the beam with different holes diameter and at different location along the

beams are analyzed in ansys14. The geometry of beams with reinforcement was created using

Revit Structure software and then imported in ANSYS14 for further analysis. The details of

different models are given in following table:

model.

No

Description Status of analysis

1 Solid beam without hole Completed

2 Beam with 80mm(0.4D) circular hole at L/2 distance Completed



5 Beam with 80mm(0.4D) circular hole at L/4 distance In progress



Model No.1: Solid beam without hole

(A)Geometry:

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(B): Importing the model into ANSYS workbench:

(C)Analysis of the model in ANSYS:

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(D) Results:

Deflection:

Stress:

Strain:

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All the models are analyzed as same as for solid beam as shown in above procedure.

And then for the loading of total load of 20KN, 40KN, 60KN, 80KN, 100KN. The deflection,

stress and strain are noted and graph of load vs. deflection, stress and strain are plotted.

3.3 Experiment of the beams for validation: I will cast the beam with opening of different

diameter and at different location for the validation of the software simulation. The experimental

set up of the beam is as shown in figure below:

3.4 Interpretations of results and conclusion

Analysis process in ansys gives total deformation, stress and strain at different values of the load

from which graph of LOAD VS DEFLECION, LOAD VS. STRESS should be drawn as

following:

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0

20

40

60

80

100

120

0 0.5 1 1.5 2 2.5 3 3.5

To

tal

loa

d i

n K

N

deflection in mm

LOAD VS DEFLECTION

solid beam

80mm opening

100mm opening

120mm opening

0

20

40

60

80

100

120

0 5 10 15 20 25 30 35 40

Solid beam

80mm opening

100mm opening

120mm opening

stress in N/mm2

Lo

ad

in

LOAD VS STRESS IN CONCRETE

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CONCLUSIONS FROM LOAD VS. DEFLECTION GRAPH:

SR NO TYPES OF

OPENING

SIZE OF

OPENING

SRENGTH

OF

CONCRETE

LOAD

CARRYING

CAPACITYOF

BEAM

% INCRESE

IN

DEFLECTION

1 Solil beam -- 20Mpa 67.5KN 0%

2 Circular 80mm 20MPa 67.5KN 0.71%

3 Circular 100mm 20Mpa 62KN 1.83%


CONCLUSION FROM LOAD VS. STRESS GRAPH:

SR NO TYPES OF

OPENING

SIZE OF

OPENING

SRENGTH

OF

CONCRETE

LOAD

CARRYING

CAPACITYOF

BEAM

%

DECREASE

IN LOAD

CARRYIN

CAPACITY

1 Solil beam -- 20Mpa 67.5KN 0%

2 Circular 80mm 20MPa 67.5KN 0.%



3.5CONCUSION:

1. When the diameter of opening is up to 40% of the overall depth the load carrying

capacity does not affected.

2. The diameter of opening of 50% of the overall depth decrease the load carrying capacity

up to 8.14%.

3. The diameter of opening of 60% of the overall depth decrease the load carrying capacity

up to 18.52 %.

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analysis of effects of openings on rcc beam