UNIVERSITI PUTRA MALAYSIA

STRUCTURAL BEHAVIOUR OF PRESTRESSED CONCRETE HOLLOW BEAMS

ROSLI MOHAHAD ZIN

FK 1993 1

STRUCTURAL BEHAVIOUR OF PRESTRESSED CONCRETE HOLLOW BEAMS

By

ROSLI MOHAHAD ZIN

Thes i s Submitted in Par t i a l Fulfilment of the Requireme n t for the Degree of Master of S c i ence in the Fa culty o f Engi neering,

Universiti Pertanian Ma laysia

April 1993

ACKNOWLEDGEMENTS

The a u t hor wi shes to expre ss h i s many t hanks to Dr . S.A

Salam, formarly the cha irman of supervi sory comm i t t e e of t h i s

projec t , t h e newly appointed mai n s upervi sor Enc i k Mohd. Salleh

Jaafar, and co-supervi sors Pro fessor Abang Abdullah Aban g Ali

and Encik Zakaria Che Muda for their con t i nuous support.

g uidan ce , encouragement and help t o make this s t udy possible.

Thanks are also due to En c i k Mohd. Ha l i m Othman , En c i k

Baharuddin, a n d Tuan Haji Ghazali for their help 111 la bora tory

works , Puan Rogayah and Puan Za i n e for thei r help i n typing the

manuscri pt, and those whose names a re not men t i oned here who

he lped i n the prepara t ion of the manuscript .

Last but not t he least , my s i n c ere gra titude is due to

Un ivers i t i Pert anian Ma lays i a , Serdang , for t he perm i s s ion to

undertake M.S programme at the Faculty of Engineeri n g and t o

unde r t ake this research.

ii

ACKNOWLEDGEMENTS

LIST OF TABLES

LIST OF FIGURES

LIST OF PLATES

LIST OF NOTATIONS

ABSTRACT

ABSTRAK

CHAPTER

1. INTRODUCTION

TABLE OF CONTENTS

Prestressed Concrete

ii

vii

viii

x

xi

xiv

xvi

1

General Principle of Prestressed Concrete 2

Advantaees of Prestressed Concrete Hollow Beams . 4

Scope and Objective

2. LITERATURE REVIEW

Selection of the Best Shapes for Prestressed Concrete Under Flexure

Prestressed Concrete Hollow Beams

5

7

9

Design of Prestressed Concrete Hollow Beams 1 1

Arrangement of Steel of Prestressed Hollow Beams . 12

Summary of Literature Survey 14

iii

3. THEORETICAL CONSIDERATION

Beam Subjected to Third Point Loading 15

Prediction of Cracking Load and Hending Moment 16

Prediction of Ultimate Load and Bending Moment 17

Location of Neutral Axis

Ultimate Moment Carrying Capacity

Ultimate Load

Prediction of Service Load

Calculation of Deflection

4. MATERIALS

Concrete

Cement,Sand and Aggregates

Prestressing Wires

Stirrup Reinforcement

Plastic Tubing, Grease etc.

5. TEST PROGRAMME

Beam Specimen Preparation

The Beam Mould

Casting and Curing of Concrete

Test Cubes

Removal of Rectangular Hollow Section and

18

23

24

24

25

27

27

29

29

30

32

32

33

34

Plastic Tubing . . . . . . . . . . . . . . . . . . . • . . . . . . . . 35

Prestressing 36

Grouting 37

iv

The Test

Testing Ma chine

Setting of the Beam

Beam Testing

Testing the Cubes

Details of Beam Tested

6. RESULTS AND DISCUSSIONS OF BEAM TESTS

Beam Test Results

Beam SOl

Beam U 0 1

Beam U02

Beam U#l

Beam U#2

Beam Ul3

Beam U00 1

Beam U00 2

Beam B01

Beam B02

Discussion

Ultimate Load

Cracking Load

Crack Width and Crack Propagation

Strain Gauge Results

Service Loa d

v

36

36

37

37

3 8

39

46

46

47

4 8

50

51

52

54

55

56

57

58

59

63

64

65

66

Deflection

7. CONCLUSIONS AND RECOMMENDATIONS

Con c l usi ons

Ultimate load

Cracking

De f l ection

Recommendations for Further Research

BIBLIOGRAPHY

APPENDIX A

APPENDIX B

APPENDIX C

APPENDIX D

CURRICULUM VITAE

vi

67

97

97

98

qg

100

111

113

117

149

161

172

LIST OF TABLES

1. Properties of Concrete Beam Specimens

2. Details of Beams Tested

PAGE

3 1

4 3

3 . Experimental Against Ca l c ulated Ultimate Load 7 0

4. Experimenta l Against Ca lculated Cra cking Load 7 1

5 . Ultimate Strength of Prestressed ijo l l ow Beams compared to Solid Beam Under Identical Circ umstances . . . . . . 7 2

6. Cal cu lation of Deflection at Service Loa d

7 . Experimenta l Against Theoretic a l Maximum deflection

7 3

at Service Load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 4

8. Experimenta l Aeainst Theoretical Maximum Deflection at Actual Cracking Loa d . . . . . . . . . . . . . . . . . . . . . . . . . . 75

9. Experimenta l Aeainst Cal c u l ated Cracking Load and Ultimate Loa d for Unhonded Beams With Variable Weight Reduction . . . . . . . .. . . .. .. . . . . . . . . . . . . . . . .. . 76

10. Experimental Agains t Ca l cul ated Cra cking Load and Ultimate Load for Bonded Beam With Variable Weight Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

vii

LIST OF FIGURES

1 . Pres tressed Con c r e t e

2 . Beam Subjected t o Th ird Po i n t Loading

3 . S t ress-s t ra i n Rel a t i onsh ip a t U l t ima t e (Neutr a l axis w i t h i n t h e flange)

4. Stress-s t r a i n Diagram for Prest ressing S t eel

5. St ress-stra i n Re lat ionship at U l t imate (Neutral axis wit h i n the web)

6 . Grad i n g Curve for Coarse , Fine and Combined Aggregat e Mc Intosh and En t roy Type Gra d i ng

7. Det a i l s of Beam Tes t ed

8 . Load Aga inst De f l e c t i on Beams UO l,U02,SO l

9. Load Against Def l e c t i on Beams Ull , UI2 , UI3

1 0 . Load Aga inst Def l e c t i on Beams U001 , U002

1 1 . Load Aga i nst Def l e c t i on Beams UOl , Ull , UOOI

1 2. Load Aga inst De f l e c t i on Beams U02 , UI2 , U002

1 3. Load Agai n s t Deflect ion Beams B0 1,B02

1 4. Load Aga i n s t Max . Crack Width Beams UO l , U0 2 , 801

1 5 . Load Against Max . Crack Width Beams Ull , UI2 , UI3

1 6 . Load Again s t Max . Crack Wid th Beams U001 , U002

17 . Load Aga i n s t Max . Crack Width Beams B0 1 , B0 2 , S0 1

1 8 . Beam Depth Against S t ra in, Beam UOI

19. Beam Depth Against S t arin , Beam U0 2

20. Beam Depth Against S t r a i n , Beam Ull

2 1 . Beam Depth Against Stra i n , Beam UI2

vi i i

PAGE

3

1 5

1 9

21

22

28

44

77

78

79

8 0

8 1

8 2

8 3

84

85

86

87

88

89

90

22 . Hearn Depth Agains t

23 . Beam Depth Against

24 . Beam Dep th Agains t

25 . Beam Depth Against

26. Beam Depth Against

27. Beam Depth Against

28. Beam Detai ls U01

29 . Beam Det a i l s UI1

30 . Beam De tails BOI

Stra in, Beam

S t r a in , Beam

S t ra in, Beam

S t rain , Beam

S t ra in, Beam

S t r a in , Beam

U#3

U00 1

U002

B0 1

BOI

S02

.. . . . . " . " . " " . " .

" . . " " " . . ,, . . . .

" " " . " . . " . " . ,, .

. . " " " " . . . ,, . ,, " "

" . . " " . . " " . . . . .

" " . " . " . . . . . . " .

. . " . " . .. . . " . .. . . . " . . " . . . . . . .. " .. " " . .. .

. . " " " " " " " .. . " . . " " " " " " .. " .. .. .. .. .. " .. .. .. .. ..

.. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. ..

ix

PAGE

9 1

92

9 3

94

95

96

1 27

1 3 7

148

1 .

2 .

3.

4.

5.

6 .

7 .

8 .

LIST OF PLATES

The Cast ing Mou l d

Cas t i n g o f Beam

Beam Ready for St ress ing

Crack Measur ing Mic roscope and Demec Gauge

Grou t i n g Pump

Prestressing i n Progress

The Beam Tes t i n g Frame and Test Set up . . . . . . . . . . .

Crack i n g Pa t t ern of Beam U0 1 . .. . . . . . . . . . . . . . . . . . .

9 . Cracking Pat t ern of Beam U02

10 . Cracking Pat t ern o f Beam U#l

1 1 . Cracking Pat t ern o f Beam UI2

1 2 . Cracking Pa t t ern of Beam UI3

13. Cra cking pa t t ern of Beam U00 1

14. Cracking Pa t t ern of Beam U002

1 5 . Cracking Pat t ern of Beam BOl

1 6 . Cra c k i n g Pat tern o f Beam BO�

1 7 . Cracking Pat t ern of Beam SOl

x

PAGE

1 0 1

102

1 0 2

1 0 3

104

104

105

105

106

106

107

1 07

1 08

108

1 09

1 0 9

1 10

a

A

b

C

d

e

LIST OF NOTATIONS

Distance from top flange to centre of top steel

Area of cross-section

Area of prestressing

Area of prestressing steel in the tension zone

Cross-sectional area of the two legs of a links

Width or effective width of the section or flange in the compression zone

Width of flange

Width of web

Total compression force

Compressive force in concrete block (flange)

Compressive force in concrete block (web)

Compressive force of top prestressing steel

Effective width to the centroid of steel area

Eccentricity of prestressing force

Young's modulus of elasticity of concrete

Young's modulus of elasticity for prestressing steel

Overall flexural tensile stress in concrete

Characteristic strength of concrete

Design compressive stress at the centroidal axis due to prestress

Design tensile stress in the tendon

Characteristic strength of prestressing wires

xi

h

L

Mo

p. 1

T

x

Maximum des i gn pri n c i p l e tens i l e stress

Depth of cross-sect ion

Depth of top fl ange

Secon d moment of area of the sec t i on

Effe c t i ve span

Cracking moment

S e l f wei ght moment

Girder moment

Moment necessary to pr oduc e zero st r ess in the concrete at the ext reme fibre

Tota l moment

U l t ima t e moment

Cracking load

Effect ive prest ressi n g force

I n i t i a l prestressing force

Spa c i n g of the links

Tensi l e force

Desi gn ult ima te shear resis tance o f the con c rete

Des i gn u l t ima t e shear resistance of a s ec t i on cracked i n f l exure

Desi gn ultimate shear resistance of a section un cracked in f l exure

Locati on of n eutra l axis from top flange

Lever arm to Cl

Lever arm t o C2

Lever arm to st op steel

First moment of area (below neut r a l axis)

xii

First moment of area (above neutral axis)

Strain in compression steel

Strain of top steel due to bending

Total strain

Deflection at mid span of the beam due to two

point loading

Deflection due to self weight U.D.L at mid span

Deflection due to effective prestress (bottom steel)

Deflection due to effective prestress (top steel)

Total deflection

xiii

An abstract o f the thesi s presented to the Senate o f Uni vers i ti Pertanian Malaysia i n partial fulfilment o f the requi rements for the degree of Master of Sci ence .

STRUCTURAL BEHAVIOUR OF PRESTRESSED CONCRETE HOLLOW BEAMS

BY

ROSLI MOHAMAD ZIN

APRIL 1993

Chairman Dr. S . A . Salam {December 19 8 8 - July 1992}

Mr . Mohd . Salleh Jaa f ar (July 1992 - April 1 99 3 )

Faculty Engi neering

Thi s thes i s is concerned with the primary objective of

studying the structural behavi our o f prestressed concrete

hollow beams . Ten simply supported recta ngular hollow beams and

one rectangular solid beam were tested on an effectjve span of

2 . 80 m subjected to two third point loadi ngs. The variables i n

th e study were th e percentage of self weight reduced and the

amount of prestress i ng w ires. Eight beams were testpd unbonded

while the other two beams were f ully bonded.

xiv

Ultimate loads, cracking loads, crack widths and

deflections were recorded at various loadings and crack

propagations were observed. The results obtained were compared

with theoretical values.

It was observed that due to the absence of material in

the hollow portion, compared to a solid beam with similar

outside dimensions, the ultimate moment carrying capacity of

prestressed hollow beam is reduced if neutral axis of the beam

at failure is located below the top flange. However, if the

neutral axis of the beams at failure is located within the top

flange, then the ultimate moment carrying capacity is at least

equivalent to that of a solid beam. It was also observed that

the theory on the ultimate moment carrying capacity presented

in this thesis gives a fairly good prediction. However, the

theory used to predict cracking load as well as deflection was

found not suitable for unbonded beams as it greatly

underestimates the deflection and overestimates the cracking

load. It was also observed that bonding has a great influence

on crack widths and deflections. Bonded beams show more uniform

crack distribution with reduced maximum crack width and

increased ultimate load capacity. From test results, it is

recommended that prestressed hollow beams should be made bonded

in order to achieve at least the predicted cracking load.

xv

Sebuah abstrak tesis yang dikemukakan kepada Senat Universiti Pertanian Malaysia bagi memenuhi sebahagian daripada kelayakan Ijazah Master Sains.

KELAKUAN STRUKTUR RASUK KONKRIT PRA-TEGASAN BERONGGA

OLEH

ROSLI MOHAMAD ZIN

APRIL 1993

Pengerusi Dr . S.A. Salam (Disember 1988 - Julai 1992)

En. Mohd. Salleh Jaafar {Julai 1992 - April 1993}

Fakulti Kejuruteraan

Tesis ini bertujuan menjalankan kajian mengenai kelakuan

rasuk-rasuk konkrit pra-tegasan berongga . Sepuluh buah rasuk

berongga dan sebuah rasuk padu tersokong secara mudah diuji di

atas jarak berkesan Z.80m dengan dua beban titik ketiga.

Pemboleh ubah terdiri dari peratus pengurangan beban diri serta

jumlah dawai pra-tegasan. Kesan ikatan relah juga dikaji . Lapan

buah rasuk telah diuji tanpa ikatan sementara dua buah lagi

terikat penuh melalui turapan tekanan.

xvi

Beban muktamad , beban retak , l ebar retak dan l en t uran

dir ekodkan pada t ahap beban ber lai nan dan pemben t ukan retak

diperh a t ikan . Keput usan yang diperolehi di bandi n gkan dengan

ni l a i-ni l a i teor i .

Ada l a h diperha t ikan , den gan kehadiran r uang berongga , j i ka

d i bandingkan dengan rasuk padu yang mempunyai s a iz kera tan

rentas yang serupa , kemampuan mena n ggung beban mukt amad bagi

rasuk berongga akan berkurangan j i ka paks i nutral pada keadaan

muktamad t er l e t a k di bawah beb i b i r a t a s . Walaubaga imanapun ,

j i ka paksi nut r a l t e r l e t ak di beb i b i r a t as ,

menanggung beban muktamat bagi rasuk berongga

kemampuan

sekurang-

kurangnya akan sarna dengan kemampuan menanggung beban muktamad

bagi rasuk padu. Dapat j u ga diperhat ikan teori beban mukt amad

yang d i g unakan d a l a m t es i s ini memberikan s a t u ramalan yang

agak b a i k . Namun begi t u teori -teori yang d i gunakan bagi meramal

beban retak beg i t u j uga lenturan bagi ra suk t i dak terikat

di dapa t i t i dak sesuai memandangkan rama l a n yang diberikan

adalah j auh dari yang s ebena rnya . Adalah diperha t i kan j uga ,

ikatan memberi kesan ke at as lebar retak dan lenturan. Rasuk

yang teri kat mampu mengagi hkan r etak dengan

seterusnya dapa t mengurangkan l ebar retak

beban muktamad . Berdasarkan keputusan

lebih berkesan

dan menin gkat kan

u j i a n , a d a l ah

disyorkan supaya rasuk-rasuk berongga d i bua t secara t erikat

untuk mencapai sekurang-kuran gnya set aka t ketahap beban retak

yang d i r amalkan.

xvi i

CHAPTER 1

INTRODUCTION

Pres t ressed Concrete

Prestressed concrete is a very important construction

material and its use will continue to grow in the next century.

In 1939 Freyssinet introduced an economical way of producing

prestressed concrete. Since then, more and mOTe research was

done for developing a better understanding of prestressed

concrete.

As mentioned by Bate and Bennett (1980), prestressing

can be defined as a technique whereby the performance of a

structure is improved by the introduction of permanent stress

(prestress) so as to cancel some of the st ress produced by the

dead and imposed load. Mosley and Buneey (1987) defines

prestressing as the artificial �reation of stresses in a

structllre before loading, so that the stresses which then exist

under load aTe more favourable than would otherwise be the

case.

The principle of prestressing when applied to �on�rete

will result in what we called prestressed �oncrete. Perhaps one

of the best definitions of prestressed concrete is given by

1

2

the ACI Committee on prestressed concrete. This de finition is

"Prestressed concrete : concrete in which there have been

introduced internal stresses of which magnitude and

distribution that the stresses resulting from given external

loading are counteracted to a desired degree" .

British Standard BS 8110 Part 1: 1985 , Clause 4 . 1.3

classifies prestressed concrete structures i nto three

categories which are as follow:

a. Class 1

b. Class 2

c. Class 3

no flexural tensile stress

flexural tensile stress but no visible cracking

flexural tensile stresses but surface width of cracks not exceeding O. lmm for members in very severe environments and not exceeding 0. 2mm for all other members.

General Principle of Prestressed Concrete

The basic concept of prestressed concrete is i lustrated in

Figure 1. A very high stren gth steel tendon has been placed in

the duct. After concrete has achieved required strength, the

tendon will be stressed prior to external l oading. Resulting

from the prestressing of tendon (Figure 1. a), the stress in the

beam varies from a maximum compression in the bottom fibre to a

small tension in the top fibre and causes the beam to deflect

upward . When external loads are applied, the stress

distribution will be as shown in Figure l(b). Combined with the

stress due to prestress produce a state of stress as in Figure

3

l(c). where maximum compression or a small tension in the

bottom fibre.

COMPRESSIon

(a) STRESS DUE TO PRESTRESS

w

TENSION

( b I STRESS DUE TO MOMElfT

w

ctJMPRESSION

( c ) COMBINED STRESSES

Figure.l- General Principle of Prestressed Concrete.

4

Advantages of Prestressed Concrete Hol low Beams

The advantages of considering prestressed concrete hollow beams

in concrete design are :

i ) The reduced weight of the member will help in economi zi ng

the section; the smalle r dead load and depth of members

will res ult in saving materi als f rom other secti ons o f

structure, e.g foundation. I n pr ecast members , a reducti on

o f weight saves handling and transportation costs.

ii) Excellent torsional s trength , and rigidity combine with

good flexural strength as prestress ed hollow beams a r e

closed section.

i i i } Low maintenance cost mainly due to i ts durabili ty,

Fati gue test on hollow beams indi cate that million of

cycles of load applications i n excess o f desi gn load does

not result in any sign of distress (Bender and Kriesel,

1969).

It is also known that when the ratio of dead to li ve load

is large, the use of structural hollow s ection become

signi ficant s i nce the saving in weight i s substantial. For

prestressed hollow beams the amount of weight that can be

reduced depending on width/wall thickness ratio and percentage

of wei eht reduction. It has been known tha t works on the effect

of width/wall thickness ratio on the behaviour of prestressed

6

beams are fully prestressed where two of them will be grouted

with cement grout to make it bonded . Deflection and crack width

will be measured at various load levels . Results will be

compared with the predicted values . Some conclusions will be

drawn, as to what extent the reduction in weight is feasible so

far as the increase of deflection and cracking is concerned .

CHAPTER 2

LITERATURE REVIEW

Parr and Maggard (1972) mentions that a survey of

b r i d ges b u i lt or proposed in the last few years in the United

States revea ls a growing awareness of at l east two items :-

i) The uti l ization of mate r i a ls and cross-section wh ich

may be more e f f icient and econom ica l than those used

in the past.

i i ) a mor e ser ious consi deration of aesthetic

requi rements.

For that reason the use of thi n webbed or hol low

structur a l secti ons has increased si g n i f i cantly throughout the

l ast d ecade .

Selection of the Best Shapes for Prestressed Concrete Under

Flexure

The simplest form of shape is rectangu l a r and is the most

economical as far as formwork is concerned . Lin and Burns

(1982) expla ins that rectangu lar section has sma l l kern

dist ance and the a va i l ab l e l ever arm for s t eel is l i mi t ed.

Rectangu l a r section is not as e fficient in the use of concrete

as nonr ectangular section such as the I-shaped sect i on . Hence

7

8

other shapes whi ch are frequently used for prestressed

concrete are :

i ) The symmetrical and unsymmetrical

I-section

i i ) The T-sect ion

i i i ) The inverted T-secti on

i v ) Box secti on.

Lin and Burns ( 1982) added that the suitability o f the

above shapes depend on certain requ irements . The I-sec t ion has

its concrete conc entrated near the extreme fibre so that it can

most ef fectively furnish the c ompression force. The more the

concrete is concentrated near the extreme f i bre , the greater

will be the lever arm f urnished for the internal resisting

couple . If the ratio of moment d ue to self wei ght to total

moment M 1M is suf fi c iently large , there is li ttle danger o f G T

o verstressing the flanges at transfer , and concrete in the

bottom flange can be accordingly d iminished . It may not be

economi c ally used, however where M 1M rat io is small , because G T

the center o f pressure at transfer may l i e below the bottom

kern po int . Then tensile stress may result in the top fl ange

and h i gh compressive stresses in the bottom flange.

The box secti on has th e same properti es as the I-section

in resisting moment . For ec onomy in steel and concrete it is

best to put the c oncrete near the extreme f i bres of the