Development and Structural Investigation of Monocoque Fibre Composite … · 2010. 6. 9. · 3.3.2...

Development and Structural Investigation of Monocoque Fibre

Composite Trusses

By

Matthew Humphreys BEng Civil (Hons), MIEAust, CPEng, RPEQ

A thesis submitted to the School of Civil Engineering Queensland University of Technology

in partial fulfilment of the requirements for the degree of

Doctor of Philosophy

December 2003

Development and Structural Investigation of Monocoque Fibre Composite Trusses iii

Abstract

Fibre composite materials are gaining recognition in civil engineering applications as

a viable alternative to traditional materials. Their migration from customary

automotive, marine, aerospace and military industries into civil engineering has

continued to gain momentum over the last three decades as new civil engineering

applications develop. The use of fibre composite materials in civil engineering has

now evolved from non-structural applications, such as handrails and cladding, into

primary structural applications such as building frames, bridge decks and concrete

reinforcement. However, there are issues which are slowing the use of fibre

composite materials into civil engineering. Issues include high costs, difficulties in

realising potential benefits, general lack of civil engineers’ familiarity with the

material and relatively little standardisation in the composites industry. For

composites to truly offer a viable alternative to traditional construction materials in

the civil engineering marketplace, it is essential that these issues be addressed. It is

proposed that this situation could be improved by demonstrating that potential

benefits offered by composites can be achieved with familiar civil engineering forms.

These forms must be well suited to fibre composite materials and be able to produce

safe and predictable civil engineering structures with existing structural engineering

methods.

Of the numerous structural forms currently being investigated for civil engineering

applications, the truss form appears particularly well suited to fibre composites. The

truss is a familiar structural engineering form which possesses certain characteristics

that make it well suited to fibre composite materials. In this research a novel

monocoque fibre composite truss concept was developed into a working structure

and investigated using analytical and experimental methods. To the best of the

author’s knowledge the research presented in this thesis represents the first doctoral

research into a structure of this type. This thesis therefore presents the details of the

development of the monocoque fibre composite (MFC) truss concept into a working

structure.

Development and Structural Investigation of Monocoque Fibre Composite Trusses iv

The developed MFC truss was used as the basis for a detailed investigation of the

structural behaviour of the MFC truss elements and the truss as a whole. The static

structural behaviour of the principal MFC truss elements (tension members,

compression members and joints) was investigated experimentally and analytically.

Physical testing required the design and fabrication of a number of novel test rigs.

Well established engineering principles were used along with complex finite element

models to predict the behaviour of the tested truss elements and trusses. Results of

the theoretical analysis were compared with experimental results to determine how

accurately their static structural behaviour could be predicted.

It was found that the static structural behaviour of all three principal truss elements

could be accurately predicted with existing engineering methods and finite element

analysis. The knowledge gained from the investigation of the principal truss elements

was then used in an investigation of the structural behaviour of the MFC truss. Three

full-scale MFC trusses were fabricated in the form of conventional Pratt, Howe and

Warren trusses and tested to destruction. The investigation included detailed finite

element modelling of the full-scale trusses and the results were compared to the full-

scale test results. Results of the investigation demonstrated that the familiar Pratt,

Howe and Warren truss forms could be successfully manufactured with locally

available fibre composite materials and existing manufacturing technology. The

static structural behaviour of these fibre composite truss forms was accurately

predicted with well established engineering principles and finite element analysis.

A successful marriage between fibre composite materials and a civil engineering

structure has been achieved. Monocoque fibre composite trusses have been

developed in the familiar Pratt, Howe and Warren truss forms. These structures

possess characteristics that make them well suited to applications as primary load

bearing structures.

KEYWORDS: civil engineering, construction, structural engineering, finite element

analysis, fibre composites, truss structures, Pratt truss, Howe truss, Warren truss,

glass fibre, carbon fibre, epoxy resin, particulate filled resin, fibre composites in civil

engineering.

Development and Structural Investigation of Monocoque Fibre Composite Trusses v

Publications

Humphreys, M.F., van Erp, G.M., and Tranberg, C.T. (1999), “Monocoque Fibre

Composite Truss Joints”, ACUN 1 – Proceedings of the First International

Composites Meeting, University of New South Wales, Australia, pp 247 - 251.

Humphreys, M.F., van Erp, G.M., and Tranberg, C.T. (1999), “An Investigation into

the Structural Behaviour of Monocoque Fibre Composite Truss Joints”, Proceedings

of ICCM12 – International Conference on Composite Materials, International

Committee on Composite Materials, Paris, France, Paper 274.

Humphreys, M.F., van Erp, G.M., and Tranberg, C.T. (1999), “Structural Behaviour

of Monocoque Fibre Composite Trusses”, Mechanics of Structures and Materials,

Edited by Bradford M. A., Bridge, R. Q., Foster, S. J., Balkema, Rotterdam, The

Netherlands, pp 501 - 506.

Humphreys, M.F., van Erp, G.M., and Tranberg, C.T. (1999), “The Structural

Behaviour of Monocoque Fibre Composite Truss Joints”, Advanced Composite

Letters, Vol 8 No. 4, Adcotec, London, UK, pp 173 - 180.

Humphreys, M.F., (2003)”, “Extending the Service Life of Buildings and

Infrastructure With Fibre Composites”, PRRES9 - Proceedings of the Ninth Pacific

Rim Real Estate Society Conference, Brisbane, Australia.

Humphreys, M.F., (2003), “The Use of Polymer Composite in Construction”,

SASBE2003 – Proceedings of the Smart and Sustainable Built Environment

conference, Brisbane, Australia, pp 585 - 593.

Development and Structural Investigation of Monocoque Fibre Composite Trusses vi

Contents

Statement of original authorship i

Acknowledgements ii

Abstract iii

Publications v

Contents vi

List of Figures xii

List of Tables xviii

Notation xxi

Chapter 1 – Introduction

1.1 Background 1.2 Aims 1.3 Scope 1.4 Thesis structure

1 - 1

1 - 2

1 - 6

1 - 7

1 - 8

Chapter 2 – Fibre Composite in Civil Engineering

2.1 Introduction 2.2 Fibre composite materials in construction and civil

engineering

2.2.1 Rehabilitation and retrofit 2.2.2 Concrete structures reinforced with fibre

composites

2.2.3 New fibre composite civil structures 2.3 Issues affecting the use of fibre composites in civil

engineering applications

2.3.1 Cost 2.3.2 Structural performance 2.3.3 Durability 2.3.4 Familiarity and education 2.3.5 Specification and standardisation 2.3.6 Compatibility 2.3.7 Temperature and fire performance

2 - 1

2 - 1

2 - 2

2 - 3

2 - 4

2 - 7

2 - 8

2 - 9

2 - 15

2 - 24

2 - 29

2 - 31

2 - 32

2 - 33

Development and Structural Investigation of Monocoque Fibre Composite Trusses vii

2.4 The need for a new approach 2.4.1 Lessons from history 2.4.2 Current approach

2.5 Summary

Chapter 3 –Monocoque Fibre Composite Trusses

3.1 Trusses 3.1.1 Truss definition 3.1.2 Brief history of trusses 3.1.3 Characteristics of trusses suited to fibre

composite materials

2 - 36

2 - 36

2 - 38

2 - 40

3 - 1

3 - 2

3 - 2

3 - 3

3 - 6

3.2 FRP trusses 3.2.1 Concrete filled CFRP tubes 3.2.2 Experimental transmission towers with serrated

joints

3.2.3 CFRP roof truss 3.2.4 Expandable space trusses 3.2.5 Pultruded section pedestrian bridge 3.2.6 Areas for potential improvement of existing

approaches to FRP trusses

3 - 8

3 - 8

3 - 9

3 - 11

3 - 11

3 - 12

3 - 13

3.3 The monocoque fibre composite (MFC) truss 3.3.1 Configuration 3.3.2 Form 3.3.3 Materials 3.3.4 Fabrication 3.3.5 Static load response

3.4 Adopted configuration of the MFC truss

Chapter 4 – Static Structural Behaviour of MFC Truss Tension

Elements

4.1 Introduction 4.2 Preliminary investigation of tension elements

4.2.1 Material properties

3 - 15

3 - 17

3 - 31

3 - 38

3 - 51

3 - 56

3 - 63

4 - 1

4 - 1

4 - 3

4 - 4

Development and Structural Investigation of Monocoque Fibre Composite Trusses viii

4.2.2 Specimen geometry 4.2.3 Prediction of static structural behaviour 4.2.4 Fabrication of test specimens 4.2.5 Testing 4.2.6 Results 4.2.7 Discussion 4.2.8 Summary

4.3 Detailed investigation of tension elements 4.3.1 Proposed approach 4.3.2 Specimen design 4.3.3 Prediction of stiffness and strength 4.3.4 Fabrication 4.3.5 Test setup 4.3.6 Results 4.3.7 Comparison of prediction with test results 4.3.8 Discussion

4.4 Conclusions

Chapter 5 – Static Structural Behaviour of MFC Truss Compression

Elements

5.1 Preliminary investigation of compression elements 5.1.1 Specimen details 5.1.2 Analysis and prediction of behaviour 5.1.3 Testing 5.1.4 Results and discussion

5.2 Detailed investigation of compression elements 5.2.1 Member design 5.2.2 Analysis 5.2.3 Fabrication 5.2.4 Test setup 5.2.5 Results 5.2.6 Discussion

5.3 Summary and conclusions

4 - 4

4 - 5

4 - 7

4 - 8

4 - 9

4 - 11

4 - 16

4 - 17

4 - 17

4 - 18

4 - 20

4 - 35

4 - 36

4 - 37

4 - 41

4 - 45

4 - 50

5 - 1

5 - 1

5 - 2

5 - 3

5 - 7

5 - 8

5 - 11

5 - 12

5 - 16

5 - 22

5 - 26

5 - 29

5 - 34

5 - 39

Development and Structural Investigation of Monocoque Fibre Composite Trusses ix

Chapter 6 – Monocoque Fibre Composite Truss Joints

6.1 Joint strength 6.1.1 Calculating joint strength 6.1.2 Preliminary experimental investigation 6.1.3 Joint testing

6.2 Joint rigidity 6.2.1 Secondary stresses 6.2.2 Truss deflection

6.3 Simplified joint analysis 6.4 Conclusions

6 - 1

6 - 1

6 - 2

6 - 12

6 - 20

6 - 33

6 - 34

6 - 39

6 - 42

6 - 43

Chapter 7 – Static Structural Behaviour of MFC Trusses

7.1 Design of truss specimens 7.2 Analysis

7.2.1 Approach 7.2.2 Elements and mesh 7.2.3 Material properties 7.2.4 Loading and restraints 7.2.5 Results of finite element analysis 7.2.6 Discussion of finite element analysis

7.3 Fabrication of truss specimens 7.4 Testing of MFC trusses

7.4.1 Test description 7.4.2 Test results

7.5 Comparison of predictions with test results 7.5.1 Stiffness 7.5.2 Strength

7.6 Summary

7 - 1

7 - 1

7 – 6

7 - 6

7 - 7

7 - 9

7 - 29

7 - 30

7 - 36

7 - 37

7 - 44

7 - 44

7 - 46

7 - 54

7 - 54

7 - 56

7 - 57

Chapter 8 – Conclusions and Recommendations

8.1 Conclusions 8.1.1 Development of truss concept into working

structure

8 - 1

8 - 1

8 - 2

Development and Structural Investigation of Monocoque Fibre Composite Trusses x

8.1.2 Investigation of structural behaviour of principal truss elements and demonstration of accuracy

with which structural behaviour can be predicted

8.1.3 Design and construction of prototype Pratt, Howe and Warren trusses

8.1.4 Evaluation of the structural performance of the three prototype trusses to characterise their

behaviour in terms of stiffness, strength, failure

mode, predictability and warning of failure

8.2 Recommendations for future research

8 - 4

8 - 9

8 - 9

8 - 10

Appendix A – Material Properties

A - 1

Appendix B – Tension Specimen Graphs

B - 1

Appendix C – Compression Specimen Graphs

C - 1

Appendix D – T-joint Graphs

D - 1

References

R - 1

Development and Structural Investigation of Monocoque Fibre Composite Trusses xi

List of Figures

Figure 1.1 – Applications of fibre composites in civil engineering

structures

1 - 4

Figure 1.2 – Examples of fibre composite trusses 2 - 5

Figure 2.1 – Simply supported beam with UDL 2 - 18

Figure 2.2 –Behaviour of hybrid FRP 2 - 22

Figure 2.3 – a) Veirendeel load path b) “Long” truss form 2 - 24

Figure 3.1 – Truss terminology 3 - 2

Figure 3.2 – Early truss structures 3 - 3

Figure 3.3 – Nail-plate 3 - 6

Figure 3.4 – Reinforced concrete jointed CFRP tube bridge (Karbhari,

1998)

3 - 9

Figure 3.5 – Serrated joints (Goldsworthy, 1998) 3 - 10

Figure 3.6 – CFRP roof truss (Agematzu, 1998) 3 - 11

Figure 3.7 – Telescopic space truss (NASA, 2002) 3 - 12

Figure 3.8 – Pultruded section pedestrian bridge (Milcovich, 2002) 3 - 13

Figure 3.9 – Conceptual configuration of monocoque truss 3 - 17

Figure 3.10 – Truss joint showing web member lapped onto bottom chord

at panel point

3 - 17

Figure 3.11 – Lapping fibres from adjacent members 3 - 18

Figure 3.12 – Pratt truss fibre architecture with arbitrary fill layers 3 - 19

Figure 3.13 – Pratt truss fibre architecture with aligned fill layers 3 - 20

Figure 3.14 – Joint layup types 3 - 22

Figure 3.15 – Type 1 fibre architecture 3 - 23

Figure 3.16 – Type 2 fibre architecture 3 - 24

Figure 3.17 – Joint type 3 fibre architecture 3 - 25

Figure 3.18 – Typical MFC truss cross-sections 3 - 27

Figure 3.19 – Frame analysis truss configurations 3 - 33

Figure 3.20 – Typical frame analysis truss 3 - 34

Figure 3.21 – Typical displaced shape 3 - 35

Figure 3.22 – Typical combined stress 3 - 36

Figure 3.23 –Buckled shape of polystyrene foam core MFC truss from 3 - 44

Development and Structural Investigation of Monocoque Fibre Composite Trusses xii

FEA

Figure 3.24 – Truss test configuration 3 - 44

Figure 3.25 – Foam and balsa truss core configuration 3 - 45

Figure 3.26 – Foam truss failure mode 3 - 45

Figure 3.27 – Load-displacement graph for polystyrene foam truss member 3 - 46

Figure 3.28 – Initial balsa investigation 3 - 47

Figure 3.29 – Initial balsa investigation 3 - 48

Figure 3.30 – PFR 3 - 50

Figure 3.31 – Use of pre-fabricated sandwich panel to produce MFC truss 3 - 53

Figure 3.32 – Fabrication of PFR core 3 - 55

Figure 4.1 – Typical MFC truss member 4 - 2

Figure 4.2 – Representative tension element 4 - 5

Figure 4.3 – Bi-linear load versus strain curve 4 - 7

Figure 4.4 – Representative tension element specimens 4 - 8

Figure 4.5 – Typical test configuration 4 - 9

Figure 4.6 – Typical load - displacement curves 4 - 9

Figure 4.7 – Type 1 specimen failure modes 4 - 12

Figure 4.8 – Transverse cracks in PFR specimens 4 - 14

Figure 4.9 – Transverse cracks in PFR specimens 4 - 15

Figure 4.10 – Tension specimen geometry and fibre architecture 4 - 19

Figure 4.11 – Two-dimensional FE model 4 - 24

Figure 4.12 – Principal stress distribution at maximum load 4 - 26

Figure 4.13 – Deformed FE model of RVE 4 - 27

Figure 4.14 – Two-stage prediction of load vs strain behaviour (PFR core) 4 - 27

Figure 4.15 – Incremental prediction of load vs strain behaviour (PFR core) 4 - 29

Figure 4.16 – Predicted load vs strain curve for plaster core specimens 4 - 30

Figure 4.17 – Predicted load vs strain curve for foam core specimens 4 - 31

Figure 4.18 – Predicted load vs strain curve for neat resin core specimens 4 - 32

Figure 4.19 – σ11 stress distribution of RVE 4 - 34

Figure 4.20 – Typical tension specimens 4 - 36

Figure 4.21 –Test set-up 4 - 37

Figure 4.22 – Typical tension specimen failure modes 4 - 37

Figure 4.23 – Predicted and Experimental load vs strain graphs for tension 4 - 43

Development and Structural Investigation of Monocoque Fibre Composite Trusses xiii

elements

Figure 4.24 – SEM of typical transverse crack 4 - 45

Figure 5.1 – Location of cut line 5 - 2

Figure 5.2 – Typical preliminary test specimens 5 - 3

Figure 5.3 – Typical MFC truss compression member section geometry 5 - 5

Figure 5.4 - Test set-up 5 - 8

Figure 5.5 – Typical compression load vs axial shortening curves 5 - 8

Figure 5.6 – Typical failure modes 5 - 9

Figure 5.7 – Compression member failure zones 5 - 12

Figure 5.8 – Proportions of stocky specimens with a non-compact cross

section

5 - 14

Figure 5.9 – Stocky / non-compact member section 5 - 15

Figure 5.10 – Typical slender member cross section 5 - 16

Figure 5.11 – First critical buckling mode (λ = 77.9, applied load = 4 kN) 5 - 19

Figure 5.12 – Slender member (symmetric about x-x neutral axis) cross

section

5 - 21

Figure 5.13 – Locations of extracted stocky compression specimens with

compact cross section

5 - 22

Figure 5.14 – 5 mm thick PFR core 5 - 23

Figure 5.15 – Stocky specimen with non-compact cross section 5 - 24

Figure 5.16 – Surface profile measurement of stocky specimen with a non-


5 - 24

Figure 5.17 – Production of slender specimen cores 5 - 25

Figure 5.18 – Out-of-straightness measurement of slender specimens 5 - 26

Figure 5.19 – Finished slender specimens 5 - 26

Figure 5.20 – Test rig for stocky member with a compact cross section 5 - 27

Figure 5.21 – Test setup for stocky member with non-compact cross

section

5 - 28

Figure 5.22 – Test setup for slender specimens 5 - 28

Figure 5.23 – Typical load vs strain graph for Pratt and Warren specimens 5 - 30

Figure 5.24 – Typical Pratt specimen failure modes 5 - 31

Figure 5.25 – Typical Warren specimen failure modes 5 - 31

Development and Structural Investigation of Monocoque Fibre Composite Trusses xiv

Figure 5.26 – Strain gauge and dial gauge readings 5 - 32

Figure 5.27 – Typical slender specimen load vs strain measurements 5 - 33

Figure 5.28 – Typical flexural test specimen 5 - 37

Figure 5.29 – Revised local buckling model (applied load = 4000 N, λ =

161.

5 - 38

Figure 6. 1 – Sensitivity of lap strength to lap length 6 - 5

Figure 6. 2 – Failure loads of lap specimens 6 - 6

Figure 6.3 – Simplified Goland and Reissner model 6 - 6

Figure 6.4 – Stress distribution of Goland and Reissner model 6 - 7

Figure 6.5 – Stress distribution of Goland and Reissner model 6 - 8

Figure 6.6 – Section through truss joint 6 - 9

Figure 6.7 – Section A-A 6 - 9

Figure 6.8 – FE model of joint section 6 - 10

Figure 6.9 – Stress distribution along bond plane of model 6 - 11

Figure 6.10 – Modified lap-shear specimen 6 - 13

Figure 6.11 – Modified lap-shear test configuration 6 - 13

Figure 6.12 – Average tensile failure stress of modified lap specimens 6 - 14

Figure 6.13 – Test specimen configuration 6 - 15

Figure 6.14 – “T” joint specimens 6 - 16

Figure 6.15 – Test fixtures 6 - 17

Figure 6.16 – “T” joint test set-up 6 - 17

Figure 6.17 – Typical failure mode of Type 1 (lap joints) 6 - 18

Figure 6.18 – Typical load vs displacement curve for Type 1 (lap joints) 6 - 18

Figure 6.19 – Typical failure mode of Type 2 (loop joints) 6 - 19

Figure 6.20 – Typical load vs displacement curve for Type 2 (loop joints) 6 - 19

Figure 6.21 – Extracted Pratt joint 6 - 21

Figure 6.22 – Joint geometry (solid areas hatched) 6 - 21

Figure 6.23 – Full casting mould 6 - 23

Figure 6.24 – Finished PFR core 6 - 24

Figure 6.25 – Finished Type 1 joint 6 - 24


Figure 6.27 – Actual PFR core section 6 - 25

Development and Structural Investigation of Monocoque Fibre Composite Trusses xv


Figure 6.29 – Schematic test rig 6 - 27

Figure 6.30 – Test set-up 6 - 27

Figure 6.31 – Typical failure mode of Type 1 joint 6 - 28

Figure 6.32 – Typical failure mode of Type 2 joint 6 - 29

Figure 6.33 – Typical failure mode of Balsa core 6 - 29

Figure 6.34 – Failed PFR core 6 – 30

Figure 6.35 - First noise location 6 – 32

Figure 6.36 – Finite element mesh, loading and restraints 6 – 34

Figure 6.37 – Finite element model section geometry and material

properties

6 - 35

Figure 6.38 – Bending moment distribution in simply supported, four-

panel, rigid jointed Pratt truss

6 - 36

Figure 6.39 – Member stress distribution with secondary stresses 6 - 37

Figure 6.40 – Effect of stress reversal on member strain in primary tensile

member

6 - 38

Figure 6.41 – Relaxation of primary stress 6 - 39

Figure 6.42 – Deflection of fixed-end beam 6 - 40

Figure 6.43 – Deflection of truss panel 6 - 40

Figure 6.44 – Four-panel MFC Pratt truss frame model with UDL 6 - 41

Figure 6.45 – Truss deflection 6 - 41

Figure 6.46 – Typical 2D Plate element Pratt joint model 6 - 42

Figure 7.1 – Truss geometries and dimensions 7 - 2

Figure 7.2 – Typical joint details 7 - 3

Figure 7.3 – Nominal truss core cross section dimensions 7 - 3

Figure 7.4 – Void extents 7 - 4

Figure 7.5 – Fibre architecture 7 - 5

Figure 7.6 – Typical truss member cross section 7 - 5

Figure 7.7 – Strain measurement locations 7 - 7

Figure 7.8 – Strand 7 plate element types used in finite element analysis 7 - 8

Figure 7.9 – Final finite element meshes for Pratt, Howe and Warren

trusses

7 - 8

Figure 7.10 – Number of layers per face in MFC truss tension members 7 - 11

Development and Structural Investigation of Monocoque Fibre Composite Trusses xvi

Figure 7.11 – Typical MFC truss member RVE 7 - 12

Figure 7.12 – Symmetric FEA model of RVE 7 - 12

Figure 7.13 – Deformed RVE, εeq = 6.3E-5mm/mm 7 - 13

Figure 7.14 – Typical material properties for tension members (Pratt truss

shown)

7 - 16

Figure 7.15 – Number of layers per face in MFC truss compression

members

7 - 18

Figure 7.16 – Typical material properties for compression members (Pratt

truss shown)

7 - 19

Figure 7.17 – Material properties and orientations in a typical Pratt truss

joint

7 - 21

Figure 7.18 – Typical detailed and simplified truss joints 7 - 23

Figure 7.19 – Typical detailed and simplified truss joints 7 - 24

Figure 7.20 - ε11 Max for different joint forms 7 - 25

Figure 7.21 – Typical displacement points (Pratt truss shown) 7 - 27

Figure 7.22 – Typical isotropic material properties used in truss joints 7 - 29

Figure 7.23 – Typical external loading and constraints (Pratt truss shown) 7 - 30

Figure 7.24 – Truss midspan top chord deflections 7 - 31

Figure 7.25 – Pratt truss FE model – fibre direction strain distribution 7 - 33

Figure 7.26 – Pratt truss local compressive strain concentration 7 - 34

Figure 7.27 – Howe truss FE model – fibre direction strain distribution 7 - 34

Figure 7.28 – Howe truss local tensile strain concentration 7 - 35

Figure 7.29 – Warren truss FE model – fibre direction strain distribution 7 - 35

Figure 7.30 – Warren truss local tensile strain concentration 7 - 36

Figure 7.31 – Typical mould configuration 7 - 38

Figure 7.32 – Typical void former restraint 7 - 39

Figure 7.33 – Typical finished MFC trusses 7 - 40

Figure 7.34 – Typical MFC member cross section 7 - 42

Figure 7.35 – Typical MFC truss joints 7 - 42

Figure 7.36 – Peel ply at strain gauge locations 7 - 45

Figure 7.37 – Typical strain gauge locations 7 - 45

Figure 7.38 – Typical test setup (Pratt truss shown) 7 - 46

Figure 7.39 – MFC truss load versus displacement graphs 7 - 47

Development and Structural Investigation of Monocoque Fibre Composite Trusses xvii

Figure 7.40 – Strain versus load curves for Pratt, Howe and Warren trusses 7 - 49

Figure 7.41 – Failure zone of Pratt truss 7 - 51

Figure 7.42 – Location of first noise in Pratt truss 7 - 51

Figure 7.43 – Failure zone of Howe truss 7 - 52

Figure 7.44 – Location of first noise in Howe truss 7 - 52

Figure 7.45 – Failure zone of Warren truss 7 - 53

Figure 7.46 – Location of first noise in Warren truss 7 - 53

Development and Structural Investigation of Monocoque Fibre Composite Trusses xviii

List of Tables Table 2.1 – Material cost comparison between traditional materials and

fibre composites

2 - 10

Table 2.2 – Indicative Strength/dollar ratio for different materials 2 - 11

Table 2.3 – Comparison of specific strength and specific elastic modulus

between traditional materials and fibre composite materials

2 - 16

Table 2.4 – Issues of emerging materials 2 - 36

Table 3.1 – Common characteristics of mechanical and adhesive joints 3 - 14

Table 3.2 – Member length and number of joints 3 - 34

Table 3.3 – Truss midspan deflection 3 - 35

Table 3.4 –Maximum combined stress in each truss 3 - 36

Table 3.5 – Percent increase in stresses due to joint rotation in each truss 3 - 37

Table 3.6 – Mechanical properties of ADR246 / West Systems 105 3 - 40

Table 3.7 – Mechanical properties of LB100 polystyrene foam typical 3 - 43

Table 3.8 – Mechanical properties of end-grain balsa (Diab, 2002) 3 - 47

Table 3.9 – PFR (44% SL150 filler by vol) typical mechanical properties 3 - 49

Table 3.10 – Summary of materials adopted 3 - 51

Table 3.11 – Contribution to member compressive stiffness by core

materials

3 - 57

Table 4.1 – Material properties for preliminary tension element tests 4 - 4

Table 4.2 – Tension test results 4 - 10

Table 4.3 – Additional tension test observations for Type 1 specimens 4 - 11

Table 4.4 – Core and hardpoint materials 4 - 19

Table 4.5 – Four core / laminated face systems 4 - 20

Table 4.6 – Material properties for prediction of PFR tension member pre-

cracking behaviour

4 – 23

Table 4.7 – Material properties of RVE used in FEA 4 - 25

Table 4.8 – Incremental equivalent elastic modulus 4 – 28

Table 4.9 – Material properties for prediction of plaster core tension

member behaviour

4 - 30

Table 4.10 – Material properties for foam core tension member behaviour 4 - 31

Table 4.11 – Material properties for neat resin core tension member 4 - 32

Development and Structural Investigation of Monocoque Fibre Composite Trusses xix

behaviour

Table 4.12a – PFR core specimen results 4 - 39

Table 4.12b – Additional PFR core specimen results 4 - 40

Table 4.13 – Plaster core specimen results 4 – 40

Table 4.14 – Foam core specimen results 4 - 41

Table 4.15 – Neat resin core specimen results 4 - 41

Table 4.16(a) – Comparison of predicted results with experimental results 4 - 42

Table 4.16 (b) – Comparison of predicted results with observed continued 4 - 42

Table 4.17 – Difference in average ultimate properties of PFR and plaster /

foam core specimens

4 - 49

Table 5.1 – Mechanical properties of 720gsm UD Colan E-glass and BASS

Pacific 420gsm heatset UD E-glass

5 - 5

Table 5.2 – Adopted material properties 5 - 6

Table 5.3 – Predicted capacity of compression specimens 5 - 7

Table 5.4 – Comparison of test results with predictions 5 - 10

Table 5.5 – Slenderness ratio of test specimens 5 - 13

Table 5.6 – Calculation of predicted failure load for specimen W1 5 - 17

Table 5.7 – Specimen predicted failure loads 5 - 17

Table 5.8 – Material properties used in FEA of stocky member with non-


5 - 18

Table 5.9 – Test results for stocky specimens with a compact cross section 5 - 29

Table 5.10 – Slender specimen results summary 5 - 33

Table 5.11 – Comparison of predicted and test values 5 - 34

Table 5.12 – Flexural elastic modulus 5 - 37

Table 6.1 – Estimated joint and member strengths 6 - 3

Table 6.2 – FE model properties (refer Figure 6.7) 6 - 11

Table 6.3 – Stress / capacity ratio 6 - 12

Table 6.4 – Lap lengths 6 - 13

Table 6.5 – Failure loads of Type 1 (lap joints) 6 - 18

Table 6.6 – Failure loads of Type 2 (loop joint) 6 - 19

Table 6.7 – Test joint materials 6 - 22

Table 6.8 – Joint fibre architecture 6 - 22

Table 6.9 – Joint ultimate capacity 6 - 28

Development and Structural Investigation of Monocoque Fibre Composite Trusses xx

Table 7.1 – RVE FEA model properties 7 - 13

Table 7.2 – Gross tension member tensile elastic moduli 7 - 14

Table 7.3 – Apparent tension member core tensile elastic moduli 7 - 15

Table 7.4 – RVE FEA model properties 7 - 18

Table 7.5 – Comparison of actual joint strains with simplified joint strains 7 - 26

Table 7.6 – Comparison of IP deflection 7 - 28

Table 7.7 –Truss deflections and strain gauge readings at 100kN load 7 - 32

Table 7.8 – Truss stiffness results from FE models 7 - 32

Table 7.9– Derived load carrying capacity 7 - 36

Table 7.10 – MFC truss weight, first noise load and crack spacing 7 - 48

Table 7.11 – Truss deflections at intermediate loads and ultimate load,

stiffness at failure and span-to-deflection ratio at failure

7 - 48

Table 7.12 – Summary of member strains at ultimate load 7 - 50

Table 7.13 – Truss stiffness comparison 7 - 54

Table 7.14 – Comparison of deflection and strain for Pratt Truss1 7 - 54

Table 7.15 – Comparison of deflection and strain for Howe Truss1 7 - 55

Table 7.16 – Comparison of deflection and strain for Warren Truss1 7 - 55

Table 7.17 – Strength / failure mode comparison summary (Pratt) 7 - 56

Table 7.18 – Strength / failure mode comparison summary (Howe) 7 - 56

Table 7.19 – Strength / failure mode comparison summary (Warren) 7 - 57

Development and Structural Investigation of Monocoque Fibre Composite Trusses xxi

Notation

Abbreviations

AFRP = aramid fibre reinforced plastic (polymers)

CERF = Civil Engineering Research Fund

CFRP = carbon fibre reinforced plastic (polymers)

COV = coefficient of variation

CTE = coefficient of thermal expansion

DB = double bias fibres

FCDD = Fibre Composites Design and Development

FEA = finite element analysis

FRP = fibre reinforced plastic (polymers)

GFRP = glass fibre reinforced plastic (polymers)

gsm = grams per square metre

HFS = high failure strain

HSHF = high strength / high failure strain

HSLF = high strength / low failure strain

LFS = low failure strain

LSHF = low strength / high failure strain

LSLF = low strength / low failure strain

MFC = monocoque fibre composite

NASA = National Aeronautics and Space Administration

PFR = particulate filled resin

QUT = Queensland University of Technology

RVE = representative volume element

UD = unidirectional fibres

UDL = uniformly distributed load

USQ = University of Southern Queensland

Development and Structural Investigation of Monocoque Fibre Composite Trusses xxii

Symbols

k = stiffness

σ = stress

ε = strain

δ = deflection

φ = ultimate limit state capacity factor

ν = Poisson’s ratio

σ11.c.ult = ultimate compressive stress in the fibre direction

ε11.c.ult = ultimate compressive strain in the fibre direction

ε11.t.ult = ultimate tensile strain in the fibre direction

σ11.t.ult = ultimate tensile stress in the fibre direction

ι12 = shear stress

σ22.c.ult = ultimate compressive stress perpendicular to the fibre direction

ε22.c.ult = ultimate compressive strain perpendicular to the fibre direction

σ22.t.ult = ultimate tensile stress perpendicular to the fibre direction

ε22.t.ult = ultimate tensile strain perpendicular to the fibre direction

εfirst noise = strain at which first noise occurs

σu = ultimate limit state stress

A = area

Acore = area of core

Afaces = area of faces

Ag = gross area

b = cross sectional width

d = cross sectional depth

E = elastic modulus

Ixx = second moment of area about the X-X axis

E11.c = compressive elastic modulus in the fibre direction

E11.t = tensile elastic modulus in the fibre direction

E11.t.core = tensile elastic modulus of core in the fibre direction

E11.t.eq = equivalent tensile elastic modulus in the fibre direction

E11.t.faces = tensile elastic modulus of faces (nominally 20 GPa for E-glass)

Development and Structural Investigation of Monocoque Fibre Composite Trusses xxiii

E22.c = compressive elastic modulus perpendicular to the fibre direction

E22.t = tensile elastic modulus perpendicular to the fibre direction

EAF1 = elastic modulus of face material in area 1

EAF2 = elastic modulus of face material in area 2

EBP = elastic modulus of “BASS Pacific” laminate

Eeq = equivalent elastic modulus based on transformed cross section

Ef = final elastic modulus

Ei = initial elastic modulus

EL = elastic modulus in the member’s longitudinal direction

EPFR = elastic modulus of PFR

G12 = shear modulus

h = depth of truss (between panel points)

I = second moment of area

Imin = second moment of area about the minor principal axis

Ixx.AF1 = second moment of area of face area 1 about the X-X axis

Ixx.AF2 = second moment of area of face area 2 about the X-X axis

Ixx.BP

= second moment of area of “BASS Pacific” laminate about the X-X

axis

Ixx.PFR = second moment of area of PFR about the X-X axis

Iyy = second moment of area about the Y-Y axis

L = length

Lav = average debond length

Nc = nominal member capacity in compression

Ns = nominal section capacity of compression member

P = point load

PE = Euler critical buckling load

Sav = average crack spacing

T = tension force in truss member

t = thickness

w = uniformly distributed load

δ = deflection

ε = strain

ε11.c.ult = ultimate compressive strain in the fibre direction

Development and Structural Investigation of Monocoque Fibre Composite Trusses xxiv

ε11.t.ult = ultimate tensile strain in the fibre direction

ε22.c.ult = ultimate compressive strain perpendicular to the fibre direction

ε22.t.ult = ultimate tensile strain perpendicular to the fibre direction

εfirst noise = strain at which first noise occurs

φ = ultimate limit state capacity factor

ι12 = shear stress

ν = Poisson’s ratio

σ = stress

σ11.c.ult = ultimate compressive stress in the fibre direction

σ11.t.ult = ultimate tensile stress in the fibre direction

σ22.c.ult = ultimate compressive stress perpendicular to the fibre direction

σ22.t.ult = ultimate tensile stress perpendicular to the fibre direction

σu = ultimate limit state stress

Development and Structural Investigation of Monocoque Fibre Composite Trusses 1 - 1

Chapter 1 - Introduction

Fibre composite materials are gaining recognition in civil engineering applications as

a viable alternative to traditional materials. Their migration from customary

automotive, marine, aerospace and military industries into civil engineering has

continued to gain momentum over the last three decades as new civil engineering

applications develop. The use of fibre composite materials in civil engineering has

now evolved from non-structural applications, such as handrails and cladding, into

primary structural applications such as building frames, bridge decks and concrete

reinforcement.

In recent years, researchers around the world have been seeking to develop new and

innovative structural forms that can successfully exploit the benefits offered by these

materials (Karbhari & Zhao, 2000; Gowripalan, 1999; van Erp, 1999d; Hooks et. al.,

1997 and Creative Pultrusions, 2002a). One structural form that appears to have

significant potential is the truss. The framed nature of trusses can be used to

minimise the shortfalls of fibre composite materials and maximise their benefits. To

date, much of the development of fibre reinforced polymer (FRP) trusses has

focussed on connection of continuous profile members using mechanical joints or

secondary bonding (Creative Pultrusions, 2002b; NASA, 2002; Goldsworthy 1995;

Morsi & Larralde, 1994a and Strongwell, 2002). Mechanical joints can introduce

performance compromises into fibre composite trusses while the use of continuous

profile members tends to reduce the freedom of the designer to use materials

efficiently and effectively.

This dissertation presents an investigation into a novel fibre composite truss

proposed by The University of Southern Queensland Fibre Composite Design and

Development. The truss uses monocoque construction to eliminate the use of

mechanical fasteners or secondary bonding to connect members, whilst allowing the

designer freedom to locate material where it will perform most efficiently and have

the greatest effect. It will be shown that structures of this type can be developed with

predictable structural behaviour and a level of strength and stiffness suitable for civil

engineering applications.


1.1 Background

Composite materials combine and maintain two or more distinct phases to produce a

material that has properties far superior than either of the base materials. Fibre

composites are two-phase materials in which one phase reinforces the other. High

strength fibres are used as the primary means of carrying load and a polymer resin

binds the fibres into a cohesive structural unit. The combination of fibres and resin

produces a bulk material with strength and stiffness governed primarily by the fibres

and chemical resistance provided by the resin.

Evidence has been found to suggest that fibre composite materials have been used in

construction for thousands of years. Straw has been used to reinforce bricks for over

3000 years and this method is still used today. Chinese bridge builders used timber as

early as 1600 BC and Greek builders were apparently the first to reinforce masonry

with metal around 1000 BC. By comparison, the development of modern synthetic

fibre composites is relatively new, beginning in the early 20th century.

In the first half of the 1900’s the development of synthetic resins which could cure at

room temperature, combined with the serendipitous discovery of a method to

manufacture fine glass fibres, led to an increased use of synthetic fibre composites.

The unique properties offered by glass fibre reinforced composites made them

particularly desirable for marine, aerospace, military and automotive applications.

Material characteristics such as low weight, tailorable durability, good fatigue

resistance and manufacturing versatility were some of the characteristics initially

recognised as potential advantages over existing materials. Over the last sixty years

fibre composites have enjoyed widespread use in these industries and have evolved

significantly. Developments include production of advanced reinforcing fibres, such

as carbon and aramid, which offer improved strength and elastic modulus and better

impact performance over glass, and the evolution of modern polyester, vinylester and

epoxy resin formulations which offer improved performance over the original high

temperature curing phenolic resin in areas such as mechanical properties, chemical

resistance and bonding.


As these materials find new applications their significance to engineering fields with

which they were previously unfamiliar is becoming more pronounced. In the last

twenty years fibre composite materials have established themselves as a viable and

competitive option for rehabilitation and retrofit of existing civil structures. By

externally bonding fibres, pre-fabricated strips and jackets to deteriorated or obsolete

structures, strength and stiffness can be re-introduced or improved to increase service

life. Fibre composites are also used to replace steel as a reinforcing and stressing

material in concrete for some specialised applications. To a lesser extent new civil

structures have been created almost entirely from fibre composite materials by

joining standard structural sections or modular components to produce complete

structures.

Civil and structural engineering is seeing an increased push for the use of these

materials in mainstream structures. This push is being driven by both composites and

civil engineering groups. Civil engineers are driven primarily by the desire to realise

the potential performance benefits offered by these materials. Potential benefits

include high strength, low weight and environmental durability. For the civil /

structural engineer high strength and light weight may translate into reduced

construction times and costs through the use of products which can be brought to

near-complete state in an off-site factory and then easily transported to site for rapid

deployment. The advantages of a material with superior durability include potential

reductions in maintenance over the life of a structure and hence lower cost for the

owner.

The composites industry, on the other hand, is driven by a desire to participate in

what is arguably the world’s largest industry. The global civil engineering and

construction market turnover has been estimated at US$800 billion per annum

(Marsh, 2000). With the global composites market in the year 2000 being valued at

only around US$8 billion (Marsh, 2000), even a 1% stake in the construction market

would double its current size.

Regardless of the reasons, the international pressure to use composites in mainstream

civil engineering has been growing over the past decade. During this time a large

number of experimental and commercial structures have been constructed around the


world to demonstrate the potential of composites in major civil engineering

structures. Figure 1.1 provides examples of these structures.

While these applications help to demonstrate that fibre composite materials are

structurally capable, there are issues which are slowing the ingress of fibre composite

materials into the civil engineering industry. One such issue is that there is currently

a significant cost premium associated with their use.

b) Road Bridge - AUS (Source: FCDD, 2002)

a) Lattice Towers - USA

(Source: Strongwell, 2002)

c) Dome roof structures – Libya

(Source: Hollaway, 2002)

Figure 1.1 – Applications of fibre composites in civil engineering structures

Other issues exist such as difficulties in realising potential benefits, general lack of

civil engineers’ familiarity with the material and relatively little standardisation in

the composites industry. For composites to truly offer a viable alternative to

traditional construction materials in the civil engineering marketplace, it is essential

that these issues are addressed. It is proposed that this situation could be improved by

demonstrating that potential benefits offered by composites can be achieved with

hallaThis figure is not available online. Please consult the hardcopy thesis available from the QUT Library




familiar civil engineering forms that are well suited to fibre composite materials and

whose structural behaviour is consistent with safe civil engineering structures and

predictable with existing structural engineering analysis and design methods.

One structural form that would appear to offer significant potential for composites in

civil engineering is the truss. Trusses have been accepted as efficient structural

elements for centuries and offer a number of advantages over solid web members.

Typically they use much less material than solid web members and their framed

nature allows material to be located where it has the greatest effect. Given the

significantly higher costs of fibre composite laminates in comparison to traditional

structural materials such as steel and concrete, this low material usage may address

some of the cost disparities between traditional and composite structures. In addition

to this, the loading within truss elements is largely axial. It is thought that this would

enable the easy tailoring of reinforcement along the load paths, resulting in a high

level of efficiency in material usage.

The concept of a fibre composites truss is not entirely new. Several examples of this

type of structure have been constructed around the world using a variety of joint

configurations (see Figure 1.2). However, these trusses are very much a reflection of

traditional technology developed for timber or metal members. As a result they fall

short of fully exploiting the potential offered by composites.

a) FRP truss bridge with pultruded

members (Source: Berenberg, 1997)

b) Bolted connection of pultruded

members (Source: Berenberg, 1997)

Figure 1.2 – Examples of fibre composite trusses




c) Inserts used to connect FRP truss

members (Source: Hollaway, 2002)

d) Serrated FRP truss joint

(Source: Goldsworthy 1995)

Figure 1.2 – Examples of fibre composite trusses

In 1998 the USQ proposed a new type of fibre composite truss. Unlike previous

composite trusses, the new concept was based on monocoque design and avoided the

need for secondary joints. Initial investigations into this truss were extremely

promising and the concept was identified as one with significant development

potential. However, there is a need to fully understand the structural behaviour of this

new monocoque fibre composite (MFC) truss before it can be used confidently.

1.2 Aims

The primary aim of this thesis is to develop and improve the fundamental

understanding of the structural behaviour of monocoque fibre composite trusses and

to advance the civil engineering community’s knowledge in the use of fibre

composite materials in civil engineering structures.

In fulfilling the broad aim above, the study will:

develop the MFC truss concept into a working structure

investigate the structural behaviour of the principal truss elements (tension

and compression members and joints)




demonstrate that the structural behaviour of principal truss elements can be

predicted using well established structural engineering analysis and design

methods

design a prototype truss in Pratt, Howe and Warren configurations using

understanding gained from truss element and joint investigations

evaluate the structural performance of the three prototype trusses through

physical testing and computer analysis

characterise the behaviour of the MFC trusses in terms of stiffness, strength,

failure mode, predictability and warning of failure

conclude on the ability of the MFC trusses to provide a safe, predictable and

adequate structural solution

1.3 Scope

In order to maintain focus on the primary aims of this investigation, adhere to

budgetary constraints of this research project and work within limitations of available

fabrication and testing equipment, the following restrictions were imposed on the

scope of this project:

Truss tests were undertaken for a single load / support system with

defined restraint conditions and loading points

A single prototype of each full-scale MFC truss design was fabricated

and tested (in this thesis the term “full-scale” refers to trusses that are

the same size as the finite element model)

Structural behaviour investigations were limited to short-term,

pseudo-static loading at ambient temperature in a non-aggressive

environment

Long-term durability was not considered


1.4 Thesis structure

Chapter 2 – Fibre Composites In Civil Engineering: presents a more in-depth

discussion of the background to this project. It examines the history of composites

and the potential benefits that make them attractive to civil engineers. Three basic

areas of fibre composite use in civil engineering type applications are discussed,

followed by an examination of the issues that affect the use of fibre composite

materials in civil engineering.

Chapter 3 – Monocoque Fibre Composites Trusses: introduces the MFC truss

concept and the methodology used to develop the concept into a working structure.

The Chapter begins with a brief discussion of the history of trusses and some of the

characteristics that saw them become the structure of choice for bridge construction

during mid 1800’s. Focus is then shifted to existing FRP trusses, a selection of which

are critically examined to identify potential shortfalls of the current approaches. The

philosophy behind the MFC truss is then presented and compared with existing FRP

truss construction, in particular how the MFC truss has the potential to overcome

some of the shortfalls of existing FRP truss technology. Chapter 3 then examines and

evaluates a number of materials and structural form options for the MFC truss,

finally presenting the adopted MFC truss configuration that will form the basis of the

study.

Chapter 4 - Static Structural Behaviour of MFC Truss Tension Elements:

presents the results of an investigation into the behaviour of axial MFC truss tension

elements constructed using the prototype cross-section developed in Chapter 3. MFC

truss tension members with a variety of core materials are investigated

experimentally and analytically to determine their load response, failure mode,

predictability and warning of failure. An incremental method based on established

structural engineering principles is developed to predict the tension member

behaviour. These predictions are compared to results obtained from testing to

determine their accuracy.


Chapter 5 – Static Structural Behaviour of MFC Truss Compression Elements:

investigates the structural behaviour of typical MFC truss compression members.

Three classes of compression member are investigated, namely; stocky members

with a compact cross-section, stocky member with a non-compact cross section and

slender members. Established closed form solutions as well as finite element analysis

are used to predict their structural behaviour. These predictions are then compared to

results obtained through testing to determine the accuracy of the predictions.

Chapter 6 - Monocoque Fibre Composite Truss Joints: shifts the focus of the

investigation to the MFC truss joints. In this Chapter the work is primarily concerned

with the ability of the joint to provide adequate connection to members. Joint

strength is studied experimentally and analytically and the effect of the rigid nature

of the joint on connected members and truss deflection is investigated. Finally, the

suitability of a proposed simplified approach to joint analysis is discussed.

Chapter 7 - Static Structural Behaviour of MFC Trusses: applies the findings of

Chapters 4, 5 and 6 in the examination of a series of prototype truss designs. These

designs are analysed using finite element analysis techniques and predicted

behaviour is evaluated through fabrication and testing of several sample trusses.

Observations including strength, failure mode, stiffness and provision of warning of

imminent failure are made.

Chapter 8 – Conclusions and Recommendations: draws together key findings on

the structural behaviour of MFC trusses and methods for predicting such behaviour.

Finally, several recommendations for future work are identified and discussed.


Chapter 2 - Fibre Composites in Civil Engineering

Synthetic fibre composite materials have existed for over a century and have been

used widely in military, aerospace, sporting and automotive applications. In the last

three decades they have been increasingly considered for civil applications mainly in

retrofit of existing structures, reinforcement for concrete and to a lesser extent

complete fibre composite structures. Their use in civil engineering applications has

been the focus of worldwide research by both the composites industry and the civil

engineering industry. This research has highlighted a number of potential benefits as

well as some concerns with the use of this material in civil engineering applications.

To date the bona fide use of fibre composite materials as load bearing structural

elements in civil applications remains rare and it is unclear why this is the case. This

chapter examines key fibre composites issues in relation to civil engineering, draws

conclusions on their viability as civil engineering materials and recommends a way

forward.

2.1 Introduction

Modern fibre composites originated in the late 19th century when the first man-made

polymer, phenol-formaldehyde, was reinforced with linen fibre to make Bakelite. In

1936, DuPont patented the first room temperature curing resin, unsaturated polyester.

The first epoxy resin system was produced in 1938 and Ciba introduced the widely

recognised Araldite epoxy resin system in 1942. At the same time reinforcing fibres

were undergoing rapid development and in 1941 Owens-Corning began production

of the world’s first woven glass fabric.

The defence and marine industries were amongst the first to exploit some of the

potential advantages of reinforced polymer composites such as relatively high

strength-to-weight ratio, good durability and fatigue performance and radiowave

transparency. The automotive industry was able to capitalise on characteristics such

as efficient production techniques leading to inexpensive tooling, rapid turn-around

production and high quality surface finishes. The Cold War prompted significant


effort by the military in the development of fibre composite materials. This led to

improvements in composites processing technology and mechanical properties of

laminates. The reader is directed to Gibson (1994), Herakovich (1997), Swanson

(1997), Kaw (1997), Vinson & Chou (1995), Mallick (1997), Johnson (1994), Lubin

(1982), Ayers (2002) and Owens Corning (2002) for more detailed information.

The use of fibre composite materials in civil structures can be traced back to the

1960’s when glass reinforced plastic rods were used to reinforce concrete. In the

1970’s civil FRP structures included roofs, pedestrian bridges, pipes, in-ground tanks

and phone boxes (see Yoosefinejad and Hogg, 1997; Liao et al, 1998; Holloway,

2002 and Sharjah Airport Corporation, 2002). However, the end of the Cold War in

the late 1980’s contributed to a glut of fibre composite resources and the composites

industry began a concerted effort to migrate fibre composites technology to

infrastructure applications.

The 1990’s saw significant development in the application of fibre composite

technology to civil infrastructure led by rehabilitation and retrofit projects.

Developments were also made in the use of fibre composite materials as concrete

reinforcement and to a lesser extent civil structures comprised primarily of fibre

composite materials. These developments will be discussed in more detail in the next

section.

In the last few years the use of fibre composites in civil engineering applications has

made some progress. However, these materials have not enjoyed the level of

widespread acceptance predicted by its proponents over the last decade. This slow

ingress appears to be due to a number of issues, which are discussed later in this

chapter.

2.2 Fibre composite materials in construction and civil engineering

Non-structural fibre composites have enjoyed widespread use in the construction

industry for many years in non-critical applications such as baths and vanities,

cladding, decoration and finishing. However, the use of structural fibre composites in

critical load-bearing applications remains rare mainly consisting of rehabilitation and


retrofit of existing infrastructure, reinforcement of new concrete structures and new

civil structures constructed predominantly of fibre composite materials. These are

discussed next.

2.2.1 Rehabilitation and retrofit

Rehabilitation and retrofit currently represents the largest structural use of fibre

composite materials in civil engineering applications. The widespread deterioration

of infrastructure in Canada, the USA and Europe is well documented (Head, 1994;

Karbhari, 1997, 1998, 2000; Rizkalla, 1999 and Green, 2000). The estimated cost to

rehabilitate and retrofit existing infrastructure worldwide is around CAD$900B

(ISIS, 1998). In Australia it is estimated that $500M per annum is required to repair

and upgrade concrete structures (Oehlers, 2000). In addition to this a large amount of

infrastructure is reaching the end of its design life as revisions in structural codes and

loading codes combined with increased traffic demands are raising load limits on

existing infrastructure. Earthquakes in Loma Prieta (1989), Northridge (1994) and

Kobe (1995) have demonstrated the vulnerability of many of the existing concrete

structures to the effects of earthquake (Karbhari, 1998; Rizkalla, 1999). In many

cases demolition and rebuilding of these structures is difficult to justify in

economical terms so engineers seek inexpensive and effective methods to strengthen

them.

Traditional rehabilitation and retrofit methods use concrete or external steel sheets to

re-introduce or improve structural properties such as strength and ductility. The

ability of concrete to form complex shapes and its suitability to submerged

installation has seen it used for encapsulation of elements such as bridge piers

(Carse, 1997). However, concrete’s relatively low stiffness, high density, frequent

requirement of complex formwork and difficulties in achieving sufficient bonding to

the substrate and sufficient compaction to properly protect reinforcing steel are seen

as drawbacks in general retrofit applications. Steel, on the other hand, can be bonded

or bolted to deteriorated concrete structures to provide strength and stiffness

improvements with relatively little additional weight. However, steel plates can be

difficult to use on complex shapes and protective coatings required by steel plates

can be compromised during installation. Both the concrete and steel systems tend to


inherently provide additional stiffness to the structure which can attract additional

load. This can be a disadvantage in some cases, particularly if foundation capacity is

limited.

Fibre composites are often used as a surface layer that either protects and/or

improves the response of the encapsulated element. In these cases the materials are

usually bonded externally to the structure in the form of tows (fibre bundles), fabrics,

plates, strips or jackets. The strength of circular or near circular concrete members

can be improved through confinement provided by tangentially oriented reinforcing

fibres without the introduction of significant stiffness. Strength or stiffness

improvement in bending members, such as beams and slabs, can be achieved by

bonding laminated strips to the underside of the member.

The advantages offered by composites in these forms include their ability to bond

well to many substrate materials and to follow complex shapes. Composites also

offer a potential benefit over isotropic retrofit materials, such as steel, by allowing

enhancement of strength without increasing stiffness and vice versa. This can be an

advantage for strength enhancement of bridge piers where increasing stiffness could

attract unwanted extra load.

An important consideration in the decision to use traditional or FRP rehabilitation

methods is cost. FRP rehabilitation methods are available, but they are currently

carried out by specialist subcontractors and tend to be expensive. However, Carse

(2003) provides an example of an application in which FRP rehabilitation was

competitive on cost and provided a more desirable solution in terms of aesthetics and

protection compared to traditional methods. In this case bridge piers were

rehabilitated with concrete below the waterline and FRP above the waterline.

2.2.2 Concrete structures reinforced with fibre composites

Concrete reinforced with FRP materials has been under investigation for decades.

Unstressed FRP reinforcement has been developed in a number of forms including

ribbed FRP rod similar in appearance to deformed steel reinforcing bar, undeformed

E-glass and carbon fibre bar bound with polyester, vinylester or epoxy resin, E-glass


mesh made from flat FRP bars and prefabricated reinforcing cages using flat bars and

box sections (Shapira et al, 1997; Ko, 1997; Harris, 1998 and Gowripalan, 1999).

Stressed FRP reinforcement is also available, usually consisting of bundles of rods or

strands of fibre reinforced polymer running parallel to the axis of the tendon. These

are used in a similar fashion to conventional steel tendons (El Kady et al, 1999 and

Gowripalan, 2000).

The durability performance of FRP reinforcements is considered by Ko (1997),

Harris (1998) and Gowripalan (1999) to offer a possible solution to the problem of

corrosion of steel reinforcement, a primary factor in reduced durability of concrete

structures. Other reported advantages of FRP rebar include enhanced erection and

handling speeds (Karbhari, 1999) and suitability to applications which are sensitive

to materials which impede radiowave propagation and disturb electromagnetic fields.

However, in many cases corrosion of reinforcing steel can be traced back to

deterioration of concrete resulting from poor design, materials or workmanship.

Well-designed steel-reinforced concrete can produce extremely durable structures

and examples exist of concrete elements found to be in excellent condition despite

continuous tidal zone wetting and drying in a saline marine environment for over 70

years (Carse, 1997). Good design and construction is likely to be a more

economically feasible approach than use of expensive FRP rebar which can be up to

eight times (Tilco, 2002) as expensive as uncoated reinforcing steel and around one

and a half times as expensive as stainless steel reinforcement (Arminox, 2003).

In terms of erection and handling speeds, FRP rebar can sometimes prove more

difficult to work on site than traditional reinforcing steel. An example of this is the

common requirement for reinforcing steel to be bent on site, a characteristic not

possessed by thermoset FRP rebars. In relation to radiowaves and electromagnetic

fields, carbon fibre rebar would offer little benefit over traditional steel

reinforcement as it is not transparent to radio waves or electromagnetic radiation. In

these cases E-glass bars are often used as they are able to combine good radiowave

and electromagnetic transparency with adequate structural performance. However,

GFRP rebar has disadvantages such as relatively low strength, which could require

up to seven times the area of steel to satisfy deemed-to-comply requirements of


AS3600 for minimum strength (SA, 2001) and susceptibility to stress rupture which

can require a large amount of reinforcing bar to keep stresses low. In addition to

this, the relatively low stiffness of GFRP rebar can require deeper sections and

greater reinforcing areas to achieve serviceability limits.

Applications that are not sensitive to radiowave or electromagnetic transparency may

use stronger and stiffer carbon fibre rebar. CFRP rebar can offer up to three times the

stiffness and three times the strength of GFRP rebar. However, these bars are usually

more expensive than GFRP rebars and in most cases will never be able to develop

their claimed high strength due to stringent serviceability limits which restrict the

maximum strain developed at serviceability failure. For example, members

supporting masonry commonly adopt a deflection limit of around span/500 (SA,

2001). Based on this deflection limit, a concrete member with a typical span-to-depth

ratio of around 12 will develop approximately 0.1% strain in the rebar. Carbon fibre

typically has an ultimate strain of around 1% and as CFRP rebar is predominantly

unidirectional, the material will be used to approximately 1/10 of its capacity at the

serviceability limit state. The result is inefficient use of FRP material and difficulties

in producing a serviceable concrete member. It is likely that unstressed FRP rebar

will be limited to applications not governed by serviceability and will need to

consider the material cost fibre composites compared to other available traditional

materials. This is discussed further in Section 2.3.1.

On the other hand, stressed FRP rebar can allow more efficient materials usage at

serviceability limit states. The action of pre-stressing can take up some of the excess

strain capacity of FRP materials allowing fuller development of the material’s

characteristic high strength and production of a large area of concrete in compression

resulting in a stiff and strong concrete member. However, tendons are required to

carry sustained load for long periods of time and issues such as stress rupture and

creep must be addressed. Materials such as CFRP and AFRP are generally favoured

over GFRP as they are not susceptible to stress rupture, except at higher stress levels,

and their tendency to creep can be accommodated at the design stage by over

stressing. A significant difference between FRP stressing tendons and steel tendons

is that unidirectional FRP tendons are predominantly brittle and do not cope well


with stress concentrations at crack locations, unlike ductile steel. It is therefore

advisable for FRP tendons not to be bonded to the concrete.

The use of stressed FRP reinforcement can provide a more economical use of

material than unstressed FRP and therefore will probably be favoured in applications

requiring FRP reinforced concrete for reasons of cost.

2.2.3 New fibre composite civil structures

A small number of new load bearing civil engineering structures have been made

predominantly from FRP materials over the last three decades. These include

compound curved roofs (Hollaway, 2002 and Sharjah Airport Corporation, 2002)

pedestrian and vehicle bridges and bridge decks (Hazen and Bassett, 1998; Karbhari,

2000; Kollar, 1998 and FHWA, 2002), energy absorbing roadside guardrails (Bank

and Gentry, 2000), building systems (Barbero and GangaRao, 1991), access

platforms for industrial, chemical and offshore (Hale, 1997), electricity transmission

towers, power poles, power pole cross-arms and light poles (Goldsworthy, 1998;

ISIS, 1998 and Weaver, 1999), modular rooftop cooling towers (Barbero and

GangaRao, 1991) and marine structures such as seawalls and fenders (Weaver,

1999). The benefits most often claimed to be offered by fibre composites include

high specific strength and specific stiffness, tailorable durability, good fatigue

performance and the potential to reduce long-term costs. However, in many cases

these claims are difficult to substantiate and are often based on sparse and irrelevant

data. Currently many civil engineers are sceptical of the material’s ability to provide

a viable alternative to traditional materials and bona-fide applications are scarce.

Many of the existing applications are experimental in nature and are aimed at

demonstrating the ability of fibre composite materials to perform in certain

applications. To this end they are often successful in terms of structural performance,

but offer little by way of meaningful financial performance data. In most cases

groups of interested parties combine to design, fabricate and install the structure at a

reduced cost. Examples of this cooperation are evident in projects such as the

Bennett’s Creek crossing on New York State route 248 (Allampali et al, 2000) and

the Tech 21 road bridge (Farhey, 2000).


Allampali et al (2000) provides some financial data for the Bennett’s Creek crossing.

These figures indicate a cost of around US$400,000 to produce the temporary bridge,

based on reduced rates and omitting some items such as deck wearing surface and

some engineering costs. Based on average costs to produce a similar pre-stressed

concrete plank bridge in Australia, at around $1000/m2, it is likely that a complete

permanent bridge could have been built for half the cost of the FRP option.

However, the extra cost of an FRP option could be amortised over the expected

service life of a project. This would offer little justification in the case of a temporary

bridge but, as will be discussed later, may be a significant consideration in structures

with a long service life, or a temporary structure which can be re-used. This would

only be an advantage if potential benefits such as high durability could be realized to

keep maintenance costs down and provided that the material and structure were

durable enough to allow multiple re-use.

New civil structures may also benefit from the ability to produce large modular

components allowing rapid deployment of an FRP structure, although the benefit of

this would be most significant where the cost of public inconvenience and traffic

management is high. Other potential benefits such as high specific stiffness and high

specific strength may exist, but at a cost which makes them uncompetitive with

traditional materials. These issues suggest that fibre composites would mainly suit

non-standard civil applications or those which can balance extra cost against some

unique composites property.

2.3 Issues affecting the use of fibre composites in civil engineering applications

To date fibre composite materials have not enjoyed widespread use in civil projects.

To understand the reasons for this it is necessary to examine the key issues affecting

the use of fibre composite materials in structures in a civil engineering context. A

number of issues affecting the use of fibre composite materials in civil engineering

applications have been highlighted by research undertaken over the last decade

(Ballinger, 1990; Meier, 1991; Gangarao, 1993; Morsi and Larralde, 1994; Karbhari,


1996; Scalzi et al, 1999; van Erp et al, 2000). The following key issues will be

examined in the context of application to civil engineering:

1. Cost

2. Structural performance

3. Durability

4. Familiarity and education

5. Specification and standardisation

6. Compatibility

7. Temperature and fire performance

2.3.1 Cost

In most civil engineering structures, good design requires provision of a solution

which can satisfy design requirements for the lowest cost. Cost can be considered in

terms of short-term costs, such as design, construction and installation, and long-term

costs such as maintenance, modification, deconstruction and disposal. These can be

further grouped into direct costs, such as materials and production, and indirect costs,

such as interruptions to traffic, depreciation, resale value and impact on the

environment.

Short term costs of fibre composites

Currently, fibre composite materials are expensive when compared to conventional

construction materials on an initial cost basis. This is demonstrated in Tables 2.1 and

2.2 which compare the cost per stiffness and cost per strength of FRP materials with

traditional construction materials. In engineering terms stiffness can be expressed as

LEAk =

Where E is the Modulus of Elasticity of the material (N/mm2), A the area (mm2) and

L the length (mm).

Strictly speaking this equation applies to axial loading only but it can also be used to

compare materials loaded in bending assuming the dimensions of the cross section

are fixed.


Determining how much “stiffness” can be bought for a dollar is a realistic way to

compare the stiffness capabilities of different materials. The cost of a material can

be expressed as

( ) ( ) densitylengthareakgAcost ×××= /$

Assuming a length of 1 m, the stiffness in N/mm per dollar is given by

( ) densitykgAEk

×=

/$1000 .

Table 2.1 – Material cost comparison between traditional materials and fibre

composites

Material E

(N/mm2)

Cost

(A$/kg)

Density

(kg/m3)

Stiffness/dollar

(N/mm per A$)

Pultruded glass composites 30,000 7.00 1,800 2,381

Carbon composites 90,000 30.00 1,400 2,142

Standard Construction Steel 200,000 2.50 7,850 10,190

Steel Rebar 200,000 1.20 7,850 21,230

Stainless Steel Rebar 200,000 6.00 7,850 4,246

50 MPa concrete 30,000 0.10 2,500 120,0001

Hard wood timber 16,000 2.50 650 9,846

New polymer concrete 11,000 0.75 1,900 7,719

1. Compression only

A similar table can be assembled for strength. For a 1 m long bar, the load carrying

capacity per dollar can be expressed as:

( ) densitykgANu

××

=/$

106φσ

where φ is an indicative ultimate limit state capacity factor and uσ is the ultimate

limit state stress (N/mm2).


Table 2.2 – Indicative strength/dollar ratio for different materials

strength/dollar

(N/A$)

Material uφσ

(N/mm2)

Cost

(A$/kg)

Density

(kg/m3)

tension compression

Pultruded glass

composites

0.6x600(T*)

0.6x480(C)

7.00 1,800 17,143

13,714

Carbon composites 0.7x900(T)

0.6x720(C)

30.00 1,400 15,000

12,000

Standard Construction

Steel

0.9x300 2.50 7,850 13,758 13,758

Steel Rebar 0.9x500 1.20 7,850 47,770 47,770

Stainless Steel Rebar 0.8x500 6.00 7,850 8,492 8,492

50 MPa concrete 0 (T)

0.8x50(C)

0.10 2,500 0

160,000

Hard wood timber 0.6x40(T)

0.6x50(C)

2.50 650 14,770

18,462

New polymer concrete 0.6x10(T)

0.6x50(C)

0.75 1,900 4,210

21,053

(*T is tension, C is compression)

These results clearly show that fibre composites struggle to compete financially with

traditional construction materials both in terms of stiffness and strength. Although

these performance criteria are rarely considered alone, they are often fundamental in

“material-evaluation”.

There are a number of factors contributing to the high cost of composite materials

including; high cost of raw materials and processing, the use of imported materials,

the general acceptance of high prices in markets such as marine and aerospace and

occasional low availability of material (Goldstein, 1996). The production of

materials locally is likely to reduce material cost, however with America and Europe

making up 35% and 27% of the worldwide market respectively (Weaver, 1999),


there appears to be little incentive for material manufacturers to provide production

facilities in Australia.

In line with the evolution of other composites uses, such as sporting equipment,

many researchers and composites commentators believe it is likely that production

volume increases resulting from the use of fibre composites in civil engineering

applications will lead to decreased cost of materials (Karbhari, 1998a; Weaver, 1999;

Hastak and Halpin, 2000). However, in the Author’s opinion, this should be viewed

as an optimistic outlook. The majority of fibre composites materials used in Australia

are imported and are therefore subject to a range of international economic variables.

For example, fluctuations in the local price of imported materials would be affected

by overseas production costs, transport and import costs and fluctuations in the

exchange rate between Australia and countries such as Europe, United States and

Japan, which supply us with carbon, aramid, E-glass fibres and many resins.

When this is considered in conjunction with the tendency of suppliers to provide

price reductions for purchase of large quantities of some materials, accurate costing

of an FRP civil project can seem difficult. The uncertainty that exists in our ability to

accurately cost FRP civil project suggests that it could be some time before

anticipated price drops could significantly influence project cost.

Fabrication cost

In addition to relatively high material costs, the short-term cost of FRP materials is

dependant on fabrication. Most fibre composite manufacturing techniques were

originally developed for the aircraft, marine and/or car industries. The civil

engineering industry is vastly different to these as civil and structural engineers tend

to be concerned with the design and construction of rather large-scale structures.

Most of these structures have to meet different design specifications and therefore

very little duplication of design solutions occurs. As a result most civil engineering

projects tend to be ‘one-off’ jobs. This situation is in contrast with the manufacturing

industries, where mass production of one design solution is common. As a result,

design and manufacturing methods which are highly successful in the manufacturing

industry are often not viable in civil engineering.


Most civil engineering applications of fibre composites are currently based around

the pultrusion process (similar to extrusion of aluminium). The pultrusion process is

ideal for the continuous production of elements of constant cross-sectional geometry

and moderate complexity. The advantages are relatively low labour cost, minimal

material wastage, consistent quality and high production rates. However, the

pultrusion process has some serious disadvantages such as high initial costs of setting

up for a production run and relatively few producers. Consequently, pultruders offer

a limited range of standard structural sections similar to those available in steel. A

non-standard section would require a significant volume to justify the high setup

cost. In many civil engineering projects, this high volume simply does not exist and

for these situations, a cheaper, more flexible manufacturing approach is required.

One of the most flexible methods of FRP fabrication is hand-layup. This method

allows individual placement of fibres and resin onto surfaces of virtually any

topography. However, this method tends to be inefficient and laborious and is

usually avoided as much as possible where large volumes are required. The most

likely way forward is through use of modular components fabricated using a

combination of manual and automated manufacturing procedures such as embedment

of standard pultruded sections in cast components, filament winding, automated tape

laying and resin transfer moulding. These procedures can be computer controlled to

produce accurate components with lower labour costs. Integration of these methods

could be particularly suitable for civil applications, which often require highly

aligned fibres and fabrication versatility.

Some short-term costs, such as transport and erection, may benefit from production

of large, lightweight, modular components. Lower weight can translate into reduced

transport and cranage costs, while the use of fewer large modular components can

reduce erection time. The implications on indirect short-term costs such as consumer

inconvenience and traffic management can be substantial. Meiers (2000) points out

that although it is difficult to quantify indirect savings, they have a cost that is

present. He believes that savings can be accrued at the systems level due to faster

construction thereby causing less distress and disruption to the community, lower

dead weight requiring smaller and l

Development and Structural Investigation of Monocoque Fibre Composite … · 2010. 6. 9. · 3.3.2...

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