INVESTIGATION OF GLASS AS A STRUCTURAL MATERIAL AND COMPARATIVE STUDY ON DIFFERENT TYPES OF GLASS. Report prepared for. M/s Glazing Society of India

Transcription

1 For restricted use only Month & Year May 2012 INVESTIGATION OF GLASS AS A STRUCTURAL MATERIAL AND COMPARATIVE STUDY ON DIFFERENT TYPES OF GLASS Report prepared for M/s Glazing Society of India Nungambakkam Chennai, Tamilnadu By K Rajgopal S. Arul Jayachandran STRUCTURAL ENGINEERING DIVISION DEPARTMENT OF CIVIL ENGINEERING INDIAN INSTITUTE OF TECHNOLOGY, MADRAS MAY 2012

2 ACKNOWLEDGEMENT The authors of this report take this opportunity with immense pleasure to thank Dr V.Kalyanaraman, Professor, Structural Engineering Division, Department of Civil Engineering, Indian Institute of Technology Madras, who has been a constant source of inspiration to us throughout our study. GSI officials in general and Mr.R.Subramanian, Mr.Mahesh and Mr.Gohul Deepak in particular have always showed a spirit of great hospitality during the entire tenure of this project from its conception to the delivery of the needful. Our sincere gratitude to Dr S.R Gandhi, Head of Civil Engineering Department, IIT Madras, for providing necessary facilities in laboratory for carrying out this study. Special thanks to Sri T. Rajkumar, Senior Technical Superintendent of Structural Engineering Laboratory, IIT Madras and all other assisting staff of the laboratory for helping in smoothly conducting the experimental part of this study. Finally we thank all the people who were directly or indirectly helpful and supportive at all times during the course of this work at Indian Institute of Technology, Madras. KAVIKONDALA RAJGOPAL Dr S.ARUL JAYACHANDRAN

3 IMPORTANT NOTE This report has been prepared for the exclusive use of M/s Glazing Society of India, Nungambakkam,Chennai and shall not be passed on to a third party without written approval from the Director, Indian Institute of Technology Madras, Chennai. DISCLAIMER The responsibility of Indian Institute of Technology Madras, Chennai is limited only to the technical content of this report. All the procedural/legal/operational matters would be the responsibility of the client. IIT Madras, Chennai, shall not in any way be responsible for the consequential effects if any, due to the usage of this report contents. Report Prepared by: K Rajgopal (Signature) (Name) Approved by: Dr S.Arul Jayachandran (Signature) (Name)

4 Indian Institute of Technology Madras, Chennai Document Sheet Class : Restricted Project Title Investigation of glass as a structural material and comparative study on different types of glass Report Title Investigation of glass as a structural material and comparative study on different types of glass Project Team/Author(s) K.Rajgopal, S.Arul Jayachandran Area Glass structures Category : Research/Consultancy/T esting/others Name & address of client Glazing Society of India, Haddows road, Nungambakkam, Chennai Keywords Structural glass, types of structural glass, tests on glass Contents Pages Figures Tables References Annexure Abstract From the ancient times glass was used as a decorative item or in windows, doors or partition walls just as a non-structural element and it never contributed to any load carrying phenomenon of the building components and acted as an appendage to the building. But now this trend is slowly changing. The recent developments and innovations in the field of architecture is demanding more contribution from glass mainly as a structural material. In other words, glass components are also expected to contribute to the overall load carrying capacity of the structure. This resulted in the need for standardised design codes which can be used to design glass components ensuring safety and serviceability. But till now there are no proper design codes available in India which exclusively deals with design of glass structural components. Though there are few international codes available on glass but most of them deal with glass that are mainly subjected to the lateral loads like wind pressure Presently structural engineers, architects and consultant engineers depend on manufacturer s specification to design load bearing glass members which is not a healthy practice as different manufacturers will have different specifications for their products and it is always difficult to arrive at a common understanding with all the stake holders in the construction industry. Hence the need for a dedicated code of practice for design of glass structures is felt by structural engineers. This study aims to investigate the structural behaviour of glass components by conducting few simple experiments like compression, flexure and tension tests and understanding the variation of the material properties for various types of glass with varying thickness & aspect ratios.

5 TABLE OF CONTENTS 1. Introduction General Objectives of the study Scope Necessity for code of practice for glass Glass as a structural material Common properties of soda lime silicate glass 8 2. Review of Literature General Fracture mechanics approach towards glass Findings of other researchers on behaviour of glass Provisions in international glass codes Manufacturing of glass Types of glass Design procedures for glass structural components General Simple design methods Experimental investigation of glass as structural material General Experiments carried out Three point flexure test Description of flexure test setup In-plane compression test Description of compression test setup Tension test Description of tension test setup 24

6 4.5 Experimental results Flexural test results Compression test results Tension test results Analysis and discussion General Comparison of Young s modulus for different types of glass Comparison of Young s modulus for same specimen in different experiments Application of mechanics Discussion on tension tests Results of percentage variation in Young s modulus 5.6 Breakage pattern Summary and conclusions General Conclusions 66 References 68 Appendix A 69

7 List of Figures Figure Title 1.1 Plot of available number of glass standards in various countries Change in trend of glass usage from the past Glass staircase ( Glass staircase in Glasstec Exhibition, Düsseldorf, Germany (Overand) Load bearing glass walls at the Reinbach pavilion, Germany. (Overand) Post-tensioned cable net façade with glass panels at the Kempinski Hotel, Munich,Germany (Schlaich Bergermann) 1.7 Glass parameters of interest to an architect & a structural engineer Plot of tensile strength of glass with effective flaw depth Chart to determine non factored load for different dimension of glass panel Chart to find thickness of glass panel according to IS Variation of stress across thickness of glass panel after heat strengthening or tempering 2.5 Different types of glass Loading of the flexure test specimen Flexural strength test setup End supports for flexure test Loading of the compression test specimen Compression strength test setup End supports for compression test Loading of the tension test specimen Tensile strength test setup Arrangement of tension specimen between the holding frames Load vs deflection for Annealed Glass 200 mm X 600 mm in flexure of various thicknesses 4.11 Load vs deflection for annealed glass 200 mm X 1200 mm in flexure of various thicknesses 4.12 Load vs deflection for HS Glass 200 mm X 600 mm in flexure of various thicknesses Load vs deflection for HS Glass 200 mm X 1200 mm in flexure of various thicknesses Load vs deflection for Tempered Glass 200 mm X 600 mm in flexure of various 29 thicknesses 4.15 Load vs deflection for Tempered Glass 200 mm X 1200 mm in flexure of various 30 thicknesses 4.16 Load vs deflection for various types of glass of size 200 mm X 600 mm X 6mm in 30 flexure 4.17 Load vs deflection for various types of glass of size 200 mm X 1200 mm X 6mm in 31 flexure 4.18 Load vs deflection for various types of glass of size 200 mm X 600 mm X 8 mm in flexure 31 Page

8 4.19 Load vs deflection for various types of glass of size 200 mm X 1200 mm X 8 mm in flexure 4.20 Load vs deflection for various types of glass of size 200 mm X 600 mm X 10 mm in flexure 4.21 Load vs deflection for various types of glass of size 200 mm X 1200 mm X 10mm in flexure 4.22 Load vs deflection for various types of glass of size 200 mm X 600 mm X 12 mm in flexure 4.23 Load vs deflection for various types of glass of size 200 mm X 1200 mm X 12mm in flexure 4.24 Load vs deflection for AG (200 mm X 600 mm) for different thickness in compression Load vs deflection for AG (200 mm X 1200 mm) for different thickness in compression 4.26 Load vs deflection for HG (200 mm X 600 mm) for different thickness in compression Load vs deflection for HG (200 mm X 1200 mm) for different thickness in compression 4.28 Load vs deflection for TG (200 mm X 600 mm) for different thickness in compression Load vs deflection for TG (200 mm X 1200 mm) for different thickness in 40 compression 4.30 Load vs deflection for 6 mm glass (200 mm X 600 mm) in compression Load vs deflection for 6 mm glass (200 mm X 1200 mm) in compression Load vs deflection for 8 mm glass (200 mm X 600 mm) in compression Load vs deflection for 8 mm glass (200 mm X 1200 mm) in compression Load vs deflection for 10 mm glass (200 mm X 600 mm) in compression Load vs deflection for 10 mm glass (200 mm X 1200 mm) in compression Load vs deflection for 12 mm glass (200 mm X 600 mm) in compression Load vs deflection for 12 mm glass (200 mm X 1200 mm) in compression Load vs axial elongation for 8 mm annealed glass (100 mm X 400 mm) in tension Variation of Young s modulus with thickness for annealed glass 200 mm X 600 mm Variation of Young s modulus with thickness for annealed glass 200 mm X 1200 mm Variation of Young s modulus with thickness for HS glass 200 mm X 600 mm Variation of Young s modulus with thickness for HS glass 200 mm X 1200 mm Variation of Young s modulus with thickness for toughened glass 200 mm X 600 mm Variation of Young s modulus with thickness for toughened glass 200 mm X mm 5.7 Load resisting phenomenon in heat strengthened or toughened glass in three stages Variation of ultimate loads for different edge distances for annealed glass Variation of ultimate loads for different edge distances for heat strengthened glass Variation of ultimate loads for different edge distances for tempered glass a Photograph showing failure pattern in annealed glass under flexure b Photograph showing failure pattern in annealed glass under flexure Photograph showing failure pattern in annealed glass under tension a Photograph showing failure pattern in heat strengthened glass under flexure b Photograph showing failure pattern in heat strengthened glass under flexure

9 5.14 Photograph showing failure pattern in heat strengthened glass under tension a Photograph showing failure pattern in tempered glass under flexure b Photograph showing failure pattern in tempered glass under flexure 65 List of Tables Table Title 1.1 Common properties of glass (Adopted from A.Lago & TJ. Sullivan) Maximum load and deflection for annealed glass in flexure Maximum load and deflection for heat strengthened glass in flexure Maximum load and deflection for tempered glass in flexure Maximum buckling load for annealed glass in compression Maximum buckling load for heat strengthened glass in compression Maximum buckling load for tempered glass in compression Ultimate load for annealed glass in tension Ultimate load for heat strengthened glass in tension Ultimate load for tempered glass in tension Average Young s modulus obtained for different types of glass in flexure Average Young s modulus for different types of glass in compression Variation of final stress in 6 mm annealed glass in flexure across thickness Variation of final stress in 6 mm HS glass in flexure across thickness Variation of final stress in 6 mm tempered glass in flexure across thickness Variation of final stress in 8 mm annealed glass in flexure across thickness Variation of final stress in 8 mm HS glass in flexure across thickness Variation of final stress in 8 mm tempered glass in flexure across thickness Variation of final stress in 10 mm annealed glass in flexure across thickness Variation of final stress in 10 mm HS glass in flexure across thickness Variation of final stress in 10 mm tempered glass in flexure across thickness Variation of final stress in 12 mm annealed glass in flexure across thickness Variation of final stress in 12 mm HS glass in flexure across thickness Variation of final stress in 12 mm tempered glass in flexure across thickness Poisson s ratio obtained in different tensile specimens Percentage variation of Young s modulus A.1 List of flexural test specimens 69 A.2 List of compression test specimens 71 A.3 List of flexural test specimens 73 Page

10 List of Symbols t - Thickness of the glass panels Put Ultimate load in tension P uc - Ultimate load in compression P uf - Ultimate load in flexure δ f Mid deflection in flexure E af Young s modulus of elasticity for annealed glass calculated in flexure test E hf - Young s modulus of elasticity for heat strengthened glass calculated in flexure test E tf - Young s modulus of elasticity for tempered glass calculated in flexure test E ac - Young s modulus of elasticity for annealed glass calculated in compression test E hc -Young s modulus of elasticity for heat strengthened glass calculated in compression test E tc - Young s modulus of elasticity for tempered glass calculated in compression test e - Edge distance of centre of hole for tensile specimen σ at -Pseudo tensile strength of annealed glass σ ht - Pseudo tensile strength of heat strengthened glass σ tt - Pseudo tensile strength of tempered glass C - Initial pre compression force applied to heat strengthened or tempered glass C 1 - Compressive force due to external load in stage 1 C 2 - Compressive force due to external load in stage 2 C 3 - Compressive force due to external load in stage 3 T 1 - Tensile force due to external load in stage 1 T 2 - Tensile force due to external load in stage 2 T 3 - Tensile force due to external load in stage 3 K 1 - Stress intensity factor K 1c - Critical stress intensity factor σ n - Normal tensile stress a c - Root radius of the flaw

11 Chapter 1 INTRODUCTION 1. General The use of glass in buildings has undergone a rapid transformation over the last few decades. Its traditional use as infill panel is still popular, but an alternate structural use of glass is emerging in which glass elements contribute to the overall load bearing capacity of the structure. Glass is to be handled very carefully as it is very brittle and even small surface crack can lead to failure of glass panel under tension. It was always used exclusively as infill material largely due to the uncertainty in its structural behaviour. Glass shows an almost perfectly elastic isotropic behaviour and it fails in a sudden brittle manner at ultimate load. It does not yield plastically. The tensile strength of glass is largely affected due to the presence of mechanical flaws on the surface of glass which may not be visible to the naked eye. The main idea of this work is to study the structural behaviour of glass component aided with appropriate experimentation to prepare some sort of design basis document which can be used as a reference for the preparation of structural glass codes of practice Objectives of the study To investigate the behaviour of glass as a structural material at component level subjected to tension, compression and flexure. To conduct experiments and compare the behaviour of raw/annealed glass and different types of processed glass as a structural material. To investigate and comprehend the variation of Young s modulus of glass specimens made of annealed, heat strengthened and toughened procedures. To investigate the behaviour of glass tension members by varying the edge distances of the spider connectors. To correlate the breakage pattern of glass with respect to the types of loads those are applied on the glass specimen.

12 1.2 Scope The scope of this work is limited to only component level testing of annealed, heat strengthened and tempered glass specimens. The plate action of the glass specimens is not investigated. In column specimens plates are treated as onedimensional flexural case. For tension members only the connection limit state is investigated. The primary objective is to comprehend the variation of Young s modulus in raw and processes glass. 1.3 Necessity for code of practice for glass structures Due to its unique properties, e.g. high compression strength and modifiable transparency, glass has great potential among building materials. This has made it popular with engineers and architects alike and has contributed to its expanding use for primary and secondary structural elements used in energy efficient buildings and in applications related to the preservation and enhancement of built cultural heritage. In the absence of common design rules, the safety of citizens in the built environment is at risk because of possible sub-standard applications. The code will provide a common understanding between owners (who ask for structural solutions supported by widely-accepted common rules), manufacturers of construction products and kits (who will specify the product properties in compliance with harmonised design procedures) and designers. It will also enhance the competitiveness of the Indian construction industry in the global market. European (EN), ISO and National standards for glass products exist along with guidelines produced by International Technical and Scientific Organisations, but none covers explicitly the design of glass products for structural applications. This proposal is motivated by the expanding use of glass products in the construction sector as primary and secondary components and the lack of a proper set of design rules. Following figure gives the number of available glass standard in various countries across the world.

13 Available number of glass standards in various countries Figure 1.1 Plot of available number of glass standards in various countries This ever-increasing marketing of said glass products sets the background for the need of a new design code. Presently structural engineers, architects and consultant engineers depend on manufacturers specifications and guidelines and some other national standards to design the primary and secondary load-bearing glass elements. Also there is no common understanding regarding the design of structures between owners, operators and users, designers, contractors and manufacturers of construction products as different manufacturers have different specifications and grouping of their own glass products. So a new code will provide a common basis for contractors to compare pricing for construction products that conform to a specific structural capacity and function and thus promote a more competitive market for glass products. It will also provide a common approach, similar to that of traditional materials, and will facilitate the selection process for designers when choosing between widely differing construction materials. The lack of standardisation leaves opportunities for deficient glass products and services to slip through the usual, prescriptive, quality-assurance processes established in the construction sector. It is possible that an unregulated market, currently characterised by the potential to deliver wide profit margins, could result in the implementation of sub-standard applications that one day may result in failures. Glass structural design standards are essential to limit the number of structural failures that arise from avoidable

14 negligence resulting from bad design or construction practice due to the lack of a harmonised standard. 1.4 Glass as a structural material In olden days glass was exclusively used as a material for windows, sometimes for doors and partition panels etc. In all these applications glass played only a nonstructural role. It was just seen as an appendage and its basic requirement was limited to resist its self-weight and more importantly lateral & vertical loads like the wind & the snow loads. But the recent trends in architecture and the changing mindset of various stake holders in construction industry towards innovation has led to a thought of using glass as a structural material so that it also takes the in plane loads in addition to the out of plane loads for which it was primarily designed for. The following figure shows how the trend has slowly changed and how glass, which was previously treated as a non-structural element, has gained the structural importance. Some of the examples of glass as a structural member are also shown. Windows, doors, partition walls, glazing etc. Load bearing members, stairs, cable net façade etc. 4o yrs ago 30 yrs ago 20 yrs ago 10 yrs ago Recent times Timeline Axis Figure 1.2 Change in trend of glass usage from the past

15 Figure 1.3 Glass staircases ( Figure 1.4 Glass staircases in Glasstec Exhibition, Düsseldorf, Germany (Overand)

16 Figure 1.5 Load bearing glass walls at the Reinbach pavilion, Germany. (Overand) Figure 1.6 Post-tensioned cable net façade with glass panels at the Kempinski Hotel, Munich,Germany (Schlaich Bergermann,Overand) From the structural engineering point of view the most important parameters of glass that are of interest are the strength and stiffness parameters.

17 Figure 1.7 Glass parameters of interest to an architect & a structural engineer Unlike an architect who is more interested in transparency, heat absorption and colour properties of glass, a structural engineer is more worried about the strength & stiffness parameters. Glass is a brittle material. It behaves in a perfectly linear elastic manner before undergoing brittle failure. And the failure is a sudden failure. Hence the design of glass members should be done with utmost care in a more reliable manner so that the probability of failure is very less. The raw material used to make glass is silica and the bond between the silicon atoms and oxygen atoms gives glass a theoretical tensile strength which is very high compared to other conventional materials like steel and concrete. It has been found theoretically that based on the molecular bond strength the tensile strength of glass is almost hundred times more than that of conventional steel. So far many authors have mentioned about strength of glass & deflection limits for glass for mainly a concentrated or distributed lateral pressure. Even many international codes that exist for design of glass panels consider a scenario where in

18 glass has to resist mainly wind loading. This work is mainly aiming to investigate the structural behaviour of glass by conducting some experiments in which the in plane compression & tension strength of different types of glass will be determined. 1.5 Common properties of soda lime silicate glass Some of the common properties related to ordinary float glass are given below Table 1.1 description of the picture. (Adopted from A.Lago & TJ. Sullivan) Modulus of rupture 41 Annealed 83 Heat strengthened 165 Tempered Design stress for 0.8 % breakage 19 Annealed 39 Heat strengthened 77 Tempered Young s modulus of elasticity 72 GPa Modulus if rigidity (shear) 30 GPa Bulk modulus 43 GPa Poisson s ratio 0.23 Density 2530 Kg/m 3 Coefficient of linear expansion 8.3 x 10 6 / C Hardness (Mohr scale) 5-6

19 Chapter 2 REVIEW OF LITERATURE 2. General Understanding structural behaviour of glass is quite difficult as most of the behaviour of the glass is dependent on many factors of which distribution and size of its surface cracks is the predominant factor. Although the strength parameters of glass depends on other factors including loading duration, moisture, temperature stresses etc. but surface crack parameter is the one which cannot be avoided as it invariably forms during the manufacturing process. 2.1 Fracture mechanics approach towards glass Fracture mechanics is the science that deals with the problems and conditions of cracks propagating through the material and the mechanism of fracture in the presence of these cracks. The occurrence of failure in conditions of low stresses in high strength materials induced the development of fracture mechanics. Griffith proposed the concept of fracture mechanics in glass based on surface energy theory in the year 1920 which was widely accepted by everyone. Griffith presented experimental results on glass with introduced flaws of various sizes to show that it was the flaws which determined the strength of the glass. Later Irwin extended this concept of Griffith and proposed a concept based on critical intensity factor. Now unlike steel or concrete, for glass there doesn t exist a particular fixed material strength, as strength of glass varies from glass to glass based on the distribution of flaws on the surface. Steel is a very reliable material as its design stress which is denoted by its yield stress can be well defined as there is no much variation from different experimental results. Such a dependable value is not available for glass. It is difficult to accurately define the strength of the glass and to arrive at an allowable design stress value for glass. There is no unique, minimum strength for glass. Manufacturers have charts which give probabilities associated with given stresses. The stress which the glass

20 manufacturer gives the engineer is no longer a material constant as it is for steel. The designer finds that the allowable stress is now combined with a probability of failure, and both vary with time. Yet the method of allowable stresses and the process of borrowing steel design philosophy persist, despite the fact that the fundamental material basis has changed. Considering the molecular forces alone in the glass material the theoretical tensile strength of glass turns out to be as high as 32 GPa. Now this strength would have been really fascinating if it could have been achieved in reality. But the inevitable surface cracks produced on the glass surface during its manufacturing process causes the glass to fail early at a strength as low as 10 or sometimes even less based on the geometry of flaw and its position on the glass surface. The strength of the glass geometrically decreases with the increase in the flaw size Tensile strength () Effective flaw depth (mm) Figure 2.1 Plot of tensile strength of glass with effective flaw depth Glass surface has got many surface flaws mainly formed during production. Every crack (or flaw) has tendency to grow when its tip experiences a stress equal to the critical stress. The stress intensity factor is defined as K 1 = Y σ n πa c where K 1 = K 1c critical stress intensity factor (0.75 for glass) When the tip of the crack experiences such a critical stress or when the stress

21 intensity factor reaches to the critical stress intensity factor an immediate failure occurs according to Irwin s fracture theory. 2.2 Findings of other researchers on behaviour of glass Salvatore et al presented a paper in the year 2011 on compressive behaviour of laminated structural glass members In this paper he presents the experimental variation of strength of glass panels & column with varying slenderness ratios & cross sections Three different glass panel specimens of size 400 X 300 X 9 mm, 500 X 300 X 9 mm & 600 X 300 X 9 mm were selected and subjected to in plane compressive loading. The failure load was recorded as 58.2 kn, 48.8 kn & 32.9 kn respectively. The variation of strength with the slenderness ration was then discussed. Also the results of few bending tests on monolithic and laminated glass panels were also shown with an evaluation of the level of connection with the glass sheets. Similarly glass columns of T & X shapes were subjected to compressive loading and comments were made on the results. Finally a comparison of the results was made with the existing analytical model. Amadio et al investigated the load bearing capacity of laminated glass beams in outof-plane bending through a simple analytical model developed on the basis of Newmark s theory for flexural response of composite beams with deformable connection. But unlike Newmark who ignored the torsional stiffness of the deformable connection the author has taken into account the torsional stiffness of the interlayer in the analysis of laminated glass beams. The effect of mechanical properties of the interlayer and other loading conditions on the critical buckling moment of the glass beams has been discussed. Some buckling curves were also presented to illustrate how a combination of weathering variations, initial imperfections or particular load conditions can simultaneously affect the buckling response of laminated glass beams. Mehmet et al investigated the effect of various factors like ambient temperature, geometric parameters on the strength of laminated glass beams. The role of two crucial factors in the estimation of the relative strength of laminated glass which are the PVB interlayer (plasticizer content) used & the effect of nonlinearity were

22 observed in the analysis of this study. The PVB interlayer is a elastomeric polymer whose shear modulus increases with falling temperature and decreases with the rising temperature. These elastic properties of the interlayer depend on the glass transition temperature which is generally degrees. So at room temperature it is very stiff & hard and makes the glass too brittle. To avoid this plasticizer is added to soften the interlayer so that it can take additional impact due to its ductility and it can very well adhere to the glass material. In this paper the effect of variation of plasticizer content on the strength of the laminated glass is discussed. Aki Vuolio studied some specific problems related to glass structures and also used non-linear finite element analysis to analyse glass panes with four point support. Also some of the methods used by European standard to design glass panels were introduced. But even the European standard considered wind load as the major load coming on to the glass panel & has given expressions for the same. S L Chan compared the linear and non-linear analyses on glass panels and also compared the different stress values used in different national glass codes. He gave a short review for design consideration of glass and aluminium structures which are basically used for light weight façade systems. He compared these materials with conventional materials like steel and concrete and summarised the practices followed by other international codes in designing such façade system which are majorly subjected to wind loads. He also emphasized the use of non-linear FEM analysis to be carried out for analysing such systems due to high geometric non linearity. Overand aimed to redress the issue faced by glass as a structural member relying upon original rules of thumb for sizing infill glass panels by reviewing and extending the principal methods for determining the strength of glass. His paper focuses more on accurate analytical and numerical methods & prototype testing to be used to validate designs in addition to the original rules of thumb which can be used as a check at a later stage. The paper starts by mapping out the design process and structural performance requirements for glass elements. This is followed by a review of the mechanical properties that underpin structural behaviour, and how these

23 fundamental material properties affect the tensile strength and post-fracture performance. The paper describes how this knowledge may be deployed in design methods, in particular, how multiple load combinations may be taken into account through a simple stress history interaction equation. The paper concludes with an overview of the use of prototype testing in structural glass design. 2.3 Provisions in international glass codes ASTM E a code of practice for design of glass structures The ASTM code talks about procedures to determine the lateral load resistance of different glass panels for different loading rates for a probability of breakage of 8 in 1000 glass panels. The lateral loads considered are wind load and snow load only. The maximum combined load is 10 kpa. The code considers only monolithic, laminated and insulated glass types with rectangular shapes only. The procedure for determining the load resistance of the glass panels consists of the following For the given dimension, thickness and support conditions of the glass panel the non-factored load value (NFL) is determined from the chart. Then the glass type factor (GTF) is determined from the table provided for the type of glass being used (whether Annealed glass, fully tempered or heat strengthened glass) and duration of loading. The load resistance (LR) is obtained by multiplying NFL with GTF. LR = NFL X GTF For laminated glass types and additional term of load sharing (LS) is also determined from the table which has to be multiplied along with NFL & GTF to get the load resistance. LR = NFL X GTF X LS The lateral deflection is also found from the table for this load resistance and the given aspect ratio of the glass panel.

24 Load resistance in ASTM code refers to the uniform lateral pressure on the glass panel. Figure 2.2 Chart to determine non factored load for different dimension of glass panel Indian standard IS Code of practice for glazing in building The Indian standard code gives some guidelines for installing glazing in buildings (for doors, windows & partitions). For finding the effective minimum thickness required, the code gives a Nomogram chart which depend on the dimension of the glass panel and the total wind load coming on to the panel calculated from IS 875 part 3.The thickness can be directly found from this chart which is for a glass panel supported on four edges. But the code doesn t talk about glass panels of different types and with different support condition & loading duration.

25 Figure 2.3 Chart to find thickness of glass panel according to IS 3548 AS , Australian standard for glass in buildings-selection & installation Australian standard uses limit states of design for designing and installing glass panels in buildings. The possible load combinations along with appropriate load factors are taken from AS/NZS /1/2- Structural design actions. The ultimate design capacity of the panel is calculated by multiplying the characteristic tensile strength of glass with a capacity reduction factor & four more factors which depend on the type of glass, geometry of panel, type of surface and duration of load respectively. As tensile strength of glass is very less compared to its compressive strength, the governing factor for glass design is its tensile strength. European standard EN , Glass in building-thermally toughened soda lime silicate safety glass- Part 1 This European standard mainly talks about the geometric tolerances of a glass panel used in the structures. Also it mentions the factors on which the position of a circular hole in glass panel (which are meant for clamping the panel) mainly depends upon.

26 These factors include the nominal glass thickness, the dimensions of the pane, the hole diameter, the shape of the pane, the number of holes etc. The main point to note here is that unlike the steel code where in the end distances of a hole are dependent on the diameter of the hole, this code gives the end distances in terms of the glass thickness. 2.4 Manufacturing of glass The manufacturing of float glass is an interesting process where in different raw materials fuse together to give a transparent product of fusion. The main raw material used in the preparation of glass is silica sand. Other raw materials include soda ash, dolomite, sodium sulphate, calcite, cullet etc. All these raw materials in a fixed proportion are added into a kiln and heated to around 1600 C. At this high temperature the materials melt and fuse together in the molten state. In this state it becomes glass in molten form. This molten material is poured on to a tin surface on which glass floats and forms a smooth surface. It is then cooled and drawn on to the rollers. The speed of the rollers is adjusted depending on the thickness of the glass panel required. It is then put into a annealing lehr where it gradually cools. The glass panel thus formed is then sent to quality control inspection. Depending on the colour of glass panel required chromates and sulphides can be added based on the amount of these elements already present in the raw materials used. 2.5 Types of glass There are different types of glass available of which the basic type of glass is called the float glass, ordinary glass or the annealed glass. Float glass is generally used in huge amounts in various sectors like building, automobile, industries etc. Automobile industries generally use coloured, tinted, laminated glass or processed glass. The main focus in this work will be towards the annealed glass and the processed glass as they are more relevant from structural point of view. Although laminated glass is very useful in building sector, the study on its structural performance is out of the scope of this work. Now float glass or the ordinary annealed glass is the one which is not pre-processed. Its structural performance vastly depends on the size and distribution of surface flaws present on its surface. As

27 the cracks open in tension and grow progressively under small loads, it generally leads to an early failure of the material. Although the theoretical tensile strength calculated based on the molecular forces is much high, in reality that much tensile strength is not attained owing to the effect of surface flaws. To avoid this problem glass is generally heat strengthened or tempered based on the requirement. Figure 2.4 Variation of stress across thickness of glass panel after heat strengthening or tempering Tempering or heat strengthening is a process in which the outer layer of glass is heated such that there is a pre compression in the outer layers and a tension in the middle layer as shown. The advantage of such a pre-treatment with temperature is that it prevents the surface cracks to open up because cracks generally open and propagate in tension. So for the surface cracks in the pre compressed outer layer of glass to grow and propagate, this compression should be overcome and then only the critical crack may experience net tensile stress and fail. As the cracks on the surface of float glass experience no stress before load is applied even a slight tension leads to its failure. Heat Strengthened Insulated glass Float glass Tempered glass Laminated glass Figure 2.5 Different types of glass

28 Chapter 3 DESIGN PROCEDURES FOR GLASS STRUCTURAL COMPONENTS 3. General There are quite many design methods available for design of glass structures. As the strength of glass material depends on many factors it is often more difficult to arrive at a unique method which can be used for the design. Many countries have identified methods which are best suited to their way of using glass & the environmental factors existing in their countries. 3.1 Simple design methods The most common way of designing glass panels so far has been the use of thumb rules. This method of designing glass structures using thumb rules often leads to an uneconomic design. In thumb rule design it is made sure that the maximum in plane principal stress is less than or equal to the permissible in plane stress which is obtained after applying a global factor of safety on the strength obtained in the experiments. The design method which is most commonly used in European countries is the damage equivalent load resistance method in which it is seen that the maximum principal design stress is less than a resistance value which is based on many factors like surface stress distribution, area, load duration, load combination and environmental factors, the actual strength values obtained in the experiments etc. Another model which is used for designing glass structures is glass failure prediction model. This model takes into account all the statistical data of surface flaw parameters, stress-time relationship and gives a prediction of probability of failure. But for this method also the exact location of critical flaw has to be determined for finding the stress-time relationship which is not easy. Based on the experimental values & statistical data of the surface flaw parameters the probability of failure is determined.

29 Since there is so much of dependence of design strength on surface flaw parameters it is suggested to avoid float glass for structural purposes. Instead, the use of heat strengthened and tempered glass makes the design process quite easy. As the cracks or flaws do not open up in compression the failure strength greatly increases. The only problem that is encountered in the usage of heat strengthened and toughened glass is that there is large variation in the amount of pre compression done for specimens from the same manufacturer. Most of the manufacturers simply produce float glass or annealed glass. The pre compression or the heat treatment is done by other agencies who take the float glass, process it and sell them in the market. According to the Australian code heat strengthened glass should have minimum pre compression of and tempered glass should have a minimum pre compression of 70. These values should be checked for all the specimens using any of the non-destructive tests.

30 Chapter 4 EXPERIMENTAL INVESTIGATION OF GLASS AS STRUCTURAL MATERIAL 4. General Different specimens of glass panels with different glass types with varying thickness for two different aspect ratios were selected for conducting simple compression, flexure and tensile strength tests. A total of 120 samples were tested. 48 specimens were selected for flexure, 48 specimens were selected for compression and the remaining 24 specimens were selected for conducting tension tests. 4.1 Experiments carried out Appendix A gives the dimensions and specifications of the glass that were used in flexure, compression and tensile strength testing. The parameters that were varied in the tests were type of glass, thickness and the aspect ratio. 4.2 Three point flexure test Different sizes of glass panels are tested for three point flexure setup as shown in the figure. 48 specimens were tested for flexure. The parameters varied for different tests were aspect ratio, type of glass and thickness. Figure 4.1 Loading of the flexure test specimen A load controlled test was carried out. The experimental setup for three point flexure test is as shown below.

31 Figure 4.2 Flexural strength test setup Description of flexure test setup The roller & the hinge supports were made using steel plates & cylindrical rods and fixed on two concrete cubes of 600 mm size as shown in the figure. Two plywood plates were placed on these supports so that it forms a point of contact between glass specimen and steel support. The centre of the specimen is marked and a strain gauge of 10 mm capacity is placed at the centre of the specimen to measure the longitudinal strain on the tension side of the specimen. An LVDT of 100 mm capacity is placed at the centre to measure the deflection of glass specimen under flexure. The loading arm of the machine, as shown in the figure, is connected to a load cell of 10 tonnes capacity. Load is applied on the specimen through load cell. A transparent cello tape is pasted on to the compression portion of the heat strengthened & tempered glass specimen so that the broken pieces of glass do not fly off after failure. The entire machine is hydraulically operated. Both the LVDT & the strain gauge are connected to the computer and calibrated accordingly to get the deflection readings directly. Following is the figure that shows the roller and hinge support used for the flexure experiments.

32 Hinge Roller Welded Figure 4.3 End supports for flexure test 4.3 Compression test Different glass panels with varying thickness and aspect ratio are tested for in-plane compressive load. The load is applied as shown in the figure below. Figure 4.4 Loading of the compression test specimen The experimental setup for in plane compression tests is as shown below. Figure 4.5 Compression strength test setup

33 4.3.1 Description of compression test setup A steel plate is firmly fixed on the ground with the help of plaster-of-paris and over this plate the hinge support is placed. The two end supports provided in this experiment are hinged. The other hinge support is fixed to the load cell. Each hinge support is welded with two small angles so that they form a groove in which the specimen can rest in without moving away from the position. The specimen is then placed in the two grooves on top and bottom and with the help of a laser it is made sure that the specimen is standing truly vertical. Two LVDTs are placed at the centre of specimen to measure the mid deflection. Both the LVDTs are connected to a computer and calibrated accordingly. The load is applied through load cell of 10 tonnes capacity. Following hinge supports were made and used for conducting all the compression tests. Figure 4.6 End supports for compression test 4.4 Tension test This test was carried out to get an estimate of the tension capacity of the glass panel. Bolts are used to hold glass through the holes near the edge of the panel and tension is applied to the glass through the frame connected to the bolts as shown in the figure. To avoid direct contact of bolt and glass panel a rubber sheet is wrapped to the bolt.

34 Figure 4.7 Loading of the tension test specimen The experimental setup for tension test is as shown below. Figure 4.8 Tensile strength test setup Description of tension test setup The test setup is similar to the compression test set up. The only change is that instead of the two hinge support a holding frame is made using steel plates so that it can hold the glass specimen with the help of a bolt passing through the circular hole provided in the specimen. Also a steel beam is anchored to the ground over which the bottom holding frame is welded to allow for applying tension to the specimen. Two strain gauges are placed at the centre of the specimen in two perpendicular directions to measure both longitudinal strain and the lateral strain. A piece of rubber sheet is placed in between the contact surface of bolt and the glass material to avoid stress concentration.

35 Glass specimen Holding frame Figure 4.9 Arrangement of tension specimen between the holding frames 4.5 Experimental Results The experiments were conducted on glass specimens for flexure, compression & tension. In the flexure tests & the compression tests conducted on glass specimens the parameters varied were thickness, type of glass & aspect ratio. In most of the failure patterns observed, there was a similarity found in the way the glass specimen fails. In case of flexure tests the failure appeared as if the strain energy stored suddenly releases in the form of a wave and shatters the entire glass specimen Flexural test results Following results were obtained for the three point flexure tests Table 4.1 Maximum load and deflection for annealed glass in flexure Annealed Glass Sl no Dimension (in mm 2 ) Thickness t Max Load Max Mid Deflection Young s Modulus (mm) P uf X 10-2 δf E af kn mm X X X X X X X

36 8 200 X X X X X X X X X Average Value Standard Deviation 8716 Table 4.2 Maximum load and deflection for heat strengthened glass in flexure Sl no Dimension (in mm 2 ) Heat Strengthened Glass Thickness Max Load t P uf X 10-2 (mm) kn Max Mid Deflection δ f Young s Modulus E hf mm X X X X X X X X X X X X X X X X Average Value Standard Deviation 2489

37 Table 4.3 Maximum load and deflection for tempered glass in flexure Sl no Tempered Glass Dimension Thickness Max Load (in mm 2 ) t P uf X 10-2 (mm) kn Max Mid Deflection δ f Young s Modulus E tf mm X X X X X X X X X X X X X X X X Average Value Standard Deviation Load P X 0.01 in kn Mid deflection in mm 6 mm 8 mm 10 mm 12 mm Figure 4.10 Load vs deflection for Annealed Glass 200 mm X 600 mm in flexure of various thicknesses

38 Load P X 0.01 in kn Mid deflection in mm 6 mm 8 mm 10 mm 12 mm Figure 4.11 Load vs deflection for Annealed Glass 200 mm X 1200 mm in flexure of various thicknesses Load P X 0.01 in kn Mid deflection in mm 6 mm 8 mm 10 mm 12 mm Figure 4.12 Load vs deflection for HS Glass 200 mm X 600 mm in flexure of various thicknesses

39 Load P X 0.01 in kn Mid deflection in mm 6 mm 8 mm 10 mm 12 mm Figure 4.13 Load vs deflection for HS Glass 200 mm X 1200 mm in flexure of various thicknesses Load P X 0.01 in kn mm 8 mm 10 mm 12 mm Mid deflection in mm Figure 4.14 Load vs deflection for Tempered Glass 200 mm X 600 mm in flexure of various thicknesses

40 Load P X 0.01 in kn mm 8 mm 10 mm 12 mm Mid deflection in mm Figure 4.15 Load vs deflection for Tempered Glass 200 mm X 1200 mm in flexure of various thicknesses Load P X 0.01 in kn Mid deflection in mm Annealed Heat Strengthened Tempered Figure 4.16 Load vs deflection for various types of glass of size 200 mm X 600 mm X 6mm in flexure

41 Load P X 0.01 in kn Mid deflection in mm Annealed Heat strengthened Tempered Figure 4.17 Load vs deflection for various types of glass of size 200 mm X 1200 mm X 6mm in flexure Load P X 0.01 in kn Annealed Heat strengthened Tempered Mid deflection in mm Figure 4.18 Load vs deflection for various types of glass of size 200 mm X 600 mm X 8 mm in flexure

42 Load P X 0.01 in kn Mid deflection in mm Annealed Heat strengthened Tempered Figure 4.19 Load vs deflection for various types of glass of size 200 mm X 1200 mm X 8 mm in flexure Load P X 0.01 in kn Mid deflection in mm Annealed Heat strengthened Tempered Figure 4.20 Load vs deflection for various types of glass of size 200 mm X 600 mm X 10 mm in flexure

43 Load P X 0.01 in kn Mid deflection in mm Annealed Heat strengthened Tempered Figure 4.21 Load vs deflection for various types of glass of size 200 mm X 1200 mm X 10mm in flexure Load P X 0.01 in kn Mid deflection in mm Annealed Heat strengthened Tempered Figure 4.22 Load vs deflection for various types of glass of size 200 mm X 600 mm X 12 mm in flexure

44 Load P X 0.01 in kn Annealed Heat strengthened Tempered Mid deflection in mm Figure 4.23 Load vs deflection for various types of glass of size 200 mm X 1200 mm X 12mm in flexure

45 4.5.2 Compression test results Table 4.4 Maximum buckling load for annealed glass in compression Annealed Glass Sl no Dimension Thickness (in mm 2 ) t Max Load P uc X 10-2 kn Young s Modulus E ac (mm) X X X X X X X X X X X X X X X X Average Value Standard Deviation 9795

46 Table 4.5 Maximum buckling load for heat strengthened glass in compression Heat Strengthened Glass Sl no Dimension Thickness Max Load Young s Modulus E hc (in mm 2 ) t (mm) P uc X 10-2 kn X X X X X X X X X X X X X X X X Average Value Standard Deviation 7348

47 Table 4.6 Maximum buckling load for tempered glass in compression Tempered Glass Sl no Dimension Thickness Max Load Young s Modulus E tc (in mm 2 ) t (mm) P uc X 10-2 kn X X X X X X X X X X X X X X X X Average Value Standard Deviation 6939

48 Axial Load P X 0.01 kn Mid Lateral Deflection in mm 6 mm 8 mm 10 mm Figure 4.24 Load vs deflection for AG (200 mm X 600 mm) for different thickness in compression Axial Load P X 0.01 kn Mid Lateral Deflection in mm 6 mm 8 mm 10 mm 12 mm Figure 4.25 Load vs deflection for AG (200 mm X 1200 mm) for different thickness in compression

49 Axial Load P X 0.01 kn Mid Lateral Deflection in mm 6 mm 8 mm 10 mm 12 mm Figure 4.26 Load vs deflection for HG (200 mm X 600 mm) for different thickness in compression Axial Load P X 0.01 kn Mid Lateral Deflection in mm 8 mm 10 mm 12 mm 6 mm Figure 4.27 Load vs deflection for HG (200 mm X 1200 mm) for different thickness in compression

50 Axial Load P X 0.01 kn Mid Lateral Deflection in mm 6 mm 8 mm 10 mm 12 mm Figure 4.28 Load vs deflection for TG (200 mm X 600 mm) for different thickness in compression Axial Load P X 0.01 kn Mid Lateral Deflection in mm 6 mm 8 mm 10 mm 12 mm Figure 4.29 Load vs deflection for TG (200 mm X 1200 mm) for different thickness in compression

51 Axial Load P X 0.01 kn Mid Lateral Deflection in mm Annealed Heat strengthene d Figure 4.30 Load vs deflection for 6 mm glass (200 mm X 600 mm) in compression Axial Load P X 0.01 kn Annealed Heat strengthened Tempered Mid Lateral Deflection in mm Figure 4.31 Load vs deflection for 6 mm glass (200 mm X 1200 mm) in compression

52 Axial Load P X 0.01 kn Mid Lateral Deflection in mm Annealed Heat strengthened Tempered Figure 4.32 Load vs deflection for 8 mm glass (200 mm X 600 mm) in compression Axial Load P X 0.01 kn Mid Lateral Deflection in mm Annealed Heat strengthened Tempered Figure 4.33 Load vs deflection for 8 mm glass (200 mm X 1200 mm) in compression

53 Axial Load P X 0.01 kn Mid Lateral Deflection in mm Annealed Heat strengthened Tempered Figure 4.34 Load vs deflection for 10 mm glass (200 mm X 600 mm) in compression Axial Load P X 0.01 kn Mid Lateral Deflection in mm Annealed Heat strengthened Tempered Figure 4.35 Load vs deflection for 10 mm glass (200 mm X 1200 mm) in compression

54 Axial Load P X 0.01 kn Heat strengthened Tempered Mid Lateral Deflection in mm Figure 4.36 Load vs deflection for 12 mm glass (200 mm X 600 mm) in compression Axial Load P X 0.01 kn Mid Lateral Deflection in mm Annealed Heat strengthened Tempered Figure 4.37 Load vs deflection for 12 mm glass (200 mm X 1200 mm) in compression

55 4.5.3 Tension test results Table 4.7 Ultimate load for annealed glass in tension Annealed Distance of centre of hole from edge e mm Ultimate Load P ut X (kn) Pseudo tensile strength σ at Table 4.8 Ultimate load for heat strengthened glass in tension Heat strengthened Distance of centre of hole from edge e mm Ultimate Load P ut X (kn) Pseudo tensile strength σ ht

56 Table 4.9 Ultimate load for tempered glass in tension Tempered Distance of centre of hole from edge e mm Ultimate Load P ut X 10 (kn) -2 Pseudo tensile strength σ tt A typical load versus axial deformation curve for the tension specimen is as shown below. All graphs showed this similarity. Following graph is for annealed glass of size 400 mm X 100 mm and the holes are 65 mm away from the edge of the panel.

57 Axial load in kn Axial elongation in mm Figure 4.38 Load vs axial elongation for 8 mm annealed glass (100 mm X 400 mm) in tension

58 Chapter 5 ANALYSIS AND DISCUSSION 5. General All the results obtained from the three tests were tabulated. The results were then processed and interpreted in the form of graphs and table to understand the behaviour of glass. 5.1 Comparison of Young s modulus for different types of glass From the results obtained from the flexural tests for different types of glass it is observed that the average modulus of elasticity value calculated using the flexure formula for all the three types of glass is almost same but the standard deviation of Young s modulus value for tempered glass is less than annealed and heat strengthened glass. Table 5.1 Average Young s modulus obtained for different types of glass in flexure Flexure Tests Young s Modulus in Parameter Average Value Standard Deviation Annealed Glass Heat Strengthened Glass Tempered Glass Similar pattern was observed in compression tests. The average Young s modulus values calculated using Euler s buckling equation for all the three types of glass were almost same but the standard deviation is found to be more for annealed glass than compared to heat strengthened or tempered glass.

59 Table 5.2 Average Young s modulus for different types of glass in compression Compression Tests Young s Modulus in Parameter Average Value Standard Deviation Annealed Heat Strengthened Tempered Glass Glass Glass This implies that variation of Young s modulus is found less in tempered glass than in annealed glass. So a higher factor of safety is to be used for annealed glass to account for this variation in Young s modulus values. 5.2 Comparison of Young s modulus values obtained in different experiments The variation in Young s modulus values were also found for the same type of glass determined through different experiments i.e. from flexure and compression. Following graphs show the variation of Young s modulus in flexure and compression.

60 Young's modulus, E Flexure Compression Thickness 't' mm Figure 5.1 Variation of Young s modulus with thickness for annealed glass 200 mm X 600 mm Young's modulus, E Flexure Compression Thickness 't' mm Figure 5.2 Variation of Young s modulus with thickness for annealed glass 200 mm X 1200 mm

61 Flexure Compression Young's modulus, E Thickness 't' mm Figure 5.3 Variation of Young s modulus with thickness for HS glass 200 mm X 600 mm Young's modulus, E Flexure Compression Thickness 't' mm Figure 5.4 Variation of Young s modulus with thickness for HS glass 200 mm X 1200 mm

62 Young's modulus, E Flexure Compression Thickness 't' mm Figure 5.5 Variation of Young s modulus with thickness for toughened glass 200 mm X 600 mm Young's modulus, E Flexure Compression Thickness 't' mm Figure 5.6 Variation of Young s modulus with thickness for TG 200 mm X 1200 mm From the graphs we can clearly see that for the same type of glass and for samples of size 200 mm X 600 mm, the Young s modulus values calculated for flexure tests are relatively higher than that calculated for compression tests. But for samples of size

63 200 mm X 1200 mm the Young s modulus values calculated for flexure tests are relatively lesser than those calculated for compression tests. This implies that same Young s modulus values cannot be used for a given glass material if the loading actions on it are different. Also the variation of Young s modulus was found for different aspect ratios of the same type of glass and hence a factor should be used to take care of this size effect as well. 5.3 Application of mechanics The annealed glass resists the moment applied to it due to external load by flexure action. As the stress values obtained for annealed glass from flexure formula and those obtained from the strain readings are very close the flexure formula can be used for glass. The same phenomenon can be extended to heat strengthened and tempered glass. As both heat strengthened and tempered glass are pre compressed near the edges and pre tensioned at the mid surface, assuming the variation of this pre stress as linear across the thickness of glass for our understanding, the moment resisting mechanism can be interpreted as shown below with the help of a stress diagram of the glass across its thickness. Stage 1: Initial application of stress due to external loads C C 1 C + C 1 C T 1 C - T 1 Stage 2 :An intermediate stage where in bottom fibre stress becomes equal to zero C C 2 C + C 2 C T C -

64 Stage 3 Final stage of stress just before failure occurs C C C + C T C - Figure 5.7 Load resisting phenomenon in heat strengthened or toughened glass in three stages Following are the tables that show the variation of final stress across thickness in all the three types of glasses for different thickness and of dimension 200 mm X 600 mm. It is to be noted that as the initial stress values for heat strengthened and tempered glass that are used in the experiment are not known exactly hence the minimum stress values that characterise the glass as heat strengthened or tempered according to the European code have been taken for understanding. Table 5.3 Variation of final stress in 6 mm annealed glass in flexure across thickness Thickness mm Initial stress 6 mm Annealed Glass of 200 mm X 600 mm Stress from Load Stress from Strain reading Total Stress

65 Table 5.4 Variation of final stress in 6 mm HS glass in flexure across thickness Thickness mm 6 mm Heat Strengthened Glass of 200 mm X 600 mm Initial stress Stress from Load Stress from Strain gauge Total Stress Table 5.5 Variation of final stress in 6 mm tempered glass in flexure across thickness Thickness mm 6 mm Tempered Glass of 200 mm X 600 mm Initial stress Stress from Load Stress from Strain gauge Total Stress

66 Table 5.6 Variation of final stress in 8 mm annealed glass in flexure across thickness Thicknes s mm 8 mm Annealed Glass of 200 mm X 600 mm Stress from Load Initial stress Stress from Strain gauge Total Stress Table 5.7 Variation of final stress in 8 mm HS glass in flexure across thickness Thickness mm 8 mm Heat Strengthened Glass of 200 mm X 600 mm Initial stress Stress from Load Stress from Strain gauge Total Stress

67 Table 5.8 Variation of final stress in 8 mm tempered glass in flexure across thickness Thicknes s mm 8 mm Tempered Glass of 200 mm X 600 mm Stress from Load Initial stress Stress from Strain gauge Total Stress Table 5.9 Variation of final stress in 10 mm annealed glass in flexure across thickness Thicknes s mm 10 mm Annealed Glass of 200 mm X 600 mm Initial stress Stress from Load Stress from Strain gauge Total Stress

68 Table 5.10 Variation of final stress in 10 mm HS glass in flexure across thickness Thicknes s mm 10 mm Heat Strengthened Glass of 200 mm X 600 mm Initial stress Stress from Load Stress from Strain gauge Total Stress Table 5.11 Variation of final stress in 10 mm tempered glass in flexure across thickness Thicknes s mm 10 mm Tempered Glass of 200 mm X 600 mm Initial stress Stress from Load Stress from Strain gauge Total Stress

69 Table 5.12 Variation of final stress in 12 mm annealed glass in flexure across thickness Thicknes s mm 12 mm Annealed Glass of 200 mm X 600 mm Initial stress Stress from Load Stress from Strain gauge Total Stress Table 5.13 Variation of final stress in 12 mm HS glass in flexure across thickness Thicknes s mm 12 mm Heat Strengthened Glass of 200 mm X 600 mm Initial stress Stress from Load Stress from Strain gauge Total Stress

70 Table 5.14 Variation of final stress in 12 mm tempered glass in flexure across thickness Thicknes s mm 12 mm Tempered Glass of 200 mm X 600 mm Initial stress Stress from Load Stress from Strain gauge Total Stress Discussion on tension tests The following table shows the Poisson s ratio values captured at the ultimate load for different glass specimens. Table 5.15 Poisson s ratio obtained in different tensile specimens Specimen Name Poisson s ratio from strain gauge reading Specimen Name Poisson s ratio from strain gauge reading 9a a b b a a b b a a b b a a b b a a b b Average = The variation in the ultimate load capacity of glass for different edge distances is plotted below. It can be seen that the ultimate capacity increases as the edge distance increases.

71 Distance of centre of hole from edg mme Annealed Glass Ultimate Load in kn Figure 5.8 Variation of ultimate loads for different edge distances for annealed glass Distance of centre of hole from edge mm Heat Strengthened Ultimate Load in kn Figure 5.9 Variation of ultimate loads for different edge distances for heat strengthened glass Distance of centre of hole from edge mm Toughened Glass Ultimate Load in kn Figure 5.10 Variation of ultimate loads for different edge distances for tempered glass

72 5.5 Results of percentage variation in Young s modulus The following table shows the percentage variation of Young s modulus for different types of samples. Table 5.16 Percentage variation of Young s modulus Percentage variation of Young's modulus Specimen Annealed Glass Heat Strengthened Toughened Glass Flexure Compression Flexure Compression Flexure Compression 200 X 600 X X 600 X X 1200 X X 1200 X X 600 X X 600 X X 1200 X X 1200 X X 600 X X 600 X X 1200 X X 1200 X X 600 X X 600 X X 1200 X X 1200 X Breakage pattern The failure pattern exhibited by different types of glass is different. This difference is mainly attributed to the variation of pre compression stress across the thickness of the glass material. Annealed glass breaks into very large fragments after its failure and often may cause injury due to its sharp long failure edges. Also annealed glass doesn t give much deflection before failure. Following are some of the figures that show the failure pattern of annealed glass during the experiments.

73 Figure 5.11a Photograph showing failure pattern in annealed glass under flexure Figure 5.11b Photograph showing failure pattern in annealed glass under flexure Figure 5.12 Photograph showing failure pattern in annealed glass under tension It should be noted that the failure that occurred was only limited to the point of application of the load. In case of heat strengthened and tempered glass the failure

74 was abrupt and as soon as the ultimate load is reached the entire glass shatters and breaks into pieces. Heat strengthened glass breaks into fragments of considerable size. Based on the broken glass fragments size the effectiveness of heat strengthening is assessed. Following is the failure pattern of heat strengthened glass. Figure 5.13a Photograph showing failure pattern in heat strengthened glass under flexure Figure 5.13b Photograph showing failure pattern in heat strengthened glass under flexure

75 One noticeable pattern observed for heat strengthened glass during tension experiments was that instead of shattering into pieces, the glass just broke near the hole through which tension was applied. It behaved as an ordinary annealed glass. This pattern was observed in almost all the specimens that were tested. Following is the pattern observed for heat strengthened glass in tension applied through two holes which were provided in the glass Figure 5.14 Photograph showing failure pattern in heat strengthened glass under tension The tempered glass showed the typical failure pattern in all the experiments conducted. The glass broke into small fragments once the ultimate load is reached. It seemed as if the strain energy stored till the failure is suddenly released in the form of a wave that shatters the entire glass and breaks it into pieces. Following is the failure pattern for tempered glass. Figure 5.15a Photograph showing failure pattern in tempered glass under flexure

76 Figure 5.15b Photograph showing failure pattern in tempered glass under flexure Chapter 6 SUMMARY AND CONCLUSIONS 6. General The main aim of this work is to investigate glass as a structural material and evaluating the principal engineering property the Young s modulus. From the literature it is seen that there are no proper standard structural design codes/ guidelines available for the structural design of glass. It has been well documented that the strength of glass is largely affected by the presence of surface flaws and it is not feasible to carry out a complex finite element analysis every time to design a glass structure. The codes which