Development of a structural element of glass, with glass welding processes.

Literature

Cecile Giezen Delft, Januari 2008

Preface

Within the scope of my graduation project at Delft University of Technology, department of the Building Technology, this literature study is done. This report is part of the graduation report, “development of a structural element of glass, with glass welding processes”. It is a summery of the literature about glass I have read in the beginning of this graduation process. To understand the value of the designing structural applications of glass, it is important to give insight into the background of glass. This report helps you to understand the basic principles of designing with glass.

ii Index

H1. History of glass ...... 1 1.1 Development of glass...... 1 1.2 Glass in architecture...... 3 1.3 Development of structural glass...... 5 H2. Production of glass ...... 6 2.1 Primary manufacture...... 6 2.1.2 Glassblowing ...... 6 2.1.3 Casting ...... 7 2.1.4 Drawing ...... 7 2.1.5 Float glass process ...... 9 2.1.6 Glass tube production ...... 10 H3. Material properties of glass...... 11 3.1 Molecular structure of glass...... 11 3.2 Glass types...... 14 3.2.1 Quartz glass (fused silica) ...... 14 3.2.2 Soda lime glass ...... 14 3.2.3 Borosilicate glass ...... 15 3.2.4 Lead glass...... 16 3.2.5 Aluminosilicate glass ...... 16 3.2.6 Glass ceramics...... 16 3.3 Mechanical properties ...... 17 3.3.1 Mechanical quantities...... 17 3.3.2 Material properties...... 17 3.4 Thermal properties...... 20 3.5 Optical properties ...... 20 H4. Designing with glass...... 22 4.1 Surface condition ...... 22 4.2 Loading time ...... 22 4.3 Area dependence...... 22 4.4 Environment conditions ...... 23 4.5 Performing tests ...... 23 4.6 Safety ...... 23 H5. Strengthening possibilities...... 25 5.1 Introduction ...... 25 5.2 Heat threatening of glass ...... 25 5.2.2 Heat strengthened glass ...... 25 5.2.3 Tempered glass...... 26 5.3 Chemical strengthened glass ...... 26 5.4 Laminated glass...... 27 5.5 Limitations in strengthening ...... 27 H6. References of structural glass ...... 28 6.1 Glass footbridge, Rotterdam ...... 28 6.2 Yuraku-cho canopy underground, Tokyo ...... 28 6.3 Sainsbury Centre of visual arts...... 29 6.4 Glass Tense grid structure ...... 29 H7. Definitions...... 31 H1. History of glass

1.1 Development of glass

Natural glass has existed since the beginning of times. Due to volcanic eruptions and lightning strikes rock melts and afterwards cools down rapidly. The actually discovery of glass is described by Pliny (AD 23-79), an ancient- roman historian. According to him Phoenician merchants transported stones discovered glass in the region of Syria around 5000BC. The accidental discovery of glass took place while cooking pots on blocks of nitrate and in combination with the heat of the fire and the sand on the ground, the materials melted and were mixed together. This leaded to a opaque liquid.

When opening the pharaohs tombs in Egypt glass beads were discovered. This proved the intentional glass manufacture. The first handmade glass dates from 3500 BC. Glassmakers learned that adding metallic compounds and minerals could result in coloured glass. Around 1500 BC hollow glass objects are made by rolling molten glass on a slab of stone. The oldest description of the glass mixture dates from 668-626 BC. It says: “Take 60 parts of sand, 180 parts of ash from marine plants, 5 parts of chalk- and you will obtain glass”. This is still the basis of the composition of glass

Figure 1.1 Lotus goblet, Tutmosis III, 1500BC

Around 30 BC the glassblowing process was invented. Syrian craftsmen used a thin metal tube for shaping glass. Romans began to blow glass into moulds for hollow glass objects. They also introduced glass for architectural purposes, when bluish green transparent glass was discovered around AD 100. These glass panes were cast and drawn. Molten glass was poured on a table sprinkled with sand and then stretched out by drawing with iron hooks.

The technique that the Romans used was spread to northern Europe regions. During the middle ages the glassmaking technology in Europe changed. Due to difficulties of importing raw materials in northern Europe, soda glass was replaced by glass made out of potash from the burning of threes. Between Northern Europe and the Mediterranean area the glass differed from composition.

Around AD 1100 a German craftsman discovered how to make flat glass by making a hollow cylinder of 3m and a diameter of 45cm. A hollow sphere was blown and put in a pod for shaping the cylinder. The ends of the pod where cut off, the cylinder was cut in its longitudinal direction and spread out.

1

Another technique was blowing a glass ball and then opening it at the opposite of the pipe. When spinning the semi melted glass it flattened and increased in diameter size. This glass is called crown glass. In contrast to cylinder glass, crown glass was more even and more lustrous. This was also the time when stained glass windows are made. Every glass pane was surrounded by lead strips and joined together. The gothic cathedral with his big window panes is the best example of that time.

Figure 1.2 Glass blowing in early years

Figure 1.3 Crown glass

In the 15th till the 17th century the most important glass industry was built in Venice and especially on one of the islands of Venice, Murano. Still famous about its glass craft. It was the major producer of drinking glasses, bowls and mirrors. The success of the Venetian glass was due to the absence of colour. Adding brownstone and soda from the sea weed to the raw materials made the glass transparent and soft.

In the 17th century an English glassmaker Ravenscroft developed lead crystal. This glass has a high refractive index and therefore a brilliant surface. In France a new process was developed for the production of glass plates. The molten glass was spread out on a preheated copper table. It was pressed by a water-cooled metal roller. The pane sizes were 1.2mx2m. In this period glass endured a booming phase. Glass was not only sold to churches and monasteries, but also for houses and palaces.

During the industrial revolution, mainly in the later states, the mechanical technology of glass developed. The relationship between the composition of glass and the physical qualities became clearer. On of the most important glass maker is Otto Schott. He did research on the optical and thermal properties of glass by adding chemical elements to the basic mixture of glass.

Toward the beginning of the 20th century Michael Owens invented the automatic bottle blowing machine. And the Belgian Fourcault developed a way of drawing sheets for window glass. This glass was drawn vertically, while in the end of the first world war, another Belgian Bicheroux, developed a method where molten glass was pored through two rollers. This leaded to a more constant thickness.

In 1910 laminated glass was invented, by a French scientist Benedictus. And in 1928 the Pittsburgh process was developed by Libbey-Owens. In 1959 the float glass process was taken into use. This is how nowadays 90% of the glass panes is produced. This process was developed by Britain’s Pilkington Brothers. These production processes are explained in chapter H2.

2

1.2 Glass in architecture

Up until the 18th century architecture was made by natural available building materials. Buildings were made out of stone wood or clay. Glass was until that time too expensive for common citizens to use for windows. The inhabitants built their own houses. The styles and principals of the building depended on the location, climate, local resources, techniques and traditions. Daylight was essential for indoor life. An opening in the outer wall was the weakest spot, so that is why they were made pretty small. It leaded wind and weather to go through. To create an acceptable indoor climate, materials were used to create some kind of closure. This defers form region to region. In China for example they used paper to close and in Europe wooden frames and shutters.

During the gothic period (1100 AD), light became the interaction between the interior and exterior and between God and man. Between the load bearing columns of the cathedral the openings were filled with stained glass windows. The coloured pieces of glass were joined together by lead and created a bright mystic space.

The Baroque period light played a different role. Light was used to create space and dissolving the limits of space. The light church walls and ample windows and doors leaded to bright interiors. The increasing trend for opening up architecture augmented the demand for glass panes.

Figure 1.4 Gothic style, Notre Dame Paris

Figure 1.5 Baroque period, Cathedral de Salamanca

In houses also came a distinction in structure between load bearing and non- load bearing elements that made it possible to create bigger openings. However glass was still rare and precious and it was exclusively used for monasteries and churches.

The first iron used by mankind dates from the prehistory, but iron still was not used as an independent building material. Late 18th century the discovery of cokes to smelting iron ore, leaded to a better quality of iron. During the industrial revolution, due to the invention of the steam engine, the ability to produce cast iron boosted. The mechanical properties of steel to accommodate tensile stresses created new construction possibilities. The mass construction of stone walls now could be replaced by slender skeletons of columns and beams. The “opening of the wall” made the passage of light easier. The principle of iron in architecture leaded to enclosed spaces that were as transparent as possible. The palm house characterized this innovative architecture in glass and iron. The most famous examples are the Palm House and Crystal Palace in London.

3

Figure 1.6 Palmhouse London

Early 20th century architects took of the image of designing the box. Traditional solid cubic buildings were replaced by columns, planes and glazing facades that became autonomous elements in the façade. Around 1914 a new vision of architecture was introduced by poet Scheerbart. The world would no longer be determined by the mass and compactness of brick, but of openness and lightness of steel and glass.

Mies van der Rohe was the first architect who designed a office building, of which the façade was completely made out of glass and steel, Glass skyscraper Berlin. The skin slipped over the building as a curtain. This new concept of the skin was a revolution for the 20th century office building.

Figure 1.7 Mies van der Rohe Berlin Skyscraper

The real glass architecture boom started after World War II. Because of the development of the float glass process, glass became a more explored product. The dream of the glass house took over some architects life. Small residential buildings where built for experimentation of autonomy and transparency. It was Mies van der Rohe who could make his concept into reality. The curtain wall principle consists of a stainless steel frame that, detached from the load bearing structure, transfers the wind load. His first skyscraper with the glass skin was built in Chicago in 1951.

4 The conviction of enclosing complete buildings in a uniform glass skin became fashionable for the American skyscrapers. Mid 1960s new methods of fixing external glazing appeared, like the structural sealant façade. Glass panes were fixed with a sealant to its back construction. These kinds of skins were rarely transparent. The glass panes covered with a colour or a reflective coating did not permit a view trough the inside during the day.

Figure 1.8 Pudong Skyline, Shanghai

Because of the oil crisis during the 1970s glass facades was thrown into question of energy wastage. Colored and mirror glass were less in desire. Clear and transparent was the theme for architecture that time. The expressionist forms took over the rectangle form of the skyscrapers. And to minimize the impact of the joint glass fins or even steel wire frames behind the glass skin were used to make new kind of façade. This system was called the suspended glazing. To maximize the transparency point fixing element were designed to connect glass panes to the background frame. The glass skin became a symbol of prestige.

During the 90s the creative possibilities of glass were much more explored. The aesthetic possibilities of reflection, translucence, colour, sandblasting, coatings and changing of lights were mixed together to create a new kind of image. Even giving a media function to all-glass facades was not an exception anymore.

Figure 1.9 Fire and Police Station Berlin

1.3 Development of structural glass

Nowadays the accent of the development of glass lays in a more structural way. Engineers are trying to build the same way with glass as we build in steel and concrete. Glass columns and beams are developed to make a construction as transparent as possible. Even metal joint to connect glass parts are being replaced by glued joints to improve the transparency. Because of the brittle character of glass, it behaves unpredictable to failure and therefore for it is unreliable for structural use. The safety is one of the most important considerations and now the search for solutions to improve the safety aspects is underway.

5 H2. Production of glass

2.1 Primary manufacture

The most important ingredients for making glass is simply pure sand (silicium oxide). To meld sand, a high temperature is required. For sustainability reasons sodas (Na2CO3) are added to lower the melting temperature, glass splinters are added to lower the viscosity and calk (CaO) and oxides like Al2O3 are added for better resistance to weather influences.

The ingredients, sand, soda, calk and glass splinters, are melted and mixed together at a temperature of +1500ºC in an oven surrounded by fire stones. Afterwards the mixture will be cooled down till 1100ºC. At this point it is possible to deform glass in every shape. As a liquid is cooled its viscosity normally increases. Annealing is a critical process in the manufacture of glass. Annealing is cooling down the glass product, for soda-lime glass form 600ºC to 100ºC, in controlled conditions, so it won’t crack due to temperature differences and cutting process. The relationship between the viscosity and the working temperatures of different kind of glasses is shown in Figure 2.1

Figure 2.1 Viscosity of a few glass types at different temperatures

The most important processes of forming glass are: blowing, casting, drawing and the float process.

2.1.2 Glassblowing

From the start of the Christian era, Syrians discovered a blowing method for shaping glass. It is a method of forming glass, while the glass is in a molten/ semi-liquid state. The major tools for blowing glass are the furnaces, the blowpipe and instruments of shaping and cutting the glass. Three furnaces are needed in this process. The first is to melt the glass ingredients together; the second is used for reheating and the third for annealing the shaped glass. For the glassblowing process, the blowpipe is preheated at the top and tipped in the molten glass mixture. The molten glass will stick to the pipe like honey to

6 a spoon. The glass blob is rolled upon a steel sheet and air can be blown into the pipe to create a bubble of glass. A rotating movement of the pipe is needed to make sure that the semi-liquid glass will not drop of the pipe so a symmetrical form is created. This is how in early days window-glass was made. A cylinder with a cross-section of 0.5 m was blown and then cut in its longitudinal direction and spread-out. The thickness of the plate was pretty irregular. Nowadays glassblowing is only used for luxury objects.

2.1.3 Casting

Casting is a process where a liquid material is formed by a mould. The material should be allowed to harden in the mould and than be detached. The glass mould is made of resin, silica sand and a catalyst. Designs are carved into the mould and melded glass is scattered in the mould. The glass takes the textures of the mould. For the (mass) production of bottles and glasses there are a few principles developed; the press-blow process, the suck-blow and the blow-blow process. Every bottle will be preformed and put in a different mould for final shaping. Afterwards the glass is press-relieved annealed.

Figure 2.2 Press-blow process

Figure 2.3 Blow-blow process

2.1.4 Drawing

Around 1900 a new production method was developed, drawing of glass. The intension of this process was to find a method to manufacture cheap flat glass for windows. The first drawing process set into action was the (1) Fourcault process. Later on some other processes were developed, such as the (2) Libbey-Owens process, (3) the Pittsburgh process and (4) the Bicheroux process. The drawing processes are nowadays rarely used for the production of flat glass.

7 Fourcault process This process was introduced in 1904 and it is vertical drawing process. A fire clay bar is floating on the molten glass and pushed down into the glass while the molten glass wells up and is taken by a long stick to which it sticks. The stick is pulled up and a film of the molten glass is formed. During drawing through rollers, the glass is cooled en annealed. The glass sizes varied form width of 1,9 to 2,3m and the thickness depends on the drawing speed. The main problem in this process is the crystallisation of glass when cooling down is not controlled. Next to that the glass surface is undulating.

Figure 2.4 Fourcault Process

Libbey-Owens process This process is also called the Colburn process and was introduced in 1905. In this process there is no problem with crystallization of the glass because the glass is drawn into an annealing lehr. The glass surface shows less drawing stripes then the Fourcault process. The drawing process is done through knurled rollers and after reheating and softening it is bent over a roller to get the glass into a horizontal position. Due to the contact with the rollers damage or degradation of the glass surface occurred. Figure 2.5 Libbey-Owens process

8 Pittsburgh process This is the vertical version of the Libbey-Owens process. The rollers aren’t needed anymore so contact with the surface is avoided. The problem with vertical drawing is gravity. While drawing the glass gravity can cause differences in thickness of the glass panes.

Bicheroux process This process was developed in the 1920s. Glass was poured between two rollers to maintain a constant thickness. This process was only for the production of glass panes and the lack continuity of the polishing and grounding leaded to new developments in the glass industry.

2.1.5 Float glass process

This method was introduced by Sir Alastair Pilkington in 1959 and now 90% of the window-glass is produced this way. The name of the process derives from the method, where a continuous ribbon of glass moves out of the melting furnace (1500 ºC) and floats along a bath of tin (1000ºC). Because the density of glass is smaller than the density of tin it floats. It forms a ribbon of 3210mm width and a thickness between 3 and 25mm. The thickness of the glass panes is dependent on the velocity of the transportation of the glass over the tin bath. Because tin has a flat surface, the glass becomes, because of its viscosity, flat too. When leaving the bath of tin (600ºC), glass is cooled down under controlled temperatures. The glass is now hard enough and can be transported by rollers. After cooling the glass will be controlled, cut in pieces of 6000x3210 and stocked by quality range.

The floated sheets do not have the brilliant fired quality as the older sheets processes; the side which has been in contact with the air (facing up) is flatter and smoother than the tin side. The process runs continuously, 24 hours a day and 7 days a week. Stopping the process will lead to energy loss when reheating the oven and solidification of the tin. Another disadvantage of this process is the lack of flexibility in this process. Using different kinds of mixtures, like for colouring glass, will delay the production and its efficiency. A float line production for every different colour could be the only alternative.

Figure 2.6 1. The basic ingredients of glass are mixed together Float glass 2. The batch materials is put into the furnace and are melted at a temperature of +1500ºC production 3. A continuous ribbon of glass is floated on a bath of tin 4. The glass is moved along rollers and coatings are applied 5. Glass is annealed and gradually cooled down to a temperature of 200ºC 6. Cutting the glass at sizes of 3,2m width and 6 m long ready for transportation.

9 2.1.6 Glass tube production

The production of tube glass is also done by drawing. Tube glass is mainly used for the lighting and laboratory industry. In the beginning hand drawn glass was used, but in 1917 the American Danner developed a mechanical drawing machine. Molten glass flows slowly down on a rotating ceramic drawing pen. The molten glass distributes and is couth and finally drawn by the drawing machine. The sizes of the tubes vary form 1-70mm. In 1929 another drawing process was developed by Vello. The molten glass flows from the feeder over a tapered pin. The air is blown through this pin to form the pipe. The glass is drawn at the end of this pin and bended to horizontal position. The production of big glass tubes with a diameter of 350mm and bigger, are drawn vertically. This process is developed by Corning. The molten glass flows into a slowly rotating plate. In the middle of this plate is an opening with a drawing edge. The glass flows over the edge and is drawn into the right tube diameter. A rotating cylinder in the glass controls the temperature and next to the drawing edges are coolers installed to influence the thickness.

Figure 2.7 Danner process

Figure 2.8 Danner process

Figure 2.9 Vello Process

Figure 2.10 Corning Process

Another technique is extrusion. This technique is derived form the metal and polymeric industry. These materials have a yield point so that cold extrusion is possible. Because glass can not be deformed plastically in solid state, it is heated to a temperature so that the viscosity lowers. In contradiction to other processes crystallization could occur, but with this process glass is not heated till its crystallization temperature. When glass is plastic enough the extrusion takes place under high compression.

Figure 2.11 Extrusion process

10 H3. Material properties of glass

3.1 Molecular structure of glass

Glass is transparent and it breaks fast.

But why does glass behave this way? Glass is transparent and brittle because of the organisation of the molecules. The most important ingredients for making glass are simply pure sand (silicium oxide). Quartz and all other types firstly consist of silicium and oxygen atoms, which form a covalent bond. A covalent bond is a form of chemical bonding in which both atoms share a minimum one electron. In case of quartz and quartz glass, a silicium atom has covalent bonds with four oxygen atoms. These atoms can’t be organized in a plane. They form a three- dimensional structure, called a tetrahedron.

Figure 3.1 The covalent bond

Figure 3.2 Schematic reproduction of a tetrahedron of silicium and oxygen bond.

In general there is a difference between the molecular structure of solids, liquids and gasses. There is a change in the properties of the material in these different phases such as density and strength. When a material is transformed from a liquid to a solid state, molecules will form a regular lattice and the volume will decrease. This process is called crystallisation. Crystallisation does not appear in a glass melt, because of the fast and controlled cooling down process of the melted glass. The molecular structure in liquid state will be as if frozen. No formation of crystals occurs which would destroy the transparency. The molecular structure of glass is motionless, like a solid, but it contains gaps, because of the random constitution, like a liquid. Glass has a so-called amorphous structure.

11 Figure 3.3 Relation between volume and temperature which shows the crystallization process

Figure 3.4 Crystal structure

Figure 3.5 Amorphous structure A

o

t

All light is either transmitted, absorbed or reflected:

I o = I t + I a + I r

Complete transparency means no absorption and no reflection. We consider light as a stream of photons. Each photon caries an amount of energy. The electrons in different materials vary in the range of energy that they can absorb. Per example when light hits a bulk metal, the energy of the photon is absorbed and reflected. Glass absorbs light of particular wavelengths. Common glass is opaque to wavelengths at the infrared and ultraviolet ends of the spectrum, but does not absorb visible light. The structure of glass or a liquid are irregular and the energy level of the covalent electrons are not corresponding by the energy level of photons of visible light, so the electrons can not absorb the energy of the photon. Light waves are not obstructed by the glass molecules. Imagine per example like water can flow through pebbles.

Figure 3.6 Light transmission of visible light and absorption of ultraviolet light

12

Deformation of a material is possible by breaking the atomic bond. Plastic deformation is accompanied by displacement of the molecular structure. Plastic deformation of glass is not possible because of its three dimensional amorphous structure. When glass is heated up, the bonds between the atoms can easily be broken and easily be fixed, like what happens with liquids. Elasticity means that a material can accommodate loads, deform and afterwards return into its original form. Per example metals have an elastic and plastic region. Metals are relatively soft, because it has a metallic bond. The electrons form an “electron cloud” and therefore can easily change position. While loading steel till its yield point, it will behave elastically. Loading it after its yield point, it will behave plastically. The material first gives a warning and permanent distortion occurs and fracture occurs when the material has experienced a significant degree of permanent deformation. This last part, the plastic behaviour, is the big difference to glass. At room temperature glass will break with no warning, it can not deform plastically. When heating the glass till its softening point it can be formed plastically.

The softening point of quartz is about 1700ºC. From a practical perspective, additions of modifiers and intermediates lower the melting point and the viscosity of glass. It makes it easier to form at lower temperatures.

Table 3.1 Modifiers and intermediates Modifiers Intermediates Melting and refining Preventing crystallisation

Sodium oxide Na2O calcium oxide CaO Sodium sulfate Na2SO4 Aluminium oxide Al2O3 Potassium oxide K2O Aluminium oxide Al2O3 Sodium nitrate NaNO3 Boric oxide B2O3 Magnesium oxide MgO Sodium chloride NaCL Zinc oxide ZnO Arsenious oxide As2O3 boron trioxide B2O3 Calcium fluoride CaF Carbon C Lower melting and Improve chemical Avoiding tiny gas bubbles Increases the viscosity while working temperatures stability cooling

13

3.2 Glass types

There are more than a thousand different kinds of glass, each with its own composition and properties. In this paragraph most common glass types are listed.

3.2.1 Quartz glass (fused silica)

SiO2 : 99.5%

Quartz (sand) is the most common material on earth. It consists of silicium dioxide (SiO2) and can be found in rock-crystals. Quartz glass is formed by melting pure quartz. It has an uninterrupted covalent bond of SiO4-monomers.

Figure 3.7 Structure of Quartz glass

Quartz glass had a high softening point (1665ºC) and is therefore difficult to blow. It is strong, doesn’t extend much and has a low coefficient of thermal expansion. This is why it can withstand high temperatures and has an excellent thermal shock resistance. Next to that quartz glass has a high transmission of ultraviolet (UV) radiation. Most applications of quartz glass are in optical branch, per example for a spectroscope (the mirrors in telescopes).

Advantages Disadvantages Table 3.2 Extremely low thermal coefficient of More expensive than other glasses Advantages and expansion disadvantages of The best shock resistance Due to high melting point more difficult to quartz glass work, higher fabrication costs Very low thermal conductivity Not standard available in flat sheets Can be melted, bent, drawn, welded into tubes and rods Harder than most glasses

3.2.2 Soda lime glass

SiO2 : 70% Na2O: 15% CaO: 10%

14 Soda lime glass is the kind of glass we use for most purposes. It consists of quartz, the most important ingredient for all glasses, soda (Na2O) to lower the melting point and calcium (CaO) for weather resistance. The softening point of soda lime glass is about 730ºC.This glass is widely used for windows, bottles, mirrors, light bulbs ect.

Figure 3.8 Structure of soda-lime glass

Advantages Disadvantages Table 3.3 Inexpensive and easy for mass production Poor thermal shock resistance Advantages and Low melting point, stay soft for a longer Sags easily at relative low temperatures disadvantages of time, so longer working time soda-lime glass Because of softness easier to cut Bad chemical resistance Easily tempered, due to high coefficient of Not as scratch resistant as borosilicate expansion and quartz

3.2.3 Borosilicate glass

SiO2 : 60-80% B2O3: 10-25% Al2O3: 1-4%

Borosilicate glass was developed in the late 19th century by Otto Schott, a German glassmaker. Nowadays it is better known as Pyrex, Kimax or Endural. Due to the boric oxides which are added to the glass mixture, borosilicate glass has a high resistance to thermal impacts and temperatures. It also has a excellent resistance to chemical attack. Borosilicate glass is mainly used for oven products and for laboratory ware in the chemical industry. It has a better resistance to acids than soda lima glass, but the resistance to alkalis is worse.

Advantages Disadvantages Table 3.4 Easier to work with and cheaper than Costs 3x more than soda lime Advantages and quartz disadvantages of Low coefficient of thermal expansion Can not be fully tempered like soda lime borosilicate glass compared to all glasses, except quartz Made by the float glass process Easily mouldable

15 3.2.4 Lead glass

SiO2 : 30-70% PbO: 29-82% Na2O: 5-20%

Lead glass is glass containing lead oxides (PbO). The addition of lead to the mixture increases the durability, lowers the melting point and decreases the hardness and it gives a high refractive index. This high refractive index gives a high brilliance to the glass. This type of glass is mainly used for decoration purposes. Glass with high lead oxide contents is used as a radiation shield in the nuclear industry. It absorbs gamma radiance. Lead glass does not withstand high temperatures or temperature changes.

3.2.5 Aluminosilicate glass

SiO2 : 62% Al2O3: 9% CaO: 7% B2O3: 0-10%

This kind of glass contains 10 times more aluminium than soda lime glass. It is also known as E-glass. E-glass is a type of borosilicate glass which is used for reinforcement of plastics with glass fibres for applications of high electric resistance. Due to the presence of the boric oxide it has a slightly better chemical durability, but has a slightly greater thermal expansion than borosilicate.

3.2.6 Glass ceramics

Glass ceramic materials have both the properties of glass as the properties of ceramics. When glass is overheated, small crystals are formed in the amorphous material, while remaining perfectly transparent. The production of glass ceramics involves the same techniques for preparing normal glass. After the production of the glass, the product is heated to a temperature of 750/1150C. A part of the structure will transform into fine-grained crystalline material. It is possible to achieve a partial or almost completely microcrystalline structure. Glass ceramics have a high resistance to thermal shock. It is hardly used in de construction industry, but popular as cookware.

16 3.3 Mechanical properties

3.3.1 Mechanical quantities

Density is defined as a mass per unit volume. m ρ = (3.1) V

The price of a material might be an important facture when choosing a material. Prices fluctuate dependent on the world’s economy.

Stress is the force per unit area within a body. F σ = (3.2) A0

Strain is de deformation of a body caused by a force. ΔL ε = (3.3) L

Yield strength, σy, is the stress at which the material strain changes from elastic deformation to plastic deformation. It is the point where permanent deformation occurs. Because glass does not have a plastic behavior the yield strength for glass is the strength at which it fails the ultimate strength.

Compressive strength, σc, is the normal stress at which a material, loaded in compression, crushes. Tensile strength, σt, is the normal stress at which a material breaks while pulling it.

Hardness is the resistance to permanent deformation. It is measured by a diamond point into the surface of the material. The hardness is reported in different units, but most commonly used is Vickers scale, Hv.

3.3.2 Material properties

The theoretical strength of a material is dependent on its chemical structure, the bonds between the atoms. If a glass body is subjected to a force, it will fail when reaching its maximum stress or it will deform and after removing the force, it will return into its original form. This is only valid at a temperature at which glass behaves elastic below Tg, the transformation temperature. Above Tg glass will show plastic deformation. The relationship between the applied force and the elongation at the linear region is defined by Hooke’s law. The young’s modulus is defined as the slope of the linear part of the stress-strain curve. It is a measure of the stiffness of a material.

F ΔL = L (3.4) EA σ E = (3.5) ε

17 In an isotropic material, like glass, the E-modulus is related as follows:

3G E = (3.6) 1+ G 3K E G = (3.7) 2(1+ v) E K = (3.8) 3(1− 2v)

When a material gets stressed, it will get thinner in one direction. The Poisson’s ratio, ν, is the negative of the ration of the lateral strain, ε2, to the axial strain, ε1, in axial loading. ε ν = − 2 (3.9) ε1 Generally the young’s modulus decreases at increasing temperatures, accept for quartz glass and some borosilicate glasses.

Figure 3.9 Young’s modulus at different temperatures.

The theoretical tensile strength of glass is according to Griffith 10.000- 30.000N/mm2. Due to the surface condition, material defects and flaws is the real strength significant smaller. Griffith showed that the critical size of the flaw, c, is dependent on the material property E and the specific surface energy, Gc.

EG σ = c (3.10) c πa

σc stress on body Gc facture toughness E Young’s modulus

σm stress at tip of the crack a half length of the elliptical crack

18 Stress at the tip of a crack can be defined by :

a σ = 2σ (3.11) m ρ

σ stress on body Gc facture toughness σm stress at tip of the crack a length of the crack ρ radius of the crack

Figure 3.10 Stress at crack tips

Fatigue means failure of a material by repeated stress. Glass always fails by brittle fracture. Cracks in the surface of the glass pane can grow at cyclic loading. Cracks only propagate during tensile stress; a compressive stress will block the cracks. When small flaws grow into bigger flows and grow till their critical crack length, glass can no longer stop the crack from moving to the

material. The glass falls into several pieces. The fatigue ratio, Fr, is the ratio of the fatigue limit to the yield strength.

σ e Fr = (3.12) σ y

σ e ≈ 0.9 ⋅σ t (3.13)

σe endurance limit σy yield strength σt tensile strength Fr fatigue ratio

The speed of crack grow is influenced by the air humidity, they grow faster in a humid environment. When the speed of crack grow is related to stress in a logarithmic graph, represents the slope the stress corrosion constant, n. The relation between the time of failure and the attend stress can be defined by:

σ nT = Constant (3.14) n stress corrosion constant T time σ stress

19

3.4 Thermal properties

Thermal conductivity, λ, is the rate at which heat is conducted through a solid at steady state. dT q = −λ (3.15) dX

Most materials expand when they are heated. The thermal strain per degree of temperature change is measured by the linear thermal-expansion coefficient, α. l dL α = (3.16) L dT

Specific heat, Cp, is the energy needed to heat a certain mass of material by 1ºC.

Thermal shock resistance, ΔTs, is the temperature difference at which the material can be quenched suddenly without damage.

Softening point is the temperature at which glass will elongate under its own weight.

Annealing point is the temperature at which the internal strain in glass is reduced to an acceptable limit.

3.5 Optical properties

The transmission coefficient, in optical way, is a measure of how much of an electromagnetic wave (light) passes through a surface.

The refractive index of a transparent material is defined as the speed of light though air divided by the speed of light in the material. When a light wave travels through different kind of media it will undergo a speed change. This will lead to a change of direction unless it encountered the boundary at the right angles. The light is bended in another direction. This effect is what we also see when we are laying in bath and looking through the water to our body. You will see that it deforms. The light striking the surface at an angle α and the light entering the material at an angle β.

c sinα n = = (3.17) v sin β

20 Figure 3.11 Refractive index

Table 3.5 Glass types and Quartz glass Soda lime Borosilicate Lead glass Alumino- their properties glass glass silicate glass

Density 2170-2200 2440-2490 2200-2300 3950-3990 2490-2300 kg/m3 Price 5140-8580 1160-1370 3430-5150 3400-5100 1170-1370 EUR/kg Young’s modulus 68-74 68-72 61-64 53-55 85-89 Gpa Hardness 450-950 440-485 84-92 475-525 68-75 Kg/mm2 Tensile strength 45-155 30-35 22-32 23-24 40-44 Mpa Yield strength 45-155 30-35 22-32 23-24 40-44 (elongation 0%) Mpa Compressive strength 1100-1600 360-420 264-348 232-244 400-440 MPa Coefficient of 0,55-0,75 9,1-9,5 3,2-4 8,82-9,18 4,11-4,28 10-6/K thermal expansion Thermal conductivity 1,4-1,5 0,7-1,3 1-1,3 0,82-0,86 1-1,5 W/(m.K) Poisson's ratio 0,15-0,19 0,21-0,22 0,19-0,21 0,23-0,24 0,23-0,24 Softening point 1665 726 820 631 ºC Annealing point 1140 540 560 437 ºC Strain point 1070 510 510 ºC Refractive index 1,46 1,52 1,47 1,56 1,55 Specific heat 680-730 850-950 760-800 850-950 700-800 J/kg.K

21 H4. Designing with glass

4.1 Surface condition

Glass has been used as a window pane for many years, but the popularity has grown to use glass as a load bearing construction. Designing with glass demands a detailed knowledge of the mechanical properties of glass. Glass has a very strong atomic bonding and therefore it has a very high compressive strength and a theoretical high tensile strength. Glass behaves elastically till it breaks and due to the irregularities of the surface, caused by moisture and contact with hard objects. There is a huge difference in the theoretical and practical strength of glass. The computable value of the strength will be around 1% of its intrinsic value. Therefore the maximum stress should be derived from a statistical variable, dependent on the degree of damage on the surface. The size and the distribution of the microscopic cracks determine the maximum stress. A glass surface with inherent damage has in average lower strengths, but a narrower distribution. When calculating with this average value means a lower risk of failure.

Figure 4.1 Distribution strength of new, weathered and damaged glass

4.2 Loading time

A glass object is allowed to higher stresses when it is subjected to sort term loads. The strength of glass decreases through chemical attacks on the surface cracks. The crack grows with long-term loads and therefore it is necessary to decrease the maximum stress of a component.

Figure 4.2 Relationship between strength of glass and the loading time

4.3 Area dependence

The same relationship applies to the size of the glass object. An glass pane with an area of 100m2 the probability of one pane breaking is 100 greater than a pane of 1m2, due to the distribution of flaws. This relationship is important when experimental tensile stresses are determined. Experiments mostly are done with relatively small objects.

22 4.4 Environment conditions

When the relative humidity is very low, it can have a great influence on the strength of glass. Normally in buildings the relative humidity is between 30% and 100%. Within this range the effect on the bending strength is not that great. The bending strength is also dependent on the temperature, but the normal temperature range in buildings does not influence the strength. The minimum strength lies at 200°C, but at extreme low temperatures glass and above the 300°C the strength increases.

4.5 Performing tests

Performing tests are often done with brand-new glass. Error! Reference source not found.is showing that the strength of new glass can be 100 times higher than glass which has been used. While performing strength tests with glass objects, the long term loading, attack of moisture and the irregularities should be taken into account.

Intrinsic value tensile strength 10.000 MPa Table 4.1 Intrinsic value compression strength 100.000 MPa Strength of glass After etching 400 MPa New 200 MPa Used 40 MPa Damaged 10 MPa

Figure 4.3 Relationship between the strength of glass and the depth of the crack

4.6 Safety

Glass is due to its brittle character unpredictable to use as a structural element. It can break spontaneously and due to the large distribution of its strength it is not possible to assign glass to the group of materials for which safe-life is guaranteed. To obtain a reliable construction attempt for compression stresses or strive for minimum tensile stresses.

When designing with glass you should pay attention to the following lines:

A glass element should be designed is such way so that life-guarantee is possible. This means that the structural system should be intact as well as the individual parts within the structure. If one part fails the whole structure should remain stable. Next to that it has to be ensured that the damaged glass element can not come out of the structure and cause harm.

23

A structure should comply with: 1. the ability to accommodate all stresses. 2. the limits of deflection.

In damaged condition a structure should comply with: • having residential lead bearing capacity; • having residential stability; • small deflections in the damaged elements as the structure as well.

Possibilities to improve the safe-life guarantee: • Strengthening of the material; • Redundant system; • Lamination; • Pre-stressing the construction; • Packing the construction with a capture construction.

24 H5. Strengthening possibilities

5.1 Introduction

Glass that comes directly from the factory is annealed. Annealed glass is made by the common float glass process. At the end of the tin bath the glass is controllably cooled down for removing all the stresses that occurred during the manufacturing process by temperatures. With the glass blowing process, when glass has been shaped by a torch flame, stresses have been introduced on the heated parts, than annealing can be done with a soft flame or an oven.

Annealed glass is the basic product for further production. Breakage of annealed glass is usually a simple one or two line fracture.

5.2 Heat threatening of glass

When annealed glass is reheated to 650ºC and than rapidly cooled down, glass will be subjected to heat treating. Depending on the air-flow for cooling down, tempered or heat strengthened glass is generated.

Figure 5.1 Heat strengthening of glass

5.2.2 Heat strengthened glass

If the annealed glass is reheated and slowly cooled down, the glass is twice as strong as the annealed glass (100MPa). This is called heat strengthened glass. It fails the same way as annealed glass and when broken it is more likely to remain in its frame, but loses its load bearing function. This kind of glass is mainly used for applications where load cases, like wind, snow or thermal loads, occur.

25

5.2.3 Tempered glass

Tempered glass is manufactured when annealed glass is reheated and rapidly cooled down with a high airflow. Because of the fast cooling down process, the surface of the glass undergoes a thermal impact and changes quickly from a semi-liquid state to solid state. On the contrary the core of the glass has not had the time to cool down. This will result in a glass pane where the surfaces are under compression stress, while the core is under tensile stress.

Figure 5.2 Stress distribution of the cross section after strengthening

Figure 5.3 Difference of stress distribution between chemical and thermal strengthening

Tempered glass is approximately four times stronger than annealed glass, namely 200MPA. When scratches on the surface of the glass pane are deep enough so they reach the tensile zone, it breaks and falls into small pieces, like you often see at bus shelters. The frame of the glass will not retain the fractured glass. Therefore a safety factor 4 is applied to the strength. This means that the allowable strength is 50MPa. Tempered glass is used for entrances and doors, atria and bath and shower enclosures. It is not save to use tempered glass at a higher temperature than 200ºC. At this point it looses its temper.

Figure 5.4 Fracture of glass of a bus shelter

5.3 Chemical strengthened glass

Chemical strengthened glass can be achieved by a bath of molten potassium at 450 ºC. The sodium ions (Na) in glass transfer in the salt and are replaced by 30% bigger potassium ions (K). This creates a surface layer under pressure. Chemical strengthening enables the glass to be resistant to higher levels of stresses. In comparison to tempered glass the compression zone is much thinner. All most all glasses with a high sodium content can be chemical strengthened. The fully chemical strengthened glass explodes on overloading.

26 The edges of all chemically strengthened glass should be chamfered and after chemical strengthening no further edge treatment is possible.

Figure 5.5 Chemical strengthening process

5.4 Laminated glass

Glass is a brittle material and in constructional or architectonic application there is always the possibility of glass breakage. Normal float glass will break into long sharp pieces, which can cause fatal injuries. To avoid injuries laminated glass is used. Laminated glass is a construction of two or more glass panes with an inner layer of PVB (polyvinyl butyral) or resin (acrylic or polyester). The glass panes and the PVB layer are bonded together in a autoclave under heat and pressure, while the resin laminated glass is made by putting a liquid resin into the cavity between two sheets. These sheets are held together until the resin is cured. The inner layer of laminated glass retains both panes when breakage occurs. The glass splinters adhere to the inner layer instead of falling down. Laminated glass is used when high performances, long durability and high protection to failure is needed. Multilayered, it can even resist bullets and small explosions.

To produce laminated glass it is possible to use normal float glass, coated, annealed, heat strengthened and tempered glass.

5.5 Limitations in strengthening

The possibilities of strengthening are limited to flat glass and simple geometries. It is difficult to strengthen tube glass, but it is possible by giving it an oil bath. After heat or chemical strengthening it is not possible to cut or form glass anymore. Laminated annealed glass can be cut with a water jet cutter.

27 H6. References of structural glass

As written in H1, the accent of using glass in architecture lies on the constructional possibilities. In this chapter a few designs in which glass is applied as a load bearing materials are presented.

6.1 Glass footbridge, Rotterdam

Design; Urbis Engineer: ABT The glass bridge in Rotterdam form a bridge between to buildings of the Kraaijvanger Urbis Architects office. The construction is made out of laminated glass. The glass beams follow the moment line and form the support of the under plate. These beams are connected to the alongside walls. The beams are placed in a steel shoe and connected to a steel U-profile that is placed in the walls. This is done for the replacement possibilities. The glass panes that form the bridge are mutual connected by point joints.

Figure 6.1 Footbridge Rotterdam

6.2 Yuraku-cho canopy underground, Tokyo

Design: Dewhurst Macfarlane&partners Enineer: Kenji Kobayashi

This canopy shelters an 8x4.8 meter staircase to the underground of Tokyo. The supporting structure comprises cantilevered beams. These beams are made out of 4 component beams pinned at the middle and in the end. The whole structure forms an arch. The beams are made out of laminated glass and acrylic blades that reduce the number from 4 at the base to 1 base at the tip. The blades are connected with 40mm diameter stainless steal pins.

28 Figure 6.2 Yuracu-cho canopy Figure 6.3 Stress distribution glass beams

6.3 Sainsbury Centre of visual arts

Norwich, 1978 Architect: Norman Forster

The building is a hall of 150x35meter. The construction of the hall consists of a spatial frame work that covers the hall in north-south direction. The frame has a thickness of 2.4 meter and the centre to centre distance of each frame is 4 meters. This makes the cover very massive in comparison to the in east- west directed glass facade. The glass facade consists of room high glass fins in the same grid as the frame. The glass fins are designed to accommodate wind forces and buckling. The fins are hanging at the steel frame and the glass facade is standing. With a polymer profile the glass facade is connected to the steel frame, so that the frame can move freely.

Figure 6.4 Sainsbury Central of Visual arts

Figure 6.5 Detail of suspended facade

6.4 Glass Tense grid structure

Dusseldorf 1996 Architect: S. Gose and P. Teuffel

This is a structure consisting out of compression and tensile elements. Eight glass tubes form the compression rods. They are spatial spanned with steel cables. The construction is standing on four glass tubes.

29 Figure 6.6 Glass tensegrity structure

30 H7. Definitions

Term Symbol Formula Description Unit ● Annealing The process of removing stresses ● Annealing point The temperature at which the internal ºC strains in glass are reduced to an acceptable limit. ● Amorphous There is no order in the positions of the atoms -6 ● Coefficient of α 1 dL Most materials expand when they are 10 /K thermal α = heated. The thermal strain per degree L dT expansion of temperature change is measured by the linear thermal-expansion coefficient.

● Compressive σc The normal stress at which a material, MPa strength loaded in compression, crushes. ● Covalent bonding Covalent bonding is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms ● Density ρ Mass per unit volume Kg/m3 ● Elasticity Ability to take up expansion ● Endurance limit The stress level below which a specimen will withstand cyclic stress indefinitely without exhibiting fatigue failure ● Fatigue ratio the ratio of maximum cyclic stress to tensile strength ● Fracture the ability of a material containing a toughness crack to resist fracture

● Hardness Hv Resistance to permanent deformation Vickers ● Isotropy is the property of being independent of direction

● Yield strength σy the stress at which a material begins to MPa (elongation 0%) deform ● Young’s modulus E E = σ / ε Measure of the stiffness of a material MPa

● Poisson's ratio v v = - εx / εy When a material gets stressed, it will get thinner in one direction. This ratio gives

εx = ΔL/r a relative strain normal to the load, divided by the strain in de direction of the load. εy = ΔL/L ● n Ratio of speed of light through a vacuum - and the speed of light through the Refractive index material ● The temperature at which glass will ºC Softening point elongate under its own weight.

● Specific heat Cp Cp = ΔU / ΔT The energy to heat 1kg of material by J/kg.K 1ºC ● Stain ε ε = ΔL/L the deformation of materials caused by - stress ● Strain point The temperature at which the internal ºC stresses in glass are reduced to low values in approximately 4 hours

● Tensile strength σt The normal stress at which a material, MPa loaded in tension, separates. Fracture strength.

31 ● Thermal λ dT The rate at which heat is conducted W/(m.K) q = − λ conductivity dX through a solid at steady state

● Thermal shock cracking as a result of rapid temperature change

● Viscosity the resistance of a material to change in form

32 H8. References

Books

Ashby, M. Shercliff, H. Cebon, D. (2007) Materials, engineering, science, processing and design. Oxford, Elsivier Ltd.

Beranek, W.J. (1975). Vlakke Constructiedelen Elasticiteitstheorie, Technische Hogeschool Delft.

Briedé, K.J. Blok, R. (2000) Tabellen voor bouwkunde en waterbouwkunde, Leiden. Spuyt, van Mantgem&de Does B.V.

Breen, J. Olsthoorn, B. (2002) De wand. Delft, Publicatie Bureau Bouwkunde.

Heller, P. Ververst, J. Wilbrink, H. (1992) Vademecum voor de glastechniek. Deventer. Kluwer Technische Boeken

Jong, T.M. de, Voordt, D.J.M. van der. (2002) Ways to study and research, urban, architectural and technical design. Delft, DUP Science

Knaack, U. (1998) Konstructiver Glasbau. Hannover. Schlütersche Druckerei

Marpillero, S. (2006) James Carpenter, Environmental Refractions, Basel. Birckhäuser

Nijse, R.(2003) Glass in Structures, Basel. Birkhäuser

Pfaender, H.G. (1996) Schott Guide to Glass, Darmstadt, ANSI/NISO

Schittisch, C. Staib, G. Balkow, D. Schuler, M. Sobek, W. (1999) Glass construction manual, Basel. Birkhäuser.

The institution of Structural Engineers Structural (1999), Use of glass in Buildings, London. SETO

Veer, F.A.(2000) Inleiding tot het materiaalkundig onderzoek, Sector Materiaalkunde, Bouwtechnologie, Faculteit Bouwkunde, TU Delft

Wigginton, M.(1996) Glass in Architecture, New York, Phaidon Press Limited

Articles

Borom, M.P. (1978) De mechanische en chemische aspecten van glas verbindingen, Microniek No7/8 (http://www.nvpt.nl/files/80-7_8-175.pdf)

Bos, F.P. Glass-to-acrylic and acrylic-to-acrylic cylindrical adhesive bonds, Delft

Bos F.P., Veer F.A., (2007). Bending and buckling strength of borosilicate glass tubes, Delft.

Doenitz, F.D. Laminated Glass Tubes as Structural Elements in Building Industry, University of Stuttgart

Nieuwehuijzen, E.J. van. Bos, F.P. Veer, F.A. The laminated glass column, Delft

Paschke, H. Eigenschappen en toepassing van glassolderen, Mainz march 1982

Veer F.A. , Bos F.P., Zuidema J., Romein T., Strength and fracture behaviour of annealed and tempered float glass, Delft

Veer, F.A. Louter, C. Romein, T. Quality control and strength of glass, Delft

Veer, F.A. Louter, Zuidema, J, Bos F.P. The strength and failure of glass in bending, Delft

33 Veer, F.A. Zuidema, J. (2003) The strength of glass, effect of edge quality, Delft

Internet http://www.bnglass.com http://www.clt.fraunhofer.com http://www.ecu.edu/glassblowing/gb.htm http://www.m-gineering.nl/rando.htm http://www.qvf.com http://vision2form.nl

Compagnies

Leidse Instrumentmakers school, Leiden. Mr. Van As and Mr. Frans Volst Louwers Glastechniek, Hapert. Mr. Bert Schepers Schott Glass. Mr Martijn Kok QVF Engineering GmbH. Mr. Hendrik Baukens

34

35