1-35 Introduction to Structural Glass

› Note 35 Level 1

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Technical Technical Guidance Note

TheStructuralEngineer March 2014

Introduction to structural glass Introduction

As with all materials, the design of structural glass elements requires a good understanding of how the material behaves when placed under load. Glass is a very strong material, but also extremely brittle. This key attribute causes it to fail suddenly as it cannot yield, unlike more traditional materials such as steel and timber. This fact presents unique challenges to structural engineers when designing structural elements to be made from glass. This technical guidance note is an introduction to glass as a structural material. It aims to describe glass in terms of its properties, how it reacts when subjected to various forces and the methods currently being explored and adopted by structural engineers when designing structural glass elements. Much of the guidance written here reflects what is provided in the recently published Institution guide: Structural use of glass in buildings: second edition.

ICON LEGEND

W

Glass as a structural material

W Applied practice

W Further reading

W Web resources

Glass as a structural material Glass has been in use for more than 5 500 years, with the earliest examples being from Egypt in the form of coloured jewellery and small vessels to store liquids. Glass manufacture was further developed by the Romans (Figure 1) who were the first to use it as a glazing material. It was very rare to have glazing in households during the Roman era, being considered highly prestigious. The manufacture of glass changed little during the Iron Age and it wasn’t until the 19th century that technology developed to the point where large glass panes could be created. This led to its wholesale adoption as a cladding material in the 1970s, but its use as a structural material is even more recent. This is one of the reasons why a Eurocode has yet to be created for the design of structural glass elements. It was in the 1990s that the first steps towards a European wide code of practice began with the release of prEN 14174, which eventually became prEN 16612. This is a draft methodology for determining the bending strength of glass using limit state theory, and forms the basis of this technical guidance note.

Glass behaviour Glass does not yield like timber and steel as it is a brittle material. Its failure is difficult to

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Figure 1 Roman glass vessels

predict once it begins to fracture. This behaviour has been borne out from destructive tests carried out on 6mm thick sheets of annealed glass using the test method described in EN 1288-2 Glass in building. Determination of the bending strength of glass. Coaxial double ring test on flat specimens with large test surface areas. The results shown in Figure 2 show how unpredictable the failure of glass is.

With glass’s inability to yield, stress concentrations around connections are of great concern as they can become the primary cause of failure. To further illustrate this point Figure 3 shows the stress/strain curve of steel and glass. This indicates how steel extends beyond its plastic limit, yet still manages to maintain its structural integrity, whereas glass will instantly fail as soon as it exceeds its elastic limit.

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39

Number of results

Results of 740 tests on 6mm annealed glass using EN 1288-2 test method, samples were from nine European factories

Breakage stress N/mm2



N

Figure 2 Test results of failed annealed glass panes, 6mm thick

Figure 3 Stress/strain curves for steel and float glass

after the manufacturing process. In order of characteristic strength (low to high) the forms of glass are:

• wired glass • patterned glass • annealed (or basic annealed) glass • heat-strengthened (or semi-tempered) glass • thermally-toughened (also referred to as heat-toughened, fully-toughened and fullytempered) glass • chemically-toughened (also referred to as chemically-tempered) glass • laminated glass

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Figure 4 Large deflection theory vs. simple deflection theory

Another behavioural aspect of glass elements is that they typically deflect more than their own thickness. This requires the adoption of large deflection theory when designing structural glass elements, which is an unfamiliar approach to most. Historically, stresses in glass have erroneously been expressed as if small deflection theory were valid using ad hoc methods, leading to the correct thickness. This gave rise to the use of unrealistic allowable stresses and typically led to the oversizing of glass elements by making them thicker than they needed to be. Quoted design stresses for use with small deflection theory will be larger than realistic design stresses used with large deflection theory. This is described diagrammatically in Figure 4.

Glass types There is actually only one core type of soda-lime glass; basic annealed. It is from this glass that all other forms are derived, as they are essentially panes of basic annealed glass that are treated during or

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What follows is a very brief description of each. For a more detailed explanation refer to Chapter 2 of Structural use of glass in buildings: second edition. Wired glass Wired glass has a welded mesh that has been laid into the glass while in its semimolten state. It is sometimes thought of as stronger than basic annealed glass because the wires are thought to act as a form of reinforcement. In fact, the opposite is true as the wires act as crack inducers that weaken the glass. However, they do provide greater post-breakage strength as the wires reduce the risk of glass panes falling from their supports. Patterned glass Patterned glass is manufactured by passing float glass between two rollers (which is why it was formerly known as ‘rolled glass’), one of which forms an impression or pattern in the glass. It is very difficult to ascertain the base thickness (and therefore the strength) of patterned glass. This is due to the varying thickness of patterned glass panes as well as sandblasting and other causes of flaws that tend to be found in the material. Due to this uncertainty the draft methodology text prEN 16612 advises a factor of 0.75 be applied to stress limits for this type of glass.

If, however, the minimum thickness at any section is known and the quality of the glass itself is of a reasonable standard, then it can be used as a base against which the full stress capacity can be applied. Basic annealed glass Commonly made using the Float Process, and hence sometimes referred to as ‘float glass’; it is made from silica sand, soda ash, limestone and salt cake. These are blended together into a cullet, which includes recycled broken glass, and heated in a furnace to 1 500ºC until it becomes molten glass. This is then fed onto a tin bath and controlled heating allows the glass to flow into a uniform thickness. The molten glass is then slowly cooled within an annealing lehr/oven. The speed at which the glass passes through the lehr defines its thickness. Basic annealed glass has no intentional locked in stresses and breaks into large shards when it fails. Heat-strengthened glass Also known as ‘partially toughened’ or ‘semi-tempered’, this type begins life as basic annealed glass which is then heated to approximately 620ºC. It is then quenched by jets of cooled air. This has the effect of cooling and solidifying the surface, before the interior has a chance to cool. As the interior cools it tries to shrink and goes into tension. This is opposed by an equal compression in the quenched surfaces. The maximum thickness of heat-strengthened glass is around 10-12mm due to the way in which it is manufactured. Its mode of failure is similar to that of basic annealed glass, i.e. large shards. Thermally-toughened glass Thermally-toughened glass is sometimes called ‘fully tempered’, although it must be borne in mind that the strength range is different depending on the term adopted. Its creation follows a similar process to that of heat-strengthened glass, but with more pronounced and effective locked-in stresses.

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› Note 35 Level 1

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TheStructuralEngineer March 2014

Technical Technical Guidance Note

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Figure 5 Broken thermally-toughened glass

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Figure 7 Section through laminated glass indicating bending stress within plies for short-term and long-term conditions

In Europe, the surface compressive stress ranges of thermally toughened glass are usually between 80 and 150N/mm2. When thermally-toughened glass fails it breaks into small fragments, commonly referred to as ‘dice’ (Figure 5).

The interlayer can be from 0.38 - 6mm thick and usually comes in multiples of 0.38mm for PVB. Though two layers of glass is the most common arrangement, more than 25 layers have been successfully bonded in an assembly over 100mm thick.

Chemically-toughened glass A different pattern of stresses can be achieved by chemical toughening, in which the composition of the surface of the glass is altered. This is achieved by dipping the panes into electrolysis baths in which the sodium ions on the surface of the glass are exchanged for potassium ions, which are 30% bigger (Figure 6).

Laminates can incorporate many thicknesses and combinations of glass types to give a range of products with the required selection of mechanical and optical properties. Other materials such as polycarbonates can be included. Basic annealed, heat-strengthened and toughened glass can all be laminated.

The structural behaviour of laminated glass depends on the type(s) of glass used and the properties of the interlayer. Generally for the PVB and resin interlayer materials, short-term out-of-plane loads can be resisted by both leaves acting compositely. Due to creep in the interlayer long-term out-of-plane loads are generally considered to act non-compositely, with the loads being shared by each leaf in proportion to their relative stiffnesses (Figure 7). This, however, is not the case with laminated glass that has an ionoplast interlayer. Such panels exhibit some composite action even during long-term loading conditions, although their strength is diminished somewhat. This is due to the stiffness of the ionoplast interlayer decreasing over time.

Material properties The material properties for all types of glass are as follows:

• Density = 2 500kg/m • Young’s modulus = 70 000N/mm • Poisson’s ratio = 0.22 3

The two key advantages of this process over thermal toughening are that there is minimal deformation during the toughening process and thinner sheets of glass can be toughened. The disadvantage is a much thinner surface compressive layer, which is likely to be less resistant to surface damage than the thicker layer produced by thermal toughening. It is also significantly more expensive than thermal toughening.

"There is actually only one core type of soda-lime glass"

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The characteristic strength of glass increases if it is pre-stressed. The values provided in Table 1 are based on a single pane of glass. The coefficient of thermal expansion of glass depends on its chemical composition. In basic annealed glass additives such as alkalines can vary the coefficient from 8-9 # 10-6K-1. Borosilicate glass has a coefficient of 3-5 # 10-6K-1 and purer silicone dioxide glass (i.e. fused silica or quartz glass) has lower values around 5 # 10-7K-1; this makes it useful in the construction of cooking surfaces such as ceramic hobs.

Laminated glass Laminating is a process in which two or more pieces of glass are bonded by means of a viscoelastic interlayer, to give redundancy post breakage. The six materials that are used for the interlayer are:

• polyvinyl butyral (PVB) • thermoplastic polyurethane (TPU) • ethyl vinyl acetate (EVA) • polyester (PET) • resins (such as acrylic) • ionoplast

2

Design criteria

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Figure 6 Section through toughened glass showing comparison between stresses in thermal and chemical processes

Draft methodology for determining the design strength of glass (prEN 16612) is based on applying material factors on the glass itself, and coefficients that address the

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41 Table 1: Characteristic strength of common types of glass Glass type

Characteristic strength (N/mm2)

Basic annealed/wired

45

Heat-strengthened

70

Toughened

120

load duration and the way in which the glass has been manufactured. The fundamental tenet of the draft methodology is that the applied bending stress (EULS;d) must not exceed the design bending strength (Rd).

Table 2: Partial factors for variable actions (cq) on structural glass elements Type of element

Partial factor for variable actions (cq)

Primary structure

1.5

Secondary structure

1.3

Infill panel

1.2

Low risk infill panel* *An infill panel whose failure would not cause injury

1.1

The calculation of the design strength is based on the design characteristic strength for basic annealed glass (fg;d) and is determined using the following equation:

Table 3: Values for kmod Duration

Example

kmod

5 seconds

Single gust

1.00

30 seconds

Domestic balustrade load

0.89

5 minutes

Workplace/public balustrade load

0.77

10 minutes

Multiple gust (storm)

0.74

30 minutes

Maintenance access

0.69

5 hours

Pedestrian access

0.60

1 week

Snow load short-term

0.48

1 month

Snow load medium-term

0.44

3 months

Snow load long-term

0.41

50 years

Permanent (e.g. self-weight and altitude pressure)

0.29

fg;d =

Type of glass

As produced

Sandblasted

Float

1.0

0.6

Drawn sheet

1.0

0.6

Enamelled float or drawn sheet

1.0

0.6

Patterned

0.75

0.45

Enamelled patterned

0.75

0.45

Polish wired

0.75

0.45

Patterned wired

0.6

0.36

Table 5: Values for kv

1.0

Vertical toughening

0.6

Load duration has a significant impact on structural glass elements due to the microscopic flaws on its surface. As loads are applied to glass elements these flaws can grow and cause cracking to the point of overall failure of the glass. In recognition of this, coefficient kmod has been developed within prEN 16612 that is always applied when determining the design strength of glass. Table 3 is a list of values for kmod with increasing typical load duration periods. The coefficient ksp concerns what posttreatment the glass pane’s surface may have received prior to installation. The values for this coefficient are listed in Table 4. When considering pre-stressed glass (i.e. heat-strengthened and toughened) an additional expression is installed into the equation for determining the design strength of basic annealed glass:

Strengthening factor kv

Horizontal toughening

Table 6: Values for fb;k Base type

fg;d =

Values of fb;k of pre-stressed glass (N/mm2) Thermally-toughened

Heat-strengthened

Chemically-toughened

Sheet float

120

70

150

Patterned

90

55

100

Enamelled float

75

45

-

Enamelled patterned

75

45

-

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k mod k sp fg;k c M;A

where: fg;k is the characteristic strength of basic annealed glass (45N/mm2) kmod is the factor for load duration (Table 3) ksp is the factor for glass surface profile (Table 4) cM;A is the material partial factor for basic annealed glass (1.6)

Table 4: Values for ksp

Manufacturing process

With the guidance being based on limit state theory, partial factors must be applied to actions. For permanent actions the partial factor (cg) is 1.35. Partial factors for variable actions (cq) are based on EN 1990-1 and are summarised in Table 2:

k mod k sp fg;k k v (fb;k - fg;k) c M;A + c M;v

where: kv is the factor derived from the method of strengthening of the glass (Table 5) fb;k is the characteristic bending strength of pre-stressed glass (Table 6) cM;v is the material partial factor for surface pre-stressed glass (1.2)

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TheStructuralEngineer March 2014

Eurocode 0.

Applied practice British Standards Institution (2013) 13/30281354 DC: BS EN 16612: Glass in building. Determination of the load resistance of glass panes by calculation and testing (draft for public comment) London: BSI British Standards Institution (2000) BS EN 1288-2:2000 Glass in building. Determination of the bending strength of glass. Coaxial double ring test on flat specimens with large test surface areas London: BSI British Standards Institution (2002) BS EN 1990:2002 Basis of Structural Design London: BSI

Technical Technical Guidance Note

Glossary and further reading Cullet – crushed glass that is ready to be melted as part of the manufacturing process of float glass. Enamel – A glassy material which is melted into the surface of the base glass at high temperatures to form a ceramic coating. Float glass – Glass which has been manufactured by floating the molten glass on a bed of molten tin until it sets, producing a product with surfaces which are flat and parallel. Interlayer – The material used to bind plies of glass together in laminated glass.

Pre-stressed glass – method of re-heating basic annealed glass that introduces a surface compressed stress, thus making it stronger in bending.

Further Reading The Institution of Structural Engineers (2014) Structural use of glass in buildings: second edition London: The Institution of Structural Engineers Eurocode 0.

Web resources The Institution of Structural Engineers library: www.istructe.org/resources-centre/library

Errata In Technical Guidance Note No. 9 (Level 2) ‘Designing a reinforced concrete retaining wall’ (The Structural Engineer, January 2014) the worked example contained errors which impact on the calculation of the bearing stress under the wall base:

• In Figure 3, the location point about which the wall rotates should have been positioned at the level of the base slab and not at the bottom of the heel beam (see revised version below)

Assumed excavation

Corrected pivot point at toe

Incorrect pivot point as used in original Figure 3

• The surcharge should have been included in the bearing stress calculation, which equates to an additional 30kN/m of unfactored load being applied to the section of the base below the surcharge

• The corrected pivot point results in a revised calculated design bearing stress under the base of the wall of 226.06 kN/m2 (maximum) and 51.19 kN/m2 (minimum). Therefore, there is no resulting tension between the soil and the base of the wall

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