1-35 introduction to structural glass

7212019 1-35 Introduction to Structural Glass

httpslidepdfcomreaderfull1-35-introduction-to-structural-glass 15

Note 35 Level 1

991290

March 2014

TheStructuralEngineer38

Technical Guidance Note

Technical

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 wasnrsquot 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 asit is a brittle material Its failure is diffi cult to

Introduction to

structural glassIntroduction

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 structuralmaterial 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

ICONLEGEND

983127

983127 Applied practice

983127 Further reading

983127 Web resources

Glass as astructural material

Glass as a

structural material

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 howunpredictable the failure of glass is

With glassrsquos 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 stressstrain

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 itexceeds its elastic limit

983118 Figure 1Roman glass vessels



wwwthestructuralengineerorg

39

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 typesThere 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 basicannealed glass that are treated during or

after the manufacturing process In order

of characteristic strength (low to high) the

forms of glass are

bull wired glass

bull patterned glass

bull annealed (or basic annealed) glass

bull heat-strengthened (or semi-tempered)

glass

bull thermally-toughened (also referred to as

heat-toughened fully-toughened and fully-

tempered) glass

bull chemically-toughened (also referred to as

chemically-tempered) glass

bull laminated glass

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 semi-

molten 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 lsquorolled glassrsquo) one

of which forms an impression or pattern in

the glass It is very diffi cult 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 075 beapplied 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 lsquofloat glassrsquo 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 500ordmC

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 lehroven 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 lsquopartially toughenedrsquo or

lsquosemi-temperedrsquo this type begins life as

basic annealed glass which is then heated

to approximately 620ordmC 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 ie

large shards

Thermally-toughened glass

Thermally-toughened glass is sometimes

called lsquofully temperedrsquo 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 morepronounced and effective locked-in stresses

983118 Figure 4Large deflection theory vs simple deflection theory

983118 Figure 3 Stressstrain curves for steel and float glass Figure 2

Test results of failed annealed glass panes 6mm thick

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

N u m

b e r o f r e s u l t s

Breakage stress Nmm2



Note 35 Level 1

991290

March 2014



Technical

In Europe the surface compressive stress

ranges of thermally toughened glass are

usually between 80 and 150Nmm2 When

thermally-toughened glass fails it breaks into

small fragments commonly referred to as

lsquodicersquo (Figure 5)

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)

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

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

bull polyvinyl butyral (PVB)

bull thermoplastic polyurethane (TPU)

bull ethyl vinyl acetate (EVA)

bull polyester (PET)

bull resins (such as acrylic)

bull ionoplast

The interlayer can be from 038 - 6mm thick

and usually comes in multiples of 038mm

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

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 propertiesThe material properties for all types of glass

are as follows

bull Density = 2 500kgm3

bull Youngrsquos modulus = 70 000Nmm2

bull Poissonrsquos ratio = 022

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 coeffi cient of thermal expansion of

glass depends on its chemical composition

In basic annealed glass additives such as

alkalines can vary the coeffi cient from

8-9 10-6K-1 Borosilicate glass has a

coeffi cient of 3-5 10-6K-1 and purer silicone

dioxide glass (ie 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

Design criteriaDraft methodology for determining the

design strength of glass (prEN 16612) is

based on applying material factors on theglass itself and coeffi cients that address the983118

Figure 6Section through toughened glass showing comparison

between stresses in thermal and chemical processes

983118 Figure 7 Section through laminated glass indicating bending

stress within plies for short-term and long-term conditions

983123 Figure 5Broken thermally-toughened glass

There is actuallyonly one core typeof soda-lime glass




41

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 ( E ULSd) must not

exceed the design bending strength ( Rd)

With the guidance being based on limit state

theory partial factors must be applied to

actions For permanent actions the partial

factor (c g) is 135 Partial factors for variable

actions (cq) are based on EN 1990-1 and are

summarised in Table 2

The calculation of the design strength is

based on the design characteristic strength

for basic annealed glass ( f gd) and is

determined using the following equation

where

f gk is the characteristic strength of basic

annealed glass (45Nmm2)

kmod is the factor for load duration (Table 3)

ksp is the factor for glass surface profile

(Table 4)

c MA is the material partial factor for basic

annealed glass (16)

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 coeffi cient 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 coeffi cient ksp concerns what post-

treatment the glass panersquos surface may have

received prior to installation The values for

this coeffi cient are listed in Table 4

When considering pre-stressed glass

(ie heat-strengthened and toughened)

an additional expression is installed into

the equation for determining the design

strength of basic annealed glass

where

kv is the factor derived from the method of

strengthening of the glass (Table 5)

f bk is the characteristic bending strength of

pre-stressed glass (Table 6)

c Mv is the material partial factor for surfacepre-stressed glass (12)

f k k f

mod

g d

M A

sp g k

c=

Table 1 Characteristic strength of common types of glass

Glass type Characteristic strength (Nmm2)

Basic annealedwired 45

Heat-strengthened 70

Toughened 120

Table 3 Values for kmod

Duration Example kmod

5 seconds Single gust 100

30 seconds Domestic balustrade load 0895 minutes Workplacepublic balustrade load 077

10 minutes Multiple gust (storm) 074

30 minutes Maintenance access 069

5 hours Pedestrian access 060

1 week Snow load short-term 048

1 month Snow load medium-term 044

3 months Snow load long-term 041

50 years Permanent (eg self-weight and altitude

pressure)

029

Table 4 Values for ksp

Type of glass As produced Sandblasted

Float 10 06

Drawn sheet 10 06

Enamelled float or drawn sheet 10 06

Patterned 075 045

Enamelled patterned 075 045

Polish wired 075 045

Patterned wired 06 036

Table 5 Values for kv

Manufacturing process Strengthening factor kv

Horizontal toughening 10

Vertical toughening 06

Table 6 Values for f bkBase type Values of f bk of pre-stressed glass (Nmm2)

Thermally-toughened Heat-strengthened Chemically-toughened

Sheet float 120 70 150

Patterned 90 55 100

Enamelled float 75 45 -

Enamelledpatterned

75 45 -

( ) f

k k f k f f

mod

g d M A

sp g k

M v

v b k g k

c c= +

-

Table 2 Partial factors for variable actions (cq) on structural glass elements

Type of element Partial factor for variable actions (cq)

Primary structure 15

Secondary structure 13

Infill panel 12

Low risk infill panel 11

An infill panel whose failure would not cause injury



Note 35 Level 1

991290

March 2014



Technical

Eurocode 0

Applied practice

British Standards Institution (2013)

1330281354 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


BS EN 1288-22000 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 19902002 Basis of Structural Design

London BSI

Glossary andfurther reading

Cullet ndash crushed glass that is ready to

be melted as part of the manufacturing

process of float glass

Enamel ndash A glassy material which is

melted into the surface of the base glass

at high temperatures to form a ceramic

coating

Float glass ndash 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 ndash The material used to bind

plies of glass together in laminated glass

Eurocode 0

Web resources

The Institution of Structural Engineers library

wwwistructeorgresources-centrelibrary

Pre-stressed glass ndash method of

re-heating basic annealed glass that

introduces a surface compressed stress

thus making it stronger in bending

Further ReadingThe Institution of Structural Engineers

(2014) Structural use of glass in buildings

second edition London The Institution of

Structural Engineers

In Technical Guidance Note No 9 (Level 2) lsquoDesigning a reinforced concrete retaining wallrsquo (The Structural Engineer January 2014) the worked example

contained errors which impact on the calculation of the bearing stress under the wall base

bull 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 heelbeam (see revised version below)

bull The surcharge should have been included in the bearing stress calculation which equates to an additional 30kNm of unfactored load being applied to

the section of the base below the surcharge

bull The corrected pivot point results in a revised calculated design bearing stress under the base of the wall of 22606 kNm2 (maximum) and 5119 kNm2

(minimum) Therefore there is no resulting tension between the soil and the base of the wall

Errata

Incorrect pivot point asused in original Figure 3

Assumedexcavation

Corrected pivot point at toe




39

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 typesThere 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 basicannealed glass that are treated during or

after the manufacturing process In order

of characteristic strength (low to high) the

forms of glass are

bull wired glass

bull patterned glass

bull annealed (or basic annealed) glass

bull heat-strengthened (or semi-tempered)

glass

bull thermally-toughened (also referred to as

heat-toughened fully-toughened and fully-

tempered) glass

bull chemically-toughened (also referred to as

chemically-tempered) glass

bull laminated glass

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 semi-

molten 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 lsquorolled glassrsquo) one

of which forms an impression or pattern in

the glass It is very diffi cult 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 075 beapplied 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 lsquofloat glassrsquo 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 500ordmC

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 lehroven 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 lsquopartially toughenedrsquo or

lsquosemi-temperedrsquo this type begins life as

basic annealed glass which is then heated

to approximately 620ordmC 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 ie

large shards

Thermally-toughened glass

Thermally-toughened glass is sometimes

called lsquofully temperedrsquo 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 morepronounced and effective locked-in stresses

983118 Figure 4Large deflection theory vs simple deflection theory

983118 Figure 3 Stressstrain curves for steel and float glass Figure 2

Test results of failed annealed glass panes 6mm thick

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

N u m

b e r o f r e s u l t s

Breakage stress Nmm2



Note 35 Level 1

991290

March 2014



Technical


























Laminated glass











bull ionoplast


































are as follows







pane of glass























41
















where





(Table 4)


annealed glass (16)





















where






f k k f

mod

g d

M A

sp g k

c=





Toughened 120












pressure)

029



Float 10 06

Drawn sheet 10 06


Patterned 075 045











Patterned 90 55 100


Enamelledpatterned

75 45 -

( ) f

k k f k f f

mod

g d M A

sp g k

M v

v b k g k

c c= +

-





Infill panel 12





Note 35 Level 1

991290

March 2014



Technical

Eurocode 0

Applied practice


1330281354 DC BS EN 16612 Glass




London BSI






London BSI



London BSI








coating








Eurocode 0

Web resources


















Errata


Assumedexcavation




Note 35 Level 1

991290

March 2014



Technical


























Laminated glass











bull ionoplast


































are as follows







pane of glass























41
















where





(Table 4)


annealed glass (16)





















where






f k k f

mod

g d

M A

sp g k

c=





Toughened 120












pressure)

029



Float 10 06

Drawn sheet 10 06


Patterned 075 045











Patterned 90 55 100


Enamelledpatterned

75 45 -

( ) f

k k f k f f

mod

g d M A

sp g k

M v

v b k g k

c c= +

-





Infill panel 12





Note 35 Level 1

991290

March 2014



Technical

Eurocode 0

Applied practice


1330281354 DC BS EN 16612 Glass




London BSI






London BSI



London BSI








coating








Eurocode 0

Web resources


















Errata


Assumedexcavation





41
















where





(Table 4)


annealed glass (16)





















where






f k k f

mod

g d

M A

sp g k

c=





Toughened 120












pressure)

029



Float 10 06

Drawn sheet 10 06


Patterned 075 045











Patterned 90 55 100


Enamelledpatterned

75 45 -

( ) f

k k f k f f

mod

g d M A

sp g k

M v

v b k g k

c c= +

-





Infill panel 12





Note 35 Level 1

991290

March 2014



Technical

Eurocode 0

Applied practice


1330281354 DC BS EN 16612 Glass




London BSI






London BSI



London BSI








coating








Eurocode 0

Web resources


















Errata


Assumedexcavation




Note 35 Level 1

991290

March 2014



Technical

Eurocode 0

Applied practice


1330281354 DC BS EN 16612 Glass




London BSI






London BSI



London BSI








coating








Eurocode 0

Web resources


















Errata


Assumedexcavation


1-35 introduction to structural glass

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