Evaluation of risk of breakage by thermal shock 2008 Technical Advisory Service.

Evaluation of risk of breakage by thermal shock

2008

Technical Advisory Service

Thermal shock

Definition

Parameters of influence

Method of calculation

Example of calculation

Thermal shock - Definition

A breaking by thermal shock occurs when there is a too high difference of temperature between two point of a annealed glass.

Indeed, if the temperature of the glass increases, this glass expands.

Reminder : Coefficient of linear expansion : 9.10-6 m/(m °C)

Ex: T° difference of 50°C for a 2 m glass 9.10-6 m/(m °C) x 50°C x 2 m = 0,0009 m = 0,9 mm

Thermal shock - Definition

This phenomenon does not present a problem if the increase in temperature is uniform on all the glazing and if the movement is not blocked.

On the other hand, if the border of the glazing remains cold, it will prevent the hot part from dilating freely and it results the beginning traction constraints.

At a certain difference of temperature between two points of a same glazing, there is a risk of thermal breakage.

Thermal shock

Thermal shock

Thermal breakage starts allways at the edge of the glass, perpendicular to the edge and the 2 faces.

The breakage can be single or multiple.

Thermal shock

Thermal shock

Definition




Thermal shock – Parameters of influence

Climatic conditions

Parameters depending of the glazing and its setting

Outside « architectural » parameters

Inside « architectural » parameters


Climatic conditions :

– Daily difference of temperature (°C)

– Solar intensity (W/m²)



– Nature of glass Energy absorption (EA)


Parameters depending of the glazing and its setting– Type of setting and colour of the frame


Parameters depending of the glazing and its setting– Structural glazing– VGG– Alu with thermal break– Dark alu without thermal break– Middle alu without thermal break– Wood or PVC– Clear alu without thermal break– Dark steel– Clear steel– Concrete/Masonry



– State of the glass edges : damaged edges or presenting chips increase the risk of thermal breaking, these one starting from these weak points.



– Use of TV or sliding frame with Low E

Toughened or heat strengthened glass



– Glass in a roof

Outside temperature

Inside temperature


Outside « architectural » parameters– Exposure of the facade (in the Northern

hemisphere)

Solar flow in summer

0

100

200

300

400

500

600

700

800

4 6 8 10 12 14 16 18 20 22

Time (hour)

Flo

w (

W/m

²)

East West

South


0

100

200

300

400

500

600

700

800

4 6 8 10 12 14 16 18 20 22

Time (hour)

Flo

w (

W/m

²)

East

South

West

Solar flow in autumn and in spring



Outside « architectural » parameters– Eventual shadows on the glazing (eaves of a

building, blind) and letting only one part of the glazing in the shadow.

more and more

dangerous


Inside « architectural » parameters

– Presence of blinds– Proximity of heating appliances – Proximity of the inside aeration forcing air system

(hot or cold) on the glazing– Inside ceiling– Proximity of a dark object behind the glazing


Minimum distance 20 cm

Liquid temperature max 65°C


h

L

Risk if L < h

Thermal shock

Definition




1°) Basic calculation of t :

SV: t = I . AE + A . he

he + hi he + hi

With I = solar intensity (750 à 850 W/m²) A = maximum daily t° range (10 à 13 °C)

DV: te = …

ti = …

Thermal shock – « Glass in building » method

2°) Influence of the inside blinds :

t1 = t + t’


2°) Influence of the inside blinds : Values of t’

Simple glazingVentilated

spaceNon ventilated

space

Open weave 3°C 6°C

Closed weave 4°C 7°C

Venetian blinds 5°C 8°C


2°) Influence of the inside blinds : Values of t’

Double glazing

Outside glazing Inside glazing

Ventilated

space

Non ventilat

ed space

Ventilated space

Non ventilated space

Open weave

2°C 4°C 4°C 8°C

Closed weave

3°C 5°C 5°C 9°C

Venetian blinds

4°C 6°C 6°C 10°C


3°) Influence of the frame :

t2 = t1 x f1


3°) Influence of the frame :


Type of frame f1

Concrete 1

Clear steel 0,9

Dark steel 0,8

Steel with thermal break 0,8

Clear alu without thermal break 0,8

Wood of PVC 0,75

Dark alu without thermal break 0,7

Alu with thermal break 0,7

Structural glazing 0,5

4°) Influence of the outside shadows :

t3 = t2 x f2


4°) Influence of the outside shadows


Sort of shadowSV and outside

glass of DGInside glass of

DG

1,2 1,1

1,5 1,2

Criteria of acceptability :

– T3 < 30°C OK

– T3 > 30°C a thermal treatment

(toughened or heat

strengthened) is

necessary


Stress in the glass :

= E . = E . . T3 = 0,63 . T3

Notes :

– Thermal treatment can be toughened or heat strengthened glass

– Advantage of a toughned glass : it is a safety glass against injuries too

– Advantage or a heat strengthened glass : does not present a risk of spontaneous breakin and so, no heat soak treatment is needed


Notes :

– The edge treatment is not considered as a good solution to prevent of the risk of thermal shock

– The spandrels must always be tempered


Thermal shock

Definition




Example of a calculation :

– 6 mm Stopsol Supersilver Dark Blue pos.212 mm air6 mm Planibel clear

– AEe = 55

– AEi = 4

Thermal shock – Example of calculation


– 6 mm Stopsol supersilver Dark Blue pos.212 mm air6 mm Planibel clear

– Setting in an aluminium frame with thermal break and

outside shadow

Te = 33,5 °C

Ti = 13,0 °C

The glass outside must be tempered



– 6 mm Stopsol supersilver Dark Blue pos.212 mm air6 mm Planibel clear

– Setting in Structural glazing without shadow nor

blind

Te = 17,6 °C

Ti = 8,5 °C

A thermal treatment is not necessary


Version 3.0 - 08/2003

Project and locationAsked by TAS DAYS 2Remarks

Glazing characteristicsComposition 6 mm Stopsol Supersilver Dark Blue #2 - 12 mm air - 6 mm PlanibelLength 1000 mmWidth 1000 mmext EA 55 %int EA 4 %U 2.9 W/(m²K)

Climatic conditionstemp. var. 12 °Csun int. 750 W/m²

Frame characteristicsType of setting Structural Glazing 4 sidesType of window Structural Glazing

Exterior environmentInternal blind No internal blind

External environmentHorizontal mullion 0 mmVertical mullion 0 mmShadow - Blind No shadow or blind

Calculated maximum temperature difference in the glass litesBasis Correction factors

Glazing T Blinds Frame Shade T Thermal treatment

[°C] T' f1 f2 [°C]Outer 31.9 0 0.50 1.1 17.6 not necessary

Inner 15.4 0 0.50 1.1 8.5 not necessary

The calculations in this sheet are only valid for the specific project above, and not for any other project.

When drawing up the said calculations, Glaverbel used its actual knowledge and as a basis the information given to it

by the customer, who expressly recognizes that this is comprehensive and true.

The customer undertakes in addition to respect fully and in good faith the recommendations made by Glaverbel

as well as the rules of the art and the standards of approved institutes, professionnal institutes and organizations

or other similar bodies, which are in force.

This sheet may only be used or exploited by the customer who asked for this document, and exclusively

for the purposes for which is was requested.

Furthermore, in the case of sliding frame with low-e glass, the 2 central glass (compared with the 4 glasses,

when the frame is open) must be tempered.

DR Page 1 of 1

Evaluation of risk of thermal choc for vision double glazing (FIV 01)

Advantages:

– Simple, proven and safety

Thermal shock – Method « Glass in building »

Disadvantages:

– Does not allow the calculation of triple glazing and double skin façades

Evaluation of risk of breakage by thermal shock 2008 Technical Advisory Service.

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