# Evaluation of risk of breakage by thermal shock

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

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 : m/(m °C) Ex: T° difference of 50°C for a 2 m glass  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 shock

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

Thermal shock Definition Parameters of influence Method of calculation
Example of calculation

Thermal shock – Parameters of influence
Climatic conditions Parameters depending of the glazing and its setting Outside « architectural » parameters Inside « architectural » parameters

Thermal shock – Parameters of influence
Climatic conditions : Daily difference of temperature (°C) Solar intensity (W/m²)

Thermal shock – Parameters of influence
Parameters depending of the glazing and its setting Nature of glass  Energy absorption (EA)

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

Thermal shock – Parameters of influence
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

Thermal shock – Parameters of influence
Parameters depending of the glazing and its setting State of the glass edges : damaged edges or presenting chips increase the risk of thermal breaking, these one starting from these weak points.

Thermal shock – Parameters of influence
Parameters depending of the glazing and its setting Use of TV or sliding frame with Low E Toughened or heat strengthened glass

Thermal shock – Parameters of influence

Thermal shock – Parameters of influence
Parameters depending of the glazing and its setting Glass in a roof Inside temperature Outside temperature

Thermal shock – Parameters of influence
Outside « architectural » parameters Exposure of the facade (in the Northern hemisphere)

Thermal shock – Parameters of influence

Thermal shock – Parameters of influence

Thermal shock – Parameters of influence
Outside « architectural » parameters Eventual shadows on the glazing (eaves of a building, blind) and letting only one part of the glazing in the shadow.

Thermal shock – Parameters of influence

Thermal shock – Parameters of influence

Thermal shock – Parameters of influence

Thermal shock – Parameters of influence

more and more dangerous

Thermal shock – Parameters of influence
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

Thermal shock – Parameters of influence

Thermal shock – Parameters of influence
Minimum distance 20 cm Liquid temperature max 65°C

Thermal shock – Parameters of influence
Risk if L < h

Thermal shock Definition Parameters of influence Method of calculation
Example of calculation

Thermal shock – « Glass in building » method
1°) Basic calculation of Dt : SV: Dt = 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: Dte = … Dti = …

Thermal shock – « Glass in building » method
2°) Influence of the inside blinds : Dt1 = Dt + Dt’

Thermal shock – « Glass in building » method
2°) Influence of the inside blinds : Values of Dt’ Simple glazing Ventilated space Non ventilated space Open weave 3°C 6°C Closed weave 4°C 7°C Venetian blinds 5°C 8°C

Thermal shock – « Glass in building » method
2°) Influence of the inside blinds : Values of Dt’ Double glazing Outside glazing Inside glazing Ventilated space Non ventilated space Open weave 2°C 4°C 8°C Closed weave 3°C 5°C 9°C Venetian blinds 6°C 10°C

Thermal shock – « Glass in building » method
3°) Influence of the frame : Dt2 = Dt1 x f1

Thermal shock – « Glass in building » method
3°) Influence of the frame : Type of frame f1 Concrete 1 Clear steel 0,9 Dark steel 0,8 Steel with thermal break Clear alu without thermal break Wood of PVC 0,75 Dark alu without thermal break 0,7 Alu with thermal break Structural glazing 0,5

Thermal shock – « Glass in building » method
4°) Influence of the outside shadows : Dt3 = Dt2 x f2

SV and outside glass of DG
Thermal shock – « Glass in building » method 4°) Influence of the outside shadows Sort of shadow SV and outside glass of DG Inside glass of DG 1,2 1,1 1,5

Thermal shock – « Glass in building » method
Stress in the glass : s = E . e = E . a . DT3 = 0,63 . DT3 Criteria of acceptability : DT3 < 30°C  OK DT3 > 30°C  a thermal treatment (toughened or heat strengthened) is necessary

Thermal shock – « Glass in building » method
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

Thermal shock – « Glass in building » method
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 Parameters of influence Method of calculation
Example of calculation

Thermal shock – Example of calculation
Example of a calculation : 6 mm Stopsol Supersilver Dark Blue pos.2 12 mm air 6 mm Planibel clear AEe = 55 AEi = 4

Thermal shock – Example of calculation
Example of a calculation : 6 mm Stopsol supersilver Dark Blue pos.2 12 mm air 6 mm Planibel clear Setting in an aluminium frame with thermal break and outside shadow DTe = 33,5 °C DTi = 13,0 °C  The glass outside must be tempered

Thermal shock – Example of calculation
Example of a calculation : 6 mm Stopsol supersilver Dark Blue pos.2 12 mm air 6 mm Planibel clear Setting in Structural glazing without shadow nor blind DTe = 17,6 °C DTi = 8,5 °C  A thermal treatment is not necessary

Thermal shock – Method « Glass in building »
Advantages: Simple, proven and safety Disadvantages: Does not allow the calculation of triple glazing and double skin façades

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