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

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

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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

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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.

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Thermal shock

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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.

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Thermal shock

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Definition Parameters of influence Method of calculation Example of calculation

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

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

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

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

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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

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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.

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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

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Thermal shock – Parameters of influence

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

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

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Thermal shock – Parameters of influence

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

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Thermal shock – Parameters of influence

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more and more dangerous

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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

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Thermal shock – Parameters of influence

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Minimum distance 20 cm Liquid temperature max 65°C

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Thermal shock – Parameters of influence h L Risk if L < h

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

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1°) Basic calculation of t : SV: t = I. AE + A. h e h e + h i h e + h i With I = solar intensity (750 à 850 W/m²) A = maximum daily t° range (10 à 13 °C) DV: t e = … t i = … Thermal shock – « Glass in building » method

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2°) Influence of the inside blinds : t 1 = t + t Thermal shock – « Glass in building » method

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2°) Influence of the inside blinds : Values of t Simple glazing Ventilated space Non ventilated space Open weave3°C6°C Closed weave4°C7°C Venetian blinds5°C8°C Thermal shock – « Glass in building » method

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2°) Influence of the inside blinds : Values of t Double glazing Outside glazingInside glazing Ventilat ed space Non ventilat ed space Ventilated space Non ventilated space Open weave 2°C4°C 8°C Closed weave 3°C5°C 9°C Venetian blinds 4°C6°C 10°C Thermal shock – « Glass in building » method

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3°) Influence of the frame : t 2 = t 1 x f 1 Thermal shock – « Glass in building » method

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3°) Influence of the frame : Thermal shock – « Glass in building » method Type of framef1f1 Concrete1 Clear steel0,9 Dark steel0,8 Steel with thermal break0,8 Clear alu without thermal break0,8 Wood of PVC0,75 Dark alu without thermal break0,7 Alu with thermal break0,7 Structural glazing0,5

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4°) Influence of the outside shadows : t 3 = t 2 x f 2 Thermal shock – « Glass in building » method

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4°) Influence of the outside shadows Thermal shock – « Glass in building » method Sort of shadow SV and outside glass of DG Inside glass of DG 1,21,1 1,51,2

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Criteria of acceptability : – T 3 < 30°C OK – T 3 > 30°C a thermal treatment (toughened or heat strengthened) is necessary Thermal shock – « Glass in building » method Stress in the glass : = E. = E.. T 3 = 0,63. T 3

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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

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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 – « Glass in building » method

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

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

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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 T e = 33,5 °C T i = 13,0 °C The glass outside must be tempered Thermal shock – Example of calculation

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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 T e = 17,6 °C T i = 8,5 °C A thermal treatment is not necessary Thermal shock – Example of calculation

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

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