Glass can fracture when differential temperatures within the pane cause tensile stresses to exceed the glass stress limit. Temperature differences, such as those caused by partial shadowing of the glass from the sun, will cause localised expansion and subsequently cause increased stresses within the pane. The maximum stresses will typically concentrate at the glass edge due to small imperfections from the cutting process, hence the thermal fracture is recognisable by a crack propagating perpendicular from an edge.
The typical thermal stress crack has a high energy straight leg of about 75-100mm, which then extends over time with a meandering low energy crack which often bifurcates along its path.
Improving glass resistance
Glass is a brittle material and fails through crack propagation. Its stress capacity is altered by the surface conditions.
Glass heat treatments will improve the stress capacity, its design strength, but improvements are compromised by glass edge conditions and microscopic surface imperfections such as those caused by ceramic coatings and sandblasting. The theoretical design strengths of annealed, heat strengthened and toughened glass are therefore modified to take into account their detailed specification.
Annealed glass with raw cut edges has the lowest resistance to thermal stress fracture, although this may be perfectly adequate if used for a north façade with no interactions with localised heating.
On the other hand, if the annealed raw cut glass is used in conjunction with a performance coating and/or on a sunny elevation with partial shading it is subjected to greater differential heat absorption and would be at greater risk of fracture.
The risk of thermal stress can be reduced by heat treating the glass and polishing the glass edges.
Toughened glass provides the greatest resistance to thermal stress, although fritting or sandblasting the glass reduces its stress capacity to that comparable to heat strengthened glass.
There are a number of factors which have the effect of increasing the magnitude of the thermal stress in the glass. These factors contribute to the risk of the thermal fracture and therefore we work to reduce and/or manage the risk by design. These contributory factors are generally those which increase the heat absorbance of the glass, for example, glass orientation, inclination, frame colour and glass build up.
Often there is a need to approve in principle the glass type prior to finalising the façade details, particularly if processing restrictions on a glass design eliminate the possibility of heat treatment or where the client is averse to the use of toughened glass. On these occasions the most conservative factors for unknown details such as shadows, coatings, spacer bars, backing panels, ventilation and blinds should be assumed when analysing the glass thermal stress and then cross checked as the project progresses.
Analysis of thermal stress
The differential temperatures within a pane can be estimated through anticipated project conditions. We use the French Norm NF DTU 39 Part 3 as a basis for determining the maximum admissible stress within the glass, governed by glass inclination, glass type, edge treatment and the allowable basic stress within the glass. (This French Norm is used within the glass industry in the absence of any harmonised EN or BS standards dealing with this matter.) Using software to determine the largest temperature differences caused by the contributory factors, we can assess whether the anticipated maximum stress generated by the temperature differences is within the allowable for the particular product. Following review, the proposed project glazing specification can then be approved or amended.
Case study for early review
For 62 Buckingham Gate we were required to anticipate the maximum thermal stresses within the vision and shadow box glass during the Developed Design stage of the project. A number of glazing types with different coatings were explored and conservative factors were taken for the glazed shadow box and the roller blind. A minimum glass thickness was assumed based on the project information available at that stage.
We could confirm that annealed glass with machine ground edges was likely to be sufficient for the vision glass, but heat strengthened glass would be required for the shadow box glass. This kept visual distortions to a minimum for the building where clean reflections were fundamental to the overall concept.
The assumptions and output were reviewed again later post-tender when the design variables could be confirmed. It was then possible to approve various details such as the final blind type and shadow box design based on the parameters set out by the thermal stress criteria. This allowed the design to progress with confidence and provided value and more certainty in the tender process.
The table below suggests the conservative inputs that can be used for a number of key unknown variables when assessing the risk of thermal stress. These have the effect of increasing the risk of thermal stress by:
- increasing or reducing heat absorbance of the pane and therefore increasing any temperature variations within the pane
- reflecting heat on to the pane causing those areas to heat up more. Therefore increasing any temperature variations
- increasing or reducing exposure of the glass to solar radiation, Differential exposures within a pane create greater temperature variations.
- reduce the stress resistance of the glass.
Table of conservative analysis inputs when assessing the glazing thermal stress risks: