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Cracking Under Pressure

Cracking Under Pressure

We will be examining a case study that delves into the causes of glass breakage and thermal stress. Among the main causes and influencing factors on glass breakage are uniform load, impact, edge damage, poor installation, site damage, thermal stress, nickel sulphite inclusion, framing design, internal and external features and handling and installation. 

In this case, an air traffic control operator discovered a crack in the air traffic control tower’s (ATCT) glazing at 90° to the top edge of the panel, which turned through 80° and pitched at circa 30°. During the course of the crack, it formed an S shape, influenced by a microscopic imperfection. Each panel was composed of multi-laminated, tapered, glass construction and the glazing was installed in a complicated frame angled 15° from the perpendicular. Both upper and lower glazing frame recesses had air voids and drain holes for condensation and the outer panel was tinted. Whilst investigating this claim we identified that the average outside temperature on the day was 24°C, the interior of the control tower was undergoing cooling tests at the time of the incident and the temperature was estimated to have been 18°C with vents blowing cold air onto the panel. Aiding thermal stress was the fact that the interior of the panel was protected by an automatic film blind system. 

Thermally induced stress within a pane of glass can be caused by temperature differential between two areas of the pane. In hot weather, the centre of the glass warms up faster than the edge as it is within the glazing rebate and shaded from direct solar radiation. As heat absorption causes the centre of the pane to expand this increases the temperature and consequently induces tensile stress in the edge which is forced to expand by a similar amount. 

Typically, thermal stress fractures in glass can be determined by the fact that the crack extends at a 90° angle to the glass edge and to the two glass surfaces. Likewise, they originate from the glazing edge and never emerge in a radial pattern. Certain types of tinted glass and coatings are inherently at risk of thermal stress breakage, varying between types of glass. The most important property in relation to thermal stress is absorption, which is defined as “the amount of light energy converted to heat within a material that is not transmitted or reflected”. 

The thermal safety of glass is also influenced by frame design, material and colour. The risk of breakage is increased if the frame is in good thermal contact with a substantial heat sink keeping the edge of the glass cool. Conditions on the room side of the glass affect the amount of heat absorbed and lost by the glass. Internal blinds may reflect radiation and reduce heat loss by restricting air movement. Structural elements may reflect solar radiation back to the glass and radiant heaters, or blow coolers can direct additional cool air towards the glass. 

When the glass edge is shaded, by an overhang for example, greater thermal stresses occuras the shadows inhibit the temperature increasing at the glass edge. As a result of the ‘colder’ glass edge, or the greater temperature difference between the centre and the glass edge, higher tensile stresses occur increasing the risk of breakage. 

Whilst examining this case no standard exclusions applied and there was no inference of defective design, materials or workmanship. Any microscopic cracks identified in the beveled edges of the outer laminate did not fall within the definition of faulty materials as they were almost impossible to detect. There was no evidence of any impact nor reported incidents during transit, storage on site or during installation in the control tower. No physical evidence of edge damage was identified during examination and the frame mechanism appeared to be designed correctly and operating effectively in the control tower. Henceforth, this claim was concluded to be admissible against a standard Erection All Risks Insurance Policy. 

Richard Dixon can be contacted at rdixon@advantaglobal.com.