Glass & Glazing
Facade physics
W
ith the priorities for glass facades most often on aesthetics, structural integrity, thermal performance and how best to deliver the most comfortable internal environment, there is one fundamental law of physics that also has to be taken into account when designing facades. In the last 12 months, there has been a notable increase in the number of architects, engineers and specifiers seeking advice about the thermal safety of glass used in facades as they seek to prevent costly remediation work. Delivering a thermally safe glass facade means addressing the material’s natural propensity to expand with heat and contract in the cold. An unsuitable glass specification usually results in thermal
fracture – a crack in the pane of the glass from the edge due to stresses induced by differing rates of expansion and contraction. Thermal stress is a potential issue for any annealed product, especially if used in laminated, tinted and/or coated formats, which is the preference for those seeking to avoid the use of toughened glass. Whilst toughened glass is not prone to thermal fracture, it does however require careful consideration of both its optical and break characteristics. For example, toughened glass is prone to optical distortion due to the tempering/toughening process and this can impact on the visual appeal of glass facades. The most common cause of thermal fracture in annealed glass is heat from the sun or from inside the building causing the exposed areas of the glass to expand more
There has been a notable increase in the number of architects, engineers and specifiers seeking advice about the thermal safety of glass used in facades
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The most common cause of thermal fracture in annealed glass is heat from the sun
rapidly than the cooler, shaded areas. The perimeter of the glass which sits in the frame rebate, and/or any other areas of glass which are shaded, will never heat up as quickly, and/or to the same extent as the exposed areas. The risk of thermal fracture increases in annealed glass where a temperature difference of between 25-40°C, depending on the product, occurs. But this is reduced if the glass has been poorly cut, damaged during handling, or improperly glazed.
Factoring in risks Achieving a thermally safe solution, however, goes way beyond the type of glass used and the quality of its processing and installation. It requires a much more comprehensive review of the facade design at an early stage. Chiefly, what is the geographical orientation of the facade? What is its pitch? Verticals are less prone to thermal fracture than horizontal, with varying degrees of impact at all angles between. And what is the type and colour of frame, and the impact of shading from other external elements? Internal elements that reflect heat back at and/or trap heat against the glass also impact. For example, blinds, floor slabs or bulkheads, and even posters that are applied to the glass. Failure to factor-in the risk of thermal safety and specify the right glass at the design stage, leaves building owners and facilities managers open to expensive and disruptive remediation work. This is especially the case for high-rise buildings where the cost of replacing one pane of glass could run into tens of thousands of pounds.
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Thermal safety is a vital consideration in glass facade specification which architects and designers are increasingly seeking to address, according to Mike Butterick, Marketing Director at facade contractor GLASSOLUTIONS.
www.glassolutions.co.uk