Thermal stress glass breakage

AGGAThe following fact sheet has been prepared by the Australian Glass and Glazing Association Technical Sub Committee and has been reproduced with the kind permission of the AGGA.

The original technical fact sheet is available as a downloadable PDF file.

Thermal Broken Glass


Thermal stress occurs in glass when there is a temperature variance in different parts of the glass. If the stress caused by the temperature difference is greater than the strength of the glass, thermal stress glass breakage will result.
Thermal stress glass breakage is not a new phenomenon and it has been relatively well understood in commercial glazing for many years. However, given the increasing use of high performance, energy efficient glazing and the fact that some of these products can carry a greater risk of thermal stress, it is important that everyone involved in the glazing industry has a better understanding of thermal stress, its cause and how to prevent it.
As thermal stress breakage is not often due to a glass fault, but rather the result of a set of conditions that a pane of glass is exposed to, it is generally not covered by a glass supplier’s warranty.

Identifying thermal stress glass breakage

A glass fracture can be identified as a thermal stress breakage if the start of the crack is at 900 to both the edge of the glass AND the face of the glass. Depending on the magnitude of the stresses involved, the crack may only travel for a few millimetres before branching out or veering off-line, so the 90° angle of the initial fracture may not always be immediately apparent.
Breakages caused by thermal stress can be characterised as either low stress breakages or high stress breakages. Low stress breakage is characterised by a single crack that “lazily” makes its way across the glass, while high stress breakage is identifiable by the initial crack branching off into a number of separate fissures a short distance from its origin.

What causes thermal stress?

As mentioned previously, thermal stress is caused by varying temperatures in different parts of a glass pane. Absorption of the sun’s energy is one of the main causes of this temperature difference. The section of glass exposed to the sun absorbs the sun’s energy and, as a result, heats up. Any part of the glass which is shaded from the sun, for example, by the window frame, stays relatively cool.
The hot glass expands while the smaller, cooler area doesn’t. If the cool area of the glass isn’t strong enough to withstand the forces imposed by the expanding hot section of the glass, thermal stress breakage will occur. Putting the forces required to induce thermal stress into perspective, Pilkington estimate that for every degree in temperature difference between the edge of the glass and the centre of the glass, around 0.62 MPa of stress is introduced into the glass. Given that in some cases temperature differences of 20 to 30°C can occur, that equates to around 12 to 19 MPa. This is higher than the design stress used for wind loading!

Thermal stress risk factors

Given that thermal stress is caused by differences in temperature in a piece of glass and that breakage occurs when the ability of the glass to withstand this stress is exceeded, any situation or factor that increases this temperature difference, or decreases the glass’s strength, increases the risk of the thermal stress breakage.
Factors that can affect the risk of thermal stress include:

Edge Condition

Damage to glass edges during manufacture or installation is arguably the largest cause of low stress thermal breakage.
As US glass manufacturer PPG puts it: “The as-cut quality of glass edges is the single most important factor affecting the edge strength of glass. Poor cut-edges quality can reduce the glass strength by 50% or more…” “Glass edge quality and the resulting glass edge strength is particularly critical to the performance of the glass under the thermal loading …” Glass Edge FeatherThe level of thermal stress that a piece of glass can withstand is directly affected by the condition of the glass edges. The edges that are most resistant to thermal stress breakage are good quality clean-cut edges with no shells, vents or shark teeth. This may be difficult to achieve in laminated glass and edgework may be required to smooth the edges.

Glass Type

Different glass products have a different tolerance to thermal stress. Generally, the more solar energy a glass product absorbs, the higher the risk of thermal stress breakage. All glass manufacturers provide data on the solar energy absorption properties for their products.
Solar absorption can vary greatly, even between similar products. The use of a reflective coating can significantly increase the amount of solar absorption. This is because, in effect, the glass is absorbing the solar energy both on the way into the glass and then again on the way out after it hits the reflective coating.


Uncoated 6mm grey glass has a solar absorption rate of around 45%, while the addition of a reflective coating to this product increases solar absorption to 63%.
The use of transparent Low E coatings can also increase the solar absorption rate of glass. Low E coatings are designed to reduce the passage of radiant heat flow through the glass to improve energy efficiency. This leads to higher glass temperatures and increases the risk of thermal stress.
Another significant risk factor is the addition of heat absorbing films or any partial covering of a glass pane by other products (such as signs or paint) after installation. Both of these can increase the risk of thermal stress.

Glass Size

The larger a pane of glass, the greater the area of glass that is absorbing the sun’s energy, compared to its relatively narrow cooler edges. The larger area of hot glass results in higher levels of thermal stress in the edges of the glass.

Exterior Shading

Whilst glass properties are well documented and can be taken into account during the design phase of a project, accounting for exterior shading can be more problematic.
The effect that an external shading device has on thermal stress depends on a combination of its size, shape and location on the glass. Exterior shading is further complicated by its seasonal nature; as the sun’s position changes throughout the year so too does the shadows it casts.
In general:

  • Shading that covers 50% or less of the glass is more unfavourable than a device that shades a greater percentage of the glass;
  • Static shading is more unfavourable than mobile shading;
  • V or L shaped shading induces higher thermal stress, particularly if the point of the V falls on an edge of the glass.

Interior Shading

A thermal stress risk factor that is encountered more often in residential construction than commercial, is the use of curtains and blinds on the inside of a window. The impact that blinds and curtains have on thermal stress depends on the colour, type and other factors, however the effect can be significant.
While close fitting blinds or curtains help to minimise heat transmission into or out of the building, they can significantly add to the risk of thermal stress. To minimise this, the space between the glass and shade must be at least 50mm (preferably 150mm) and should be vented. Ventilation is provided by leaving a gap between the blinds and the walls, or frame, of 50mm at the head and 25mm at the sill.
The effect that blinds and curtains have on thermal stress is also dependent upon how much energy they reflect back onto the glass. Light colours are good reflectors while dark colours are not. Closed weave fabric helps trap heat more effectively while open weave lets the heat pass through. Venetian blinds are excellent heat reflectors as are metallised blinds.

Heating & Cooling

Artificial heating and/or cooling devices should be positioned so that they do not blow hot or cool air directly onto the glass surface, nor into the space between the glass and the curtains. Doing so may result in varying temperatures on the glass surface and therefore increase the risk of thermal stress.

Glazing Method

Commonly used glazing methods do not significantly affect the risk of thermal stress. The exception to this is structural glazing, which reduces the levels of thermal stress by reducing the temperature difference between the glazed edge and the centre of the glass. Care should be taken when using any glazing method that either encourages the transfer of heat away from the glass or covers unusually large amounts of the edges of the glass.

Managing the risk of thermal stress

The risk of thermal stress breakage can be eliminated by managing the factors outlined above or by heat strengthening the glass.
Heat strengthening increases the strength of the glass, which allows it to resist the thermally induced stress. Heat strengthening can be relatively expensive, particularly if required for laminated glass, so unless replacing glass, which has been broken under thermal, stress, a method of quantifying the risk level is needed.
Many glass manufacturers’ websites provide detailed information on thermal stress breakage. Some also provide online tools that enable you to perform a thermal stress analysis yourself, or they will perform a thermal stress analysis for you providing you purchase glass from them.
By using available information, the glass installer can objectively assess the risks prior to installation. In some cases redesign may be possible, eliminating the need for heat strengthening and therefore saving unnecessary expense.

Capitol Apartments

The Capitol ApartmentsCapitol Apartments has been recently constructed at 35 Peel St, South Bank, Brisbane, QLD. It is a 10 storey building designed by Kowalski and built by TMF. This project is situated in the busy west side of South Bank, along some major traffic routes – one being Queensland Rail train tracks and rail bridge.  It is an ambitious project considering its location, and had very stringent guidelines to achieve before it was allowed to be constructed.

Strict Design Criteria

The main aspect of design took into account the proximity to the adjacent train line. Acoustics is an obvious problem, but the location of the railway tracks are within a stones throw, literally. As such, protection of the railway tracks from litter being thrown onto the lines is of critical importance. Accompanying this, the architects designed a building with many differing glazing requirements to achieve a cohesive up market residential property. This building is to be used as furnished apartments for long or short term accommodation for people in the South Bank area. With venues like Rydges and other large hotel names in the vicinity, a boutique, stylish result needed to be achieved. The Capitol Apartments The initial design phase required the windows to comply with acoustic standards, or the apartment would not get approval to be built. G.James were the only glaziers that could beat all the ratings required, and provide evidence via testing that these results were guaranteed. The design initially specified opening sizes to be built to, but to ensure quality, it was actually done as a measure and fit job. As such, the lead times were brought down dramatically in the manufacture and installation scheduling required to meet the builders time line, which G.James achieved.

Design Resolution

To accommodate the requirements to protect the tracks from litter, all windows to the railways (northern) elevation were fitted with fixed Crimsafe screens. The balconies are set up as an Alfresco area, and the Lismore designed version of the 445 sliding door system was used for the operable windows overlooking the tracks and the city, with a fixed light beneath. The Lismore design, allows the sliding door to be operated from the inside, allowing the Crimsafe screen to be fitted and fixed externally. This alfresco area also helps protect the interior from noise pollution. The Capitol Apartments Bedrooms were fitted with jockey sashes to provide an adequate acoustic barrier, and living spaces had IGUs (as well as the alfresco area) to protect it from railway traffic noise. Both use the 451 system. Some balconies also have 136 Double Hung IGUs incorporated into their design. Other areas use differing glazing suites including the 165 slider vents to wet areas, 265 awning windows, 651 shop front with IGUs in the gym on level 1 with a 476 hinged door.  The main entry was a 475 auto sliding door, and the 477-300 bi-fold system with a lowlight under in  650 framing are a suitable finish in the restaurant. All framing not done on the railway elevation used various types of SGUs to suit the look required. The Capitol Apartments We have released a project map to provide the location and a summary of works.  Keep an eye out for the Capitol Apartments on this map…

Interactive Map: Building Brisbane

Brisbane construction projects by G.James Glass & Aluminium

Brisbane, being the location of our Head Office, sees many fine examples of G.James workmanship.   Here, we outline some of the biggest and best projects undertaken to showcase our capabilities in recent times.

The interactive map is designed so you can take a tour of some of our most recent and notable works.  Either at your desk looking out a CBD window, taking a stroll around town, and driving past a building or through an area you have always wanted to know more about.


G.James has contributed widely to what Brisbane looks like today.   There are buildings that have added to Brisbane’s sky line and to the diversity of looks and uses that are designed for the various parts of this fair city.  On some buildings, there are unique features that make them distinctive.  For example –

  • the ribbons of M&A,
  • the splash of red across the Australian Federal Police building,
  • the glass wall of Sir Samuel Griffith Centre,
  • the towering Aurora and Riparian plaza.

There are many buildings that have achieved the coveted green star energy efficient design,  some interesting artwork on glass designed by local artists – its worth a visit to the Anthropology Museum at UQ to see the ceramic printed window alone. Some of the buildings have specialised glass systems to suit the works being done, like the Translational Research Institute and the ABC headquarters.

There are projects that have altered the face of a tired old façade, so if you look at an old image of QIMR, you won’t recognize it.  And then theres the Suncorp Stadium which gives you a glimpse inside a place where state pride and competition is on the line.

The Interactive Map

The map is aimed to give you a glimpse into the depth the G.James knowledge base and provide an overview of the types of works that G.James is capable of.  It highlights projects done by various departments in the company, including:

  • Commercial departments
  • Residential departments
  • Gossi park and street furniture
  • Glass department

You can have a look at the map and plan out a scenic drive, or target specific jobs, or just get an idea of what we have produced, in your area.  As you can imagine, there are too many jobs to make this an all-inclusive list, but we aimed to include a range of jobs reflecting different styles and features.

A brief dossier on the project is included – a photo of what to look for, basic job data and links to further information on the project.  G.James can help you with any further information required for the jobs represented.

Explore Here…

Enjoy the exploration, and keep an eye on this space. Other areas will be released as our database of projects rolls out – Sydney, Melbourne, the Gold Coast, as well as other areas to be where you can find G.James fingerprints…

Until then, enjoy this insight into the River City.


 G.James Projects

 Gossi Designs

51 Alfred St – Efficiency in commercial design

51 Alfred St A green star accredited office block constructed in one of Brisbane’s growing commercial areas, 51 Alfred St comprises of a ground floor retail space with 8 levels of office space above.

51 Alfred St, Fortitude valley was constructed by Blackwatch Projects, to the design of Willis Greenhalgh Architects. The sustainable design intended to minimise its carbon footprint was a urban friendly solution to council and the community. It includes a smart foyer, featuring floor to ceiling full height glazing on two street frontages, and superb views to the city above level 2.

G.James Role

Development of the project went from initial “design in principle” early drawings to installation of the finalised glazing products. The project had a short time frame, and lead times for manufacture and installation had to be carefully managed.

The building incorporates shopfront glazing from ground floor to level 1 that used the 850-500 and 650-500 series framing systems. Hinged doors are 475 or 476 series, with 445 series sliding doors (as on some upper levels, also). G.James picked up the cladding package for the ground floor columns, which was made and fitted by the G.James Joinery department. QuickAlly, G.James scaffolding division assisted with providing access to entry the shop front entry glazing.

Upper levels utilizes the 651-500 curtain wall suite. The design had to allow for the addition of randomly placed aluminium horizontal and vertical fins that use different shapes in keeping with the difference in direction. There are also composite cladding positioned irregularly across the southern face, and frames the edge of the eastern face.

All of the differences in cladding and sun shading incorporated into the curtain wall, made for a wide variety of specialist panels.  This required coordinating the transportation and installation of the panels to be highly organised. Careful design, preparation and on site works were given particular attention at the corner feature to achieve the seamless angled cladding and glazed finish that spans the full height of the buildings office area.

Glazing selection

IGU’s were used in the vision areas of the tower to combat city noise and provide sufficient thermal and solar efficiency to achieve the green star accreditation.

The spandrel area uses a colourlite backing on clear glass to achieve the opaque finish. Charcoal and White were used to keep in theme with the buildings monochromatic scheme that highlights the slash of copper that makes the corner feature stand out.


There was no tower crane available for this job, so all of the framing hoisted into the floors from a crane on the street below.  Including the “Spider Hulk”, the name of the lifting crane that positions the panels into place.

51 Alfred St

“Spider Hulk” is the name of the machine that lifts the panels into position on the building.

Early design intended the framing to be fixed into cast ins – a quick and minimal fixing method that utilizes the concrete structure to enclose and support the framing. Later changes, however, meant that the frame fixings were redesigned and engineered to be fitted with bolts into the concrete.

The ground floor was site glazed as the size of the glazing was so large. There was also a curtain wall panel that needed to be site glazed. This requires extra safety measures and some specialised techniques to carry out.

Blackwatch had a tight program which was run like clockwork. It enabled overall job satisfaction with the resulting installation of the work performed by G.James, and we look forward to working on further projects together.

Access all areas with QuickAlly, G.James’ scaffolding business

QuickAlly, BrisbaneQuickAlly Access Solutions is an expanding branch of the G.James family. It manufactures and provide solutions in a wide variety of scaffolding equipment and specific custom designs, geared and engineered for unique purpose. The quality is of the highest industry standard, with safety, usability and best practise at the heart of its design.

QuickAlly – Height of Safety


QuickAlly Aluminium Scaffolds

The quickally scaffolding system was developed around the turn of the millenium by experienced Gold Coast scaffolder David Dart. Having been in the scaffolding business for about 30 years, David’s frustration at being unable to find a simple, high quality aluminium scaffolding system led him to develop, manufacture & start selling his own system. The benefits of the system were obvious to others in the industry, leading it to be quickly adopted by a number of scaffolding businesses.

G.James Access Equipment

G.James developed a simple mobile scaffolding system to use, primarily for in house jobs, in 1991 (developed in 1991, testing complete in 1992) and a Trestle Safety System in 1998. Custom products were also being made from this time.

Due to requests in the market for a scaffolding product with more features and abilities, G.James looked at ways it could expand its range. Looking at alternatives, QuickAlly impressed G.James with their smart design principles and wide product selection that would enhance the G.James range. The acquisition was completed in May 2010.

QuickAlly Access Solutions

Quickally’s original headquarters were on the Gold Coast.  However with its purchase by G.James and amalgamation with G.James Access Equipment, there were obvious benefits of moving to Brisbane.  The move took place in April 2011, and now positioned alongside G.James’ existing manufacturing facilities.  The two departments compliment each other under the one roof as Quickally Access Solutions.

QuickAlly Today

QuickAlly provides access solutions for hire and purchase, from ladders and step platforms to trestles and planks; scaffolding trailer packs through to multi storey systems. The range includes approximately 300 products – a number that grows with demand. The selection has solutions for everyone into home improvements, construction and maintenance – for the do it yourself types, tradies, builders and industry departments from aerospace to defence and emergency services.

Super Scaffolding Systems

QuickAlly provide complete mobile and systems scaffolding that can be added to with an expanding business. The scaffolding design is based on a triangulated star configuration that provides the ultimate in strength, speed and flexibility on site. The product list caters to gain access to any configuration required. Many safety features make this system attractive to today’s building site requirements. Careful attention to detail and troubleshooting common concerns on site means the systems features are designed to work for you in the best ways possible.

Innovative Custom Products

Their custom designed products are a comprehensive service. The process goes from understanding the needs of the customer, drafting up solutions, manufacture and delivery of product, providing engineering data and safety manuals. Post sales support include maintenance and future design changes for evolving use of products.

Expect The Best

All products are thoroughly engineered and rigorously tested to meet or better Australian, New Zealand and international standards. Risk assessments and safety developments are continuously monitored and adopted as business practise. After sales support assist with troubleshooting and continuing product development.

Where You Can Find QuickAlly

Currently, there are hire branches located in Sydney and Brisbane servicing the NSW and SE Queensland areas, and dry hire (product supply only) can be arranged nationwide. Purchase of the QuickAlly scaffolding products are available through G.James outlets anywhere in Australia. Custom access solutions are tailored to the needs of the customer in Australia or overseas, via their new warehouse and processing facility in Brisbane.

Meet the Team

The friendly team members all have many years experience in the industry and are happy to assist with any queries. The dedicated hotline for sales and hire is:

13 25 59

Keep in touch with the team, here: