Last update: 02.09.2019
Guidelines
2.

Glass breakage

Glass is a brittle construction material and therefore does not allow for excessive deformations. Exceeding the elasticity border due to mechanical or thermal influences immediately leads to breakage.

Glass is a brittle construction material and therefore does not allow for excessive deformations. Exceeding the elasticity border due to mechanical or thermal influences immediately leads to breakage. The defined guidelines referring to this must be followed precisely. For thermal load, the normal float glass used for facades that are partially in the shade, may be exposed to a maximum temperature difference of 40 K (EN 572) to 42 K. If the glass is exposed to temperature changes exceeding this range, then this float glass should be replaced with tempered or heat strengthened glass in order to increase this delta. This is particularly essential in the case of absorbing solar protection glasses.


Another danger that may lead to glass breakage is on the construction site when modern, coated insulating glass packages on racks are unprotected from the sun. The sun heats the glass packs and, due to the coatings, the heat is unable to dissipate. This inevitably results in glass breakage. Therefore, glass packs standing in the open must be covered with opaque material. Also, small-sized insulating glass panes with unfavourably proportioned sides and asymmetric structures need a thinner tempered glass pane to prevent breakage.


Glass breakage formerly caused by residual stress is practically ruled out, due to today’s glass production methods. But both unqualified finishing of edges with nearly invisible micro-cracks and mechanical surface damage may lead to medium-term pane failure. The same applies to incorrect transport and edge damage. In such cases, the failure may not become obvious immediately, but only at a later point in time. Breakage by the material itself can only occur with tempered glass, so-called “spontaneous” breakage occurs where nickel sulphide inclusions are present.


Generally, it can be said that glass breakage is almost 100 % preventable if glass is handled appropriately with advance planning, correct dimensioning, proper use and maintenance.

 

Thermal breakage / thermal stress

General

Thermal stress is caused when one area of glass is heated, while another part remains cold at the same time. It is not an issue if the temperature increase is uniform across the entire glass pane. Heated areas are under expansion – cold areas are under compression due to the temperature difference aspect.

Thermal breakage

Thermal cracks


Thermal breakage always starts at the edge and is perpendicular (90°) to the glass edge.

  • Different areas expand and contract differently; when they collide, energy seeks its way out towards the nearest edge and breakage starts
  • Breakage can be single or multiple depending on thermal stress build up
  • It starts straight for 2 to 5 cm and then branches out into one or more directions
  • The number of branches or secondary cracks depends on the amount of stress
Thermal cracks

 

Factors influencing thermal breakage

Thermal stress caused by temperature differences is the only ultimate factor. 

ΔT depends on:    

  • Environmental factors
    • Façade orientation
    • Solar intensity (W/m²)
    • Temperature difference between inside and outside
  • Glass-related factors
    • Glass types and glazing build up (solar energy absorption)
  • Architectural factors
    • Internal and external blinds
    • Ventilation, heating elements
    • Internal constructions, obstacles
    • Window profiles
    • Heavy shadow (surroundings, building shape)

 

Factors influencing glass strength and thermal breakage risk


The glass strength and therefore the risk of thermal breakage depends directly on the edge quality of the glazing.
The following conditions influence the edge quality:

  • Cutting → tools, oil, speed, type of table when cutting laminated glass, etc.
  • Handling / Transportation / Storage → usage of separators, avoiding cracks during handling, how glass is tied down → clean and proper storage feet
  • Project site / Installation → protection of glass pack, thermal bridges, clean profiles


Calculation and evaluation of the risk of thermal breakages


The French standard NF DTU 39 part 3 is the basis of thermal stress considerations. This standard describes the calculation methods, the influence of environmental and construction conditions, the glass properties and required glass qualities related to maximum allowed temperature differences. 


Table: Max. admissible ΔT and required glass qualities (selection) – more glass types in NF DTU 39

Max admissible glass qualities

Maximum temperature differences shown in the table are based on the theoretical resistance of a specific glass type supported along all sides. If the conditions vary, appropriate maximum temperature differences should be taken into consideration.


Glass resistance under thermal stress is significantly influenced by the quality of the edge finishing. The maximum allowable values are only valid when edge finishing has no defects. 


The Guardian Technical Advisory Centre provides a comprehensive service for the assessment of thermal breakage risk using customer and project-related data. 


IGU producers and glaziers are responsible for the quality and the installation of glazed units. In case of doubt, Guardian recommends the consideration of a lower maximum allowable temperature difference. The final decision regarding the glazing to be installed is the responsibility of the insulating glass unit manufacturer. 


Guardian does not provide any warranty regarding thermal breakage.
 

Typical glass fracture pattern

Typical glass fracture pattern