Design of Structural Glass
Structural glass has been used within buildings for many years, most commonly in the form of glazing as a source of natural light, however a recent shift has seen the appearance of more complex structural glass solutions including beams, floors, columns, roof structures as well as complete staircases made of the material.
G2 Structural are able to offer a complete structural design service if structural glass is within your scope of work with experience in the design of glass elements on numerous projects.
The appearance of large amounts of glazing in a building can often give a sense of grandeur, high cost and high standards, which is one of the reasons Architects often propose its use. Engineers however have an interesting challenge as the way it behaves as a structural material is quite different to more common materials such as steel, concrete, timber and masonry as it gives little warning prior to failure with its fracture being difficult to predict and often occurring due to imperfections invisible to the naked eye. A large amount of globally iconic structures use structural glass as the focal point, with some of these results being visually breathtaking.
Private conservatory showing glass fin beams and windposts as well as glass roof and wall panels (Source: www.pilkington .com)
Head connection of glass fin and bolted connection to glass pane. Source: Legacy Salmon Creek, Vancouver, Canada)
One major concern with the performance of glass elements has always been the presence of bolt openings and corners within a panel as the material is unable to redistribute stresses in these areas so experiences high stress concentration factors and this quite often controls the design process. However, recent advances in this area of glass design have resulted in better connection performance.
There are three types of glazing commonly available, these being annealed, toughened (or tempered) and laminated glass.
This type of glass is now almost exclusively produced by the float bath process and is no longer used as much today due to safety concerns with the way in which it fails. Annealed glass exhibits no creep and has virtually zero ductility therefore fracture will occur with no warning as the material does not have the ability to transfer local stresses in a way that steel does. Although the theoretical ultimate tensile strength of annealed glass is something in the order of 5000N/mm2, this is not the governing factor when designing using this material as the presence of minor cracks reduces the strength significantly. These cracks, which are not usually visible to the naked eye, can be as a result of manufacturing imperfections, weathering and impact damage. High stresses are generated around these imperfections which the material has no ability to redistribute.
When annealed glass breaks it tends to break in large pieces, this failure mode creates a significant safety hazard when used in building structures, however this can have a benefit in some applications as some of the material will be held in position and only a section of it breaking away, therefore allowing a certain degree of stress re-distribution globally.
This manufacturing process starts with annealed glass but is then passed through an oven once more with temperatures above 600°C, the glass is then cooled quickly by jet air or water. This process of rapid cooling induces compressive stresses at the surface through an internal parabolic stress distribution pattern, therefore increasing its load bearing capacity as minor cracks that would occur at the surface of the panel through the manufacturing process, handling and loading etc. can be reduced.
The fracture pattern of tempered glass differs somewhat to that of Annealed glass due to the inherent strength properties, tension and compression within the section thickness. As the section reaches yield point, the cracks within the compression zone propagate into the tensile region, the stresses within the matrix are destabilised which creates an intense reaction, thus shattering the glass which creates small 'dice' like elements which are considered safer than the failure pattern of annealed glass. However it should be noted that the failure of a larger tempered glass panel from height still causes a significant health and safety risk.
Internal stress distribution of Annealed and Tempered Glass
This type of glass is usually expensive, even more so with the fact that due to the locked in compression and tension, any openings within the element would need to be drilled prior to the toughening process to allow the same stress regime to occur close to these bolts, otherwise the standard glass openings would become a weak point in the panel. The stress neutral zone occurs at approximately one fifth of the thickness of the section.
Laminated glass consists of a minimum of two layers of glass separated by a layer of a thin transparent polymer material, usually polyvinyl butryal (PVB). This PVB interlayer can be bonded or unbonded to the glass but bonding the materials together offers a degree of post cracking strength. The fracture mechanism is consistent with annealed glass however if the bonding of the PVB layer to the glass gives residual strength through the shear transfer between layers. This can prevent sudden failure of a panel giving opportunity for replacement.
It is becoming apparent that the future of structural glass lies with a mixture of toughened and laminated glass products as they can both offer solutions in different environments and are safer products than annealed glass in its simplest form.
Internal stress distribution = Springer images (www.springerimages.com/Engineering/S11340)