10.01 Glass stresses
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Introduction
Glass is a perfectly elastic material giving a linear stress/strain relationship and a brittle failure mode, image. Glass does not undergo creep and on unloading shows no permanent deformation however this means that it is unable to redistribute concentrated stresses by plastic deformation.
Glass is potentially very strong with a theoretical strength of more than 10x103 N/mm2 however in practice the strength of annealed glass is less than 100 N/mm2. Invisible defects known as Griffith cracks are always present on the surface of the glass. These flaws cause stress concentrations at the tip of the crack which initiate fracture and are responsible for the reduced strength. The failure of glass is therefore initiated at the glass surface rather than in the body of the material. The strength of glass may be reduced further by the presence of visible damage.
Griffith cracks are randomly distributed and vary in size and hence the stress concentration they produce. Failure of a piece of glass will be initiated at the crack within the loaded area giving the highest stress concentration. It follows that the strength of glass is variable, the strength of two apparently identical pieces of glass can vary by a factor of 3. It is impossible to predict the strength of a piece of glass. Any quoted strength of glass will have some risk that it will not be achieved by a given piece of glass. Reducing the quoted value reduces the risk that the quoted value will not be obtained but the risk cannot be entirely eliminated.
- Duration of load.
Cracks may grow when subject to sustained loads below the short term failure load. As the cracks grow a point may be reached when the stress at the crack tip is sufficient to cause rapid growth of the crack and fracture of the glass occurs. Lower design stresses are therefore used for long term loads such as dead load and snow load compared with short term loads such as wind load.
- Age of glass
- Size of glass
The strength of glass is not directly affected by age however older glass may be more likely to be damaged by exposure to damage such as scratching during cleaning and hence have a lower strength.
As the loaded area increases the risk of a defect giving a greater stress concentration increases and the strength of the glass is reduced.
- Environment
Glass is a very durable material however the presence of moisture affects glass. Moisture may allow the rearrangement of atomic bonds at a crack tip. Under
unstressed conditions the severity of the defect may be reduced but under stress the effect is to increase crack growth.
Wired glass
It is commonly thought that wired glass is stronger than plain annealed glass due to the reinforcing effect of the wires. In fact wired glass is weaker as the wires act as crack inducers. The benefit of the wires is to allow transfer of load after cracking and hold the glass together.
Laminated glass
The strength of laminated glass is dependent on the strength of the glass from which it is made, the material used for the interlayer and the duration of loading. Under short term loading the leaves act compositely and the strength is the same as the same thickness of monolithic glass of the same type. When subject to long term load the interlayer may deform under load allowing relative slip between the glass leaves, image, and the glass will act as a set of independent layers. The load will then be distributed between the leaves in proportion to their stiffness. Laminated glass is usually chosen because of its behaviour after failure either in resisting penetration or holding the broken pieces together for safety. Composite action may not be important in achieving these aims.
Glazing units
As the cavity of insulated glazing units is sealed, deflection of one pane of a unit under applied load will affect the pressure in the cavity and apply a load to the other pane. The load will therefore be shared by the panes in proportion to their stiffness.
Toughened glass
Glass failure occurs when tensile stresses are generated sufficient to cause uncontrolled growth of surface cracks. If the surface of the glass remains in compression failure will not occur. In the manufacture of thermally toughened glass the surface is cooled and solidifies while the core is still hot. As the core cools and contracts it induces compressive stress in the glass surface which is balanced by tensile stress in the core, image. Failure of the glass will not occur until the tensile stress at the surface exceeds the induced compressive stress.
Heat strengthened glass
The principle of heat strengthened glass is the same as for toughened glass except that the induced stresses are lower due to differences in the temperature regime during manufacture.
Design stresses
The most common use of glass is for glazing supported around the perimeter in a frame. Normally support is provided on all four sides but two sided and three sided support may also be used. Design stresses are not given in British Standards for this situation due to the number of factors which affect strength and the difficulty of calculating the stresses in a glass pane under this form of support. Instead BS 6262 gives design charts which take account of wind load, area of glass, thickness of glass type of glass and support conditions, image.
When calculating stresses in glass it should be remembered that the actual thickness of the glass may differ from the nominal thickness and the minimum thickness allowed by the specification should be used. Tolerances for float glass complying with BS 952 Part 1 are as follows:
| Glass thickness (mm) | Tolerance (mm) |
| Tolerances for float glass complying with BS 952 Part 1 | |
The design charts in BS 6262 assume that the glass thickness is the minimum permitted by BS 952. If the glass does not comply with the thickness tolerances of BS 952, the design charts in BS 6262 will need to be modified.
Recent developments with the structural use of glass require design stresses to be evaluated. The recent report by the Institution of Structural Engineers quotes design strengths of glass from different sources. The following values are attributed to Pilkington Glass Consultants.
| Glass type | Allowable tensile stress for use with unfactored loads for specified type of loading | ||
| Short-term loads | Dead load | Snow load | |
| Float glass up to 6mm thick | 41N/mm2 | As the load type it is associated with or 7N/mm2 if assessed separately | Short term value / 2.6 |
| Float glass up to 8mm thick | 34.5N/mm2 | ||
| Float glass up to 10mm thick | 28N/mm2 | ||
| Patterned glass | 27N/mm2 | ||
| Wired glass | 21N/mm2 | ||
| Toughened glass | 59N/mm2 | As the load type it is associated with or 35N/mm2 if assessed separately | |
| Allowable tensile stresses in glass | |||
When using toughened glass the surface stress in the glass will affect the allowable design stress. Most European manufacturers achieve a stress of at least 85N/mm2 but American Standards allow a stress of 69N/mm2. It is therefore important to ensure that the glass used has strength characteristics at least as good as those assumed in design.
The surface stress in toughened glass can be assessed using a differential surface refractometer which measures the twist of polarised light from the tin surface of the glass.