The load a glass shelf will carry is governed by three things in a fixed order of importance: how it is supported, how far it spans between those supports, and how thick it is. Span dominates. Halving the unsupported distance does far more for a shelf than adding a few millimetres of glass, and most overloaded shelves in display cabinets fail because the span was too long rather than because the glass was too thin.<\/p>\n
We should say clearly what this report is. We have not carried out laboratory load testing, we hold no test certificates of our own, and we therefore publish no load figures in kilograms. Presenting a tested load table we did not measure would be worse than useless, because a specific number invites people to load right up to it. What follows is the engineering behaviour of glass shelving, expressed in terms of how the variables relate, together with the support and specification guidance that keeps a shelf well inside its capability.<\/p>\n
Glass behaves as a brittle elastic material. It does not yield and then hold, as steel does. It deflects in proportion to load and then, past its limit, it fails suddenly. Designing a shelf is therefore about staying comfortably within the elastic range rather than approaching a breaking point.<\/p>\n
Span is the first variable and the most powerful. For a uniformly loaded shelf supported at both ends, deflection scales with the fourth power of the span and the stress with the square of it. In everyday terms this means a shelf twice as long is not twice as demanding: it is dramatically more so. A 1200mm unsupported span asks far more of glass than two 600mm spans carrying the same total weight, which is why a centre support transforms a long shelf.<\/p>\n
Thickness is the second. Bending stiffness rises with the cube of thickness and strength with its square, so the difference between 6mm and 8mm glass is considerably greater than the 2mm suggests. This is also why very thin glass has almost no place in a shelf that will hold anything of substance.<\/p>\n
Support is the third, and it is the one most often got wrong. A shelf resting on four small corner pegs concentrates all the reaction force into four small areas of glass, and those contact points are exactly where a crack will start. A shelf carried on continuous rails or channels along its long edges spreads the reaction along the whole edge and behaves as a much stronger component.<\/p>\n
Three types of glass appear in furniture, and only two of them belong in a shelf.<\/p>\n
Annealed glass is ordinary float glass that has been cooled slowly. It is the weakest of the three in bending and, critically, it breaks into large sharp shards. It should not be used as a loaded shelf in a display cabinet.<\/p>\n
Toughened glass, made to the European standard BS EN 12150, is heat treated to put its surfaces into compression. That compression has to be overcome before the glass can go into tension at the surface, which is why toughened glass is significantly stronger in bending than annealed glass of identical thickness. When it does fail it fragments into small blunt pieces, which is the behaviour that makes it a safety glass. The trade off is that it cannot be cut, drilled or edge worked after toughening, so every hole and every edge profile has to be finished before treatment.<\/p>\n
Laminated glass bonds two or more panes with an interlayer. Its advantage is that the interlayer holds the fragments in place after breakage, so a failed laminated shelf sags rather than emptying its contents. It is thicker and heavier for a given performance, which is why it is more common in structural applications than in cabinets.<\/p>\n
One further point about toughened glass deserves attention. A small proportion of toughened panes contain nickel sulphide inclusions that can, very occasionally, cause spontaneous breakage long after installation. This is a known characteristic of the material rather than a defect in any individual cabinet, and it is one more reason to keep loads modest and to avoid placing anything irreplaceable on a high glass shelf.<\/p>\n
Rather than a load figure, the practical approach is to match the shelf specification to what will actually sit on it, with the span kept short enough that deflection is not visible.<\/p>\n
The table below sets out the combinations we consider sensible for display cabinet shelving. It describes specification and typical use, not measured capacity.<\/p>\n
Glass shelf specification against span and appropriate contents in display furniture.<\/p>\n