Glass

Structural glass tends to influence modern architecture more and more. It defines not only modernity, but also value, richness and "future technology" whatever that means. However, the kowledge about technological properties and proven construction details is less known than for any other modern building material. I try here to outline some basic design rules for the glass design reflecting the applied internal standards of my current projects. This is just informative and a personal proposal. No liability can be taken for the applicabilty of those rules in certain cases, nor for any action of somebody who follows this information.

By the way: Standard framed windows and shop front glazing in usual sizes is not subject of engineering glass design, also glass that in the case of break cannot do any harm to anybody. Those glazing ought to be designed following the traditional craftsmen rules. But all glass which prevents people from falling down (e.g. glass railings), or all glass beeing overhead of traffic areas, or all glass not beeing traditionally kept in place by window frames, have to be designed and approved by engineers.

Content

1. Redundant construction as a basic rule

2. Loads

3. Support forces and bending moments

4. Allowable stresses (as currently applied in Germany)

5. Comments

6. Reduced safety after break of redundant glass parts

7. Glass and lateral stability

8. Way of approving in Germany

9. Literature

Note: The worked-out example at the German page has not been translated into the English page - please see there.

1. Redundant construction as basic rule

Content | 1. Redundant.. | 2. Loads | 3. Forces | 4.Allowable stresses | 5. Comments | 6. Reduced safety | 7. Lateral stability |8. Way of approving | 9.Literature

Glass can always break, even if designed properly. Or, more scientific, the initial energy to break glass is very low, even a minor accident may destroy this material. Glass structures must be designed redundantly, so if one glass part braks, the rest of the structure (either steel or also glass parts) will still be safe, maybe wit an reduced level of safety. Therefore, glass construction are mostly secondary or tertiary structural elements (in term of structural hierarchie of a building), i.e. parts of the buliding envelope, but not parts of the main primary structure.

The understanding of aspect of reduced safety after glass failure is the vital element of glas design. This case can be assessed by means of analysis, but mostly by experiments.

2. Loads

Content | 1. Redundant.. | 2. Loads | 3. Forces | 4.Allowable stresses | 5. Comments | 6. Reduced safety | 7. Lateral stability | 8. Way of approving | 9.Literature

The imposed loads for glass are the same as for any cladding detail. Main loads are wind, snow and own weight, the national codes of practice ought to be used ( in Germany thats the DIN 1055 part 4). Care should be taken to consider the increased wind loads at corners of buildings. High-rise buildings or buildings with unusual forms may require a detailed wind assessment with wind channel research.

3. Support forces and bending moments / Schnittgr??en

Content | 1. Redundant.. | 2. Loads | 3. Forces | 4.Allowable stresses | 5. Comments | 6. Reduced safety | 7. Lateral stability | 8. Way of approving | 9.Literature

Since glass is typically used as plates with linear or punctual supports, bending moments and support reactions can easily be obtained by using simple FEM programs with plate elements or by literature with tabulated values for plates. The approach with the linear-elastic theory (Kirchhoff-theory) is always a good way because its at safe side. Deflections more than plate thickness indicate the limit of that theory, i.e. nonlinear calculation could here yield lower stresses, but that effort is actually seldom required.

For the allowable stress approach the forces and bending moments need no load factors included.

Most FEM programs give you already the stresses in both direction and the principal stresses. Please note that due to glass crack mechanism the principal tension stress will lead to the crack, so this maximum value ought to be compared with the allowable stress.

Laminated glass (made from layers of same thickness) ought to be modelled as one layer, imposed the reduced load = real load divided by the number of layers.The shear composite effect ought to be neglected, since its not sure, especially with long-term-loads and under hight temperature the PVB foil tend to creep.

Density / Dichte: 25 kN/m3

Youngs modulus/ E-Modul of glass: E = 70000 N/mm2 (Querkontraktionszahl / Poisson ratio: 0,23)

4. Allowable stresses (as currently applied in Germany)

Content | 1. Redundant.. | 2. Loads | 3. Forces | 4.Allowable stresses | 5. Comments | 6. Reduced safety | 7. Lateral stability | 8. Way of approving | 9.Literature

Althoug there is currently no code of practice in Germany for structural glass, the following values for maximum allowable stresses (they include already a global safety factor of 2.4 against 5%-quantil value for breaking) have been applied for numerous projects with the approval of building authorities: Glass

Allowable stress

Comment

ESG = Safety glass

50 N/mm2

also in laminated glass, but without considering shear between layers

ESG = Safety glass

30 N/mm2

if imprinted at tension side

TVG = Heat stengthened Glass

29 N/mm2

also in laminated glass, but without considering shear between layers

TVG = Heat stengthened Glass

18 N/mm2

if imprinted at tension side

Float vertikal

18 N/mm2

(slope up to 10ˇă to the vertical allowed)

Float horizontal

00 N/mm2

In overhead glazing forbidden

Float horizontal in insolating glass units

12 N/mm2

only applicable for upper glass in insulating glass units, the lower glass must be a laminated glass

laminated glass (VSG) from Float, horizontal

15 N/mm2

stresses have to be calculated without shear between layers

laminated glass (VSG) from Float, vertikal

18 N/mm2

stresses have to be calculated without shear between layers

laminated glass (VSG) from Float

25 N/mm2

for calculations of rest safety of the survived one layer when the other glass sheet is already destroyed

5. Comments

Content | 1. Redundant.. | 2. Loads | 3. Forces | 4.Allowable stresses | 5. Comments | 6. Reduced safety | 7. Lateral stability | 8. Way of approving | 9.Literature

With "Laminated safety glass" is ment : Glass, laminated with PVB foil. Other glass with acrylic compound have to be especially approved by the building authority. The thickness of the PVB foil may affect the rest safety after break of one layer. Typical PVB foil thicknesses are 0.38mm, 0.76 mm and 1.52 mm. Glass that must withstand pdestrian load by cleaners has to have at least 0.76 mm foil. Glass for regular use as pedestrian area is usually at least a 4-layer composite glass. Safety glass (ESG) is fully thermically prestressed glass, which breaks in small smooth pieces. It is used as car glass as well. Thermally / heat strength glass (TVG) is like the safety glass thermally prestressed, but less than safety glass. It breaks like normal float glass, but has higher allowable stresses. Beeing laminated, TVG has the best redundant safety properties, i.e. even if broken no parts fall down and the broken laminate acts like a carpet bewteen supports and has still astonishing loadbearing capacities. Therefore, laminates from TVG became first choice for engineered glazing meanwhile.

All stresses are to compare with Major tension stresses. Experiments have shown that in-plane stresses lead earlier to failure than plate stresses due to bending, so for the maximum allowable stress for in-plane loads (shear panel loads) 90% of the values above should be taken.

Punctual fittings consider much more detail knowledge. The common way is to test the actual fitting type with the glass type and find experimentally the maximum break load. Common fittings (Flachglas /Pilkinkton , SADEV etc.) allow break forces perpendicular to plane of about 5 to 15 kN (pull-out-test). If you have a typical glass of 1.35 x 2.7 m and a wind load of lets say 1 kN/m2, each fitting will have to carry 0.91 kN, which gives to 5 kN ultimate load an overall safety factor of about 5. Those reserves vapor with horizontally glass, where own weight and snow are imposed, so more fittings and/or such with large washers may have to be used.

Punctual fittings for in-plane-loads (e.g. load-bearing glass fins with bolted supports) are much more special. Some fitting manufacturer have fitting types in stock for those purposes. As a thumb rule you can obtain the average stress in the glass hole by

stress = bolt force /(glass thickness * hole diameter) . This value should not reach more than 1/3 of the allowable glass stress written above. The reason is that - in difference to steel, glass is not ductile, so stress peaks in the hole cannot plastify away, and tend to crack the glass very early. For that type of connection, experiments are strongly recommended. For laminated glass with drilled holes I just remember for the known problem, that the drilled holes of the layers cannot be placed exactly side by side, so some special knowledge is required to tackle this problem.

The glass itself always ought to be supported in plane statically determinated to avoid forces due to temperature.

For insulating glass of small (below 1,2 m) edge size, an additional stress component due to the pressure change inside the insulating gas volume must be considered.

Deflection of glass is limited to 1/100 ... 1/200 of span. Actually, in terms of breaking the deflection does not matter at all, because due to the low Youngs modulus glass bends astonishing wide before breaking. I suggest to choose the design deflection limit keeping in mind the elasticity of the joint sealing material to obtain waterthight sealings. More important is the deflection of the steel substructure of the glass. The allowable substructure movement can be checked by FEM analysis too.

6. Reduced safety after break of redundant glass parts

Content| 1. Redundant.. | 2. Loads | 3. Forces | 4.Allowable stresses | 5. Comments | 6. Reduced safety | 7. Lateral stability | 8. Way of approving | 9.Literature

For an first analytical assessment, loads can be reduced as following:

- wind or snow loads reduced to about 66%... 75% (aproxximately the maximum wind value in 2 years, instead of the maximum value in 50 years which gives the standard wind and snow load of the codes of practice)

- global safety factor at material resistance side reduced to 1.6 (instead of 2.4), in other words, the allowable glass stresses may be factored by 1.5 An analysis with those parameter will give a first answer if the glass dimension is viable at all.

Most building authorities in Germany want the construction to be tested to assess the remaining stabilty after break of redundant parts. There are some standard test for different situations, e.g. the "soft bump" modelling the crash of a human beeing against a glass railing in case the glass is the only thing preventing the person from falling down. Other test are made with horizontal glass over traffic areas, which has to withstand own weigt and snow load for at least 24 hours even in case one layer of the (laminated) glass is broken.- >See 8.Ways of approval in Germany.

7. Glass and lateral stability

Content | 1. Redundant.. | 2. Loads | 3. Forces | 4.Allowable stresses | 5. Comments | 6. Reduced safety | 7. Lateral stability | 8. Way of approving | 9.Literature

In general, glass should not serve as lateral stabilisating member. Actually, in most cases the loadbearing capacity of glass is sufficient for lateral stability members. But, a sudden failure of the glass could lead to a donˇämino collaps of those members who lost its lateral bracing, and the whole of the building could fail. The second reason i, that lateral bracings require a tight fit to the structure, but glass ought to be supportet without tight fit so temperature can work without adding additional force to the glass.