The desire of modern architecture for free form structures opens a large market forcurved glass. Compared to flat glass, the production of curved glass is much moredifficult because of additional parameters through the bending process. Since thereis no standard available for curved glass in construction so far, at present the rulesfor flat glass are being considered. However, several cases of damage show that theapplication of curved glass needs own regulations to avoid glass breakage on theconstruction site. Therefore, the Munich University of Applied Sciences and theRWTH Aachen University have performed a research program [1] concerning thequality control and criteria of curved glass

Bucak, Ömer

Bues, M.

Feldmann, M.

Illguth, M.

Kasper, R.

English

TU Delft Open Access Journals

Challenging Glass 2 – Conference on Architectural and Structural Applications of Glass, 
Bos, Louter, Veer (Eds.), TU Delft, May 2010. 
Copyright © with the authors. All rights reserved.  
Curved Glass – Quality and Application 
Markus Feldmann, Ruth Kasper 
RWTH Aachen University, Institute for Steel Structures, Germany, stb@stb.rwth-
aachen.de 
Ömer Bucak, Marcus Illguth, Martin Bues 
Munic University of Applied Sciences, Institute of Steel Structures, Germany, 
info@laborsl.de 
The desire of modern architecture for free form structures opens a large market for 
curved glass. Compared to flat glass, the production of curved glass is much more 
difficult because of additional parameters through the bending process. Since there 
is no standard available for curved glass in construction so far, at present the rules 
for flat glass are being considered. However, several cases of damage show that the 
application of curved glass needs own regulations to avoid glass breakage on the 
construction site. Therefore, the Munich University of Applied Sciences and the 
RWTH Aachen University have performed a research program [1] concerning the 
quality control and criteria of curved glass.  
Keywords: Glass, heat strengthened glass, thermally toughened glass, curved glass, 
tempering, material strength, breakage pattern, thermal pre-stress 
1. Problem 
So far no standards are available for the product “curved glass” and there are no 
regulation standards for glass facades or glass roofs containing curved glass elements. 
As no other rules are available the quality criteria like geometrical tolerance, optical 
distortion, material strength or breakage pattern of flat glass panels are generally 
assumed. However, during the last years several damages of curved glass constructions 
occurred. This shows that curved glass needs own regulations and a quality control is 
necessary for a product which manufacturing process is far more challenging than the 
flat glass one. 
2. Examples 
Curved glass panels are used for glass facades, glass roofs and balustrades. Besides hot 
bending, cold bending may be an option when strength is subordinated optical 
appearance.  Elements consisting of monolithic glass, laminated glass panels or 
insulated glass units can be produced. Hot bent curved glass is available in qualities 
comparable to plane annealed glass, heat strengthened glass and thermally toughened 
glass. Furthermore the refining possibility of chemically tempering through surface ion 
exchange also exists. Simple cylindrical forms as well as spherical geometries or 
sinusoidal shapes can be produced. 
 
Challenging Glass 2 
  
Figure 1a and b: possible shapes of hot bent glass. 
3. Cases of damages and disagreement concerning the quality 
Occurring Problems with curved glass in use can mainly be related to two major 
reasons: 
 
• Discussion about the geometrical tolerances and the optical quality and 
• Glass breakage on the construction site. 
 
The first mentioned discussions can be avoided by fixing the demanded accuracy in the 
contract and producing samples to define e.g. the reachable optical quality. 
 
Glass breakage on the construction site is commonly caused by large imperfections of 
glass panes and steel structures. For example railings of curved glass balustrades may 
induce  inadmissible restraints in the glass due to forced edge aligning. 
 
Restraint problems often induced glass failure occurs in conjunction with non-uniform 
pre-stress distributions. As explained below, the thermal pre-stress distribution can be 
visualized with polarization filters. Figure 2 shows the non-uniform stress distribution 
of a laminated glass unit made of heat-strengthened glass and the associated breakage 
pattern. This kind of breakage pattern is typical for curved laminated glass made of heat 
strengthened glass panels. The number of fracture lines is higher than expected from flat 
glass experiences. Until now, it has not been satisfactory evaluated if this phenomenon 
is caused by the non-uniform tempering or unintentionally induced stresses through the 
lamination process. 
 
Non-uniform pre-stress distribution is the result of non-uniform tempering. Until 1960, 
non-uniform tempering was scheduled used to produce thermally tempered glass (name 
of the product VISURIT) with a breakage pattern shown in figure 3a. VISURIT was 
used for car windshields. The well known parabolic pre-stress distribution over the glass 
thickness is superposed with a constant membrane stress (figure 3b). In the area Z the 
membrane stress causes pressure, outside this area the membrane stress evokes tension. 
In the case of a glass breakage the rupture lines are concentrating at the border of the 
area Z. If the membrane stresses are high enough compared to the parabolic stresses, 
there will be no rupture line in the area Z. 
 
Curved Glass – Quality and Application 
Until now, the quantity of the membrane stresses is unknown. The relation between 
polarization colors and the quantity of the non-uniformity of the pre-stress is not known 
yet. The acceptable non-uniformity has to be part of further investigations because of 
non-uniform tempered glass seemed to be more sensible concerning unintentionally 
restraints. 
 
  
Figure 2a and b: Laminated glass unit made of 2 panels heat strengthened curved glass – Visualization of the 
non-uniform stress distribution and associated fracture lines after breakage. 
 
 
 
 
 
Figure 3a and b: Typical fracture lines of the product “VISURIT” [2] and simplified model to explain the 
thermal membrane stresses. 
4. Production of curved glass panels 
The production of curved glass panels is much more difficult than the production of flat 
glass.  
 
For curved glass with annealed glass properties, annealed glass panels are heated up to 
the transformation temperature (circ 600°C). With the aid of gravity loading, the panels 
fall into  the mould. Afterwards, a slow and controlled cooling down is necessary to 
avoid internal stresses. For insulating glass units or laminated glass panels, two glass 
panes can be formed pairwise. Hence, a good parallelism of the surfaces can be reached. 
 
The tempering of curved glass needs a lot of experience. A glass panel is heated up to 
the transformation temperature of about 600°C and formed by pressing in the desired 
Challenging Glass 2 
shape. Afterwards the tempering stresses are induced by fast cooling. Depending on the 
process parameters, curved glass with properties of heat strengthened glass or thermally 
toughened glass can be produced. A pairwise forming is not possible, so the surface 
parallelism of tempered curved glass pairs is worse compared to curved annealed ones. 
 
In discussions with several manufacturers it was clarified that the quality of curved 
glass is not comparable with the quality of flat glass. This means that heat strengthened 
glass with good optical properties, compliance with the regarded tolerances, bending 
strength of fck= 70 N/mm² and a typical heat strengthened breakage pattern is not yet 
economical producible. That applies for curved thermally toughened glass with a 
bending strength of fck= 120 N/mm² in the same manner. 
5. Material strength tests 
5.1. Testing of curved glass panels 
Today no standardized test procedure to determine the bending strength of curved glass 
exists. Based on the existing test procedure for bending strength of flat glass [3] (Four-
Point-bending test and coaxial double ring test), a new test set-up has been developed 
[1,4,5]. The test specimens are cylindrically formed and have dimensions of 1100 mm x 
360 mm. The material strength has been determined for the two directions: “n” and “u” 
(compare figure 4). For the evaluation of the test results, the geometrical non-linearity 
of the system has to be taken into account. By using the modified Four-Point-Bending-
Test, only the glass strength parallel to the curved line can be tested. Additional large 
scale tests (dimension of the test specimens 2000 mm x 800 mm) have been carried out 
by using a new vacuum test set-up (figure 5). 
 
n-type u-type 
Laststempel
kugelgelagerte Auflagerrollen
1000
 
Laststempel
kugelgelagerte Auflagerrollen
1000
 
  
Figure 4a and b: Modified Four-Point-Bending test set-up for curved glass panels (“n”- and 
“u”-direction).  
 
support with ball-bea ings 
support with ball-bearings 
plunger
plunger
Curved Glass – Quality and Application 
 
Figure 5: Vacuum test set-up. 
5.2. Results 
The following results of the tests can be evaluated: 
 
• The material strength determined in the “u”-direction is lower compared to the strength values 
determined in the “n”-direction. Finally, all tests have been carried out in the “u”-direction. 
• The test results are depending on the edge processing. Polished edges lead to high test values, but 
also to a large scatter. Grinded edges are preferable for material strength tests due to the small 
scatter in the strength results 
• The evaluated and recommended bending strengths of curved glass in the qualities of annealed 
glass, heat strengthened glass and thermally toughened glass are given in Chapter 8.1. 
6. Breakage pattern 
The breakage pattern of small test specimens (360 mm x 1100 mm) is a quality criteria 
of heat strengthened and thermally toughened flat glass panels [6] [7]. According to the 
European Product Standards, the breakage pattern of tempered curved glass panels has 
to be determined. 
 
Within this research project the specimens were laid on a mockup form or in case of 
lager dimensions on air bags (figure 6), to avoid any external stress in the glass panels 
which is influencing the breakage pattern.  
 
The breakage pattern tests of bent heat strengthened and bent toughened glass led to 
different results when they were compared to flat glass. It could be determined, that the 
breakage pattern of bent toughened, uniform pre-stressed glass is comparable to the 
known breakage pattern of flat toughened glass (figure 8). Therefore the requirements 
according to [7] could also defined for bent glass. Unlike toughened glass, the breakage 
pattern of bent heat strengthened glass is not comparable to the flat ones. In most cases 
the bent specimens showed more fragments as it is common by flat glass (figure 7).  
 
In accordance with the manufacturers, one can say that bent heat strengthened glass 
with a breakage pattern as expected from flat glass, high bending strength and low 
geometric tolerances at the same time are not economic producible. 
 
Challenging Glass 2 
 
 
Figure 6a and b: breakage pattern tests a) small specimen with mockup form b) lager specimen with air bag 
support. 
 
  
Figure 7a and b: Breakage pattern of curved heat strengthened glass panels (7a: not conform, 7b: conform to 
DIN EN 1863). 
 
  
Figure 8a and b: Breakage pattern of curved thermally toughened glass panels (8a: not conform, 8b: conform to 
DIN EN 12150). 
7. Distribution of thermal pre-stress 
An important quality characteristic is the distribution of the thermal pre-stress. In many 
cases of damage a non-uniform pre-stress distribution over the glass panel can be 
qualitatively shown with polarizations filters, as mentioned above.  
 
Curved Glass – Quality and Application 
Quantitative pre-stress values can be determined with measurement devices like Scalp-
03 [8] and LaserGASP [9]. Here, only a local value can be determined.  
 
Because of the high thermal pre-stress variation of tempered curved glass panels both 
methods (global visualization and local measurements) have been used. Within the 
research project several polarization filter photographs of toughened and heat 
strengthened glass specimens were taken. In consideration with the breakage pattern 
tests and the local stress measurements typical interference fringes which lead to an 
abnormal breakage pattern (Figure 8b) could be determined. 
 
  
Figure 8a and b: polarization filter pictures of toughened bent glass specimens with a) uniform  and b) non-
uniform pre-stress distribution. 
8. Recommendations for the production and use of curved glass panels 
Regarding the gathered perceptions of this research project the following issues should 
be considered. 
8.1. Material strength criteria 
Based on the tests results the following strength values are recommended for the 
application of curved glass panes. For heat strengthened glass and thermally toughened 
glass the material strength should be tested object- and manufacturer-related. 
 
• Annealed glass 
 Characteristic bending strength fck = 45 N/mm2 in the plane 
Characteristic bending strength fck = 36 N/mm2 at the edges (reduction to 
80%) 
• Heat strengthened glass 
 Characteristic bending strength fck = 55 N/mm2  
• Toughened glass 
 Characteristic bending strength fck = 105 N/mm2  
8.2. Breakage pattern criteria 
Heat strengthened glass: 
Based on the research results a final breakage pattern quality criterion for heat 
strengthened glass cannot be determined. Here further experimental investigations based 
on existing flat glass accreditations for flat heat strengthened glass are necessary to gain 
an authoritative result. 
 
 
Challenging Glass 2 
Toughened glass: 
Toughened glass can fulfill the demands to the breakage pattern according to DIN EN 
12150-1:2000 
8.3. Design of curved glass panels 
Beside common load situations like  own weight, wind and snow, additional load cases 
should  be taken into account. Depending on load direction and geometrical shape of the 
curved glass pane, higher bending stiffnesses compared to flat panels may occur. So 
deformations of substructures and climate loading of curved insulated glazing units 
have to be considered separately and superposed with common loads. 
8.4. Quality control and delivery arrangements 
To control the uniformity of the thermal pre-stress distribution polarization filters 
should be used.  Furthermore, the optical quality of the curved glass panes should be 
stipulated with samples. Also the geometrical tolerances ought to be declared in 
advance.  
8.5. Further advices 
For laminated curved glass units the interlayer thickness should be high enough to 
compensate geometrical variations. Residual resistance should be tested for overhead 
glazings made of laminated glass consisting of 2 x heat strengthened glass panels. 
Generally it should be avoided to induce constructional restraints into the glass panels. 
9. Summary 
The research project “Curved glass” analyzed the material strength of curved glass 
panels and figured out possible procedures to ensure a certain product quality. 
 
The application of curved glass panels requires own design rules and benchmark 
criteria. The well known product properties of flat glass should not be assigned to 
curved glass one to one. 
 
The presented approach shall aid the quality control of the manufacturers as well as 
assist the engineer in his design tasks. Moreover, the results of this project can be taken 
as a foundation for future product accreditations. 
10. Acknowledgements 
The investigations have been carried out with the promotion of the Arbeitsgemeinschaft 
industrielle Forschungsvereinigung “Otto von Guericke” e.V. (AiF), Köln, through the 
research program FH³. 
 
The glass samples were provided by the following producers: 
Finiglas GmbH, Franz Döring GmbH, Flintermann GmbH & Co.KG, Glasid AG, Hero 
Glas Veredlungs GmbH, Adolf Schell & Otto Vittali GmbH, Semco Holding GmbH 
und Visio-Glas GmbH. 
 
We wish to thank them all for their contribution to our work.  
Curved Glass – Quality and Application 
11. References 
[1]  Forschungsbericht, AiF Nr. 1703X06: Tragverhalten von gebogenen Glasscheiben im Bauwesen, 2009. 
[2] DIN EN 1288:2000 – Glas im Bauwesen – Bestimmung der Biegefestigkeit von Glas. 
[3] Blank, Kurt, Verfahren zur Festigkeitssteigerung, In: Fortbildungskurs „Festigkeit von Glas“ HVG, 
1987. 
[4] Bucak, Ömer; Feldmann, Markus; Kasper, Ruth; Bues, Martin; Illguth, Marcus, Das Bauprodukt “warm 
gebogenes Glas” – Prüfverfahren, Festigkeiten und Qualitätssicherung, In: Stahlbau Spezial Ernst & 
Sohn, 3/2009, pp.23-28. 
[5]  Bucak, Ömer; Feldmann, Markus; Kasper, Ruth; Bues, Martin; Illguth, Marcus, Tragverhalten von 
gebogenen Gläser in Bauwesen, Glas in Konstruktiven Ingenieurbau 7, 2009. 
[6]  DIN EN 1863:2000 – Glas im Bauwesen – teilvorgespanntes Glas. 
[7] DIN EN 12150:2000– Glas im Bauwesen – thermisch vorgespanntes Einscheibensicherheitsglas.  
[8]   Instruction Manual Grazing Angle Surface Polarimeter. s.l.: Strainoptic Technologies Inc. 
[9]   Scattered Light Polariscope SCAPL-03. s.l.: Instruction Manual, Glass Stress Ltd. 
Challenging Glass 2 
 


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