Exploratory investigation on the cold bending of thin glass

105 Pages·2015·4.46 MB·English

Exploratory investigation on the cold bending of thin glass
Thierry Mainil
Supervisors: Prof. Dr. Ir.-Arch. Jan Belis, Univ-Prof. Dr. Ing. Geralt Siebert (UniBW)
Counselor: Captain Dipl.-Ing. Gordon Nehring
Master's dissertation submitted in order to obtain the academic degree of
Master of Science in de ingenieurswetenschappen: architectuur
Department of Structural Engineering
Chairman: Prof. Dr. Ir. Luc Taerwe
Faculty of Engineering and Architecture
Academic year 2014-2015

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Exploratory investigation on the cold bending of thin glass
Thierry Mainil
Supervisors: Prof. Dr. Ir.-Arch. Jan Belis, Univ-Prof. Dr. Ing. Geralt Siebert (UniBW)
Counselor: Captain Dipl.-Ing. Gordon Nehring
Master's dissertation submitted in order to obtain the academic degree of
Master of Science in de ingenieurswetenschappen: architectuur
Department of Structural Engineering
Chairman: Prof. Dr. Ir. Luc Taerwe
Faculty of Engineering and Architecture
Academic year 2014-2015
Foreword and acknowledgements

Glass is a fascinating material full of contradictions. It is very tough and durable, but a
small scratch can make it brake. It separates and protects us from the outdoor
conditions, but it is transparent, and has been defining the way our buildings’ outlook for
multiple decennia. This ambiguity makes the process of designing with glass very
interesting and never without surprises.

Even though glass has not been the broad subject during the studies of architectural
engineering, the material and its specific properties have drawn the attention of both, the
architect and the engineer in me. This master thesis has given me the opportunity to
research the material extensively with both, an experimental, and a theoretical approach.

I would like to thank Prof. Dr. Ir.-Arch. J. Belis and Univ.-Prof. Dr. Ing. G. Siebert, for
making it possible for me to do this research abroad and for guiding me in the right
direction during this time. This experience has allowed me not only to develop my
knowledge about glass, but also to get to know myself better.

I would like to thank Gordon for the counseling and motivation until the very end. I would
also like to thank Daniel and Robert, as well as the workers in the laboratory, for their
advice and help with the experimental study. It would not have been the same without
any one of them.

I wish to thank my friends from the Oskar von Miller Forum for creating a pleasant working
environment. In particular Mariano, Jan, Jaco, Christine and Phillipp for their support
during the last weeks.

I thank LiSEC for the generous contributions of time, knowhow and products, and also
the Universität der Bundeswehr München for providing the space and financial means to
perform the experiments.

I am very grateful to my parents and brothers for believing in me and being there when I
needed them. And last, but not least, I am very grateful for the unconditional and loving
support of my girlfriend, who supports my dreams even if it means being apart for so
long.

“The author gives permission to make this master dissertation available for consultation
and to copy parts of this master dissertation for personal use. In the case of any other use,
the copyright terms have to be respected, in particular with regard to the obligation to state
expressly the source when quoting results from this master dissertation.”

January 19, 2015

Thierry Mainil
Abstract

In the underlying exploratory investigation, the most important factors for the cold
bending of thin glass (t < 3 mm), have been mapped in an attempt to create an
affordable and easy alternative for the production of doubly curved anticlastic shapes.

In the first chapter, some properties and generalities of the material glass have been
presented as well as the production of thin glass. Afterwards the focus shifts to the usage
of glass in architecture.

The second chapter is about the current bending techniques for glass. The advantages
and disadvantages of warm, cold and lamination bending have been explained and two
case studies of cold bending were discussed.

In the third chapter, numerical analyses have been performed on quadratic (1 m x 1 m)
and rectangular (1.1 m x 0.36 m) monolithic thin glass panes with ANSYS Workbench
15.0. This to have a basis to compare the experiments with and to investigate specific
factors, which are not perceivable in reality, e.g. the internal stresses. Here, as well as
during the experimental research, the shaping of the doubly curved form was integrated
in the boundary conditions, being four point supported and two edge linear supported,
and achieved by lifting one corner.

The experiments were then described in chapter four. For that, the quadratic and
rectangular thin glass panes were deformed while measuring the midpoint displacement
and the edge displacement with displacement transducers, and the maximum normal
stresses with strain gauges. This was done until a corner displacement of 105 mm and
that for both the bearing principles and in a vertical and horizontal direction. In that way,
the influence of the own weight could be perceived and the horizontal setup allowed
loading the pane to observe the form-activation.

Before starting the conclusions, a small example design was made to present the
possibilities of thin glass. In the interpretation from the numerical analyses and the
experimental tests, it was noticed that the most influential factors were the boundary
conditions and the width-to-length ratio of the glass pane. It could be concluded that a
two edge linear supported pane allows greater corner displacements as a four point
supported pane. In addition, a decreasing width-to-length ratio results in an increasing
peak midpoint displacement. Next to that, it was also seen that a stiffening effect takes
place when twisting the pane.

Keywords: Cold bending, thin glass, anticlastic double curvature, numerical analysis,
experimental research
Exploratory investigation on the cold bending
of thin glass
Thierry Mainil
Supervisors: Prof. Dr. Ir.-Arch. Jan Belis, Univ.-Prof. Dr.-Ing. Geralt Siebert, Captain Dipl.-Ing. Gordon Nehring
 Next to that, the behaviour of cold bending thin glass has
Abstract – This article describes the most important factors for been investigated for multiple boundary conditions. This kind
the cold bending of thin glass (t < 3 mm). By mapping the of investigation was necessary to monitor the factors that
influential factors it is attempted to develop an affordable and cannot be perceived during the tests (e.g. the internal stresses).
simple alternative for the production of doubly curved anticlastic In addition, the investigation is proficient to validate the
shapes.
experimental research.
Keywords - Cold bending, thin glass, anticlastic double
curvature, numerical analysis, experimental research
A. Parameter study

A four point supported, quadratic shell model was
I. INTRODUCTION constructed, as it can be seen in Figure 1. A downward
imperfection load was added to the model in order to influence
The development of computer aided design technologies in
the buckling direction and to create consistent data.
the past twenty years has led to an increased freedom of forms

in contemporary architecture. It makes the use of curved glass
in building applications more than ever favored. Additionally,
it facilitates the creation of unique free-form facades that are
characterized by a combination of aesthetic appeal,
transparency and use of natural light within buildings.
Cold bending is an energy efficient method to construct
curved glass panes. It is based on the elastic deformation of
glass combined with applications of out of pane loads to
construct the required shape. The deformations still remain

limited though. Nevertheless, the limitations can be diminished
Figure 1 Four point supported, quadratic basis model in ANSYS
by applying thin glass (t < 3 mm). Since internal stresses
Workbench.
created by cold bending depend on the pane thickness, thin
glass can enable larger deformations. The result is more After that, the model was modified to examine the influence
freedom in architecture [1]. of thickness, size and width-to-length ratio of the glass pane
The internal stress is not the only factor that needs to be and the boundary conditions that support the pane.
considered. Staaks has already reported on instability as well as
on deformation modes in terms of forcing one corner out of the B. Thin glass analysis
plane to create a hypar surface. In the first mode, a curved shape Since not everything can be observed in the experimental
characterizes the diagonals and the edges preserve its initial study, a deeper analysis about the behaviour of thin glass during
shape. However, if the out of plane displacement of the corner buckling was conducted. For that, the movement of the free
is larger than 16.8 times the pane thickness, the plate buckles. edges, the middle axis and the diagonals were observed as well
It causes instability at the point where one diagonal straightens as the changes in the membrane stresses.
and the edges will become curved [2].
In this study, the influential factors of cold twisting thin glass C. Results
have been investigated for multiple pane sizes and multiple
From the numerical analyses it can be concluded that the
boundary conditions. Experiments were conducted in order to
critical corner displacement has many influence factors. It
create an affordable and simple alternative for the production
cannot be simply summarized in a thickness depending factor
of doubly curved shapes.
of 16.8. Many other factors have to be taken into account.
The boundary conditions as well as the pane’s width-to-
II. NUMERICAL INVESTIGATION
length ratio are two factors that are essential. Both have an
A geometric non-linear finite element analysis was impact on the behaviour of the pane and they affect the critical
performed with the ANSYS Workbench 15.0 software. The corner displacement. Furthermore, it can be noticed that a two
results for cold twisting normal glass (t > 3 mm) found in edge linear supported pane allows greater corner displacements
literature are compared to the behaviour of thin glass. A as a four point supported pane. Additionally, a decreasing
parameter study has been executed to evaluate the comparison.

T. Mainil is a student with the Faculty of Engineering and Architecture at
Ghent University (UGent), Gent, Belgium. E-mail: [email protected].
.
width-to-length ratio results in an increasing peak midpoint monolithic panes. It includes also the quadratic and the
displacement. rectangular panes. Two differences have to be mentioned
Subsequently, it was observed that a pure hyperbolical though. First, no strain gauges were applied on the panes.
paraboloid could not be made, since it is an unwindable shape. Because of that, the normal stresses could not be measured
Therefore, the free edges display a slight S-shaped curvature, during the sequence. Second, two different PVB thicknesses
even for the slightest corner displacement. In the case of its (0.76 mm and 1.52 mm) were available for the rectangular
prevention by the linear support, it results in higher stresses in laminates. It enabled a brief analysis of the interlayer thickness
the pane. during cold bending and more interesting during the loading
Looking at the effects of the buckling on the shape and the when bent.
internal stresses, it was noticed that in the supported diagonal
BD a plain arose after buckling. The plain can also be perceived C. Results
in the principal stresses that indicate an increasing compressive
For the experiments, the same conclusions can be made as for
zone in the middle of the pane. Next to that, the membrane
the numerical study, which validates both. Firstly, linearly
stresses undergo a shift from a double symmetric distribution
supported edges instead of solely point fixed corners allowed a
around the middle axes to a more diagonally oriented
greater corner displacement before buckling. Secondly, the
distribution.
influence of the width-to-length ratio can be seen. The
rectangular panes permit greater corner displacements than the
III. EXPERIMENTAL STUDY quadratic ones. In addition, it can be noticed that the maximum
The stability of cold bent thin glass was also a subject within normal stresses are lower for the four point supported panes as
the experiment. Two test setups were built for that, being a four for the two edge linear supported.
point supported and a two edge linear supported setup, as it can
be seen in figure 2. Both designs allowed the lifting of one Further on, the horizontal two edge linear supported tests
corner to create the desired doubly curved anticlastic shape for presented that a form-activation was achieved and the
multiple pane sizes. In addition, they were equipped with deflections for known loads decreased for increasing corner
displacement transducers to measure the midpoint displacements, although the shape was never a perfect hypar
displacement and displacement of the middle of the supported surface. The same results may be expected for the vertical
edge AD. setups as long as the buckling point is not reached, even though
it was not explicitly tested. From this point on, the pane was not
stable anymore and hence it was not capable of carrying loads.

Simply focusing on the shape, the tests with laminated thin
glass demonstrated the best results. In terms of the vertical
position and linearly supported at two edges, a shape that was
as close as possible to a hypar surface, was formed for both the
quadratic and the rectangular panes.
Figure 2 Boundary conditions of the test setups, four point supported
(left) and two edge linear supported (right).
IV. CONCLUSIONS
All performed tests concerning the research were executed in
the laboratory of the Universität der Bundeswehr München in The application of cold twisted thin glass is able to form an
a controlled environment with a constant temperature of attractive alternative for warm bending in the field of doubly
20.5°C. curved architecture. The experimental and numerical research
resulted in a valid model to predict the pattern of deformation,
which can be used for design purposes. The key influence
A. Monolithic experiments
factors that were detected in both the studies were the boundary
In these experiments, two pane sizes were tested: a quadratic
conditions and the width-to-length ratio of the glass pane.
1 m x 1 m and rectangular 1.1 m x 0.36 m (w x l) shape, both
The most promising result for shape forming was achieved
with a nominal thickness of 2 mm. This was carried out to
with the laminated panes. There, the primary deformation
monitor the effect of the width-to-length ratio.
created an almost perfect hypar surface. If these panes are
After the physical and laser-optical measurement of the entire
loaded with a constant load in a horizontal setup decreasing
test specimens, strain gauges were applied on one of each kind
deflections were perceived for increasing corner displacements.
to examine the normal stresses generated by the cold bending.
This proves the form-activation of the shape.
At that point, the testing sequence can be started for both
setups and both pane sizes. This meaning the lifting of one
ACKNOWLEDGEMENTS
corner for a total of 105 mm, divided in steps of 5 mm. After
each step, the measured data was noted to be compared with the The author would like to acknowledge the suggestions of
numerical analysis. The tests were conducted in a vertical and Prof. Dr. Ing.-Arch. J. Belis, Univ.-Prof. Dr.-Ing. G. Siebert
a horizontal direction not only to be capable of examining the and Captain Dipl.-Ing. G. Nehring during the research project.
influence of the own weight but also to apply a known load onto A lot of gratitude is also shown to the company LiSEC for
the structure. Based on the deflections it can then be defined if providing the test specimens.
the cold bending had an effect on the pane stiffness or not.
REFERENCES
B. Laminated experiments [1] Arend, S., Untersuchung zum Tragverhalten von Schalen aus Dünnglas,
Master thesis, Universität der Bundeswehr München, 2014.
After defining all pane properties, the identic tests have been
[2] Staaks, Koud torderen van glaspanelen in blobs, Master thesis,
performed for the laminated thin glass panes as for the
Technische Universiteit Eindhoven, 2003.
Table of content

1. Glass as a building material ...................................................................................... 1
A ] Overview ............................................................................................................ 1
B ] Composition and structure ................................................................................ 3
C ] Production of flat glass ...................................................................................... 4
D ] Structural application of glass ........................................................................... 6
E ] Free-form design ............................................................................................... 7
2. Bending of glass ....................................................................................................... 9
A ] Overview ............................................................................................................ 9
B ] Current technics................................................................................................. 9
B.1 ] Warm bending ......................................................................................... 9
B.2 ] Cold bending ........................................................................................ 11
B.3 ] Lamination bending .............................................................................. 13
C ] Existing designs ............................................................................................... 14
C.1 ] Single curvature .................................................................................... 14
C.2 ] Double curvature ................................................................................... 17
3. Numerical investigation ........................................................................................... 19
A ] Overview .......................................................................................................... 19
B ] Fundamentals of the finite element method .................................................... 20
C ] ANSYS Workbench .......................................................................................... 21
D ] Models in ANSYS Workbench ......................................................................... 22
D.1 ] Basic model .......................................................................................... 22
D.2 ] Convergence study ............................................................................... 25
E ] Parameter study .............................................................................................. 27
E.1 ] Thickness .............................................................................................. 27
E.2 ] Boundary conditions ............................................................................. 29
E.3 ] Size ........................................................................................................ 31
E.4 ] Shape ratio ............................................................................................ 32
F ] Analyses for experiments ................................................................................. 34
F.1 ] Free edges ............................................................................................ 34
F.2 ] Diagonals .............................................................................................. 35
F.3 ] Middle axis ............................................................................................ 36
F.4 ] Membrane stresses ............................................................................... 37
G ] Summary.......................................................................................................... 40
4. Experimental study .................................................................................................. 41
A ] Overview .......................................................................................................... 41
B ] Preliminary tests............................................................................................... 41
C ] Method ............................................................................................................. 43
C.1 ] Pane properties ..................................................................................... 43
C.2 ] Test setup properties ............................................................................ 48
C.3 ] Test setup adjustments......................................................................... 52
D ] Square experiments ......................................................................................... 54
D.1 ] Four point support ................................................................................ 54
D.2 ] Two edge linear support ....................................................................... 59
E ] Rectangle experiments .................................................................................... 63
E.1 ] Four point support ................................................................................. 63
E.2 ] Two edge linear support ....................................................................... 66
F ] Laminated glass experiments .......................................................................... 69
F.1 ] Pane properties ..................................................................................... 70
F.2 ] Square experiments .............................................................................. 71
F.3 ] Rectangle experiments .......................................................................... 74
G ] Summary.......................................................................................................... 78
5. Application of thin glass .......................................................................................... 80
A ] Overview .......................................................................................................... 80
B ] Concept ........................................................................................................... 80
C ] Design .............................................................................................................. 81
6. Conclusions and recommendations ....................................................................... 84
A ] Conclusions ..................................................................................................... 84
B ] Recommendations .......................................................................................... 85
Bibliography .................................................................................................................... 86
Appendix ......................................................................................................................... 88
A ] Deformation methods ...................................................................................... 89
B ] Laser-optical measurements ........................................................................... 90
C ] Point fixing details ............................................................................................ 91
D ] Displacements transducer details ................................................................... 92