Note: Descriptions are shown in the official language in which they were submitted.
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Composite support systems using plastics in combination
with other materials
Description of the invention
A load-bearing system is described and is composed of a
plurality of components of different materials. At
least one component that dissipates load here is
composed of plastic. A frictional bond achieves load
transmission for the various components.
The load-bearing system described here utilizes the
different strengths and specific properties of
materials in order to obtain a load-bearing structure
which is as slim as possible but nevertheless can
withstand high loads, and which, depending on the
nature of the plastics components, is in part
transparent, translucent or opaque. The load-bearing
system can be used either horizontally for example as a
transverse-load-bearing element or else vertically as a
prop.
Other load-bearing systems are also possible, examples
being frameworks, Vierendeel trusses, arches, and also
three-dimensional structures, such as sheets, plates,
folded-plate structures or load-bearing shell
structures.
Prior art
Various composite load-bearing elements are known, some
of these being transparent.
Timber-glass load-bearing elements: According to Prof.
Julius Natterer and Dr. Klaus Kreher, a load-bearing
element has been developed by combining timber and
glass at the Ecole Polytechnique Federale de Lausanne,
Switzerland. The load-bearing element is composed of a
vertical glass sheet, with a frame composed of timber
adhesive-bonded to both of its sides. The timber frame
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distributes the loads and provides tensile
reinforcement for the glass sheet in the event that the
sheet cracks when its flexural tensile strength has
been exceeded. These timber-glass composite load-
bearing elements have been used in the construction of
a hotel in Switzerland.
(SOURCE: Dissertation by Klaus Kreher, EPFL Lausanne,
2002) .
Concrete-glass load-bearing elements: Mr. Freytag has
carried out experiments with a concrete-glass load-
bearing element at the Technical University in Graz,
Austria. Glass sheets, which have the function of
dissipating stress loads, were combined with
reinforced-concrete flanges.
(SOURCE: Dissertation by B. Freytag, Technical
University of Graz, October 2002)
Timber I-beams: In 1969, Trus Joist was the first
company in the world to produce an I-beam completely
composed of timber. The load-bearing capability of the
beams is provided by their constitution composed of
laminated timber veneer as flange material and OSB as
web material. The two fundamental materials are joined
by a water-resistant glue, using heat and pressure.
(SOURCE: Internet: http://www.trusjoist.com/GerSite/)
WO 2003/023162 describes a transparent structural
element which has a sheet and which draws its load-
bearing and stiffening properties from a frame
surrounding all sides of the sheet. The sheet is a
multilayer element composed of glass and/or polymer
variants, these having been adhesive-bonded to one
another. Various plastics are mentioned (Claim 8 to
10), but only in a combination in a plurality of
layers. Specifically, polycarbonate (PC), polyurethane
(PU) and polyvinyl chloride (PVC) are used. There is no
mention of polymethyl (meth)acrylates (PMMA) or of
other transparent polymers, such as polystyrene (PS),
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acrylonitrile-butadiene-styrene copolymers (ABS),
styrene-acrylonitrile copolymers (SAN), polyolefins,
etc. Nor is there any mention of PMMA-glass laminates
as material for the sheet of, and the web of, a load-
bearing structure. The stiffening frame material was
moreover described as a material composed of layers.
Disadvantages of the prior art
When transparent load-bearing systems are considered,
composite load-bearing elements involving glass are
extremely fragile.
Glass is the stiffer material in a timber-glass load-
bearing element and also in a concrete-glass load-
bearing element. The consequence of this, on exposure
to stress, is that the relatively brittle and
flexurally stiff glass attracts the load. The stress
within the glass is therefore higher than in the
composite materials used. The glass is therefore also
the first material to fail, and fails long before the
materials in the combination can begin to exhibit their
load-bearing capability.
Lack of transparency of a timber I-beam or other
composite load-bearing elements prevents their use as a
transparent design element with advantages in lighting
and illumination, although their production is cost-
effective.
Object and achievement of object
The object of the present invention consists in
developing a composite load-bearing element in which
plastics are used in accordance with the properties of
these materials. When they are compared to the
conventional construction materials, such as timber,
steel, aluminium, glass, etc., they feature comparably
low modulus of elasticity and high ductility. The
supposed disadvantage of the low modulus of elasticity
becomes an advantage in the appropriately devised
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composite with other materials. The combination leads
to absorption of the high tensile and compressive
stresses by the stronger materials and of the
relatively small shear stresses by the softer
materials. In contrast to other, known, load-bearing
systems, the invention provides a frameless load-
bearing structure which comprises a frictional bond
between load-bearing parts of different materials.
Plastics elements can be multilayer elements, or
preferably single-layer elements composed of
homogeneous materials. Another possibility is to
develop load-bearing systems which are in part
transparent, or coloured, or indeed luminous. Lighting
elements that can be used are not only incandescent
bulbs or fluorescent tubes but also LEDs. It is thus
possible to meet almost any particular request relating
to the optical properties of the load-bearing element.
By virtue of the transparency of the plastics elements,
the load-bearing structure is perceived as very
filigree and lightweight. It is also possible to join
props and transverse-load-bearing elements together to
give a construction system.
The combination of plastics with other materials can
produce a filigree load-bearing system. A load-bearing
system here means a system involved in dissipation of
load. It can either, like a transverse-load-bearing
element or a cantilever, transmit loads in a horizontal
direction, or, like a prop, transmit loads in a
vertical direction.
In the case of a transverse-load-bearing element, the
upper and lower part, here called flange, is composed
of a stiff, conventional material, such as timber,
steel, aluminium or glass, and the central part, here
called web, is composed of one or more plastics. On
exposure to load, by virtue of the marked difference in
the stiffnesses, the conventional material attracts the
loads, and the web serves merely to achieve equilibrium
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between the upper flange and lower flange. The two
materials are bonded either via mechanical means of
bonding, e.g. various screws or bolts, plugs, rivets,
dowel pins, studs, etc., or by adhesive bonding. Other
types of frictional bond are also conceivable here. The
selection of the bonding technique is related to the
manner of force transmission and therefore also to the
load-bearing system under consideration.
In the case of a prop, the load-bearing system is
composed, for example, of a plurality of small cross
sections composed of conventional materials, prevented
from buckling via bonding of the cross section with
sheets of plastic.
Selection of materials
Examples of stiffer material that can be used are
conventional materials such as timber, timber
materials, metals, glass or concrete, or high-
performance plastics or reinforced plastics.
The less stiff material used can comprise plastics
whose modulus of elasticity (measured to DIN EN ISO
527) is at least 150 N/mm2, examples being
poly(meth)acrylate (PMMA), polycarbonate (PC), acrylo-
nitrile-butadiene-styrene copolymers (ABS), styrene-
acrylonitrile copolymers (SAN), polyvinyl chloride
(PVC) or polystyrene (PS). PMMA is marketed by Rohm
GmbH with the trade mark Plexiglas For this use,
Plexiglas GS grades are particularly suitable, these
being produced via cast polymerization. It is also
possible to use filled grades of PMMA, these being
marketed by way of example with the name Corian or
Creanit . It is also possible to use laminates of
different plastics or layered materials.
Production of composite load-bearing elements
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In the production of the inventive article, means of
bonding are used to secure the linear, conventional
materials to a sheet-like plastics component. The
sheet-like plastics component is very much longer in
the direction of loading than perpendicularly to the
direction of loading. The height:length ratio between
two adjacent retention points of the component, also
called bearing points, is by way of example from 1:1 to
1:80, preferably from 1:5 to 1:40 and very particularly
preferably from 1:10 to 1:25. The height of the
component is by way of example from 10 to 300 cm,
preferably from 15 to 120 cm and very particularly
preferably from 20 to 80 cm. The thickness of the
component can by way of example be from 3 to 500 mm.
The length of the component is selected as appropriate
for the structural requirements, and the sheet-like
plastics component can be converted to the required
length via adhesive bonding. The linear, conventional
materials are secured to the long edges. The bonding
between the plastic and the conventional material is
produced via means of bonding.
Production Example 1:
Two commercially available slating battens whose cross
section is 24 * 48 mm and whose length is 3 m are
secured to each of the longer edges of a transparent
sheet composed of Plexiglas whose thickness is 10 mm
and whose length is 3 m and whose width is 25 cm, with
the aid of screw clamps, the battens therefore having
opposite location on the sheet of plastic. Holes whose
diameter is 8 mm are drilled at regular intervals
(about 10 cm) through these two timber battens and the
intervening plastics layer. Hexagon head cap screws are
inserted through these holes and secured by a nut. The
screw clamps are removed after assembly.
Production Example 2:
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An adhesive which solvates the material is applied to
both sides along the longer edges of a transparent
sheet composed of PMMA whose length is substantially
greater than its width, an example being the adhesive
marketed as Acrifix by Rohm GmbH. Timber battens are
pressed from both sides against the adhesion areas and
fixed with screw clamps. After hardening and the
resultant coherent bonding between plastic and timber,
the screw clamps are removed.
Bonding of the material (means of bonding)
The actual bonding of the material, between the
individual elements that dissipate load, is
particularly important, since this contributes
decisively to the stability and load-bearing capability
of the load-bearing structure.
A coherent permanent bond, where the materials involved
in the bond are held together at the atomic or
molecular levels is ideal. Familiar methods here would
be adhesive bonding (welding, soldering) or
vulcanization.
Frictional- or interlock-bonding techniques are a
conceivable alternative method, and these also give
sufficiently stable bonds. Clamping methods, and
particularly screwing methods, may be mentioned here as
a frictional bonding technique.
Possibilities of producing the load-bearing systems
described by way of interlock bonding are provided via
riveting, pinning (plugging), compression, shrinking,
compressive jointing or thermoforming of the component
materials. Some advantageous bonding techniques are
described in detail below:
Partial combinations of the various bonding techniques
are also conceivable.
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Adhesive bonding
Adhesive bonding should be the preferred coherent-
bonding technique for using various materials to
produce the load-bearing structure described here.
As a function of the selection of material, there is a
wide variety of adhesive systems known in the
literature which generally cover the following
combinations:
metal-plastic, timber-plastic,
metal-glass, metal-timber, etc.
According to DIN 16920, an adhesive is a non-metallic
material which bonds adherends to one another by virtue
of surface adhesion and internal strength. Suitable
bonding adhesives therefore have to meet at least two
requirements: they must produce sufficiently high
adhesion to both the first and the second material, and
they themselves must supply strength within the
adhesive layer. Assessment of an "adhesive" bond of a
material depends on the typical type of stress
encountered in the application. In the present case of
a composite load-bearing structure, the main stress
present comprises shear, and rarely tension or indeed
delamination. A sufficiently good criterion for
assessment of adhesive bonds is therefore what is known
as the shear strength, which involves separating the
adherends from one another in a parallel direction. The
more force required here, the better the bonding of the
material.
Welding or soldering is mainly reserved for load-
bearing structures composed of unitary materials, but
can also be used as bonding technique in special cases,
e.g. in metal-lightweight metal variants or plastic A -
plastic B combinations.
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Bonding via screwing methods
Bonding via screwing methods can use almost any type of
screwing method. This produces a frictional bond.
In the case of relatively soft materials, it is even
possible to use self-tapping wood screws. If at least
one hard material is involved at the bond, bonding of
the materials involved has to be produced by way of the
bearing surfaces within a hole, or screw threads on a
screw or nut.
The force-transfer mechanism here in essence takes
place in the bearing surfaces within the hole. The
permissible stresses in the materials in the respective
regions of the bearing surfaces within the hole cannot
be exceeded, otherwise the material can break away or
crack, thus weakening the load-bearing system. The
selected size of the hole which has the bearing
surfaces is generally slightly larger than the diameter
of the screw. Appropriate screw-fixing methods have to
be selected as a function of the use of the load-
bearing system.
Pegged bonds
Pegs used here comprise either timber pegs or else any
other types of peg, such as steel pins, or springs.
These pegs are intended to produce a bond in pre-
drilled holes.
Bonding via thermoforming
Bonding can also be achieved between the plastic and
the other material via thermoforming. Here, a heated
thermoplastic material inserts itself into an irregular
groove in the conventional material. The irregularity
in the groove produces cavities into which the
thermoplastic material inserts itself and therefore
"grips".
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Bonding via shrinkage
Cooling is used to bring, for example, the plastics
part to a very low temperature. This causes shrinkage
of this plastics part. The plastics part is now
introduced with precise fit between two components
composed of conventional material. Heating the plastics
component to normal temperature causes it to expand and
thus become clamped between the conventional material.
Means of bonding: screws or bolts, studs, pegs,
adhesives, rivets, dowel pins, sintering, or any of the
known mechanical and adhesive bonding techniques.
Examples:
Timber-PMMA I-profile
One possible example of the use of the load-bearing
system described in the construction industry is a
transverse-load-bearing element with an I cross section
composed of various materials. The upper and lower
flange of the load-bearing element is composed here of
a traditional construction material, e.g. metal or
timber, while the web is produced from a plastic. The
web ideally has lower stiffness than the two flanges,
since this ensures that the majority of the normal
stresses occur in the flanges. The plastics web
transfers the shear forces between the two flanges. The
two different materials are bonded with means of
bonding in the shape of pegs. Examples of means that
can be used here are studs or plugs. Appropriate
adhesive bonding would also be possible. A transparent
plastic, such as PMMA, gives the load-bearing element
low perceived weight, which is of high aesthetic value.
The height of the load-bearing element varies from 10
to 300 cm, the thickness of the plastics webs being
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from 3 to 500 mm. The cross-sectional area of the
flanges is in the range from 5 to 3000 cm2 in the case
of timber, and from 1 to 500 cmz in the case of steel.
In the example constructed (see drawing No. 1), a load-
bearing element of height 25 cm was constructed with a
Plexiglas XT 20070 PMMA sheet of thickness 10 mm. The
flange material used in each case comprised two
commercially available slating battens of dimensions
24 * 48 mm. The means of bonding used comprised screws
whose diameter was 8 mm with about 10 cm separation. A
deflection of about 2 cm was measured on exposure to a
load of 5000 kg (as shown in drawing 6).
Underbraced load-bearing element
Another possibility for a load-bearing system composed
of transparent plastics in conjunction with known types
of structure is an underbraced load-bearing element
composed of a known material, for example aluminium or
timber, of a plastic and of a bracing cable. The load-
bearing element has an upper flange which accepts the
compressive forces and a possibly transparent plastics
web whose underside has a milled groove which serves as
guide for a cable. The load-bearing element has a fish-
belly shape, thus permitting the cable to be connected
at the end of the load-bearing element with the
pressure flange. Both the upper flange and the
underside of the load-bearing element here can have a
curved shaped.
Solid load-bearing element
The system described here can also be applied to a
solid beam. Here, two lamellae of a conventional
construction material are adhesive-bonded, or fixed
with mechanical means of bonding, to the upper and
lower side of a solid plastics beam for reinforcement.
In this case, too, the respective flange is again
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mainly responsible for the normal stresses. In
selecting materials here, glass may also be mentioned
as flange material, since this can give a completely
translucent load-bearing element in the case of
combination with a transparent plastic. Another
conceivable variation is a solid plastics beam with a
filament composed of steel on the upper and lower edge
of the load-bearing element. This steel filament
accepts the tensile forces and therefore provides a
type of reinforcement for the plastics load-bearing
element in a manner similar to that in reinforced
concrete.
Prop
Bonding of conventional materials to a plurality of
transparent plastics sheets can give a prop which is
perceived as extremely slim. In this multi-part member,
intended for compression, the compressive forces are
accepted by, for example, four metal rods, while the
sheets of plastic stabilize the individual compression
rods and thus prevent buckling. The moment of inertia
of the prop is more important here than the cross-
sectional area, and a non-solid cross section therefore
provides an alternative which is markedly more filigree
and lightweight and moreover saves material. In plan
view, there are many possible variants and shapes for
the arrangement of the individual compression members
and sheets, but for reasons of static efficiency the
location of the metal elements or timber elements
should be at maximum distance from the centre of
gravity.
Key:
1: conventional material
2: plastic
3: means of bonding
4: steel filament
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5: weight (1000 kg)
