SELECTED ISSUES IN DESIGN AND
IMPLEMENTATION OF PERFORATED METAL
CLADDINGS
Paweł ŻWIREK 1
1. INTRODUCTION
Capabilities of modern computer aided design systems (CAD) and computer aided
manufacturing process (CAM) make contemporary architects to increasingly turn to
the perforated claddings with complex geometry - both for the façades of new
buildings, as well as revitalized ones. The development of LED technology, in
particular the low electricity usage together with high reliability and long life become
an additional incentive to the use of perforated façade illuminated from the inside.
Perforated claddings may be made of different materials:
- metal, especially aluminium, steel (including stainless steel, galvanized steel,
corten), copper, bronze,
- composite materials consisting of two cover sheets, usually of aluminium and
polyethylene or mineral core (e.g. Alucobond [2], Reynobond [1])
- fibre cement boards (e.g. Equitone [3]),
- high pressure laminate HPL (e.g. Fundermax [9]),
- pressed ply made of rock wool with thermosetting bitumen (e.g. Rockpanel [4]),
- glass-reinforced plastics (with glass fibre), also known as GRP.
- prefabricated concrete.
The choice of material, in addition to aesthetic reasons is also affected by the
complexity of the perforation pattern and its intensity, and in the case of revitalized
façades, also by the unit weight. Of the above-mentioned materials, the greatest
opportunities for making patterns of perforation of the most complex shapes and
highest intensity of perforation - while maintaining low weight - are enabled by metal
claddings. Present paper further discusses issues related to the design and
performance of the metal claddings in the form of perforated sheets and expanded
metal mesh.
1
Dr inż., Politechnika Krakowska, ul. Warszawska 24, 31-152 Krakow, e-mail: pzwi@pk.edu.pl
2. PRODUCTION TECHNOLOGY
Designer of perforated or expanded metal mesh façades makes decisions
concerning the perforation pattern and type of material. Knowledge of the production
process of perforated claddings allows for the selection of solutions, which are
technically correct, economically viable and meet certain requirements in terms of
aesthetics.
The manufacturing process of flat perforated metal claddings comprises the
following steps:
- preparation of batch files for numerically controlled machines (CNC)
- perforating the planar metal sheet,
- preparation of cladding's contour element,
- forming a cladding by folding the edges of flat workpieces.
Preparation of batch files is usually carried out using specialized software provided
by the manufacturers together with the machines. There are also programs that
automatically, based on a three-dimensional model, generate batch files for
numerically controlled machines - both perforating, as well as bending ones.
The choice of technology for making perforations in the metal cladding depends
on the perforation pattern and the type of material. Commonly used are the following
technologies for making perforations in the flat sheets:
- punching on wide-perforating presses,
- punching using punch presses,
- laser cutting,
- milling.
In the case of one type of hole, disposed at a regular, usually triangular, square or
rectangular mesh (Fig. 1a) the cheapest technique of perforations is punching using
wide perforating presses. Making holes by means of punching is to use a punch and
die. After placing the material between the punch and the die press, punch presses
on creating a hole (which is analogous to the technology commonly used for office
hole punches). The wide perforating presses typically use strips with a width of the
whole metal sheet (1000mm, 1250mm and 1500mm) equipped with punches and
dies so that during one operation the holes over the entire width of the sheet are
performed.
Fig. 1. a) Perforated sheets of regular and repetitive arrangement of holes, b) rotational head of punch
press with 47-ohm connectors for punches, c) example of perforation of the irregular pattern of
identical holes, d) example perforation made with laser cutting, e ) expanded metal mesh , f)
perforated light box (source: b) - https://www.lvdgroup.com, others: P. Żwirek)
Feed material for the production of perforated metal is supplied in rolls, then it is:
unrolled, straightened by a set of rolls, perforated on wide perforating press and cut
into sheets of a specified length. The rate of perforation process, and the ability to
produce sheets of a desired length at a regular perforation pattern of high intensity
makes it currently the least expensive production technology.
If the perforation pattern consists of a small number of types of holes arranged in
an irregular manner (Fig. 1c) punch presses are used. As in wide perforating
presses, punches and dies are used. The punches and dies are set in a fixed position,
and a special worktable moves the material in a preprogrammed manner - both in
the longitudinal and transverse directions. After setting of the material in the right
position the press performs the punch movement and makes a hole. Modern punch
presses are equipped with a set number of punches (Fig. 1b), which in the process
of cutting can be changed automatically.
Fig. 2 Sample drawings created using perforation technique by punch presses (source: P. Żwirek)
Punching holes may be performed in the sheets made of various kinds of metals
including galvanized and pre-painted. If the cladding will be made of galvanized and
pre-painted steel sheets, punching technique may prove to be the most economical.
In the case of galvanized steel sheets, hole punching technique causes the anticorrosion layer of zinc sheet surface "hauled" on the edge of the hole during the
operation of punching the hole, so there is no additional protection of the cut edge
of the sheet required. Making perforations using wide perforating and punch presses
can be combined with the technique of pressing with little depth. Recently it has
become extremely popular to create façades drawings using perforation technique.
As shown in Fig. 2 interesting drawings can be achieved using a small number of
hole types (in the presented examples only two) spaced on a regular grid. Punching
using punch presses is the most often used technique in this case. Currently,
programs are available for forming groups of holes with predetermined size, shape
and the dimensions of a single opening based on an image,
Laser cutting is a thermal process in which the laser beam is produced by a laser
source (cavity resonator), and supplied to the cutting head of the machine where it is
focused with high power using a lens. Such a focused laser beam meets the sheet
causing it to melt. The cut material is melted through its entire thickness using a laser
beam of high intensity, and then blown out of the cut by the piercing gas flowing
from the nozzle with a high kinetic energy. Laser cutting technology is used in the
case of irregularly shaped holes, as in Fig. 1d. Laser cutting technology should not
be used on pre-finished steel sheets, galvanized or galvanized and painted sheets because of the damage of these coatings under high temperature. Attempts to repair
and corrosion protection of the above mentione sheets are not economically justified.
The technique of laser cutting in connection with corten type of sheets allows for
striking results (Fig. 3), and relatively inexpensive claddings - mainly due to the lack
of the need for additional corrosion protection and less stringent requirements to
protect the surface of the material during processing, transport and assembly of
finished cladding elements.
Least often, for economic reasons, the perforation technique of metal machining in
the form of milling is used. Milling is typically used in thick sheets, which puts a
further requirement for the shape of the edge of the hole.
In addition to perforated flat metal sheets, currently expanded metal meshes are
often used for claddings (Fig. 1e). These are originally flat sheets locally incised and
then stretched so as to obtain permanent deformation of the workpiece. Expanded
metal meshes are limited in terms of bending and therefore mostly used as flat mats
screwed directly to the substructure or mounted in frames by welding.
Performing laser perforation or punching using punch presses allows the
simultaneous formation of cladding's contour element, drilling assembly or hangers
integrated with the cladding element. (Fig. 1f).
In the simplest embodiments of a flat perforated metal cladding sheets mats are
attached to the frame - in this case the manufacturing process of cladding elements
ends at the stage of appropriate cutting the edge of the mat's material. In turn, the
cladding, which have been designed as ligth boxes require bending the edges of the
material (Fig. 1f). Bending is done on the edge presses or benders.
Fig. 3. Examples of laser perforations on cor-ten type sheets (source: P. Żwirek)
3. PROJECT ISSUES
Designing perforation pattern must take into account the technological limitations
resulting from the type of material and technology of punching. Both in the case of
punching and laser techniques, the minimum width of the material, which is to
remain between two adjacent holes, depends on the thickness of the material.
A separate issue is the issue of static-strength calculation. Lack of design
standards for this type of component causes the designer to be limited to the
following possibilities: computer simulations based on models of cladding or
experimental studies. Obtaining reliable results of numerical simulations can be
difficult due to occurrence of complex distributions of residual stress generated in
the process of punching holes or structural changes in the area of the holes, which
were made using laser technology - due to the presence of high temperature, which
may significantly affect the behaviour of the cladding element. An additional
difficulty may be correct modelling of wind loads.
Much more reliable results can be obtained with experimental studies. For
example, in [6] the course and the basic results of experimental tests in the wind
tunnel for the cladding of the National Stadium in Warsaw, which have been made
in the form of panels filled with expanded metal mesh, were described. Research has
allowed for verifying the behaviour of the perforated element and to determine the
values of aerodynamic coefficients [8] needed to determine the effects of cladding
on the supporting structure. Issues concerning the design of the supporting structure
for the cladding are described in the paper [10].
The specificity of perforated claddings requires taking into account specific
issues related to safety, functionality of use and aesthetics in the design process. The
use of perforated elements in the lower parts of the façade, especially in the
immediate vicinity of the routes, makes it necessary to analyse whether the same
shape and the mesh size of the perforation is not a "trap" for hands or fingers of
passers-by. The use of large holes in these places requires appropriate, aesthetic,
resistant to vandalism finishing layer underlying the perforated cladding, which is
mostly thermal insulation. In addition, oversized holes located at the communication
paths may allow for throwing garbage behind the perforated cladding. These
circumstances lead to replacement of large perforations by punching, which only
shows the contour of the opening (see Fig. 1c and 1d).
For perforated claddings to be illuminated from the inside, the issue of lightproofness needs to be analysed. Lack of light-proofness may result in arbitrary
drawing in the form of a set of lines that occur at the interface between panels on
the façade (see. Fig. 4a). The presence of the light source behind claddings makes it
necessary to be able to disassemble them. The solution which allows for easy
removal of individual elements of cladding (in this case made of corten metal sheet)
is shown in Fig. 4c and 4d. This solution contains a vertical assembly strip with
horizontal pins, which are hung on the cladding by means of hangers cut at the stage
of forming the contour of the panel. The shape of the vertical strip can be chosen in
such a way as to provide light-proofness. The panels should be protected from
uncontrolled horizontal movement caused by thermal deformation. This protection
can be performed by means of special lockable sleeve on a horizontal spindle
assembly after setting the position of the cladding panels. If the applied technical
solution allows the removal of individual panels without dismantling adjacent ones,
then the panels should have adequate security lock before removing the panels from
the mandrels by unauthorized persons - particularly in the case of panels mounted
directly above the surface of adjacent land.
The presence of relatively large perforations may require the use of special
protection, e.g. in the form of additional mesh mounted behind a perforated element,
so as to eliminate the possibility of the entry of birds into the cladding. Choosing the
appropriate size of the perforation holes can also eliminate the risk of birds entering
the holes.
If there is insulation under the elements of perforated cladding it is necessary to
analyse the possibility of entry of snow and rain through the holes, which may result
in dampening and gradual degradation of the insulation. In particular, the thermal
insulation made of rock wool is exposed to these effects. In this case, it is required
to perform the appropriate tests or change the technical solution in terms of the type
and arrangement of layers of insulation using e.g. an additional thin-layer plaster on
insulation or shielding with e.g. trapezoidal metal sheet with a low profile and a dark
colour.
For aesthetic reasons it is important that all elements under the perforated
claddings were as little visible from the outside as possible. It is recommended to
use e.g. insulation sheathed with glass veil in black (without inscirptions).
Supporting structure, at least in terms that can be visible, should be painted dark.
When planning the use of perforated cladding with relatively large holes in the
immediate vicinity of the trees, one must be aware of the need for periodic access to
the space behind the cladding for leaves removal. If between the lining and the wall
of the building there is a large enough space and it is possible to provide secure
access to the cladding it is not necessary to periodically remove the cladding.
Otherwise, it should be possible to dismantle individual components by applying a
solution in the form of hanged panels (see. Fig. 4c and 4d). Another solution is to
change the perforation pattern so as to eliminate the risk of ingress of leaves under
the cladding.
Fig. 4. a) Cladding perforated without an assured light-proofness between adjacent panels, b) view of
the façade after "sealing" the space between the panels, c), d) example solution to eliminate the
problem of light-proofness at the contact between the cladding elements that enables easy
disassembly and reassembly of a single cladding element (source: P. Żwirek)
4. PERFORATED ELEMENTS IN REVITALIZATION
Perforated elements, especially aluminium, mainly due to low weight, can be
successfully used for the revitalization of façades of buildings. An additional
advantage of perforated claddings in relation to the full ones is the fact that smaller
impact of wind is provided on a wall of an existing building. These two properties
of perforated aluminium claddings provide that they can be successfully used for
even relatively weak substrates. This type of claddings also results in relatively small
load on existing building structure.
Fig. 5. a) office building before revitalization, b) office building after the revitalization with
the use of expanded metal mesh, c) a false wall of perforated claddings above the garage
ramp (source: P. Żwirek)
In the case of a building shown in Fig. 5 [11] the exterior walls were made of
blocks of cellular concrete. The analysis of the available systems of anchoring points
to their low capacity in the substrates made of cellular concrete, which was
confirmed by subsequent studies. A small load capacity of anchors eliminated the
possibility of heavy stone or ceramic claddings, referring to the existing buildings in
the neighbourhood. Therefore, it was considered to use lightweight aluminium or
composite claddings (e.g. Alucobond). Greater opportunities for forming,
perforation (including the availability of expanded metal meshes) and the availability
of virtually unlimited palette of colours in powder coating technology, determined
the choice of aluminium. Alloyed aluminium AW-1050A was used, which is plastic
with good susceptibility to cold bending, highly resistant to atmospheric corrosion.
Fig. 5a and 5b show respectively a view of the façade of the building before and
after revitalization. The subject of the revitalization of a group of buildings
consisting of an office building and a two-level garage with entrance ramps, which
has been designed for the Provincial Committee of the Communist Party in Krakow.
The buildings are located in the historic part of Krakow, at Smolensk Street in the
Old Town district, in the urban conservation area. The buildings are from the 1970s
and did not meet modern standards - in both functional and aesthetic terms.
5. CONCLUSION
Perforated metal claddings make it possible to obtain an attractive façade, which
can impart a unique character and underline the function of building’s body.
Knowledge of production technology and specific issues related to the use of this
type of façade, such as light-proofness, the need for periodic disassembly, safety
considerations and functionality allow for the conscious design of relatively
inexpensive and functional perforated claddings.
Due to its low weight and lower wind load than the full claddings, perforated
claddings can be used successfully revitalizing the façades of existing buildings.
REFERENCES
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A b s t r a c t
The possibilities of modern CAD and CAM systems supporting the design and
manufacturing cause modern architects to be increasingly turning to perforated
claddings. The use of perforated claddings allows for creating a specific pattern,
drawing on the entire surface of the façade. In addition, it is possible to create a
remarkable light/shadow design.
The present study selected topics related to the design and implementation of
modern perforated and pressed claddings.