CPD 25: Perforated metal facades25 October 2013

The latest in our series of CPD modules focuses on the exterior and interior applications ofperforated metals, and the methods of manufacture. This module is sponsored by RMIG

How to take this module

To take this module read the technical article below and clickthrough to a multiple-choice questionnaire, once taken you willreceive your results and if you successfully pass you will beissued automatically with a certificate to print for your records.

Introduction

Incorporating precision-perforated metals within building facades allows architects not only tocreate unique decorative designs and aesthetic effects, but also offers practical solutions forissues such as solar shading and sound management.

As part of a lightweight cladding system, perforated materials can be used to provide morepleasant internal environments and reduce energy consumption by allowing natural ventilationand daylighting to enter while shading spaces from direct sunlight. They can also improvebuilding security, or be combined with sound-absorbing material to enhance acousticperformance.

Perforated, embossed or indented facades can be created from metals including aluminium,steel or copper, and manufacturers offer a range of finishing techniques such as bending,rolling, forming, coating or anodising. The shape, size and pattern of the holes in a perforatedmaterial can all be specified during the manufacturing process to meet a project’s exactrequirements. While perforations involve the complete penetration of the metal, patterns canalso be embossed or indented, either instead of or in combination with perforation. In somecases, the embossing or indenting process and the choice of pattern can also add rigidity to thematerials.This CPD module will discuss the exterior and interior applications of perforated materials, themost common metals used in perforated facades and how perforated materials aremanufactured.

RMFR has participated in the rehabilitation of an old castle for a Mediatheque at the Château deCangé in Saint Avertin, France. RMFR has delivered decorative sunscreens for the project.Rawmaterial: Corten. Thickness: 2 mm. Pattern: R15T22. Cut by laser.

APPLICATIONS OF PERFORATED MATERIALS

Facades

The nature of the precision-manufacturing processes used in metal perforation enables

complex designs to be accurately rendered within perforated facades. This allows theincorporation of geometric shapes, graphic designs and perforated representation ofphotographic images, known as “picture perforation”. In most projects, images are split acrossa large number of panels, which are then assembled on site to an installation template to createthe final photographic effect. This may be used for dramatic effect or to reflect a design theme,an aspect of a building’s use or its surrounding environment. In some cases, the perforatedmaterial performs the supplementary function of solar shading; in other applications, reducingsolar gain may be its primary purpose.

In addition to complex graphic and picture perforation, a range of standard perforation patterns,hole shapes and configurations can also be selected, which are produced by standard tooling.This approach is often used for projects where individual design and aesthetics may not beprimary considerations, such as public transport shelters, car parks and security applications,as well as balconies and balustrades.

Acoustic uses of perforated materials

A common interior use of perforated metals is within acoustic and sound managementapplications. Depending on the size and spacing of the holes, perforated materials can eitherblock sound or allow it to pass through in specific ways. Perforated materials offer buildingdesigners a solution for noise control, whether keeping external noise from penetrating within - ifa building is next to a busy road, for example - or keeping noisy activities within a building fromdisturbing the surrounding community. According to a report produced for the IndustrialPerforators Association in the US, there has been dramatic growth in acoustic applications ofperforated materials over recent decades. In construction, perforated materials are used in twomain ways: as a facing for something else or as tuned resonant sound absorbers.

Facings This is where the perforated material is used as a protective or decorativecovering for an acoustic material, which may be designed to absorb sound, or to reflect orscatter it. Using a facing protects the acoustic material and disguises it - perforatedmetals can be made to look like plaster, for example. The purpose of the perforatedmaterial in these applications is to “disappear” acoustically - that is, it must be transparentfor the sound waves to pass through it. In this case, specifiers must choose the perforatedmetal that offers the greatest sound transparency, across all frequencies. For example, ina concert hall, perforated metal surfaces can allow the sound to pass through and bereflected back into the hall by specially designed surfaces behind the metal. This is knownas a broad-band sound absorptive treatment, and is appropriate when the acoustictreatment is intended to control sound at all frequencies.Tuned resonant sound absorbers In some instances, it is desirable to absorb soundselectively, only blocking those sounds within a certain band of frequencies and allowingsounds with frequencies either side to pass through. For this purpose, a “resonant soundabsorber” is designed, where the perforated metal plays an active part in tuning theabsorber - that is, in determining which frequencies are absorbed.

The sound transparency of perforated material depends on the pattern of the hole and thepercentage of the open area. Generally, the greater the percentage of the open area, the moreeasily sound can pass through, but the spacing of the holes is also important.

For example, a solid sheet with one large hole in the centre might have the same percentage ofopen area as one with tiny perforations all over it. The sheet with the central hole would blocksound across most of its surface, whereas the finely perforated sheet is almost completelytransparent to sound because the solid areas between the perforations are too small tointercept the sound waves. For high transparency, it is therefore better to have many small,closely spaced perforations.

To create a tuned resonant sound absorber, the diameter and the spacing of the holes must bedesigned to be transparent or opaque to air molecules vibrating at certain frequencies. Thedistance between the perforated material and the surface behind is also a factor in how soundwill pass through it, and different sound absorbing materials may be contained within the cavitydepending on the required effect.Further details can be found in the report, Acoustical Applications for Perforated Metals:Principles and Applications, by Theodore J Schulz.

The

COMMON METALS USED IN PERFORATED FACADES

Résidence André de Gouvéia- Maison du Portugal.Renovation of students’ housein the

Perforated facades can be made from various metals. The choice of metal is a key designconsideration, and will depend on requirements such as aesthetics, strength, durability,sustainability, resistance to environmental factors and cost. The three most commonlyperforated metals are mild steel, stainless steel and aluminium.

Mild steel

Mild steel is the least expensive and the most vulnerable to corrosion. It is manufactured ineither sheet or coil form by passing it through rollers to flatten it to the desired thickness. This isdone either while the steel is still hot (hot rolled) or after it has cooled (cold rolled). The resultingproducts differ in cost, quality and physical properties.

Hot rolled steel is typically less expensive and produced inthicker gauges of 1.5mm and upwards, whereas cold rolledsteel offers lower tolerance and higher quality surface finishes. Itcan be produced in almost any gauge, but is most readilyavailable in thinner gauges from 1.5mm down to 0.4mm.

As mild steel is susceptible to corrosion, it is used in eitherunexposed applications or with a protective coating. Coatingssuch as paint and powder are applied after perforating andcompletely protect the material. Metals to which a zinc oraluminised-based coating is applied, such as aluminised pre-galvanised steel or galvannealed steel, are coated at the millbefore the perforation process and therefore may notcompletely protect the material where the mild steel substrate isexposed during perforation. Manufacturer RMIG recentlysupplied Corten alloy steel for a chateau refurbishment projectin France (see image, previous page), which shows how theneed for

rust protection can be used to aesthetic advantage.

Stainless steel

Stainless steel was developed to be resistant to corrosion, and can be used where coating mildsteel is impractical. Stainless steels are steel alloys containing a minimum of 10.5% chromium,steel itself being produced from iron and carbon. Chromium makes the steel more resistant tocorrosion because a chromium oxide film forms on the surface of the material, which blocksoxygen from reaching the steel and from causing oxidation to occur. This is a very thin layer,which can reform if the metal is scratched, under the right conditions.

Other elements may be used instainless steel, such asmolybdenum, nickel and nitrogen,which creates materials withdifferent molecular structures andtherefore properties. For example,nickel is used to improve theformability and ductility of stainlesssteel.

Stainless steel is recyclable, whichmeans that it can be melted downand reused when scrapped.

Aluminium

Aluminium is the third mostabundant element and the most abundant metal in the Earth’s crust, making up around 8% ofthe earth’s solid surface by weight. It is much lighter and softer than both stainless and mildsteel. It is also resistant to corrosion, due to the formation of a very thin surface layer ofaluminium oxide, which prevents further oxidation. This can however result in a dull appearanceso aluminium may be either clear or colour anodised after perforating. Unlike paint or powdercoatings, which are applied to a surface, an anodised film is built from the aluminium itself andhas a translucent appearance.

STANDARDS

There are a number of relevant British, European and international standards relevant to theproduction of perforated materials. These include:

BS 7792:1995, ISO 10630:1994 - Specification for industrial plate screensBS EN 1090-1:2009 - Execution of steel structures and aluminium structures.Requirements for conformity assessment of structural componentsBS 5502-51:1991 - Buildings and structures for agriculture. Code of practice for designand construction of slatted, perforated and mesh floors for livestockBS EN ISO 9001:2008 - Quality management systems. RequirementsBS EN 9100:2009 - Quality management systems. Requirements for aviation, space anddefence organisations.

MANUFACTURE OF PERFORATED MATERIALS

Perforated materials are manufactured by punching holes in a sheet of metal or plastic. Thereare two main methods of perforating a material: by using an all across press, or by using a turret press.

All across press

An all across press punches holes in whole sheets of material as they are fed through, using asingle tool. Because many holes are made in a single stroke, an all across press can produceup to 50,000 holes per minute, which makes this the fastest and most cost-effective way ofcreating large-scale perforated materials.

Turret press

A turret press moves quickly over the surface of a material, punching holes as it goes. The mainadvantage of turret press perforating is that up to 65

0

Follow @buildingnews 40.8K follow ers

different tools may be used throughout the process,whereas an all across press uses only a single tool.

This machine will add additional holes for fixingpurposes and can emboss and indent the material aswell as making perforations. It can also countersinkholes, which is often used to hide fixing holes onfacades.

The turret press is also cheaper - with a standard toolcost of approximately £250-1,000, compared with£2,500-20,000 for the all across press, according toperforated materials manufacturer RMIG. Its drawbacks compared to the all across press are thatthere may be greater deformation or distortion due tomultiple tool hits and there is less uniformity. It is alsounsuitable for creating very large areas of perforation. All of the burrs removed from the material can berecycled in both manufacturing methods.

To complete this CPD, read the module and thenanswer the questions online at www.building.co.uk/cpdClosing date: 6 December 2013

CPD credits: 1 hour

Related company resources

Related Company Resources

The latest building ideas & solutions from companies within the industry

Like 0