New impetus for sandwich construction

Almost unnoticed by the majority of architects and planners, during the last 50 years sandwich construction has developed into a particularly economical construction method, which today also satisfies high architectural requirements. While many building sectors are heading for rock bottom at an alarming rate with -5% to -10% p.a., sandwich construction is recording growth of +5% p.a. and even more. Today, around 64 million square metres of roof and façade panels are manufactured and installed in Europe p.a.. You can get a better idea of this area by comparing it with familiar units. For example, you would need around 10,000 football pitches to fill this area. You could also make the comparison by taking the entire housing stock of a large city like Cologne. That's around 55,000 houses and 65,000 apartments. If we estimate an average area for façade and roof of 500 m2 each, then that's around 60 million m2. However, this considerable area contrasts with the relatively small number of architects who have intensively concerned themselves with sandwich construction.

Figure 9.1.1 Symbolic comparison of sandwich construction with the lego construction kit: Administration building in Legoland Park, Windsor, Great Britain (1996)- architects: Wimberley, Allison, Torry & Goo

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In view of this trend, it might well be considered to be in the planners' own best interests and part of their duty of care to become more closely involved with sandwich construction. This was previously only possible directly through the sandwich construction industry, as there was barely a hint of the existence of sandwich construction in universities, technical journals and specialist literature. A market survey of 2,000 architects in Germany shows that, for the reasons mentioned, many architects and civil engineers have little detailed knowledge of sandwich construction, but that there is a stated desire for better information. Almost half of the architects said that they had no experience at all with sandwich construction. This shortage of experience is in no way limited to Germany; similar situations are also evident in other European countries.

Figure 9.1.2 Around half of the architects surveyed had no experience with sandwich construction

The general development means that sandwich construction already makes up around 6 to 7% of all construction methods. There are many signs that this proportion will increase in the next few years. Reliable estimates reckon that sandwich construction will make up 12 to 13% of all construction methods in the next few years. This would be roughly equivalent to a doubling of the current figures.
What are the reasons for this European-wide trend towards sandwich construction? Obviously this construction method offers advantages which are convincing both building owners and, increasingly, architects. This contribution very briefly deals with the most important characteristics of modern sandwich construction.

EA detailed treatment is provided by the reference book, "Die Sandwichbauweise", by Rolf Koschade, published in August 2000 by Ernst & Sohn, Berlin (the English edition is expected to be published in October 2001 by A. Wiley). Covering approximately 400 pages, with over 500 illustrations and numerous tables, this book provides a detailed presentation of sandwich construction with industrially pre-fabricated sandwich panels consisting of metal facings and polyurethane high resistance foam core.

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Figure 9.1.3 The first reference book on sandwich construction in the world, which will serve architects, structural support planners, static engineers, installation engineers and building clients as a useful handbook for planning and implementation, as a reference work and as a source of architectural examples.

Vassilis Sgoutas, President of the UIA (UNION INTERNATIONALE DES ARCHITECTES), describes this book in his preface: "This publication presents different applications of sandwich panels. The use of sandwich panels clearly ventures into completely new dimensions, when combined with the creative flair of the architects whose works we can admire here. The UIA welcomes the initiative for the publication of this book..."

A new generation of sandwich panels

Thanks to the conjunction of corrosion-protected metal facings and core insulation made of polyurethane high resistance foam, sandwich panels have a useful combination of mechanical and building physics characteristics. In addition, they are very straightforward to install. As a result of continuous progress in profiling, joining technique and in design, the application options and techniques have been specifically improved, so that today a brand new generation of structural elements with enduring characteristics are available. Different profiles (trapezoidal, beaded, lined, microlined and flat elements), as well as different facing materials (steel, stainless steel, aluminium, copper), surface structures and colour coatings, now provide the architect with a broad design spectrum.

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Figure 9.1.4 Variable surface structures of sandwich panels -L: lined; S: beads; TW: trapezoidal wall panel; TD: trapezoidal roof panel; W: corrugations; M: microlining; E: flat.

Building systems, preforms, doors and systems

Over a period of several decades, the market-leading sandwich manufacturers in Europe have developed from producers of individual building components into suppliers of complete modular systems.

During the last two decades, this development has also been considerably influenced by architectural design requirements. Accordingly, the range of building components, initially geared towards cost-effective elements, has been expanded by sandwich systems and structural accessories which have enabled a design-rich, aesthetic architecture. Extensive preforms, such as e.g. curved and corner elements for roof and façade, make the sandwich systems into modular systems allowing richer variations of architecture.

Figure 9.1.5 Variable joint geometry of sandwich panels:
1. Narrow shadow joint with visible fixing;
2. Narrow shadow joint with concealed fixing;
3. No joint in trapezoidal pattern - with visible fixing;
4. No joint, through overlapping of high bead;
5. Concealed fixing of a roof panel.

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Figure 9.1.6 Special preforms now enable more ambitious architecture

Figure 9.1.7 Examples of windows that can be integrated into sandwich façade panels

Figure 9.1.8 This stainless steel sliding door in sandwich construction for cold and deep-freeze stores is just one example of numerous structural elements which make sandwich construction a complete modular system.

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The functionality of these structural elements has also been further developed. Today, sandwich systems are available with integrated structural elements, such as, for example, windows, lighting elements, vent pipes, attic components, gutters, interior and exterior doors, solar technology etc.. In most cases, the sandwich manufacturers also have an extensive product range of metal components, special profiles and preforms, which can be combined with sandwich systems.

Sandwich effect - load bearing performance

If you place an insulating board and two thin metal facings loose on two bearings, then the individual elements, particularly the metal facings, will deflect enormously due to their own weight. Thanks to the sandwich technology in which these two metal facings are non-positively foamed with the polyurethane high resistance foam core insulation, a light structural element results, with excellent load bearing performance.

Figure 9.1.9 Left: Loose layers - Right: Foamed layers of a sandwich panel provide good bearing characteristics and inherent rigidity, thanks to shear-resistant connection between insulation core and facings

Figure 9.1.10 Comparison of deflection of a 50 mm thick sandwich panel with a 50 mm thick wooden beam with 3,000 mm span

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A practical comparison with the familiar material of wood shows the benefits: A three metre long solid wood lining, as used for example in truss construction for truss group 3 as truss soffit, with a point load of 1.5 kN, to observe a maximum deflection of 20 mm, needs to be around 50 mm thick and must be multiply bonded in order to fulfil this requirement. If the lining width is 500mm, this results in a weight of 45 kg. In comparison, a 50 mm thick sandwich panel only deflects 16 to 17 mm under the same conditions, and in addition it only weighs 18 kg. The sandwich panel offers a weight saving of 60%. This weight advantage becomes particularly noticeable in transport, assembly and dimensioning of the supporting structure.

Thanks to the sandwich effect, sandwich panels have an excellent flexural strength and stiffness against torsion. Their high inherent rigidity not only makes them self-supporting, but also able to bear considerable loads, depending on the static system and dimensioning, despite their light weight. For this reason, sandwich panels can be used for demanding facade design and safe roofing in a very broad range of building applications. In principle, sandwich panels are suitable for all light and composite building, and can be mounted on a very wide variety of supporting frameworks, such as reinforced concrete, steel, aluminium or wood.

Design, technology, economy and ecology

Defensive statements like: "You can't achieve sophisticated design with sandwich construction" belong either to the 60s or to the realm of prejudice. Today, hundreds of colours, a multitude of surface textures and profiles as well as a broad range of system-related design accessories are available to the architect who uses modern sandwich construction. With buildings throughout Europe, creative architects of the 90s showed the design latitude permitted by sandwich construction, if its potential is properly utilised. Three buildings with different uses are shown here, selected from a large number of architectural examples world-wide:

Figure 9.1.11 To achieve a sympathetic design for the 276,000 m3 building volume of the up to 46 m high ROTEB waste incineration plant in Rotterdam, in the immediate vicinity of a residential district in Rotterdam, the Dutch architect Maarten Struijs used a façade of curved silver-metallic sandwich panels and rounded corners. The building, constructed between 1993 and 1994, reflects the sunlight during the day and creates a constantly changing interplay between light and shadow.

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Figure 9.1.12 The airport management company SEA built the entire façade of the new Milan "Malpensa 2000" airport in sandwich construction. The wall panels, with a total area of 50,000 m2, have an outer aluminium facing in the colours light ivory yellow, fir green and cream.

Figure 9.1.13 Between 1996 and 1998, on the former site of the Muenchen Riem airport, 13 exhibition halls were erected in sandwich construction for the New Munich Trade Fair Centre. The rounded, white aluminium sandwich structures recall the aerodynamic shape of aircraft support surfaces (design architects: Bystrup, Bregenjoj & Partner, Denmark; implementation: planning team Kaup, Scholz, Jesse and Obermeyer, Germany).

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Heat insulation and moisture protection

A high level of heat insulation was required in the buildings presented here, without costly structural designs. Thanks to core insulation of the sandwich panels in PUR high resistance foam, the required heat insulation is achieved with relatively small wall thicknesses. With a thickness range of 60 to 140 mm, all requirements of the heat insulation regulation and also the tightened requirements, currently under discussion, of the draft EnEV 2000, can be fulfilled.

Figure 9.1.14: Relation between increase in heat insulation, weight increase, increase in overall costs and annual saving in fuel oil with sandwich panels

Abbildung 9.1.15 Kombination von Sandwich-Dachelementen mit Solartechnik

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It is interesting in this context that, even with a 100% increase in heat insulation, the total costs for material and installation only increase by around 10% for sandwich construction. Innovations offer additional functional and architectural possibilities. One example is the combination of solar modules with sandwich panels, which are available on the market in different design variants.

No less important for heat insulation and moisture protection, as well as for the quality of room climate, are atmospheric density, moisture proofing and protection against driving rain of the building envelope. The joining technique of modern sandwich panels now enables a quality of panel connection which is up to 100 times more impervious than high quality window designs. Due to their metal facings, sandwich panels are impervious to water vapour diffusion. If necessary, the panel connections can be made impervious to water vapours by means of suitable sealing systems. For this reason, sandwich construction has also proven to be outstanding in cooling and refrigeration technology.

Weather and corrosion protection

Weather and corrosion protection have a considerable influence on the lifetime and maintenance-freedom of the building. During the last few decades, the corrosion protection for metal facings of sandwich panels has been so perfected that today, depending on location, a lifetime spanning at least one to two generations can be expected. In the examples shown here, in addition to steel profiles with high quality duplex systems, aluminium facings with an organic coating were also used.

Figure 9.1.15 Combination of sandwich roof panels with solar technology

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Preventive fire protection

Under rigorous observance of structural fire protection as well as the applicable fire protection requirements and safety regulations, sandwich construction can also be considered a safe and proven building method from the view point of fire protection. As a rule, the entire element has low flammability (B1 as per DIN 4102). An additional protection function against the effect of flames is provided by the enveloping metal facings, including the joints.

Figure 9.1.17a - 9.1.17c
Development of an SBI test for the fire protection technology classification of sandwich panels on 4 February 2000 in MFPA Leipzig: the sandwich panels do not contribute to maintaining the fire. The fire goes out when the source of the fire is removed.

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Sound proofing

Light components naturally have a lower sound insulation than heavy components. However, this does not mean that there is no sound proofing in lightweight construction. Sandwich panels with a thickness of 60 mm have an "Assessed sound insulation value Rw" of approx. 25 dB. This sound insulation value will suffice for numerous areas of application in industrial and shed construction. In noise-sensitive zones of administrative and residential construction, the planner can achieve the required sound proofing in the sense of an optimal combination of sound insulation and sound absorption by using appropriate design measures with advantageous building materials and components.

Cost effectiveness

A free choice of supporting structure (wood, steel, aluminium, reinforced concrete) permits the planner to respond to the regional conditions and the requirements of building owners. The light but very stable elements allow the supporting structure to be economically dimensioned. Productivity and a short construction time are important in many building projects. The combined benefit of high level of prefabrication, rational grid planning with large-surface building elements, light weight and quick installation, has a very advantageous effect for sandwich construction. This can be clarified by an actual example: The Argos commercial centre in Stafford, Great Britain, a 384,000 m3, 8 m high industrial building with 48,000 m2 roof area, including supporting structure, was completely erected within five working weeks. The technically simple assembly principle (plug-in connection and screwing down to the substructure) can also provide evident cost and time savings for conversion and extension measures.

Figure 9.1.18 Ecological energy balance for core insulation of sandwich panels for a usage period of 35 years

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Ecology

With careful assessment of the energy saving potential of a sandwich panel, over the usage period of one generation at least 40 times the energy used to produce the PUR high resistance foam core insulation can be saved in fuel energy, and at least double the investment costs for sandwich construction.

The saving in resources and capital is accompanied by a correspondingly high reduction in emissions, which result from the burning of fuel oil or other organic fuels. Even after their life cycle as sandwich panels, metal and PUR high resistance foam can be used for ecologically and economically sensible recovery. For polyurethane high resistance foam, in addition to energy recycling with an energy recovery of around 34% of the entire production energy for thermal use, material recycling is also practised, which has enabled the building industry to produce interesting products with new material characteristics.

All in all, thanks to a symbiosis of design, technology, economy and ecology, sandwich construction has developed into an interesting alternative for architecture.

Author: Rolf Koschade

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