Sandwich Panels

Due to their high level of prefabrication, their outstanding thermal insulation and good load bearing characteristics, sandwich panels are enjoying increasing popularity with building owners and planners. We explain the verification procedures for practical use and will discuss the European standardisation in a next report.

The use of sandwich panels is constantly increasing due to their
simple assembly on the building site and the good load bearing
and thermal insulation core values.

The sandwich panels used in practice mainly consist of metal facings and a plastic core (for example, polyurethane, polystyrene) or mineral wool. The cover sheets which can be quasi-flat (beaded or lined) or trapezoidal, are joined shear-resistantly with the core layer by means of self-adherence or bonding, giving an overall bearing composite or sandwich cross-section with a high level of rigidity (Figures 1 and 2).

Fig. 1: Design variants of sandwich panels depending on the core layers

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Areas of application and load bearing performance
Sandwich panels are mainly used as space-creating and load-bearing wall and roof systems. They are thus essential construction elements of a building. Around 12 to 14 million m² roof and wall coverings were built from these elements in Germany in 1999.

Material Steel sheet tN = 0,40 –1,00 mm, bs = 280 – 350 N/mm² Corrosion protection according to DIN 55928 Aluminium for example: Al Mn Mg 0,5 tN = 0,50 – 0,70 mm Rp;0,2 = 140 – 220 N/mm² Copper for example: SF – Cu Nr. 2.0090, tN = 0,50 – 0,70 mm Rp;0,2 = 280 N/mm² Wood board-shaped wood materials for example: laminated particle boards according to DIN 68763 d = 8 mm

Fig. 2: Design variants of sandwich panels to facings

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The advantage of sandwich panels lies primarily in the fact that they are pre-completed building site systems, which only need to be installed and secured as wall and roof panels - prefabricated in the factory - on the building site. A further essential advantage of these simple and therefore inexpensive to install sandwich supporting structures is that, in the sense of “integrated building”, the core layer not only acts as excellent thermal insulation, but the rigidity and load capacity of the components are also considerably increased through combination with the cover sheets.

Thus one obtains, for example, for a 80 mm thick wall panel (quasi-flat, beaded cover sheets with t_N = 0.5 mm and a polyurethane core) permissible spans of 5.0 to 6.0 m (building height < 8.0 m) and for a roof panel (trapezoidal at the top, beaded at the bottom) with a snow load of 0.75 kN/m2, spans of 4.0 to 6.0 m.

Due to the use of core layers made of plastic or mineral wool, which behave so favourably with regard to manufacture and building technology requirements, a number of special features have to be noted when assessing the load capacity of the sandwich supporting structures and thus the dimensioning.

Thus, for example, when assessing the load capacity, the shear softness of the core layer (Figure 3) must be taken into account “Theory of the elastic bond” . This is a question of reliable dimensioning, as when dealing with the “rigid bond” the load capacity of the components would be over-estimated. In addition, due to the combinations of different materials (for example metal/plastic) special influences must be taken into account, which have a significant effect on the load bearing performance.

Figure 3: Theory of the elastic bond

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General building supervision certification

For the use of sandwich panels for roof and wall, in the case of which the composite structural conditions are specified and utilised through the shear resistant bonding of the cover sheets with a core, a “General building supervision certification of the Deutschen Institut für Bautechnik [German Institute for Building Technology], Berlin (DIBt)”, is required in Germany, as these components are not regulated in any standard and are not recorded in building regulation lists A and C.

With the definitions that are recorded for a particular component type of a particular manufacturer in the certification decision, practical verifications for the bearing safety and suitability for use can be performed with sufficient accuracy. For the calculations, a generally valid Appendix “A” is attached to each certification decision, which lists all necessary verifications for the stability and suitability for use, but also, above all, the influences that need to be taken into account. If the verifications, with the characteristic values and resistance values, are then performed according to Appendix “B”, and the required safety aspects are observed, it can be assumed that no reservations exist against the use of these components.

Besondere Einwirkungen
In the case of façade panels and supporting wall and roof panels, the usual influences resulting from unladen weight, snow and wind can be estimated as external loads (Figure 4). However, in addition - and this applies in principle for all sandwich panels, irrespective of their use - two further influences are of crucial importance:

- Temperature squeezing due to the varying cover sheet temperatures and (particularly with plastic) creep due to the shear stress in the core layer
- The additional stresses in the event of temperature change result from the deformations which occur due to expansions in the facings. It is proven and, above all, also prescribed for the design loads in the building supervision certifications (table), that in the case of sandwich panels with metal facings and plastic core, temperatures of 80°C can occur in central Europe due to solar radiation on the outer facing, while on the inner facings, only 25°C is estimated, due to the good insulation properties of the core layer. The temperature level on the outer layer depends on the colour. In winter, for instance, -20°C can be taken into account outside and +20°C inside as influence variables.

Figure 4: Effects (design load)
of sandwich panels

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In the case of prolonged loading, for example due to weight and snow, attention must be paid to the transposition of stress in the cover panels of sandwich components due to the shearing creep of the plastic core layer. As is well known, creep means increase in deformations without an increase in loading. Due to the additional deformations, transpositions of stress occur, which must be taken into account in the dimensioning sandwich panels.

When assessing the load capacity of sandwich panels, the “temperature burden” and the “creep burden” must never be ignored, as the stress from these “special” influences may well be in the same order of magnitude as those from external burdens. Thus they are of crucial importance for the safe dimensioning of these components.

Verification of stability and serviceability
To perform the necessary verifications of stability and serviceability, two procedures are conceivable in practice, which are expressly cited in the certifications and are thus prescribed: Verification on the basis of type-tested span tables or project-related individual verifications.

Verifications with type-tested span tables
For the practical dimensioning of components which are relatively complex due to the calculation procedures and the multitude of possible load combinations, first of all tables with (type-tested) permissible spans should be made available to each manufacturer. The spans must be calculated for the relevant use as roof or wall with the related estimated loads (for example, snow according to snow load zones or wind depending on the building height) and the relevant colour classes, so that all necessary verifications are provided.

Attention is expressly drawn to the fact that, in addition to the verification that the component itself is dimensioned with adequate safety margins, the following verifications must be performed (Figure 5):

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Verification of the bearing pressure

Verification of the fasteners
a) Verification of the load capacity of the connections
(particularly for lifting-off stresses, edge and corner areas)

b) Verification of the screw head deflection
(due to varying cover panel temperatures)

Figure 5: Verifications required in addition to the type tests

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- Verification of the fixings, particularly for lifting-off stresses due to wind pull and temperature coercion, including in edge and corner areas of the building, according to Section 7.7 of the certification, Appendix A
- Verification of the bearing pressure (compression resistance of the core layer) according to Section 7.2.1.4 and 7.3 of the certification, Appendix A, on the basis of bearing forces due to loading, including squeezing forces due to temperature
- Verification of the screw head deflection due to varying cover panel temperatures according to Section 7.7.2 of the certification, Appendix A.

Individual verifications
For special projects (for example different spans) or uses (for example deep-freeze stores), a project-related individual dimensioning must be performed for practical application, on the basis of Appendix A of the building supervision certifications. An exact representation of the individual verifications would go beyond the scope of this contribution. However, attention is expressly drawn to the fact that adequate explanations and detailed examples of the verifications are required according to the building supervision certification are presented in the literature, for example in [1] to [13].

Prof. Dr. Klaus Berner,
Professor at Mainz Technical College for steel construction,
wood construction and load bearings

Facing temperature, inner side
As a rule:: θ_I = 20°C in winter θ_I = 25°C in summer

Facing temperature, outer side

Figure 6: Temperature load (extract from the general building supervision certifications, Appendix A)

Literature

[1] Stamm, K., Witte, H.: Sandwichkonstruktionen. Springer- Verlag, 1974
[2] Berner, K.: Stahl/PUR-Sandwichtragwerk unter Temperatur- und Brandbelastung. TH Darmstadt, 1978
[3] Stamm, K.: Sandwichelemente mit metallischen Deckschichten als Wandbauelemente und Dachbautafeln im Bauwesen. Stahlbau (5 und 8) 1984
[4] Schwarze, K.: Numerische Methoden zur Berechnung von Sandwichelementen. Stahlbau (12) 1984
[5] Langlie, C.: Berechnung von Sandwichelementen mit ebenen metallischen Deckschichten. Stahlbau (10) 1985
[6] Jungbluth, O., Berner,K.: Verbund- und Sandwichtragwerke. Springer-Verlag, 1986
[7] Wölfel, E.: Nachgiebiger Verbund. Eine Näherungslösung und deren Anwendungsmöglichkeiten. Stahlbau (6) 1987
[8] Berner, K.: Erarbeitung vollständiger Bemessungsunterlagen im Rahmen bautechnischer Zulassungen für Sandwichbauteile. Forschungsvorhaben DIBt, Berlin, 1995
[9] Berner, K.; Hassinen, P.: Present Design Methods of Leightweight Sandwich Panels. Nordic Steel Construction Conference, 1995
[10] Berner, K.: Praxisgerechte Nachweise zur Trag- und Gebrauchsfähigkeit von Sandwichbauteilen. Stahlbau (12) 1998
[11] Koschade, R., Berner, K., u. a.: Die Sandwichbauweise. Ernst & Sohn, Verlag für Architektur und techn. Wissenschaften GmbH, Berlin, 2000
[12] Berner, K., Gruber P.: Handbuch zum Programm SandStat 3. Ingenieurbüro Berner & Gruber GmbH, Darmstadt, 1998 / 2000
[13] Davies J. M. (Hrsg.), Berner, K., u. a.: Lightweight sandwich construction. Blackwell Science Ltd., Oxford, 2000

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