Sandwich Components - Standardised, approved, controlled and certified

Sandwich panels are quickly gaining ground, particularly in industrial construction. The verifications required in this sector for stability and serviceability are relatively complex. A European standard is under development.
In Germany, the only valid principle for practical building use is the general construction supervision approval of the DIBt. However, in the rest of Europe, other documents are frequently used for dimensioning and practical use.

Only official European principle

The so-called ECCS/CI recommendations form the only official principle that can be cited at present, with the following title:
“European Recommendations for Sandwich Panels, Part I Design” Final Draft 1.9.2000 developed by

• ECCS (European Convention for Constructional Steelwork)
• CIB (Conseil International du Batiment pour l‘Etude).

The version of September 2000 is a revision of the so-called temporary recommendations (“Preliminary European Recommendations for the design and testing of Sandwich panels”), which were published in an initial version in 1991. The recommendations covered sandwich panels with metal facings and a polyurethane core.

Fig. 1: Classic range of application for sandwich panels - industrial construction. In this combustion plant in the Netherlands these panels were used not only for aesthetic reasons but also for reasons of sound adsorption and thermal insulation.

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A second version was developed in a joint ECCS and CIB committee with the inclusion of components with a mineral wool core. This was published in 1995 under the following title: “Preliminary European Recommendations for sandwich panels with additional recommendations for panels with mineral core material”, Part I, Design. In addition to the general sandwich technology principles, the European recommendations primarily contain the verification procedures for stability and serviceability. The verifications correspond in principle exactly to the verifications required according to the German certifications. The only differences are in the recommended safety correction values, with the “European” values being slightly more favourable.

In addition to the verification procedures, however, all the European recommendations above also describe in detail all tests which are required to determine the characteristic values (for example, failure stresses, shear and compression resistance and modulus of elasticity). In addition, these tests also correspond to the experimental tests which are performed in the context of the certification procedures.

European standardisation soon
In future, a European standard with the following title is to regulate the use of sandwich panels: “Self-supporting double skin metal faced insulating sandwich panels - Factory made products - Specification”
The standard is being prepared by sub-committee SC 11 of CEN/TC 128 on the basis of a mandate which the European Commission has given to CEN (European Committee for Standardisation). At present, the seventh revised draft is under discussion. The preparation is so far advanced that a complete version containing all the planned chapters is available.

Essentially, a number of comments and requests for changes by the national committees still need to be discussed in detail during the further treatment and taken into account in the standard where appropriate. The standard draft is to be ready for presentation to the individual nations for discussion in the year 2001. If the draft is approved by a majority, then a “harmonised European standard” will be introduced which is authoritative for all members of CEN.

Fig. 2: Sandwich panels are also enjoying ever increasing popularity with building owners and planners in roof assembly.
Photo: Berner

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The European standard is a pure product standard which in particular specifies the product “Self-supporting sandwich panels with double-sided metal facings and a thermal-insulating core”, so that standard assessment criteria are present in Europe.
This includes not only the definition of the mechanical values (Chap. 4.3.1), but a number of further important characteristics, such as, for example:

• Dimension tolerances (Chap. 4.3.2)
• Thermal insulation (Chap. 4.3.3)
  
• Ageing and long-term behaviour (Chap. 4.3.4)
  
• Safety in the event of fire (Chap. 4.3.5)
  
• Water and air tightness (Chap. 4.3.6/7)
  
• Soundproofing (Chap. 4.3.8).
  

The requirements are individually defined for the specified characteristics, for example in the form of minimum values, and documented for practice. In this respect, the European standard clearly includes more product characteristics than the German certifications. An essential constituent of the standard is above all also the definition and description of the tests (Chap. 5 and Appendix A), which are necessary to determine the information for the above-specified characteristics.

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Chapter 6 regulates evaluation of the tests with regard to defining the values for the individual requirements and with regard to checking agreement with the required values. It is remarkable that with reference to the verifications for stability and serviceability in Chapter 4.4, exclusively general information is given about the safety level and the influences. Verification procedures are not regulated “normatively” in the standard. Only in Appendix D are the required arithmetic verifications and procedures stated “informatively”. This information corresponds absolutely, in content and form, to the information in the ECCS/CIB recommendations and thus in principle also to the German certifications.

As the data for the arithmetic verifications is only defined “informatively” in the standard, the individual nations do not need to keep to this, although an informative part of a standard will also give important impetus for a uniform examination of the calculation procedure. In addition to the disadvantage that the arithmetic verification procedures are not regulated in the standard, in the author’s opinion the restriction of the planned monitoring to the certification of conformity with regard to the mechanical values is absolutely unsatisfactory.

Although the relevant discussions have not yet been concluded, at the lowest level (System 4) the manufacturer can also perform both the initial check and the in-house production control himself. In the sense of an objective quality control, as is currently performed in Germany on the basis of the building supervision certifications by means of supervision by supervisory offices (Official material research laboratories), at least System 3 should be aimed at here, with an initial inspection by an Official Test Office. However, System 2 which is planned in the European context, is to be applied, i.e. initial inspection by the manufacturer, inspection by the in-house production control and confirmation by an Official supervision office.

Fig. 3: Inspection, supervision and certification offices.

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No use without conformity-symbol
For the German market, it is currently very important for the practical use of sandwich panels that the manufacturing companies must submit to a quality control with internal and external supervision to obtain and maintain general building supervision certifications.
The relevant experimental tests on samples that are taken from routine production are precisely defined in the certifications. If the control is without complaint, the panels are provided with a conformity mark, the so-called conformity symbol [Ü-Zeichen], by an inspection and certification office homologated by the DIBt. The Ü-symbol is thus a vital sign to guarantee that the components are homologated and quality controlled. The components may not be used in Germany without the conformity symbol.

After introduction of the European standard for sandwich panels - analogous to the German conformity symbol - the so-called CE symbol will then be authoritative for practical use.

The discussion about the necessary information which must be recorded on the CE symbol for specification of the panels has not yet been concluded in the CEN committee. From a German point of view, it is essential that all values are recorded which are used for the arithmetic verifications, analogously to the German certifications or to the European recommendations, as these will also be required in Germany in future.
Since January 2000, panels from manufacturing companies which submit to a particularly intensive quality control, can also be marked by a quality symbol, which is granted by separately Official inspection offices on the instructions of the “Gütegemeinschaft Bauelemente aus Stahlblech e.V.” [Registered Quality Association for Steel Sheet Building Panels]. The relevant quality and test definitions were developed by the “RAL Deutsches Institut für Gütesicherung und Kennzeichnung e.V.” [Registered RAL German Institute for Quality Assurance and Marking] (RAL-GZ 617). Thus components with a quality mark can be assumed to have a particularly high level of quality, which is undoubtedly an important indication for the practical use of these panels. These verifications are demanded in Section 3 of the certification.

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

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