FAQ

On the one hand, the German National Annex of the Eurocode 2 specifies for sandwich walls the use of exclusively approved and corrosion-resistant materials. On the other hand, there are high requirements for the facing layers resulting from the exposure (XC3 or XC4, i. e. moderate humidity or cyclic wet and dry, as well as XF1 with moderate water saturation without deicing agents).

By using stainless steel or plastic, the concrete covers for these parts are significantly lower compared to normal reinforcing steel.

A comparison of shape and dimensions alone often gives the (false) impression that the products also have comparable technical properties. However, many more criteria are decisive for a comparison:  

• Product range and dimensions
• Material
• Concrete type and strength class
• Anchoring in concrete (embedment depths)
• Minimum and additional reinforcement
• Edge and centre distances

Verifications resp. design resistances for different load cases

Furthermore, it shall be ensured that

• the determination of the actions to the anchors and
• the necessary verifications are made.

A comparison or replacement of sandwich anchor sys-tems is therefore very complex, as there are a lot of points to consider. Our technical team will help you with this difficult question.

Sandwich elements are exposed to a variety of actions, not all of them are relevant resp. are insignificant for the design of the anchor systems.

Most software tools therefore mainly use the “external” actions temperature and wind as well as the dead weight of the facing layer plus maybe any additional load to design the anchorage.

With classic systems, the bearing anchors transfer the shear forces and the pins the axial forces from the facing to the bearing layer.

The problems with the design of sandwich anchors are wide-ranging. In particular, the lack of space for the anchoring elements as well as extreme geometric conditions often lead to a difficult design.

Very large, very small or elements with many openings often force a design beyond the limits specified. Nevertheless, a solution can be found for almost every case with our design software PHacade (see www.philipp-software.de).

Metal parts only represent small thermal bridges within a sandwich element. Nevertheless, the sum of the metallic cross-sections has an influence on the U-value of a sandwich element, but only a very small one (2nd or 3rd decimal place). In this context, the loss via the neighbouring joints shall also be considered for a correct and complete U-value calculation.

In the process of an increasing energy optimisation of concrete sandwich façades, the metallic parts therefore account for an extremely small part compared to the individual layer materials (concrete, thermal insulation). Finally, minimising the number of anchoring elements (resp. the entire metallic cross-section) and joints is an important design approach.

In the approvals for classic stainless steel anchor systems, a two-layer mesh reinforcement with at least 1.88 cm²/m per direction and layer is usually specified for thicker facing layers.

The approval is based on tests with reinforcement close to the surface, i. e. the anchoring reinforcement (additional reinforcement) of the load-bearing anchors is in the closest distance to the mesh reinforcement. This may no longer be the case with thicker facing layers. Here, the reinforcement is assumed to be a crack bridging or crack limitation, which ensures the nominal load-bearing capacity of the anchorage.

Larger crack widths can lead to an early pull-out of anchoring elements. In order to provide a sufficient resistance to the internal and external actions on the facing layer, a two-layer reinforcement is usually required for the design. Visual defects caused by over-wide cracks can be reduced by sufficient reinforcement.

At this time, the following documentation is referenced on this topic: Expert opinion from MPA Braunschweig on the fire protection assessment of concrete sandwich panels as fire walls according to DIN 4102-3:1977-09, April 2000 Final report on the research project:

"Sandwich load-bearing effect of core-insulated prefabricated wall panels under fire exposure" (File number: SWD-10.08.18.7-17.25) dated July 31, 2019, from the University of Kaiserslautern

The risk of unwanted imposed deformations in the direction of the panel plane increases the larger the dimensions of the facing layer and thus the number of pins. To reduce this risk and the related cracking, the dimensions of the facing layer should be limited.

However, possible cracking in the facing layer depends on many other factors. These include the facing and bearing layer thickness as well as height, reinforcement and insulation layer thickness.

Precast manufacturers can rely on a wide range of experience in this field and may be able to produce larger facing layers without any problems. This is merely a recommendation for the production resp. splitting of facing layers from approx. 6-7 m and not a technical regulation.

Classic systems based on stainless steel such as Flat anchors (FA) or SPA are generally designed in such a way that the parts are taken uniformly from one system. This approach has technical reasons on the one hand, but is also practical, as the precast manufacturers usually focus on one preferred system.

A basic differentiation for load-bearing anchors is made between vertical and horizontal anchors (also torsion anchor). It is not advisable to use a mixture of vertical anchors; the use of horizontal anchors from another sys-tem is generally possible and is already partially covered by the PHacade design software (see www.philipp-software.de).

Approved load-bearing anchors or pins require an em-bedment depth of approx. 5-6 cm in the concrete layers for anchoring reasons. A minimum thickness of 7 cm is currently specified in Germany for reinforced concrete facing layers.

The classic sandwich anchor systems are designed and tested to meet this requirement. Furthermore, for constructional (necessary additional reinforcement) and material-related reasons (mixture of stainless steel, reinforcing steel and FRP), the use of classic anchoring systems is not practical for this sand-wich element build-up.

In rare cases, e. g. when renovating a facing layer or when openings are subsequently inserted into a sandwich element, special anchor systems are required to secure the facing layer. These very special anchors are subject to an approval and are offered by various manufacturers.

Almost all classic sandwich anchor systems consist exclusively of metal parts, so that when reinforced concrete sandwich elements are removed, all anchors/pins and the additional reinforcing steel can be sorted and recycled without restriction.

If sandwich panels are to be reused as a complete unit, there is nothing to be said against it from the sandwich anchor system point of view. But installed transport anchors must no longer be used for lifting (exception: transport anchors made of stainless steel, see FAQ Transport anchor systems).