05 May 2010

European approval for undercut anchors in ashlar

More efficient facades with fischer FZP

Today, undercut anchors for fixing ashlar facade panels are state of the technology. The most recent generation of ETA approvals now classifies the ashlar stones into four groups of stonework. Virtually all relevant natural stones used as facing material for facades are now covered in a single approval document. The fischer FZP allows the highly efficient use with all stone grades and types listed in the approval.

The latest updated European Technical Approval ETA-05/0266 for the first time accounts for the strength values of the facade panels with their characteristic properties, also specifying the impact of ageing on the strength values of the natural stone with an exposure factor.

Ashlar panels are often used as facing material for high-grade ventilated facades, mixed and curtain facades. The requirements, specifications and dimensions of ashlar facade panels are defined in DIN 18516-3. Since 1995, there are also general construction approvals for undercut anchors used for fixing facade panels. Compared with traditional systems, the advantage of approvals for undercut anchors is that they have not been constrained by a clear-cut standard or norm, allowing the capacity and efficiency of the undercut technology to be used to the full. This means that static structural assumptions and computations can be made a great deal easier and quicker.

Clearly defined applications

The requirements involving a European Technical Approval (ETA) rest on the assumption that the useful life of the anchor is 50 years. The anchor has been approved for outside areas, its use under extremely aggressive ambient conditions, such as indoor swimming pools with chlorinated atmosphere or road tunnels in which de-icing agents are used, is ruled out. The ashlar stones must comply with EN 1469:2004 and be assignable to the following groups of stone:

• Group I: high-grade intrusive rock, e.g. granite, granitite and gabbro
• Group II: metamorphites with hard stone properties such as quartzite, granulite and gneiss
• Group III: high-grade igneous rock, e.g. basalt and basalt lava
• Group IV: sedimentary rock with “hard stone character” such limestone and sandstone.

The suitability of the stones in Group III is initially defined through their raw density. The new approvals for the first time explicitly include basalt and basalt lava. Basalt, for instance, must have a minimum raw density of 2.7 kg/dm³, basalt lava a minimum density of 2.2 kg/dm³. The group including sandstones has been defined with a minimum raw density of 2.1 kg/dm³. If anisotropic strata occur in the material, the bending strength parallel to the plane of the layer and perpendicular to the edges of the layer is not allowed to differ by more than 50%.

The facade panels must be fixed to a suitable subframe construction using four anchors set in a square configuration and without technical restraint. In addition, jambs and reveals can be fixed with flush-mounting anchors and interior brackets. The fischer Zykon panel anchor for the rear-mounted installation of facade panels made of selected natural stonework under EN 1469 has shown superior performance ever since its market launch as stand-off and flush installation anchor.

The maximum panel sizes are 3.0 m² with a maximum side length of 3.0 metres. The minimum rated panel thickness is 20 mm, for stone group IV and basalt lava, the minimum thickness is 30 mm. The fischer FZP may be used with metric connecting threads M6 and M8 and with an internal thread M6 and a rated anchoring depth range of 12 mm to 25 mm. The minimum edge spacings are 50 mm for façade panels and 40 mm for jamb panels. The minimum axial spacing between two anchors must be greater than eight times the anchoring depth to prevent the breakout cones from influencing each other. For anchors used in stand-off installations, the minimum residual wall thickness of the façade panel must be 40% of the rated panel thickness.

During the approval tests, the suitability in use of the anchoring has been demonstrated successfully in a number of different test series: centric tension, shear stress, oblique tension, building component tests, functional effectiveness under dynamic load, under permanent load, under freeze/thaw conditions, and functional effectiveness after water absorption.

Summary

The ETA for undercut anchors in natural stonework, amended earlier this year, now takes account of all required practical parameters for fixing natural stonework. For the first time, the strength values with the characteristic properties of each panel and the impact of their ageing are taken into consideration. As basalt and basalt lava are now included, all ashlar stones are now be imaged in a single approval, making design a great deal easier. Also, innovative products such as the fischer FZP undercut anchor can now be used with greater economic efficiency. In practice, both features result in quicker designs and thinner panel thicknesses and therefore in more cost-effective facades.

press pictures