Façade systems form an important element of a buildings fabric. They can be used to provide a variety of characteristics, structural support, weather resistance, acoustic and thermal protection. Just as important, they can also provide the exterior finish to a building. In this article Holger Glinde of Institute Feuverzinken, highlights new work in Germany on the use of hot dip galvanized steel for façade systems.
Field of application
In design terms, curtain-type, rear-ventilated façades represent a very versatile option for façade design. The typical structure consists of an insulating layer, a subframe, rear ventilation and the exterior finishing panel. The requirements and inspection principles for rear-ventilated external wall facings are defined by a variety of regulations across Europe. Some regions have specific standards while others tend to cover the specifics within general standards. This applies to load-bearing systems and façade facings as well as connecting and fastening elements. Virtually all of the steel sections used can be hot dip galvanized in accordance with EN ISO 1461 including: supporting sections of the subframe (1), the facing elements (2), the wall brackets (6), the connecting elements with fixed or movable points of attachment (8,12) and the fastening elements (11).
Façade facings for rear-ventilated façades can be made from numerous materials. These include, for example, hot dip galvanized steel, aluminium and aluminium alloys, high grade stainless steel, high-pressure laminates (HPL’s), fibre-reinforced construction materials, ceramic or brick elements, plastic facings and single-pane safety glass. Subframes made from hot dip galvanized steel can be used alongside many of the current façade facing elements.
Hot dip galvanized steel in comparison with other façade materials
The thermal conductivity and the modulus of elasticity (E-modulus) are among the most important technical characteristics of materials for façade subframes.
The thermal conductivity is the speed at which heat is transmitted through a material. To avoid temperature build-up, materials with the lowest possible thermal conductivity are recommended for façade subframes. While the thermal conductivity levels of steels lie between 20 and 60 W/mK, those for aluminium alloys vary from 160 up to 220 W/mK.
The modulus of elasticity is a material‘s characteristic value which describes the relationship between stress and extension when a solid body deforms in terms of its linear elastic behaviour i.e. its stiffness. The higher the modulus of elasticity, the greater the resistance of a material to deformation. The E-modulus for steel is 210000 N/mm2 for aluminium, it is 70000 N/mm2.
Steel thus has an E-modulus which is three times higher than that of aluminium, i. e., steel can be used to create the same stiffness as aluminium with considerable savings in terms of material. In the context of energy saving policies and the associated increase in insulation layer thicknesses, the cantilever arms of façade subframes are increasingly being designed to be as long as practically possible. The E-modulus is thus becoming more important.
As regards economic factors, hot dip galvanized steel has an advantage over both aluminium and high grade stainless steel. Research has shown that if aluminium is used the initial costs are approximately 20% higher by comparison with hot dip galvanized steel. The standard costs for high grade stainless steel can be as much as even three times higher than that for hot dip galvanized steel.
The stiffness characteristics of steel can enable a longer and hence more economic solution for galvanized façade subframes for most existing façade facing elements.
Durability of hot dip galvanized façade elements
Hot dip galvanizing is an extremely long-lasting corrosion prevention system. There is no need for routine maintenance or refurbishing. This is important above all, in façade subframes which cannot be inspected (or not without great expense) once the façade is in place. Under normal conditions of use, hot dip galvanizing provides protection for 50 years or more. This is shown (among other evidence) by the zinc corrosion cards which are issued by the Federal Environmental Agency in Germany, the tables headed ’Steel that is hot dip galvanized to EN ISO 1461 was included as a corrosion protection option in the German standards for façades once it had been proved to have the required durability of an estimated structural service life of 50 years.
Execution of hot dip galvanized façade subframes
Hot dip galvanized steel can be used as a façade subframe in many situations including extremely complicated façade geometries. With respect to the execution of hot dip galvanized façade construction components and elements, the regulations governing construction suitable for hot dip galvanizing must be taken into account. As with any other product it is always recommended that in order to achieve optimal results, design advice should be sought.
1. Standard construction with L-section for visible facing fastening
The subframe consisists of an L-shaped wall bracket and a continuous L-shaped mounting rail. A thermal separation must be provided between the wall bracket and the wall. The wall brackets are of standard form and require fixed and movable points of attachment in order to absorb the construction loads and to be able to counter wind suction and wind pressure.
The external façade panel is fastened to the mounting rails by a visible fixing. The dimensions of the wall bracket projection depends on the heat insulation required and the rear ventilation area. The system can increase existing tolerances through the differentiation of the wall bracket and the support profile. If the subframe is executed correctly, the façade elements can be mounted quickly and simply.
2. Construction with U-section for covered facing fastening
The subframe consists of a U-shaped wall bracket and a continuous U-shaped mounting rail. The system makes it possible to provide a hidden fastening for the façade panel via holding slots. It is worth pointing out that plastic clips must be fitted into the clasps in order to avoid the creation of undesirable noise, e. g., the rattling of attachment pins. The system can increase existing tolerances through the differentiation of wall brackets and support profiles. If the subframe is executed correctly, the façade elements can be mounted quickly and easily.
Literature sources: New options and examples of use for subframes made from hot dip galvanized steel; Daniela Ridder and Michael Heinemann, FH Dortmund, 2014
Image: seppeler.de, Graphics: Ridder/ Heinemann