If the supporting contruction is made of concrete, single dowel stays which can be adjusted, are used. This is most economical for plates with a weight of 1,2 kN/m2 or of 2,4 kN/m2, if the stays are fixed in the horizontal crevice.
Gesa Todt
Alternative systems are increasingly used for fixing natural stone facades apart from the traditional method with drift bearings. Several factors must be observed. The size, depth, weight and solidity of the applied natural stone as well as the ground, load per dowel, fixing method and external conditions such as wind force or even earthquake danger must be considered. In Germany the standards for safe fixing are regarded as being very high compared to other countries. However, the current new regulation for admission processes for the EU market prove that in other European countries application regulations also become more and more important.
If the supporting construction consists of brickwork, so-called mortar stays are inserted with a special mortar in pre-bored holes. They can carry plates with a weight of up to appr. 4 kN/m2 with cantilevers of up to appr. 300 mm.
If the supporting contruction is made of concrete, single dowel stays which can be
adjusted, are used. This is most economical for plates with a weight of 1,2 kN/m2 or of
2,4 kN/m2, if the stays are fixed in the horizontal crevice.
If facades are to be
fixed with a large distance to the supporting construction or the ground is inadequate, a
separate supporting construction made of suspension rails is usually fixed to the front
sides of the ceilings made of reinforced concrete. Adjustable carrying and fixing stays
are screwn to the supporting construction so that the assembly can be carried out quickly.
There are two basic conceptions for mounting plates on this kind of rails: slotted and
rear cut armatures.
When slotted armatures are applied, natural stone plates are usually at their upper and lower narrow edge. Rectangular profiles gear with this slot, carry or hold the natural stone element and deduct the working weight and wind load. Here the plate is not directly positioned on the horizontal bar of the holding part but on an intermediate layer of polyamide so that different heights can be equalized. This layer also guarantees a good fit without clearances and thus a more equal distribution of the load and the reduction of stress for the plate. If the holding profile must invisible in the crevice, the narrow edge of the plate should be formed as a rebate step. Then the holding part is directly screwn to the vertical aluminium supporting contruction. This construction must be adjustable in the horizontal direction so that tolerances of the bare brickwork can be equalized and the natural stone façade can be fixed evenly. The equalization can be carried out e.g. by two meshing profiles with gearing, which are fixed by means of a screwed connection.
The slot on the narrow edges of the natural stone plate preferably should be milled half-rounded to minimize the breaking risk. Natural sone plates with an edge length of more than 1,000 mm are usually only milled locally in certain areas. However, the edges of shorter plates are often milled throughout, so that the milling tool has to be applied only once and the plates can be adjusted to the side.
Rear cut armatures at an continuous supporting construction are more and more widespread. They have a much higher load carrying capacity compared to drift bearings. Cylindric blind holes, which are equipped with a rear cut by means of the special bore head, are bored on the rear side of the facade plates for rear cut dowelling. Then the armatures are inserted in the boreholes and the fixing screw is driven through the plate fixture and the base. The inserted rear cur armature fits the rear cut thanks to its spring ring so that it is fixed. Then the façade plate is mounted to the supporting construction.
If the rear side of the natural stone lies closely to the supporting construction, as the bushing of the armature aligns with the rear side of the plate, we call it aligned mounting. Thickness tolerances cannot be compensated so that natural stone plates must be calibrated completely. This treatment is expensive, so that aligned mounting is not applied very often.
In the case of distant mounting there is a gap between the plate and the supporting construction so that tolerances of the plate thickness can be equalized. The rear cut armature is inserted with a constant distance between the outer side of the plate and the bottom of the borehole in the natural stone. Tolerances of the plate thicknes are equalized by the variable gap between the rear side of the plate and the spacer nut.
The armatures can be additionally grouted in especially to increase the maximum load on the rear cut fittings, which are mounted with a distance, in more porous and softer stone. The gap between the rear side of the natural stone and the angle bracket is filled in with a special thin layer mortar, thus increasing the contact surface and improving the load distribution.
In order to adjust the individual plates as good as possible and to save time during the mounting on site, by now natural stone facades are frequently pre-manufactured at works, e.g. with the help of a cassette system. Here several single natural stone plates are mounted and adjusted with rear cut armatures on light metal sheet cassettes. Then these elements only have to be hung into the bolts of the continuous aluminium supporting construction. The fixing method is especially efficient with small plate formats.
As the connection with rear cut armatures is form-closed, they cannot be removed and as they are installed without spring pressure, splitting of the plates is avoided. What is left is breaking from the fixing underground, i.e. the facade plate. Usually it breaks in the form of a cone and the breaking load depends on the cone surface. Therefore the armatures are installed as deep as possible, so that the overlapping should be as small as possible when this plates are used.
Whereas in foreign countries normally the high breaking loads can be utilised, in Germany the application of rear cut armatures is limited by the general regulations of the building authorities. It depends on the application whether dowels which have especially been admitted by the building authorities must be used, e.g. for mounting lowered ceiling constructions or intermediate ceilings only metal dowels are admissible. Compression stress and tensile loads, which may cause cracks (frequently at first at the fixing points) work on ceilings. As plastic dowels cannot spreada afterwards in the borehole, they might fail. Also in the case of fire plastic dowels might lose their holding power due to the heat.
If special requirements regarding the physics or the architecture of a building have to be met, fixing systems for natural stone are sometimes tailored individually for certain projects, such as the Jowa Japan Centre in Frankfurt am Main (Germany), which is 115 m tall (architects Ganz + Rolfes, Berlin). Here between the 6th and the 23rd floor natural stone plates with a thickness of 40 mm made of African Red Granite were screwed in the supporting construction made of aluminium which had been mounted before. For this purpose the granite plates were equipped in the corners with through boreholes with neoprene bushes. On the outside decorative bolts in the form of screw cylinders made of aluminium with a diameter of 35 mm and an anodized surface were used.
Nowadays plates which are even thicker are not often used, on one hand because they are expensive and on the other hand because it is easier to meet the high German heat shield requirements with coverings which are rear-ventilated and have heat insulation in between. If thicknesses of more than 6 cm are desired, they are usually grouted in in the form of front brickworks mainly with the help of conventional armatures drifts or S hooks in order to guarantee a deduction of the force into the wall.
In Germany there are various providers for fixing systems. Some of them offer even their own further developments with special constructions.
The biggest problem with slot fixing is that it is difficult to exchange single broken plates. As the supporting profiles are screwed with the supporting contruction and the natural stone plates ''build up" on each other, frequently many plates must be de-mounted in order to exchange a broken one.
Therefore the German company Götz GmbH developed their one mounting system for natural stone. Here the web of the supporting profiles which are fixed at the upper part does not mesh a horizontal slot, but a vertical notch at the side faces. Additionally the supporting profile is not firmly screwed with the supporting construction but only inserted in the guide bars of the profile of the supporting construction. Then it is fixed by a screw. Now the upper supporting profile can be removed and the natural stone plate can be quickly taken out. A fixture with an angle of 120° meshes in the corresponding slot and carries the natural stone plate so that even window connections made of natural stone can be mounted invisibly.
The German company Fischerwerke Arthur Fischer GmbH & Co.KG developed already several years ago so-calle Zykon plate armatures to fix hard rock plates with a minimum thicknes of 20 mm. With this kind of armatures even intrados had not to be glued any more and expensive additional constructions became superfluous.
An example for this application is the ABN Amro headquarters (24 floors, 105 m) by the architect Henry N. Cobb in Amsterdam. The facade plates "Virginia Black" from the U.S.A. are 30 mm thick, 900 mm high and 270 mm wide. The intrados plate and the mother plate are connected by 40,000 rear cut dowels via metal angles. The average breaking load of the applied armatures in the site M8 is 9.790 N.
The German company KEIL Werkzeugfabrik is another important provider of rear cut armatures. The Keil façade armature is admissible for fixing suitable hard rocks with a thickness from at least 2 cm and plate sizes up to 1 m x 1.50 m. As little covering is required (5 mm with granite) it can be mounted very deeply. Boring a 7 mm borehole in granite with the pertaining façade boring machine lasts about 10 seconds. As the intrados plates with inner angles are fixed only mechanically, they can be mounted independent from the weather. Mother and intrados plates can be bored at the natural stone workes and be screwn together on site.
A great advantage of an invisible fixing method is the intact surface, thus avoiding the usual dirty marks which are common with visible systems (e.g. clips).
A special soft rock façade armature with a bigger diameter and even bigger fixing depths. Higher breaking loads improve the safety and reduce the required plate thicknesses.
In many countries it is essential to make buildings resistant to eathquakes. The advantages of a matured fixing system were shown e.g. during the earthquake in Taiwan in September 1999. The natural stone façade in Taichung, fixed with the Keil rear cur façade armatures was in a perfect state even after this natural desaster.
In Peking Fischer Zykon plate armatures wer tested in 30 earthquake simulations with a heaviness of 8.9 on the Richter scale and an epicentre right under the building. Granite plates with a size of 1.5 square meters and a thickness of only 20 mm as well as with a size of 2 square meters and a thickness of 30 mm were observed. Although they used only four FZP 13x24, the fixtures of the plates withsodd this load.
Since the end of 1999 in Germany the revised standards DIN 18516 "Exterior Wall Coverage with Rear Ventilation" is valid, which also deals with important requirements to natural stone facades. This includes the topics heat shunts, wind loads, fixing and material of thermal insulation, tests of natural stone plates by bending tests, breaking load at the drift hole of the armature, resistance against atmospheric conditions, determination of cramming forces in the case of differences in temperature, the wideness of the gap between the armature web and the plate edge, the length of the plug-in drift and the calculation of the armature webs. Requirements to building products, which also include fixing elements, are regulated in Germany by the Building Product Regulations and the Examplary Building Regulations (MBO). Here dowel fixing is not covered by standards (EN or DIN) and must prove its applicability. In the MBO it is mentioned that buildings must be arranged in such a way that public safety and order, especially life, health or the natural basics of life must not be jeopardized.
Since 1998 approvals are granted according to European standards. The so-called ETA European Technical Approval is a technical specification according to the building product regulations of the EU. That means this approval os valid all over Europe and the product may be equipped with the CE identification. In future this will be the condition for sales on the EU market. The ETA is mainly good for the users: the tests utilize the actual capacity of fixing elements by about 20 % better than before. That means, for comparable applications dowels with a smaller diameter may be used. Then the dowels are not only cheaper but also smaller boreholes save considerable time and money.
The possibility to utilize the existing capacity of dowels even better increases the requirements to the user, however, who has to determine exact information about the location and the prevailing conditions. The software of dowel manufacturers as well as the Fischer COMPUFIX program or the Upat Ccplan program support the planners and users when they select the suitable fixing method and its calculation.
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Last Updated: July 2008
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