FIXING OF NATURAL STONE PIECES ON VENTILATED WALLS - THE POINT TO POINT SYSTEMS

- JOSE MARIA GUILLEN

The ventilated walls is one of the new construction techniques which is demonstrating that natural stone is not only a material with a splendid past but also has more than a promising future. The incorporation of this material to the new construction system demands that the design of the holding elements be radically different to the any of those we can find in the traditional construction : the cladding.

It is clear that the cladding in themselves do not constitute a novelty in the world of construction. The novelty lies in the multiple specific functions that are demanded of these elements, mid-way between an impeccable finish of the wall cladding and a necessarily simple and rational mounting process , so as to arrive at the final result without any problems. This is to say, we find ourselves before a system formed by a multitude of small "structural" elements that should work with total static independence one from the other, and that , at the same time, will be the only ones responsible for supporting the rigid visual cladding unit.

Fig 1 For simple embedded fixings such as the fischer F5, the support of the slab, the element subject to flexion and the union with the fixing base are brought together in one piece of stainless steel.

Fig 2 The anchors with mechanical regulation offer an intermediate body that allows the displacing of the hooking point of the slab with respect to the fixing point on the cladding base.

This specialisation implies the use of industrial products that imply a careful design, from the point of view of the function and the setting on site, with maximum economy ; all this under strict quality control, both in execution as well as the material itself.

Any system of cladding of slabs on ventilated walls can be made in a scheme of three principal parts ( three functions ) : hooking of the slab ( union of the slab to the fixing), controllable intermediate element and joining to the base of the cladding ( wall, structure ,etc.). The most common cladding nowadays can be described as 1) hooking by the edge of the slab, by inserting the steel dowel and 2) individual fixing of each fixing point to the wall, be it by embedment of a drill of big diameter ( Fig.1), or mechanically, by means of an intermediate element articulated in three dimensions ( Fig.2 ) We will now give some ideas on the more common fixing elements of this type, characterised by (1) and (2), which are frequently given the generic name of "point to point fixing".

The manufacturer who is thinking of a design of one of these elements will have to consider both the correct functioning of the finished cladding as well as the inevitable incidents during the installation and the repercussion of these on the functioning. Within each of these sections the following factors will have to be taken into account :

A FUNCTIONING

1 The fixing of the slabs at a certain distance of the support ( the base of the cladding ) means that the cladding have to work at flexion between the point of hook of the slab and the rigid fixing over the support ( anchor or embedment in mortar ). This condition demands a high mechanical resistance , within the elastic phase , of the materials used in the entire area subject to flexion, as well as the particular analysis in each different resistant moment inside the said area.

2 The work in harsh weather of these claddings demands a high resistance to corrosion for each one of its components. That is why that all the metallic parts will have to be stainless steel ( AISI 304, AISI 316 ).
The frequent use of zinc steel anchors in these systems, with the excuse that the named component do not work in harsh weather constitutes a serious error , because although it is not directly affected by the atmospheric agents, it is still in permanent contact with the stainless steel of the rest of the fixing, which would lead without any remedy to a strong electrolytic corrosion, owing to the galvanic reaction between the two metals, which can mean an even bigger danger than direct atmospheric corrosion.

Fig 3 The anchor chosen for the union of the fixing on the cladding base will depend on its material as well as the value of the applied loads. The conjunction of both factors shows a considerable number of possible combinations.

Fig 4 Each one of the small fixing elements will behave like a mini - structure, independent of all other elements around it

3 The system of union of the fixing to the support ( fixing base ) that , in general will be the external wall of the building itself, has to be chosen carefully in function of the material with which it is built and the existing loads. This union will be produced by means of a embedment with mortar in simple fixings or by means of an anchor in articulated fixing with mechanical adjustment.
In the first case, the mortar will have to be show a good mechanical resistance , apart from a certain expansion during the setting. The use of ordinary mortars with retraction ends up being dangerous given that the claddings would remain loose once total strength is reached.
The choice of anchor in the mixed systems ( Fig.3) requires some thinking, since the diversity of the bases of the anchor, added to the great possible variety of static situations means that the range of anchors that can be used be more extensive than what would seem to be in the beginning. In general, one could propose the following options :

• a) Steel anchors : any fixing on concrete.
• b) Conventional anchors of polyamide : fixing over cement and solid materials in general, with light or medium loads.
• c) Anchors of resin injection : any fixing on hollow or perforated materials ( one would have to check if the actual wall is adequate for supporting a stone cladding ).
• d) Anchor of polyamide for hollow materials : fixing on these materials ( hollow or perforated bricks ), with light loads.

To calculate the total load on the anchor we will have to consider the result of figure 4, produced by the weight of the stone slab ( vectorial : weight + horizontal reaction owing to the application of the load at distance ), the wind load by suction, considered in the project plus other individual demands ( local seismic coefficient , increase of weight by water absorption , etc.) .

4 The hooking of the stone by the fixing element should not be rigid, so as to allow for the expansion movement of the slab. This can be solved in hooking systems with dowel by the edge of the slab, by means of a polyamide socket that absorbs in an elastic form these perpendicular movements to the mentioned dowel.

5 For a correct choice of the cladding element one would have to determine if the hooking of the slabs is done by the horizontal joint ( hooked through the base of the slab ) or vertical ( lateral hooking ). Before making any other proposal it is important to take into account that the resistant output for the first hooking system is theoretically double than for the second. This is for the sake of safety. As can be appreciated in figure 5, in a hooking by the base with symmetric fixing , each one of these will receive half of the vertical loads linked by a slab, while in the case of lateral hooking the distribution of vertical loads between the two fixings will remain indeterminate, with the possibility that only one of them can support the weight of the adjacent half slabs, that is, of an entire slab, case that will finally be considered, being the worst of the all the possible situations.

Fig 5 Restraint types according to the joint where the fixings are located.

B MOUNTING

1 The fixings for stone slabs in ventilated walls present two union points firmly secured between themselves, once the fixing has been done. One is the hooking with slab ( by the edge ) and other the fixing with support ( anchor in the fixing base ). It is important to take into account that , if the second case is subject to numerous installation incidences, so that it is impossible to place it exactly, the definite location of the hooking point with the slab has to be very precise if one wants a good finish, i.e., that the joints between slabs be drawn perfectly square, and that the surface of the cladding be completely plain.

This double situation, on one hand obliges us to regulate the position relative to both points of union during the installation phase, and on the other hand, to maintain them rigidly united after them. Any cladding system should be able to regulate according to the 3 space directions during the installation and, after reaching a definite position, it should remain immovable ( figure 6).

Fig 6A	Fig 6B
The position of the hooking point with the slab in relation to the anchor on the wall, variable during the installation so as to allow regulation, has to maintained firm after the final installation.

2 In a ventilated system there are fixings subject to vertical and horizontal loads and others which apply only in these cases. In any way, although they have to support a significantly lower load, it is advisable to use the same fixing element for the whole wall, because this way we can avoid the situation of the installer making a mistake. Subject gravitational forces to a fixing calculated only for handling horizontal loads can have serious consequences. Apart from the common systems used these days, practically all the fixings installed in a wall ( a pair for each slab )are fulfilling two roles ( contention and support ) .

In general, the installation of any of these systems has to be very simple to be admitted , because their characteristics may lead to committing errors , since the multiplicity of repetitive operations ( installing of the same kind of slabs ), added to the fact that fixing systems in ventilated walls are completely isostatic ( each fixing point constitutes the only support of "its slab " or of a defined part ), leads to the fact that an error in only one point can lead to serious consequences. Here we do not find " redundant " fixing systems, i.e., when one fixing fails, the adjoining ones respond with their own safety margin. Moreover, the fixings of the stone slabs in ventilated walls are calculated with a highly adjusted safety margin, as in any other steel element that works at flexion, so that one cannot permit the breakdown of any one of them.

This simplicity of installation is obtained without any problems, for example, with the Fischer system F10, thanks to the following characteristics :

• a) Use the same type of fixing both for supporting function as well as contention, as well as for lateral hooks and by the base ( Fig.7 ).

Fig 7A	Fig 7 B
The support system by means of a regulation screw stamped with dowel allows the use of the same fixing element , be it in lateral installation or by the base.

• b) The existence of unequivocal signs of limit of service : clear marking ( with paint, etc.) the limit of regulation of the supporting screw ( if exceeded it would be critical for the behaviour at flexion or for the degree of embedment in the support ).
• c) Only one pressure point, with which at the time final expansion of the anchor is produced, the fixing element remains firm in the position chosen after the regulation.
• d) The jagged washer (Fig.8) is there because, even in the case where there is no pressure on the screw, the chosen position remains unchanged due to fitting of central projection in one of the teeth of the washer. This fact, apart from preventing possible forgetfulness, allows the use of nylon anchors with the system F10, since in the expansion of these anchors no axial tension is produced over the screw, thus a pressure of the support against the wall will never take place, which means any other fixing system that does not have "geometric" system against vertical slipping as is the case with F10 which has jagged teeth, is not advisable. Any fixing system in which the firmness of the vertical quota depends purely on the friction of the screw head ( through " Grower" washers, etc. ) cannot be used with nylon anchors, and its safety with steel anchors is questionable.

3 Another of the indispensable requisites for the good functioning of one of these installations is, as already said, the absence of contact between the adjoining slabs, specially through the horizontal joint. For this reason, during the installation it should be verified that there is free space between slabs of at least 2 mm, even under the flat support in the hooking area. This system can be controlled easily with the F10 system thanks to the nylon adjusting socket that, apart from impeding direct contact of steel with the stone presents a conic elevation ( a little more than 2 mm outside the stone ) that constitutes a guage of immediate easy control for the installer ( Fig.9).

Fig 8 The jagged washer of the fischer F10 fixing system is an insurance against any vertical slipping of the fixing element and, in this way, against the overload of some fixing by support of some slabs over others. Allows even nylon anchors ( without traction tension in the screw, neither possibility of pressure against the washer ).

Fig 9 The adjusting socket of the fischer hooking systems by the edge of the slab ( F5 and F10) provide to the installer an easy way of control of the horizontal joint , since the height of the conic portion that comes out coincides with the minimum separation needed between the fixing and the lower slab to avoid contact between them.

4 As far as the anchors are concerned, each type requires its own installation technique. Therefore, within the factors mentioned before ( the most common), the nylon anchors, be they of hollow or solid material, will need a screw of adequate length and diameter, so as to produce the maximum expansion possible and, along with it, an optimum resistance.

The injection anchors of resin in hollow material, on their part, demand that the installer assures that the filling is done correctly during the injection, of the uniformity of the mixture, and above all, that he waits for sufficient time so that the resin strengthens and thus reaches its final resistance.

The stainless steel anchors, like any other steel anchor for concrete, need a precise expansion strength for optimum behaviour , i.e., sufficient pressure at the drill end to achieve maximum resistance and, at the same time, that the pressure does not communicate to the screw an excess axial tension ( specially important in anchors of small dimensions , like those used precisely in the fixing of ventilated walls ). The tool necessary for achieving this precision in the pressure of any expansion steel anchor is a dynamo-metric key. It has to be said that if we do not use this apparatus we are taking a double risk : if the torque of the pressure is insufficient, the resistance of the union with the concrete will be less than expected, but, if it is excessive ( relatively frequent when one of these tools is not employed ), there is the even more serious danger of surpassing the elastic limit of the steel and of even breaking due to excess of traction.

If any other type of anchor is employed one would have to take into account the particular mechanism of control to be sure of correct installation.

One could add much more information about systems of "point to point systems" for the cladding of ventilated walls. In any way, the aim of this exposition is to provide a relation of important requisites, demanded of any fixing element in the market so as to obtain correct functioning, such as simple, rational and ,above all, safe installation. Following the scheme we proposed in the beginning ( hooking of the slab, union with the walls, regulated intermediate element ) , we can establish other types of fixing systems that are more complicated, for example, with substructures of steel or aluminium as regulated intermediate elements, held in concrete points of the building and multiple undercut anchors on the back part of the slabs, but these will constitute the object of another proposal in the future.