Curtain walls

Curtain Walls

Mechanical fixing systems:

The techniques of applying external cladding to the wall structure generally entail using metal anchors which can guarantee:

1. lasting static stability of the cladding;
2. simplified and flexible drymounting to compensate for the wall structure’s irregularity;
3. reasonable prices for the motives specified below.

The factors which largely influence higher costs are:
1. the use of valuable materials for external cladding;
2. significant cost of metal fixing systems;
3. construction method involving chronologically sequenced execution phases (construction of wall structure, preparation of insulating materials, positioning of frames and metal anchors, dry-mounting of cladding) and differentiated teams of site operators;
4. due to the uniqueness of each building, specific solutions are required, above all concerning the architectural definition of upper and lower edges, corners, openings and any other nodal points.
Although the construction costs appear rather high, above all if compared with other more conventional solutions of wall.
structure, there are economizing factors linked to the management of he building’s entire life cycle, able to alleviate the initial burden of expenses.
With regard to the more general economizing factors, the cavity wall system guarantees:
- saving of energy consumption thanks to minimal heat dispersion from the perimeter walls;
- a reduction in thickness of the perimeter walls, which leads to smaller dimensions in load bearing structures.
From an economical point of view, it is evidently important that the mechanical fixing system, which can assume highly sophisticated technological characteristics, does not heavily increase the costs, at the same time ensuring quick and easy execution (with reduced mounting costs) and simple maintenance procedures.
The fixing system should always allow for the substitution of a damaged part without having to disassemble a large number of elements adjacent to it. Going back to the mechanical fixing systems of cladding, having briefly looked at the general costs, we can highlight the most important aspects to bear in mind before choosing the type of anchorage:
1. physical, chemical and mechanical properties of the material, together with the specific geometry of the elements, of the cladding;
2. climatic and meteorological conditions of the building’s location;
3. type and spatial form of structures on which the cladding is abutted.
With regard to the first point, Ennio Grassi, a great expert in mechanical fixing systems for cavity walls, outlines the features of the two research phases (analyses of the material and analyses of the elements’ morphology), useful for a correct definition of the type of fixing mechanism: “The definition of the fixing system involves acquiring an important set of characteristic parameters of the material to be fixed, in this case terracotta, which familiarize one with its physical, chemical and mechanical reactions to certain climatic conditions and to the building’s location and relative conditions of load.

Such research should not be limited to merely determining traditional data regarding compressive and tensile stress on samples of core material in their natural state. It is also necessary to effect special tests on the actual size of the cladding element to be installed, to determine its mechanical resistance after having applied cycles of freezing and thermal shock and tested its resistance to breaking in the points of fastening, which is strongly influenced by the geometry of the metal fixing element’s housing on the terracotta element”.
1 Before illustrating the functional articulation of the mechanical fixing systems and different types of anchors for cladding, we feel it necessary to highlight two general methods adopted in the creation of cavity walls. The first issue concerns the cladding’s mounting flexibility. All the recently most developed mechanical systems, aimed at compensating for any irregularity in the wall structure, have mechanisms of internal/external regulation (+/-3cm) to enable the teams of builders to regularly align the network of joints and obtain smooth cladding surfaces. In fact, it is important, also in the case of side lighting, that the elements forming the building’s outer skin present a surface without depressions or any other irregularity. Another aspect that has become predominant in all the normative laws of countries most attentive to the constructional problems of cavity walls is that of singling out the materials most suitable for mechanical fixing systems.
The field is gradually narrowing down to stainless steel, pushing aside materials like galvanized steel, copper (and alloy derivatives), aluminium and brass, which only until some 5 or 10 years ago were widely used. The series of mechanical systems used to attach the outer cladding to the wall structure can be subdivided into two technical and functional subsystems:
1. fixing devices (anchors) of the cladding elements;
2. structural frames necessary for transferring the general load of the outer skin to the wall structure. The fixing devices in point 1, which indirectly influence the design and distribution of notches made on the inner face of the cladding components, can be subdivided into two fundamental categories. The first type (Local Fixing), undoubtedly the most commonly used both in Italy and abroad for its easy application and reasonable cost, involves isostatic type devices made up of a minimal number of anchors, but 0sufficient in guaranteeing the static stability of cladding elements.
An example of this is the method of fixing through means of four pins lodged into four holes located on two opposite edges of the cladding, whether square or rectangular. These fixing devices require an accurate study which contextually bears in mind the cladding’s specificities (morphology of elements, mechanical characteristics, etc.) and those of the mechanical fixing devices.
Generally, they require thicker cladding, so as to prevent the yielding of even a single fixing point from causing the relative component to disjoin. The Externa elements produced by Sannini, thanks to the notable thickness (5 cm) of the various system components,

prove ideal for the use of mechanical local fixings. On top of reasonable costs, the local fixing systems have two further advantages:
1. the cladding elements are independent from each other as well as from the package of underlying layers, radically reducing any phenomenon of coercion (following thermal variations, stress from load, displacement caused from outside and often unanticipated by simulations and structural calculations);
2. operations of mounting and disassembling the cladding are simplified and speeded up, with the added possibility of substituting faulty or damaged elements in the life cycle of the cavity wall. For more slender elements, as in the case of the Cottostone slabs produced by Sannini-Stone Italiana, these fixing systems allow for metal inserts on the back of the slabs. The second type (Spread Fixing) involves hyperstatic devices. Here, the cladding elements are tied to the load bearing wall structure with anchors in a scattering of points (or through means of continuous section bars along the edges). These types of fixing make it possible to use cladding of reduced thickness. However, it should be stressed that the creation of a greater consolidation and interdependence between cladding and wall structure could cause the latter to strain, with possible overstress in the fixing points, especially in the presence of elastic load bearing structures.

The second subsystem of mechanical fixing systems - or rather, structural frames - generally consists of a continuous mesh of geometrically de-fined section bars, essential for transferring cladding stress (weight) to the wall structure, as well as stress induced by wind pressure (in either direction) transmitted through the anchors of the cladding itself. Normally, the structural mesh is formed by stainless steel section bars arranged to orm a principle frame which develops vertically (usually fixed to the load bearing wall structure or perimeter beams of each floor through regulating mechanisms, necessary for correcting out-of-plumb walls). To the main ertical frame a secondary horizontal one is connected, with elements (plates) linking the fixing devices used for anchoring the cladding elements. The specific morphological and geometric configuration of the structural frames influence the quantity of fixing elements needed to connect cladding to wall structure. Elements consist of stainless steel screw anchors or dowels which consider the specific material and conformation of the wall structure (whether in reinforced concrete, solid brick, perforated brick or perforated concrete blocks, etc.). To avoid accumulation of static energy in the frame, it is advisable to adopt an earthing system.