Tamsin Pickeral, business development manager, Szerelmey, explains how a combination of technology and tradition is overcoming the challenges of stone cladding for the 21st Century.
The relationship between a contemporary visual aesthetic and fulfilling structural and legislative requirements is one that is becoming increasingly complicated. Stone, the age old building material and the most traditional, certainly in this country, is still in many cases a preferred material for architects and one often demanded by planners.
Taking this traditional material and making it work in fantastic, contemporary designs can be part challenging and part exhilarating – the latter more common on completion of the building! As stone specialists, it is essential to us that solutions are found to facilitate the design intent of today’s architects and that show how stone remains aesthetically relevant in the current marketplace.
One of the most frequent challenges facing engineers and installers is measuring the visual expectation of architects requiring increasingly large stone panels, against the technical issues of fixing to lightweight building structures. Lightweight construction systems speed up construction significantly and reduce the load on foundations, in turn reducing sub-structure costs.
The development of increasingly sophisticated materials testing programmes has helped to overcome this particular challenge. Software platforms can facilitate finite element analysis applied to fixings and standard structural analysis for lightweight structures. And by inputting parameters such as loads, flexural strength, break out around fixing points, material proof strength and modulus of elasticity, the stress and deflection of building materials can be demonstrated, which in turn dictates the stone size and thickness, as well as that of any supporting bracketry.
Designs incorporating large stone elements projecting from the building envelope with a seemingly weightless appearance are currently ‘on trend’. The often apparently simple aesthetic of these buildings when finished, belie the extensive, or even exhaustive, materials testing and engineering that facilitates them.
Further challenges within this design requirement of achieving a monolithic appearance include the desire by architects for minimal joint sizes and fewer movement joints. Installers will provide a joint size range, normally between 3-5mm for stone cladding. Smaller joint sizes can be achievable, but in this case it is probable that the movement joints will need to be larger to compensate. One option is to disguise joints as much as possible by colour matching the grout or mortar to a very close end. Movement joints, which are an absolute requirement, can also be ‘hidden’ under string courses and within the context of the design.
Another ‘cladding challenge’ has arisen due to increased performance requirements under BREEAM, leading to much larger cavities between the substructure and cladding to account for insulation requirements and prevention of thermal heat loss – this has seen cavities increase to up to 500mm in some instances. This means that the stone cladding is being fixed back on fixings that are having to span a large distance, therefore the fixing systems undergo rigorous testing. One solution for this is the use of a secondary metal framework within the cavity to facilitate the fixing system and reduce the distance the fixing has to span. However, this of course has added cost implications.
Another solution, and one favoured by stone specialists, is a return to the traditional method of construction using load bearing, self-supporting stone facades.
A further implication of BREEAM is the thermal performance of the building envelope, which is commonly quoted as a U-value and is adversely affected by large steel bracketry. These areas, where heat is potentially conducted out of the building through the fixings or other projections, are called ‘cold bridges’. Full thermal analysis of the complete wall build up, from substructure out to determine the areas where cold bridging might occur, can be undertaken. Once identified, this issue is overcome through the use of thermal breaks – non-conductive pads placed behind the fixings. There is currently extensive, ongoing research and development into less-conductive materials suitable for use as fixings, although this is still in its early days.
The development of 3D modelling and BIM represents a significant (and not wholly accepted) technological change in the construction process, particularly for clash detection and to aid the prevention of any construction issues.
An increasingly popular modern aesthetic is the requirement for stone columns and fins – fins in particular provide both exciting visuals and can act as solar shading. Fins, despite their apparent simplicity are extremely complex to design and install. They may cantilever out from the building, supported on one side only. This brings into play eccentric and substantial loading and the potential for movement – both these challenges are overcome through extensive engineering and structural design. Fins are typically built onto a steel supporting framework, clad in stone or as a pre-tensioned unit. The more slender the requirement of the fin, the harder it is to engineer given that space must be allowed for the framework within.
Another popular design seen in several new buildings is that of a stone ‘exoskeleton’, a slender stone frame and columns that ‘embrace’ the glazed building within. These again present technical challenges in engineering, fabrication and installation. One of the biggest challenges with columns is modulus of gyration, which is essentially twisting movement. This is overcome through detailed engineering work using programmes such as Inventor as well as traditional hand calculation methods.
The obvious other issue with the column is the size, width and composition (solid stone, stone on precast, stone on metal) which is based on calculations of load and the compressive strength of the material itself. A popular construction method, due to time efficiency, is to install prefabricated columns that are post-tensioned off site and installed in one piece (fixed top and bottom). This does require substantial lifting equipment.
To conclude, the balance comes in achieving the architect’s vision within an engineered and workable framework. Stone specialists will strive to find a solution to accommodate the visual intent.