11.01 Introduction

Categories: Stone Cladding

Introduction
In recent years natural stone materials have experienced a marked revival in use, most obviously as external cladding to steel or concrete framed structures.  The Broadgate and Canary Wharf developments in London are notable examples of such construction.  One of the reasons for this renaissance in the use of stone has been the improved cutting technology used by stone processing companies which allows stones to be cut into thin panels, making it both affordable and relatively light-weight.  Another reason is the ease with which stone is available from around the world.

BS 8298: 1994 gives guidance on stone cladding based on experience.  This guidance includes limitations on the size and thickness of the stone units. Where the stone units do not conform to the dimensional constraints of the Standard, testing is required however the Standard does not include guidance on how to carry out such testing.

Against this background there is a growing requirement from construction professionals for an increased understanding of the physical, mechanical and durability properties of any stone proposed for use in building facades.  In addition to construction professionals, reassurance about the fitness for purpose of the stone is frequently being sought by building owners and tenants of large office buildings who will find themselves responsible for the maintenance of the cladding for perhaps a twenty five year period.  Under such circumstances questions concerned with the strength and durability of the stone are no longer an academic consideration but one with serious commercial implications.

The cladding system developed for a specific project depends upon a combination of economic, structural and architectural factors.  This interaction clearly indicates that close co-operation between the client, architect, engineer and manufacturer are necessary at an early stage of the design process.

The construction professionals involved in the design of the facade must take account of the height and size of the building, environmental conditions, appearance and building design life before considering the selection of the stone.  This will affect the choice of stone and also the method of fixing to the primary frame and of forming and sealing the joints

It is important that construction professionals understand and work within the appropriate strength range of the material, in view of the fact that:

• The cost of stone cladding frequently represents a significant proportion of the total building cost,
• Stone is used in a variety of different ways and supported on the building by a variety of different fixing methods,
• There is a significant safety hazard and financial risk associated with failure of the stone.

Support systems for stone cladding panels
Stone cladding panels may be supported on a building facade in a number of ways.  Stone panels are limited in size by the nature of the stone, the loads that are to be carried on the panels, the ability to cut, transport and lift large panels, and the need to accommodate building and cladding panel movement.  Invariably the panels are too small to span the grid of the primary building structure and some form of secondary support structure is required.

There are three principal ways in which stone panels can be supported:

• Attachment to a precast concrete panel,
• Support from an inner wall of masonry or concrete,
• Support from a metal structural framework.

Precast panels
The attachment of natural stone cladding panels as a facing to precast concrete panels is normally achieved by casting the precast panel onto the back of the stone panels.  This method produces finished cladding units that may even have windows and glazing installed before they leave the factory.  This method of construction places the stone panels in close proximity to the precast support panel to which they are fastened at close intervals by inclined interlocking dowels, image.  Adhesion between the stone and concrete is prevented to allow relative movement to take place.

This use of natural stone panels is different from the other methods of attachment where the stone is fastened at only a few points at or near to its perimeter.  Stone panels fastened in this way behave differently under thermal and moisture movement and carry loads more as a veneer than as a structural element. Impact loading may be a consideration but wind loading and selfweight for each panel generally are not.  This method of construction is well understood in the UK where a number of principal suppliers exist and guidance on design and specification has been published, BS 8298, Architectural Cladding Association (1990) and Taylor (1992).
 

Support walls
Stone cladding panels supported directly from inner walls are site fixed using fixings either into the edge of the stone panels or near to the edge of the panel, image.  In the latter case fixings may be into the rear face of the panel or through the panel.  This form of support is comparatively rigid, the inner wall undergoes only small deflections and the stone cladding has to accommodate only limited movement of the support system.  The stone panels themselves, however, may exhibit considerable moisture and temperature movement.  This method of construction can allow for fixings to be attached to the support wall at any point.  This obviates the need to co-ordinate the joints between the stone panels with the support system except that movement joints must occur at the same places.  The support wall may be load bearing brick or block masonry or an infill panel to a primary structural frame.  Note that not all infill panels are structural and brick or block masonry may have to be strengthened with a ‘wind pole’ or similar reinforcement.
 

Metal support frames
Stone panels may also be supported from a metal framework in which case they are again attached by fixings at or near their perimeter.  The metal support frame takes one of two basic forms although both comprise support rails and some method of hooking the panels on or bolting them to the rails.

Support rails may be attached to an inner structural wall at intervals along their length in which case the wall will contribute to their stiffness, image.  Deflections of the support system are a function of both the wall construction and the rail construction.  In this use the rails may be little more than an alternative way of fixing the panels directly to the inner structural wall.  In curtain walling construction a metal framework is used to span between the edges of the floor slabs.  This metal framework transmits all loads from the cladding to the primary structure and is assumed to receive no structural assistance from the cladding, image.  In this form of construction the deflections of the support rails are greater and the panels and their fixings often have to accommodate greater movement of the fixing points

A further form of curtain walling uses lightweight support rails for the stone but these are supported from a substantial structural steel sub-frame that forms a cladding unit spanning one or two storeys in height and up to a full structural bay in width.  This panellised construction enables the manufacture of finished units in the factory, image, and offers the benefit of swift erection on site that is also achieved by using factory finished precast concrete units.

The use of a curtain walling grid often imposes a fixed rectilinear pattern to the panel fixings but it is normally possible to co-ordinate the frame grid with the pattern of joints in the stone cladding.  It is however possible to construct support frames of more complex geometry in two or three dimensions.  The principal constraint to such constructions is the ability to achieve the required tolerances for the frame and the stone panels.
 

Load transfer
Loads on the stone cladding panels can be separated into those that act vertically such as self weight and those that act normal to the face of the panel such as wind loading.

For vertical facades the self weight loads act in the plane of the wall.  It is therefore possible to support each and every stone panel independently by transferring its self weight directly through the fixings of that panel to the support system image.  Alternatively the stone panels may be stacked one on top of the other so that the self weight of several panels is eventually transferred back to the support system by a more substantial fixing typically at each floor level, image.

Common usage describes fixings that transfer wind loading only as restraint fixings, while those that transfer self weight and wind loading are known as support fixings.  In practice all fixings offer restraint in one, two or all three translational degrees of freedom.  This document refers to support fixings, - those that offer restraint and carry load in the plane of the panel - and non-support fixings.  For panels in a horizontal or inclined plane the distinction between support fixings and non-support fixings is more complex.

Stone cladding supported from block or brick masonry infill panels is commonly stacked from a support fixing attached at each floor level.  Wind loading is then transferred from the stone panels to the back wall by non-support fixings connecting to each stone panel.  Allowance for movement is made at the top of each bay of cladding immediately below the support fixing for the bay above.

Stone cladding panels supported from a metal framework or support rails in front of a block or brick masonry wall are often attached as separate panels with the self weight of each panel transferred through its own fixings.  This is done to allow the use of identical fixings across the whole facade and to eliminate the need for large support fixings.  Supporting each panel separately means that both horizontal and vertical joints can be left between the panels to accommodate movement more uniformly across the facade.

Properly identified load paths and simple bracketry and fixings facilitate design and testing and eliminate redundancy that may cause unanticipated loads as a result of movement and lack of fit.
 

Wall construction
The external envelope of the building is required to fulfill a number of functions including provision of the external appearance, weathertightness and thermal insulation.  Stone cladding is usually chosen for its appearance.  It will act in conjunction with the other layers of the wall construction to provide weathertightness but will contribute little to the thermal insulation.
 

Weathertightness
Stone cladding may be used to form a sealed façade or a rainscreen, Section 02.04 .  In a face sealed wall, the joints between the stone panels are sealed forming the primary barrier to water leakage and the cavity behind the stone panels is only required to drain any unintended water leakage.  A secondary seal is provided behind the drained cavity to ensure that air leakage is controlled and that no water passes through the wall.

The use of natural stone in a face sealed curtain wall may place limitations on the allowable movement of the stone panels in order to permit effective sealing of the wall.  Relative movement of adjacent panels will influence the design of the sealed joints, CIRIA, 1998.

Rainscreen facades are a layered form of construction in which the outer layer of the wall is not necessarily sealed but acts as a rainscreen.  This layer contains lapped and drained joints that prevent entry to most of the rainwater and allow any water that passes the rainscreen to drain outward from the wall.  The building is then sealed by an inner wall and air barrier, which control air and moisture movement through the wall. Between the inner wall and the outer rainscreen there is a cavity which allows any water passing the rainscreen to drain and not cross the cavity onto the inner wall.  The performance of rainscreen systems depends upon the way in which joints, air gap and backing wall are created.

In the ‘drained and ventilated’ method, the air gap is left continuous, perhaps running through several storeys, and across several panel widths. With this approach, water penetration is allowed to drain downwards and outwards, and air movement is encouraged to dry out any water that enters the cavity.  To enable this ventilation the rainscreen may have all joints left unsealed or open, or may only have unsealed joints at the top and bottom of the air gap.

In the pressure equalised system, the air gap is compartmentalised and the openings are sufficiently large for air to move behind the stone panels and create a pressure on the back of the panels.  The reduced pressure difference across the joints limits water penetration.  The inner and outer face pressures do not balance exactly and the stone panel will still experience a load from the wind.  However a correctly designed pressure equalised rainscreen can reduce the wind loads on the panels by up to thirty percent, CWCT, 1998.

The presence of water in the cavity of rainscreen cladding may affect the extent of saturation of the stone units and hence their moisture movement.

Natural stone facades constructed as rainscreens are growing in popularity in the UK and their use is well established in Central European countries.
 

Thermal insulation
The requirement to use less energy in buildings has led to improved thermal insulation of facades.  This means that the outer layer of the wall may experience lower temperatures than used to be the case.  Mechanically fixed stone may also be subject to large and more rapid changes in temperature than stone used in more traditional ways.  This arises because the stone is thinner and has less thermal mass itself and because it is thermally isolated from any inner wall by an insulated cavity and sparse connections.  The resulting thermal movements of the stone have to be accommodated by the fixings and appropriate movement joints.