11.07 Fixing performance

Categories: Stone Cladding

Introduction
When assessing the integrity of fixings for thin stone cladding it is important to fully understand the behaviour of the stone panels and fixings as constructed on the face of a building.  This may differ greatly from the idealised performance seen in some tests and from the behaviour assumed in some calculations.

The behaviour of the fixings has to be considered as part of a system; neither integrity of the stone panels nor that of the fixings should be considered separately.  The support frame or back wall, fixings, method of attachment to the stone panels and the stone panels themselves should be considered as a whole.  Only then is it possible to separate out the behaviour of individual components.

Loss of integrity of the system may occur as a result of:

• Over stress and fracture of the stone panels;
• Over stressing or excess deformation of the fixings;
• Failure of the support wall or frame, or the connections to it.

Effect of induced stresses
Stone panels, fixings and fixing systems have to provide the necessary movement accommodation but also provide the required restraint and carry the applied loads.  The integrity of the panels and the fixings under load will depend on both the externally applied loads and the loads induced as a result of restrained movement.  The stress fields set up in the stone panels result from both forms of load and fracture will be determined by the combined effects.

It follows that the integrity of the fixing system cannot be predicted unless the induced stresses are known.  It is only possible to test the integrity of the fixings if the induced stresses that occur in the finished construction are also present in any specimens used for strength testing.

When evaluating integrity it is necessary to consider the worst combinations of load.  This process is complicated if induced loads are to be considered during the test.  In order to test all combinations it would be necessary to test for load carrying capacity at both extremes of temperature and with the stone panels both dry and saturated.  The numerous combinations of load and environmental conditions make it impractical to take account of induced stresses in this way.

It is good practice to detail stone fixing systems so that movement can take place freely and no stresses are induced in the stone panels.  This maxim of ‘keeping things simple’ aids the understanding of load transfer and of the stresses in the stone.  Only if this is done and no stresses are induced in the stone panels is it possible to assess the panels and fixings for integrity with confidence. Movements are discussed in Section 04.03.
 

Loads and load paths
The stresses in a stone panel will depend on the magnitude of the applied loads and their positions.  The positions of the loads are clearly affected by the deviations induced during manufacture.  The magnitude of the loads is also affected by the induced dimensional deviations.  The positions and magnitude of the loads may also change as a result of deflections of the stone panel or fixing.  It is important that the load and load paths are known for the purpose of stress calculation.  Any integrity tests performed should correctly model the loads and load paths that occur in the completed wall.
 

Effect of induced deviations
When calculating forces and stresses or testing for integrity under self weight loading, it is desirable to use stone panels of the greatest expected dimension in order to maximise the self weight.  It may however be desirable at the same time to use the thinnest stone panel expected in order to maximise the flexural stresses arising from wind loading.

The strength of many attachments to natural stone depends on the geometry of the stone.  Embedment depths of anchors affect the pull out strength and the strength of kerf and mortice fixings will always depend on the distance of the kerf from the face of the panel.  This is best understood in each case by considering the performance and failure modes of different fixings.

The self weight load of the stone is offset from the support frame or back wall and this leads to bending of the fixings.  The exact off set from the wall will depend on the position of the structural frame and the finished plane of the stone cladding.  The maximum off set that can be expected in practice will lead to the greatest bending moments in the fixing and greatest loads in the connection to the support frame or back wall.  The smallest offset that can be expected will give the least movement accommodation at the fixing.
 

Effect of movement
Load is transferred from the stone panels to the fixings through the point of contact of the fixing on the stone.  Under the influence of self weight or wind load it is possible that a fixing will deflect so that a different point of contact is made and a different load path is set up.  This will change the stresses in the stone panel and the fixing and invalidate any analysis that does not take account of the new load path.  This is best understood by considering the alternative load paths and failure modes that can be set up for each type of fixing. Fixing systems for stone panels should be designed to avoid the setting up of alternative load paths.  This simplifies stress calculations and the maxim ‘keep it simple’ again applies.

If load paths are clearly and uniquely determined by the detailed design of the panels and support systems then it is possible to calculate loads and conduct tests on components of the fixing system, for instance pull out tests on resin anchors or tests of individual kerf fixings.  If load paths are not clearly defined then whole panels of cladding may have to be tested to create the movement that sets up the correct load paths, loads and stresses.
 

Failure modes - Failure of the stone
Kerfs and mortices
Failure of the stone at a kerf or mortice detail will occur as a fracture surface and pulling away of part of the stone as shown in this image.  The strength of the fixing depends on the strength of the stone and the area of the failure surface.  A deeper kerf, thicker shoulder or greater width of contact on to the kerf plate or cramp will all increase the load carrying capacity of these details.  Use of a kerf plate or cramp too close to a corner may mean that the failure surface is reduced in area and that the strength of the fixing is reduced.

Rotation of the edge of the stone panel or rotation of the fixing both have the potential to change the load path through the kerf or mortice.  This image shows a kerf plate or cramp that has rotated and come into contact with the shoulder of the kerf.  Under these circumstances the shoulder of the kerf breaks away at a much reduced load.  It is important when detailing fixings into kerfs and mortices that the correct failure mode is identified to avoid over estimating the strength of the fixing.  Similarly it is important to achieve the correct failure mode under test.

The load carrying capacity of a kerf or mortice may vary as a result of thermal or moisture caused dishing of the panel.  This image shows two details for a kerf plate or cramp.  In the first case the load paths are clearly identified and are independent of small rotations of the panel or fixing as indicated.  The second detail has no clearly defined contact points to transfer either in-plane or out-of-plane loads.  The load paths are totally dependent on the relative rotations of the panel and the cramp.  If fixing strength depends on deflection and rotation of components then the worst assumptions have to be made in any calculations.  When testing it is essential to achieve the correct failure mode.
 

Dowels and wire ties
Failure of dowels and wire ties is very similar to the failure of kerfs and mortices.  This image shows a failure of a dowel in the edge of a stone panel.  The load capacity of this detail is dependent on the strength of the stone, embedment depth of the dowel and the distance from the dowel to the face of the stone panel.  As with cramps and mortices, rotation of the panel edge or the fixing may lead to a different form of failure that is more localised, image, and to a considerably reduced load carrying capacity.

With softer stones, holes are often drilled on site to accommodate dowels.  These holes are positioned to give the correct face alignment of the stone and may be positioned closer to one face than specified.  The strength of the fixing is then reduced below the design capacity.  Factory drilled holes are usually positioned at a constant distance from the outer face of the stone panels so that the outer faces are correctly aligned when installed.  The distance from the dowel hole to the inner face of the panel is then dependent on the overall thickness of the panel.  The design load capacity of the fixing is governed by the minimum anticipated distance from the dowel hole to either face of the stone panel.

Site drilled dowel holes may not be exactly aligned with the plane of the stone or of the correct diameter.  Attempts to force fit dowels into holes will induce stresses in the stone around the dowel.  These will lead to a reduction in the load carrying capacity of the fixing.
 

Bolts and anchors into stone
When bolts and anchors into stone fail as a result of fracture of the stone, failure takes the form of a cone of stone pulled (or pushed) from the face of the panel. This image shows a section through a failure cone pulled by a resin stud anchor.  The load capacity of the fixing is determined by the strength of the stone and the size of the cone that is pulled from the stone.  Greater embedment depth of the anchor will give a greater load capacity but use of the anchors too close to the edge of the panel will lead to a reduction in load capacity, image.

The load capacity of through bolts and undercut anchors will depend on the diameter of the washer or the diameter of the bottom of the undercut anchor.  Omission or substitution of washers for through bolts may reduce the load capacity of these anchors.

Fixing bolts and anchors are normally designed to resist pull-out loads however wind loading on panels is reversible and loads on bolts and anchors may be in either axial direction.  Either panels and anchors should be designed to resist compressive loads, image, or an alternative load path should be set up as shown in this image.  In this case the bracket transfers compressive loads direct to the inner face of the panel and the anchor carries only tensile loads.  Through bolts will have no strength in compression and should only be used with brackets as shown in this image.

Rotation of the bolts as a result of bending of the fixings or flexure of the panel can lead to a reduced load capacity of the fixing.  For thin stone panels and structurally assisted stone panels consideration should be given to testing complete panels and not just single fixings.
 

Failure modes - Failure of the fixing
Angles and shelves
Failure of an angle or shelf support may be manifest as bending of the support as shown in this image.  This may lead to gross deflections and unacceptable movement of the stone panel on the facade.  However failure may occur at lower deflection of the bracket.  For instance the bracket may deflect so that the inner edge of the stone panel bears on the bracket and local spalling occurs.  Another failure mode occurs when the support deflects and contacts the top of the panel below.  This transfers load to supports further down the wall, which may in turn fail.

Angles may also deflect as shown in this image  where the spacing between fixings to the primary structure is too great.  The combined effects of flange bending, torsion and overall bending between supports should not lead to unacceptably large deflections that change the load paths in the system and stress distribution in the stone.
 

Brackets and ties
Brackets, whether used as edge supports or attached to the inner face of the stone panels, may deflect under load and cause failure of the system.  This may occur because the bracket induces stresses in the stone or increases the load in the anchors.

Brackets attached to the inner face of a stone panel will attempt to move with the panel as it rotates, image.  If the bracket is free to rotate relative to the wall neither the bracket nor its attachment to the panel will experience any additional loads.  Otherwise flexure of the bracket will occur or, if there is flexible packing between the bracket and the stone, the bracket will rotate relative to the stone panel.  In the former case failure may occur as a result of fatigue of the bracket. In the latter case flexure of the anchor may take place and premature failure occur as a result of fatigue.  The correct failure modes have to be identified before testing can be undertaken.

Brackets and ties may deflect under out-of-plane loading as shown in this image.  This is a common failure mode with ties, particularly thin metal cramps.  These are often supplied with the intention that they are bent on site to set the stone panels the correct distance from the support structure.  They are made flexible to allow site adjustment but depending on the position of the anchor to the back wall will allow different amounts of movement and excessive movement may occur.  Deflections of this form may be classed as failure or they may be the cause of some other failure mode, for instance they may allow a greater offset of the dead load from the support wall that leads to overload of the supports.
 

Anchors
Anchors and bolts frequently fail by pulling out of the stone panel or other substrate into which they are fixed.  However, movement of the stone panels, both rotation and in-plane movement can lead to flexure of bolts and studs.  This will occur where a nut is used to stand a panel away from a bracket but also occurs inadvertently where shims are used to position a panel in the correct position.  The failure mode of an anchor is dependent on the extent of any shimming and the method of attachment of the anchor to a bracket or other fixing point.  In general anchors should be used to hold components in close contact so that no flexure occurs or they should be part of a system that is articulated or flexible so that no flexure of the anchor takes place.