< SUPPORTS, FIXTURES & FITTINGS
MORE ABOUT STRUCTURAL SUPPORT ARMATURES
Most cast sculptures are adequately self supporting and stable, which is to say that they do not fall down of their own accord when in a neutral position. On rare occasions, the design or the location of a cast artwork requires it to be given some additional support to lend strength or stability. Any added support is usually in the form of an internal ARMATURE, which is mostly or all hidden within the hollow cavity of the cast, perhaps exiting through the base or other aperture where it can be secured to a foundation or mounting feature.
A permanent armature fixed within a cast artwork, is normally intended to lend structural support for the duration of the sculpture’s serviceable life. Given that a cast artwork can have a potential ‘life-span’ of many hundreds of years, serious consideration should be given to the longevity of any permanent feature affixed to a sculpture’s cast fabric. The design of a structural armature is normally the product of collaboration between the founder, who has long experience of working with casting materials/processes and the structural engineer, who is primarily responsible for ensuring that any proposed support feature is safe and adequate for it’s intended purpose.
The overall stability of a cast and it’s structural armature is determined by the calculation of it’s COLLAPSE LIMIT STATE, that is to say, the predicted load value at which either the cast, and/or the armature would fail. Once this value has been established, the structural engineer can then determine a second value which takes into account the likely loadings the completed cast will endure once installed plus a generous safety margin, this value is referred to as the SAFE WORKING LOAD (or SWL). The safe load allowable on a structure is usually at least 30-50% below the value of it’s collapse limit state. Provided the maximum predicted load applied to the cast or it’s support armature does not exceed the value of the SWL, the design could be said to be safe, and therefore approved for installation.
In addition to the consideration of any external loads on the static cast, the deadweight load of the artwork itself on a supporting groundwork or wall, must also be calculated and allowed for. In short, any heavy cast must be installed on suitable foundations. It is also essential to make some provision for exposure to incidental loads. These temporary stresses can arise for any number of reasons, for instance by the weight of persons climbing on to the artwork (whether the sculpture is designed for this or not), or through intermittent dynamic forces, such as wind, vibration, suction, or even earthquake.
In most cases, the need for a permanent structural support to be fixed inside a cast artwork will be determined by a supervising architect or project engineer. Structural armatures for cast artworks should always be specified and designed at the earliest possible opportunity – forward planning here can help avoid later difficulties in integrating the armature into the fabric of the cast, and also enable the founder to accurately determine the full cost of works.
In conjunction with the founder, the project engineer should supply a full design specifications for the support structure, together they should also evolve a workable method of securing the armature both to the fabric of the cast, and to the external support feature (plinth, floor, wall etc). Detailed information on materials and any relevant standards or codes that the construction must comply with (including corrosion protection measures), are also usually included in any specification.
Although permanent structural armatures and supports obviously vary considerably in detail according to individual circumstances, the following points broadly encompass the general issues involved.
• The materials used in the cast and it’s support structure should be compatible with each other and serviceable for the expected life of the work. Armatures and fastenings are often constructed from steel alloys which are incompatible in contact with cast copper alloys. Under these circumstances, measures must be taken to ensure that any interaction does not interfere with the structure’s overall integrity and longevity.
• Direct surface to surface contact between dissimilar metals must be avoided. Without the insertion of a insulating ‘buffer’ (such as a non-conductive washer or sleeve), contact between dissimilar metals can initiate a small electrical current across the affected surfaces. This type of reaction between incompatible metals is referred to as ELECTRO-CHEMICAL GALVANIC CORROSION – an effect which is accelerated in the presence of moisture, especially salt water (which functions as an ELECTOLYTE to enhance conductivity). The end result of a electro-chemical galvanic reaction is deterioration of either the cast, or the armature. When a copper alloy is in direct contact with a non-copper based metal, it is usually the non-copper metal that will show evidence of corrosion, (preserving the copper cast). In the case of an aluminium cast, the reverse effect is expected with evidence of corrosion likely in the aluminium and likely preservation of the dissimilar metal [ref]. (See also: EMF & CORROSION)
• When planning the division of a large scale master pattern for casting in sections, the eventual positioning of an internal support armature should be taken into account. If needs be, the anticipated cut lines on the master pattern should be modified to enable a more straightforward assembly of the cast parts around the armature.
• Care should be taken to avoid INTERFERENCES These arise when one internal feature inadvertently occupies a space intended for another. Interference is normally only a potential problem if a number of different services (armature, water pipe, electric conduit for example), are run through a common, or constricted section of a cast.
• If an armature support cannot be easily inserted and then later withdrawn from the interior of the cast, careful consideration should to be given to the application of any corrosion resistant coatings. A schedule of progressive protection applications (in step with the cast’s construction), may need to be scheduled to avoid access difficulties as the work progresses.
• The amount of planning required to successfully integrate an armature into a cast artwork can be easily underestimated. Even then, the best planned projects may encounter unforeseen difficulties and simple oversights (such as not allowing for ‘down time’ as layers of paint protection dry). Events like this can have an adverse impact on tight completion schedules.
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