PRACTICAL ASPECTS OF DESIGN & MATERIAL SPECIFICATIONS
Dr. Ken Price, BSc (Hons), DIC, PhD, FIStructE, FICE, PrEng, CEng
Director, Tension Structures, Cape Town
Concept design & criteria for shape finding
The concept design is the most important stage of the design process – a bad concept will reverberate throughout the design, manufacture and installation process, and will impair the appearance and performance of the final product. A number of factors need to be taken into consideration, including, but not necessarily limited to, the following:
a) Geometric constraints of the site and adjacent buildings
b) Sun shading levels required & sun angles
c) Air flow & ventilation of the space
d) Light transmission requirements for the space below
e) Availability & positions of anchorage points
f) Need for continuously sealed perimeter anchorages
g) Aesthetic considerations & compatibility with adjacent elements
h) Achievement of adequate curvature to minimise fabric stresses & movements
i) Drainage of rainwater & avoidance of ponding
j) Suitable fabric slopes to ensure adequate self-cleansing
k) Nature of supporting structure & tensile elements
Methods used for concept design
a) Wire frame computer models – these simple computer models are the starting point for investigating various fabric forms and developing the concept design into a practical form to ensure that the design criteria are satisfied.
b) Fully rendered computer models – models of the final design of the fabric structure can be added to the model of the whole building, and are thus very useful for final presentations to clients.
Anchors, foundations & supporting structures
The provision of adequate anchorages and supporting structures must be addressed at the earliest stages of the design process. Failure to address these issues early on could result in the tensile structures being impossible to include once the design and construction has progressed beyond a certain stage. Even small canopy structures impose significant forces on the sub-structures, and large structures will require special design features to be able to resist the tensile forces. Tensile fabric structures impose forces which have significant horizontal and vertically upward components, and these are not the types of forces which buildings are normally designed for. If special provision has not been made for these forces, it is crucial that the tensile structure be designed taking this into account.
Fabric options – PVC and PTFE
There are basically two generic types of fabric which are commonly used for permanent outdoor structures, i.e. PVC-coated polyester and PTFE-coated fibreglass. Both of these fabrics are available in various types of mesh format. For short term and interior structures, many more options are available, including various types of cotton and other synthetic materials, including stretch fabrics such as Lycra or Spandex.
Factors influencing fabric performance
Observation of fabric structures in different situations in various parts of the world show that the particular environment in which the fabric is situated has a very significant effect on its performance.
The main factors which appear to influence the fabric performance are the following:
a) Geographic latitude and thus the temperature of the fabric
b) UV radiation level reaching the fabric
c) Humidity level
d) Pollution level, as well as the type of pollutants
e) Dust level
f) Frequency and nature of cleaning operations
g) Deposition of vegetable matter (leaves, etc.) onto the fabric
h) Staining resulting from rainwater run-off over other building materials
i) Exposure to direct rainfall to assist in dirt and dust removal
The effect of items a) to e) cannot be influenced by the designers, contractors or owners, but items f) to i) should be assessed by the design team.
PVC/Polyester fabric – properties & characteristics
PVC/Polyester fabric consists of a woven polyester base cloth, which is then coated with PVC and another top coating. They can be classified into 2 basic categories, depending on the type of protective top coating, as follows:
a) Acrylic lacquer
b) PVDF/acrylic lacquer alloy coatings in varying proportions
These top coatings have a large influence on the performance and appearance of the fabric, because they not only provide the fabric with some of its UV resistance, but they also vastly improve its self-cleaning characteristics.
In general, fabrics with acrylic coatings have not performed that well long term in tropical countries, and most examples appear to attract and retain significant amounts of dirt and dust after relatively short times in service. On sites with high UV levels, the acrylic coatings break down fairly quickly, and thus the deterioration in appearance can occur within a few years after installation. If the humidity levels are high, the deterioration in appearance can be even worse, as black mould growth can occur within the fibres soon after the coating breaks down.
The fabrics with PVDF/acrylic alloy coatings are the most commonly used, and have now been in service about 25 years. The PVDF/acrylic coating is heat-fused onto the base fabric as part of the manufacturing process. The top surface of the fabric has a smooth slippery feel to it, and it is thus very effective in repelling dirt and resisting mould growth. Useful lives of 15-25 years are achieved with these fabrics, with the anticipated life being directly proportional to the amount of PVDF in the top coating.
PTFE / Fibreglass fabric – properties & characteristics
The first outdoor PTFE/fibreglass structure was erected in California, USA 40 years ago and is still performing well in service. It is therefore likely that structures which are made with today’s PTFE/fibreglass fabrics will achieve useful lives in excess of 50 years. PTFE/fibreglass fabric is very effective in repelling dirt and some structures which have been inspected after years in service have perfectly clean surfaces even though they have never had cleaning maintenance.
Three of the new stadiums built for the Soccer World Cup in South Africa in 2010 had PTFE/fibreglass roofs, but the main disadvantage of PTFE fabric is its high cost – the overall cost of structures using this fabric are almost twice that of PVC structures.
Light & heat transmission
One of the main advantages of fabric is its translucent properties – on average, architectural fabrics transmit about 13% of the light falling on the top surface. This results in a very pleasant light and airy feel to the space below, and can also result in significant cost savings on lighting.
Fabric is also very effective in reducing the transmission of radiant heat from the sun, and it is a material which has been significantly underutilised in the climatic conditions prevailing in South Africa. However, global warming is likely to result in increased usage of tensile fabric structures in future.
Lighting of fabric
Lighting is very effective in emphasising the aesthetic appearance of fabric structures and should always be included whenever possible. Both back-lighting and front-lighting can be used depending on the effect that is desired.
Fabric & fire
PVC has fire retardant properties, and achieves a class 2 fire rating – meaning that the fabric is self-extinguishing and does not produce drips of molten fabric. This fire rating is generally accepted by most approval authorities for use as a roof enclosure.
PVC has an added advantage in a fire situation in that the fabric seams will separate at about 100ºC, thus allowing the very early venting of toxic fumes and smoke.
This is a major advantage in saving lives of people who may be trapped in the building.
Fabric is easy to clean and can be done using similar methods to those that one would use to clean a car, i.e. soft brushes, light duty, non-acidic detergents and copious rinsing water. Personnel can access the fabric by means of ropes and use soft soled shoes to walk on the fabric.
Structural materials – choices for tensile structures
All the usual structural materials (steel, concrete, aluminium, composites, etc.) have been used for tensile structures, but generally the emphasis is on the achievement of a minimalist appearance for the supporting structures. As steel has the highest strength and stiffness values, and is also relatively cheap, it is the most commonly used material.
Tensile systems – design & materials
The choice and design of the tensile systems is one of the most important aspects of tensile fabric structures. A variety of materials and coatings are available, ranging from marine-grade 316 stainless steel, through hot-dip galvanised steel, to simple painted steel. The particular environment in which the structure is sited is of prime importance in selecting the materials to be used, because the tensile components are generally fully exposed to the vagaries of the weather. Unfortunately, there are numerous examples of inadequately specified materials being used for short term cost benefits.
The design and selection of tensile connections is an area which requires specialist expertise and this work should not be undertaken without adequate engineering experience and knowledge. The main aspect of the design which requires careful attention is the secondary effects – these often have a major bearing on the performance of the components in practice. Local wind load effects, eccentricities, unequal load sharing in multiple connections, angular discontinuities (resulting in bending stresses), fatigue loading, etc. need to be taken account of. The visual impact of these connections on the overall appearance of the structure is significant and due attention needs to be given to ensuring that they contribute to enhancing the inherent aesthetic appeal of tensile structures.