Back to Basics: Shadecloth

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Gregory Collins
6 years ago
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Fabrication The following content is based on our own general knowledge, research and advice obtained from consultants and experienced fabricators in the industry.

When cut, shadecloth will fray if the edge is not seamed particularly if the exposed edge is allowed to flap in the wind.

Although the latter has more give, it can have an abrasive effect on the sewing thread and in failure tends to un-zip along the underside.

1 Fabrication The following content is based on our own general knowledge, research and advice obtained from consultants and experienced fabricators in the industry. The information is provided in good faith, but no warranty is given or is to be implied with respect to its accuracy or applicability. When cut, shadecloth will fray if the edge is not seamed particularly if the exposed edge is allowed to flap in the wind. For strength and aesthetic reasons, the edge and join seam should be overlapped twice and preferably double stitched. A lockstitch is preferred to a chain-stitch. Although the latter has more give, it can have an abrasive effect on the sewing thread and in failure tends to un-zip along the underside. Although once widely used, cotton thread, or a poly-cotton blend is not recommended. Cotton is not a particularly durable material for use in an outdoor environment. It does not have a very high strength and without treatment, frays, rots and is susceptible to mildew. This is why one sees evidence of seam failure on jobs where the fabric is still in good condition. SUNGUARD thread ideal choice for shade structures The use of a UV stabilised, 100% polyester thread (such as SUNGUARD ) has proved to be durable in the field. Polyester, however does lose strength under the action of sunlight. Although tests show that while size B138 is probably adequate, the heavier size B208 is recommended. Recently, PTFE (Poly/Tetra/Fluro/Ethylene) thread, (such as PROFILEN ) has started to become more widely used. Although costly, it has far more durability and a much longer life than other types of thread.

Non-Tension Applications Back to Basics: Shadecloth Even in non-tension applications the cloth will be subjected to wind-load, which will induce stress in the cloth and the isolated fixing points.

Caution should be exercised when connecting to a stud frame or wall of a building as these have limited resistance to overturning and there is a real risk of structural damage occurring in a high

2 Non-Tension Applications Back to Basics: Shadecloth Even in non-tension applications the cloth will be subjected to wind-load, which will induce stress in the cloth and the isolated fixing points. For this reason, one must ensure that all fixing points are structurally adequate and that fittings are soundly secured. Caution should be exercised when connecting to a stud frame or wall of a building as these have limited resistance to overturning and there is a real risk of structural damage occurring in a high wind. Preferably the sail should be supported independently. It should also be taken down in the likelihood of strong wind conditions to avoid the potentially high loads at the fixing locations. Ideally the cloth should be supported uniformly along the whole of its edge. Even with isolated connecting points, it is possible to convert the concentrated load to a more uniformly distributed load in the fabric. There are two recommended ways of doing this. Method 1: Method one is to fold and sew 50mm of UV stabilised, reinforced rayon seat belt webbing along the edge, and punch SP4 eyelets through this reinforcing at the desired locations. Folded 50mm Seat-belt Webbing Stitch 25mm SP 4 Eyelet Method 2: Method two is to sew a six millimetre nylon cord along the edge trim, and use Plaspaline Ropeclamps at the desired fixing locations. This method is more flexible, as the clamps do not penetrate the fabric and can be easily relocated. 25mm Double Stitching Plaspaline Clamp 6mm Diameter Cord

Non-Tension Applications (continued) Where the cloth is to be fixed using timber battens, the supporting edge should be wrapped at least once around the batten before nailing.

3 Non-Tension Applications (continued) Where the cloth is to be fixed using timber battens, the supporting edge should be wrapped at least once around the batten before nailing. Even then, reliance is being placed on the friction induced between the batten and the substrate. Any loosening of the batten over time will concentrate stress at the nail locations, possibly leading to tear. Tensioned Applications Only products that have been specified for high tension structures (i.e. ProTEX Parasol and Polyfab Architec 400) should be used. It is important that it is not over-tensioned if the shape and elasticity of the material is to be retained. The higher the load, the greater the risk of deformation or sagging over time. The maximum recommended pre-stress is 0.25 kn/m in both the warp and weft directions. Tension structures rely on applied force for both shaping the fabric and structural integrity. The applied forces and wind-load on the supports can be significant and professional engineering advice should always be sought at the design stage. Pre-stress in the cloth is invariably induced by tensioning a cable along the edge of the fabric which is cut in a curved shape. The flatter the curve, the higher the tension in the cable which is equal to the radius of curvature, multiplied by the reactive stress of the cloth. The reactive stress applied by cloth is dependent on its deformation or strain, the characteristics of which can be different in the warp and weft directions. This can be a complicating factor in design. To minimise lateral loading at supports, it is desirable that the tension in the cable is constant (which it will be anyway if it is continuous). Most tension structures are designed for the condition of equal stress in both warp and weft directions, where pretension strain response is similar, thus making patterning much simpler. Under these design conditions, the finished radius of curvature will be equal for each span and a theoretical engineering relationship exists between cable tension, reactive stress and tensioned chord depth for any given span. This may prove useful as a guide for setting out, or estimating the applied stress or cable tension in the finished structure.

Tensioned Applications (continued) There is a formula for the same design conditions, where the ratio of depth to span can be approximated; d = kl² Where: d = depth at mid-point L = span and k is a

4 Tensioned Applications (continued) There is a formula for the same design conditions, where the ratio of depth to span can be approximated; d = kl² Where: d = depth at mid-point L = span and k is a constant. For the longest span, if the ratio of d to L is assumed to be 0.1, the proportional value of the expected finished depth of other spans can be determined by multiplying this longest span depth by the ratio of the square of the lengths as follows; dn = d1 x (Ln/L1)² Where dn and Ln refer to the depth and length respectively of any other span. This rule of thumb may prove useful for initial shaping, determining expected shading envelopes of proposed designs or estimating the relative quantities of material required. There are a number of ways of making a sheath for a tensioning cable. The simplest is to incorporate it into the edge seam. Another method is to create a pouch using seat belt webbing, or use the webbing itself as the tensioning medium. 50mm UV stabilised rayon seat belt webbing has a breaking strength of 20kN. It is therefore suited for use in smaller structures (i.e. <50m2). The webbing can be continuous and neatly looped at the corners to provide an attachment point to a turnbuckle or other tensioning device. One problem with this method is that the webbing is a lot stiffer than the shadecloth itself, and because of this, the sail can become baggy as the shadecloth deforms with time.

Design and Installation of Supports The force of wind should not be underestimated and is the reason the supporting structures and cables of large area shade sails appear so robust.

5 Design and Installation of Supports The force of wind should not be underestimated and is the reason the supporting structures and cables of large area shade sails appear so robust. Professional engineering advice should always be sought on the design of the supports. Wind design stresses are in the order of 1.5kN/m, which is still very much less than the ultimate stress of the cloth. Typically then, the wind failure mode is loss of elasticity rather than tear or burst. Failure can also occur for other reasons, such as the accumulation of hail. Shadesails are invariably attached by a corner to a column or other fitting and the treatment of these requires some thought. It is crucial that the tensile load in the cable is transferred directly to the supports. The reactive stress in the fabric tends to drag it away from the supports and reinforcement at these corner points is recommended. Connection to a column without a backstay results in significant bending or overturning being applied to the column, which is resisted by the bolts at the base, causing reactive forces on the side and bottom of the footing. In this cantilevered loading case, deflection in the column can be very noticeable and is usually the basis for determining column size. Staying the column can reduce both deflection and overturning, however the axial load is higher. A pin jointed base ensures that the applied bending is further reduced, which means an elegantly slim column can be used. However consideration must be given to lateral stability. This can be attained by using a tripod as support, or additional guy wires. Shading The principal objective of constructing a shade structure is to provide shade, but it is surprising how often the planning in this context is overlooked. The degree of risk due to exposure to Ultra-violet Radiation (UVR) is dependent on the season, time of day and geographic location. The single most important factor is the height of the sun. The higher the sun, the higher the levels of solar UVR experienced. This rather obvious fact usually dictates the base design parameters of a shade structure, but what is often ignored is the potential degree of exposure at earlier and later times of the day. For example at 4.00pm in summer, when the sun is well below its noon elevation, the UVR is still suffcient to cause sunburn to the average fair-skinned person within twenty minutes. However, the shade cast by the structure at that time of day may not be providing the protection where it is required.

Back to Basics: Shadecloth Shading Four particular days per year are important yardsticks for defining the boundaries of the extent of shadow cast by a structure; The summer

On this day, the sun is at its highest summer zenith at noon; and rises and sets at its furthest point south.

On these days, the sun at noon is midway between its summer and winter zeniths and rises and sets due east and west respectively.

On this day, the sun is at its lowest winter zenith at noon; and rises and sets at its furthest point north.

6 Back to Basics: Shadecloth Shading Four particular days per year are important yardsticks for defining the boundaries of the extent of shadow cast by a structure; The summer solstice: This occurs on the 21st December and is the longest day of the year. On this day, the sun is at its highest summer zenith at noon; and rises and sets at its furthest point south. The equinox: This occurs twice a year on the 21st March and 23rd September. On these days, the sun at noon is midway between its summer and winter zeniths and rises and sets due east and west respectively. The winter solstice: This occurs on the 21st June and is the shortest day of the year. On this day, the sun is at its lowest winter zenith at noon; and rises and sets at its furthest point north. The diagrams seen in Figure 1 show the shade provided by a typical Hypar structure of 36m ² with its supports aligned north to south, in Sydney. The actual shaded area is shwon as a green outline and the effective shade beneath the structure is shown in grey. Please note that the shade provided is not coincident with the structure itself, which has ramifications for its efficiency. Figure 1: Standard Shade Diagrams Sydney, 22nd December Shade Time of day 10am Time of day 12pm Time of day 2pm Total Shade 36m ² 36.06m ² 36.12m ² 36.32m ² Shade below the structure 36m ² 23m ² 32m ² 22m ² Nolans.com.au info@nolans.com.au

Nolan Warehouses

TECHNICAL GUIDE NUMBER ONE ProTEX SHADECLOTH E1 October 2007 Nolan Warehouses www.nolans.com.au COMPANY PROFILE Nolan Warehouses, established in 1920, is a merchant wholesaler whose products can be segregated