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. At 4.00 pm in summer, when the sun is well below its noon elevation, the UVR is still sufficient to cause sunburn of the average fair-skinned person in twenty minutes. However, the shade cast by the structure at that time of day may not be providing the protection where it is required.
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 respective sun angles are dependent on the latitude of the site. The angles for Sydney, Australia (latitude 33 degrees, 53 minutes South) are illustrated in Figure Three. Clearly, in terms of designing for shade protection, the summer solstice and equinox are of primary interest.
Figure Three – Solar Azimuth angles for Sydney, Australia
Shading diagrams for a six metre by six metre hypar structure aligned with its low points in a North / South direction, for various times of the day during the Sydney summer solstice are shown on Figure Four (a). As one would expect, the shading area is maximised at noon, but is significantly less at 10.00am and 2.00 pm. Note that the net area beneath the fabric that is in full shade for the whole period between these latter times is relatively limited, but can be identified. This has important ramifications in determining the specific location of the structure relative to the fixtures below it (such as a park dining table or play equipment), or vice versa, as illustrated in figures four (b), (c) and (d).
Figure Four (a)– Shade created by a six metre x six metre hypar structure in Sydney at various times of the day on the summer solstice.
Figure Four (b) – An example of where the design of shade sails may provide only limited shading in mid-morning to children’s play equipment, an area they are intended to protect.
Figure Four(c) A series of overlapping triangles providing effective shading when the sun is directly overhead. Triangles are not as structurally efficient as hypars.
Figure Four (d) An excellent example of shadesails integrated with vegetation to maximise shading at most times of day