The higher these seats are raised above playing field level, the better viewing standards will be, but the steeper the rake will be (Figure 11.15). The chosen method of separating the crowd from the playing
6 m 9 m
7.3 m 9.2 m
Maximum angle of rake
Point of focus
Figure 11.13 Quality of vision is improved by increasing the heights of seats above pitch level; and by bringing the seats closer to the pitch (point of focus).
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in Chapter 9) will influence this decision. It is recom-mended that eye height above the pitch should not be less than 800 mm, with 700 mm as an absolute minimum.
11.4.3 Final design
By juggling all the above factors against each other (Figures 11.16 and 11.17), and against site constraints and construction costs, a theoretical stadium geometry will emerge. Some tests that must be carried out on the hypothetical profile are the following.
Angle of rake
Choosing a stadium profile that minimizes the dis-tance between spectators and playing field may give a rake that is too steep for comfort or safety.
It is generally accepted that an angle of rake steeper than 34 degrees (approximately the angle of a stair) is uncomfortable and induces a sense of vertigo in some people as they descend the gang-ways, even if regulations in some countries do allow steeper angles. In Britain the Guide to Safety at Sports Grounds (see Bibliography), also known as
the ‘Green Guide’, recommends a maximum angle of 34 degrees. In Italy up to 41 degrees is allowed, but this extremely steep rake is usually found only towards the backs of the upper tiers. Handrails are then provided in front of each row of seats for safety and to counteract the sense of vertigo.
Not all countries have specific regulations, but in all cases the local codes of practice and legislation must be checked. Where no specific regulations exist the angle of rake will normally be determined by staircase regulations.
Varying riser heights
The calculated rake for a deep stadium will not be a constant angle, but a curve (Figure 11.18), with each successive riser one or two millimetres greater than the one in front. The building process tends to favour standardization, and constructing a stand in this way could be more expensive than straight tiers.
Therefore it is customary to divide tiers into facets which provide optimum viewing angles while redu-cing the variety of riser heights. In Europe and North America, where precision in pre-cast concrete work is relatively easily obtained, the changes in stepping
6 m 9 m seats is to the point of focus, the steeper the rake will be, and the higher the back of the stand for a given ‘C’ value.
Point of focus
Figure 11.15 The higher the first row of seats above pitch level, the better viewing standards will be, but also the back of the stand will be higher.
This may cause problems of building cost and of appearance, and may obstruct access of sunlight to a grass pitch.
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heights could be as little as 10 to 15 mm. In regions with less sophisticated technologies it would be wise to increase the stepping differences to 20 or 25 mm.
When varying seating tier heights cause stair ris-ers also to vary, there might be a conflict with the local building regulations, which sometimes prohibit variations in stair riser heights. This should be checked. In England and Wales the most recent reg-ulations take note of the situation, and waivers are usually obtainable.
When the general design has been completed the view from each seat can be checked by creating a computer image of the playing field from the eye
position. If tickets are being sold in advance such images can also be used to inform potential pur-chasers of what they will see from any given seat.
11.5 Obstructions to viewing
This factor is more critical for some sports than others. In motor or horse racing, a few columns in front of the spectators may be acceptable because the cars or horses are large objects whose move-ment past the columns is easily tracked. In tennis, by contrast, the repeated invisibility of a small-diameter ball as it speeds to and fro behind an obstructive column would be intolerable. The structural aspects of column-free roof design are discussed in Section 5.8.
Point of focus
C=120 45 degrees
C=90 39 degrees
C=60 33 degrees
Higher than acceptable
Figure 11.16 One example of the varying results obtained by juggling the factors identified in the foregoing figure.
Point of focus
8.2 m 7.0 m 5.9 m C=120
C=90 C=60
Figure 11.17 The effect of changing
‘C’ values on the angle of rake.
Point of focus
Consistent viewing standard produces a curved tier Figure 11.18 The riser heights required to
maintain a specified ‘C’ value in each row of a tier will not be constant, but will vary from each row to the next. In practice such a curved profile will be built as a series of facets which strike a balance between optimum viewing angles and standardization of construction.
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