Spectators’ aerothermal comfort assessment method in stadia

ARTICLE IN PRESS

Building and Environment 42 (2007) 2227–2240 www.elsevier.com/locate/buildenv

Spectators’ aerothermal comfort assessment method in stadia Agota Szucs, Sophie Moreau, Francis Allard France-Centre Scientifique et Technique du Baˆtiment (CSTB), Universite´ de La Rochelle, 11, rue Henri Picherit BP 82341-44323 Nantes Cedex 3, France Received 18 September 2005; received in revised form 8 March 2006; accepted 24 March 2006

Abstract Spectators’ aerothermal comfort in stadia is affected by the stadium morphology. The morphology signifies not only the global building form, but also the dimension, the position, the porosity of the different building elements, such as the roof, the fac- ade and the spectators’ terrace. The airflow modifying effects of these architectural parameters have been investigated in a boundary layer wind tunnel using a stadium model of a variable geometry. The present article illustrates a method of spectators’ aerothermal comfort assessment based on the results of the wind tunnel tests, and a comfort zone of flexible outlines. The last blends the wind comfort assessment method used by the CSTB Nantes, the bioclimatic chart revised by Arens et al. [Thermal comfort under an extended range of environmental conditions. ASHRAE Transactions 1986;92(Part 1):18–26] and the new wind chill concept. r 2006 Elsevier Ltd. All rights reserved. Keywords: Stadium; Aerothermal comfort; Wind; Outdoor temperature; Wind tunnel test; Morphology

1. Introduction Stadia are sculptures of urban structure with a tradition inherited from the antiquity. They have a modifying effect on the microclimate inside and around them due to their global morphology and great dimension. This paper presents the methodological approach to evaluate the thermal comfort of spectators in stadia. First, the conception of a comfort zone with flexible limits has been developed using an applicable thermal comfort index and a chart to semi-outdoor spaces. The comfort zone is based on the wind comfort assessment method used by the CSTB Nantes [1], the bioclimatic chart revised by Arens et al. [2] and the new wind chill concept. Analysing the cooling effect of air movement at lower temperatures, the maximum admissible wind speeds have been determined by the new wind chill index (WCI). Besides the structural and aesthetic aspects, canopies considerably influence the wind-generated airflow patterns on both the spectators’ terrace and the playfield—and consequently the thermal and wind comfort of visitors. Several particular stadia have already been investigated Corresponding author.

E-mail address: [email protected] (F. Allard). 0360-1323/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2006.03.009

through wind tunnel tests all over the world. However, no generalised information on the relation of morphology and airflow patterns has been obtained so far because the results of these tests apply to specific cases in given locations taking into account the prevailing wind conditions. To bridge this gap, a series of systematic wind tunnel tests have been carried out in one of the boundary layer wind tunnels of the CSTB Nantes. A variable model has been developed in order to allow the modification of the geometrical parameters. Average wind speed and its standard deviation have been measured at several points of the spectators’ terrace by hot wire anemometer and expressed as the ratio to those of the reference wind for three wind directions. The measured data of each (morphological) configuration have been evaluated using statistical method. Interrelations have been found between the main morphological features, such as the slope, the overhang of the roof, the porosity of the fac- ade and the characteristics of the airflow. Based on the output, correlation between airflow patterns and geometry has been established. Combining the theoretical and experimental methods, we aim to provide information for architects on

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climate–architecture–comfort context of stadia that can be used at the conceptual stage of the design. An adaptation method is presented, facilitating the transposition of the wind tunnel tests’ results to a given real building site in order to assess the expected spectators’ wind comfort conditions. 2. Environmental parameters influencing aerothermal comfort The thermal sensation can be related to changes in body temperature due to heat loss or gain. Numerous factors effect heat dissipation from the body, thus also thermal comfort. They can be grouped into three sets [3] (Table 1): Air temperature is the most important environmental factor measured by the dry bulb temperature (DBT). It determines the convective heat dissipation together with any air movement. Next to air temperature, radiation has the greatest effect on thermal sensation. Radiation falling on the body surface activates the same sensory organs as the warmth of the air. Conversely if the body is facing cold surfaces, a significant amount of heat is emitted in form of radiation towards these surfaces. Such radiation emission causes a sensation of cold. Humidity of the atmosphere has little effect on thermal comfort sensation at or near comfortable temperatures, unless it is extremely low or extremely high. It does, however, play an important part in the evaporative regulation zone. At comfortable temperatures, there is no need for evaporative cooling but at high temperature, this is the most important heat dissipation channel. Saturated air at 100% RH will hinder any evaporative cooling. The highest temperatures under which thermal balance can be effectively maintained by evaporative cooling largely depend on the humidity: 100% RH: 31 1C; 50% RH: 38 1C; 18% RH: 45 1C. In warm situation, humid air above 60% (and more pronouncedly above 80%) feels warmer than it actually is [4]. Air movement produces thermal effects, even without any change in air temperature. It increases heat dissipation in two ways:



It increases convective heat loss as long as the temperature of the moving air is less than the skin temperature. In some climates, the air temperature can reach above 40 1C and more in which case the moving air actually warms the skin.

Table 1 Parameters influencing thermal sensation Environmental Personal Contributing factors

Air temperature, humidity, radiation, air movement. Metabolic rate, clothing. Food and drink, acclimatisation, body shape, subcutaneous fat, age and gender.



It accelerates evaporation providing a physiological cooling. In low humidities (below 30%), this effect is significant as there is an unrestricted evaporation even with still air. In high humidities (about 85%), the evaporation is restricted; thus, even the air movement cannot adequately increase cooling effect. Evaporation is most significantly accelerated in medium (40–50%) humidities. Here, the evaporation potential is good but if the air is stagnant, the layer next to the skin soon becomes saturated, preventing further evaporation. Air movement would break this saturated air envelope and would ensure a continuous maximum evaporation rate.

Wind represents one of the main differences between outdoors and indoors. Even a light wind will far exceed the typical air velocities experienced indoors. The effects of wind on people can be divided into two categories. The first, the mechanical, is the direct effect of the wind force on people and items such as umbrellas, dust and loose papers. The second, the thermal effect, is the indirect influence of wind on thermal comfort when combined with air temperature, humidity and solar radiation. The mechanical effects of various wind velocities and the corresponding wind forces have been described by the Beaufort scale [1]. A particular question is the effect of the wind speed fluctuation, the turbulence effect on the thermal sensation. On one hand, the heat transfer phenomenon is to be mentioned: each change of flow direction and velocity is accompanied by the disruption of the previous and the development of a new boundary layer. In its first part, the heat transfer is more intensive. On the other hand, there is a physiological effect: each change means a new stimulus for the receptors of the human thermoregulatory system, preventing the saturation of the receptor system by the dynamic effect [5]. Each comfortable situation becomes less comfortable and even slightly uncomfortable if it is maintained during a long period of time due to monotony [6]. In the research of spectators’ thermal comfort in stadia, the cumulative effect, the interaction of the climatic factors with special emphasis on the combination of air movement and DBT have to be taken into account. The effect of radiation and relative humidity can be assessed, too, or even calculated for particular cases. 3. Comfort zone Stadia are semi-outdoor spaces—according to the classification of Spagnolo and de Dear [7], as they are open as well as covered. They contain building elements that provide some protection from the outdoor environmental conditions, such as a wall acting as windbreak or a roof providing shade. In these semi-outdoor spaces besides the solar radiation, the wind is an environmental factor of prevailing importance that can be altered to a great extent by the stadium architecture.

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The effect of wind on heat perception

16.0 14.0 DBT−WCT (°C)

To approach the definition of comfort in stadia, thermal comfort indices and charts provide a basis; the former by describing certain physical conditions of comfort by a sole parameter, the latter by integrating the different climatic parameters and showing the contribution of each separately. In the literature, numerous indices of comfort expressing the effect of air movement can be found. Nevertheless, several of them cannot be applied to outdoor environment because only a few tenths of m/s of air velocity values can be substituted. Through the bibliographic research, the following indices and charts are assumed to be applicable to evaluate thermal comfort in stadia:

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-20°C

12.0 -10°C

10.0 8.0

0°C

6.0 4.0

+10°C

3K

2.0 0.0 2.0

4.0

6.0

8.0

10.0

Wind velocity (m / s)

new WCI corrected effective temperature (CET) Olgyay’s bioclimatic chart and its revised version by Arens et al.

Besides the thermal, the mechanical effect of the wind has also to be concerned using conventional form and numeral value. Following a literature overview, integrating the effect of different environmental factors, in particular that of the wind and the outdoor temperature, a comfort zone has been derived [8]. It is based on the new WCI, the bioclimatic chart revised by Arens et al. [2] and the wind comfort assessment method used by the CSTB Nantes [1]. The WCI is the measure of the cooling effect of the wind on exposed skin combined with low temperatures [3]. The temperature of the air cannot be felt directly by the human beings. They feel the temperature of the skin. The last varies among others, in function of the wind velocity. In consequence, in the presence of wind, the convective and evaporative heat losses from the exposed skin are intensified, and the temperature is perceived as lower than the actual DBT. The old version of the WCI has been ameliorated from numerous aspects, among others it uses wind speed calculated at the average height of the human face (about 1.5 m) instead of the standard anemometer height of 10 m. Furthermore, it is based on a model of the human face, and incorporates modern heat transfer theory, i.e. the theory of how much heat is lost by the body to its surroundings during cold and windy days [9]. Fig. 1 illustrates the cooling effect of the wind for DBT values ranging from 20 to +10 1C and for wind speeds up to 10 m/s using the new wind chill formula. The parameter of the curves is the DBT from the top to the bottom 20, 10, 0, +10 1C. The WCI is meaningless for wind speeds lower than about 2.5 m/s. According to the graph, the chilling effect of wind on exposed skin is greater in case of lower DBT. When assessing spectators’ thermal comfort in stadia, a conventional value of illusory temperature drop of 3 K can be defined that illustrates the admissible difference between the actual DBT and the perceived temperature. This difference is on one hand due to thermal tolerance of humans outdoors, and on the other

Fig. 1. Illusory temperature drop (DBT-WCT) versus air velocity—using the new wind chill concept.

Admissible wind speeds in case of low temperatures 5.0 4.0 Wind velocity (m /s)

  

3.6 m /s

3.0 COMFORT ZONE

2.0 1.0 0.0 0.0

2.0

4.0

6.0

8.0

10.0

12.0

DBT (°C) Fig. 2. Mechanical threshold and admissible wind speeds using the wind chill concept.

hand, it can be compensated by taking on a pullover or a jacket [10]. In this manner, the tolerable wind velocities in case of lower temperatures can be read from the diagram. Fig. 2 illustrates the admissible wind velocities in case of lower outdoor temperature. It can be seen on the figure that the admissible wind velocity increases, as the DBT augments; however, the wind velocity is not allowed to exceed 3.6 m/s, the mechanical threshold value. This conventional value has been defined by the CSTB Nantes based on wind tunnel experiments. It takes into account the mechanical effect of the wind that is felt on the human face. The first discomfort events such as feeling the wind on the face, or disturbed hair are due to wind speeds belonging to the Beaufort scale numbers 2 and 3. The 3.6 m/s corresponds to an ‘‘equal wind speed value’’ in between these two classes and integrates the average wind speed and its standard deviation. The admissible wind velocity for DBT values lower than about 4 1C stays under the mechanical threshold value.

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Contrary to cold conditions, air movement is required to maintain thermal comfort in case of high temperatures— up to they reach the core temperature, about 38 1C [3]. The required air movement has been defined based on the bioclimatic chart revised by Arens et al. [2] that integrates the effect of all environmental parameters influencing thermal comfort outdoors. The basis of the chart is the comfort index DISC (discomfort index), as predicted by the J.B. Pierce two-node model of the thermoregulatory system. DISC is a function of skin temperature in cold conditions and of skin wettedness alone in hot conditions. On the chart, a comfort zone is outlined with a minimal air movement (0.1 m/s) assuming that the air temperature is equal to the mean radiant temperature. Outside the comfort zone, the chart has contour lines along which selected levels of added radiation or air velocity restore the body to the nearest boundary of comfort zone. The radiation temperatures offset temperatures that are too low and the air velocities offset temperatures that are too high. An innovation is the use of the effective radiant field (ERF) to describe the additional long-wave radiation energy received by the body when surrounding surface temperatures are different from the air temperature. Using the results of the experiments carried out by Spagnolo and de Dear [7] at semi-exterior spaces, the maximum comfortable DBT with a minimal air movement (0.1 m/s) is set to 26.2 1C with an RH of 60%, when defining the required air movement for high temperatures. This combination of climatic factors is taken as a reference situation that is to reach by more intensive air movement in case of higher temperatures. Fig. 3 illustrates the required air movement corresponding to higher temperatures. Nevertheless, this air movement should not exceed the mechanical threshold. A comfort zone that integrates the admissible and the required air movements in function of the DBT with an

v

7.0

threshold

3.6

"Minimal" threshold

"Mechanical" threshold

3.0 ERF 2.0

ERF

COMFORT ZONE

1.0

4

26

33

DBT (°C)

Fig. 4. Comfort zone of flexible outlines.

RH of 60% and for sedentary position and an adequate clo value has been established. The outlines of the comfort zone illustrated by Fig. 4 are of three kinds:





 Minimal and mechanical threshold lines of the comfort zone

"Maximal"

(m /s)

Maximal threshold line that shows the maximal admissible air movement at low temperatures (below 10 1C) based on the new WCI [10]—it cuts the left corner of the comfort zone where the DBT is close to 0 and the air movement is close to the mechanical threshold. Mechanical threshold line at 3.6 m/s, that equals to 3 m/s average wind speed and its 20%, as standard deviation to express the wind speed fluctuation (conventional threshold value used in CSTB Nantes) [12]. According to the Beaufort scale, this value is associated with the first discomfort events: the wind is felt on the face and the hair is disturbed [12]. Minimal threshold line that defines the required air movement at high temperatures, based on the bioclimatic chart revised by Arens et al. [2] that illustrates the cooling effect of air movement. Again, this air movement should not exceed the mechanical threshold.

Air movement (m /s)

6.0

The comfort zone is bordered by these outlines that should be shifted if additional short- or long-wave radiation is present. The last is expressed by the ERF, when the surrounding surface temperatures are different from that of the air [2]. The cooling effect of wind can also be modified by radiation; however, for the first approach, only the wind speed and the outdoor temperature have been investigated.

5.0 3.6 m /s 4.0 3.0 COMFORT ZONE

2.0 1.0

4. Wind tunnel tests

0.0 25

26

27

28

29

30

31

32

33

34

DBT (°C) Fig. 3. Mechanical threshold and required air movement at high temperatures for RH ¼ 60%.

A scale stadium model of variable geometry (Fig. 5) has been constructed in order to investigate the airflow in one of the boundary layer wind tunnels of the CSTB Nantes, for numerous architectural configurations of stadia.

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Fig. 5. 1@300 scale model of stadium.

A series of parametric wind tunnel tests have been carried out in order to study the airflow modifying effect of the following architectural parameters:

    

5. Results

the slope and position of the roof, overhang of the roof, dimension of the central roof opening—oculus, fac- ade porosity, stadium layout.

5.1. Analysis method

Fig. 6 illustrates the characteristic stadium configurations studied during the parametric wind tunnel tests. Average wind speed and its standard deviation have been measured at selected points of the spectators’ terrace (Fig. 7) for three wind incidences: longitudinal (parallel to the longitudinal axis of the stadium), transversal (perpendicular to the longitudinal axis) and diagonal (forming an angle of 301 by the longitudinal axis)—using hot wire anemometer. The measurements have been carried out for 52 points in case of diagonal wind and 29 (dispersed on the half of the bowl surface) for longitudinal and transversal wind. The airflow has been considered symmetric for longitudinal and transversal wind; in consequence results have been mirrored to the other, non-measured half of the stadium bowl. The results have been expressed in form of a dimensionless parameter denoted by c, introduced by Gandemer [12]. The index c integrates the average wind speed and its standard deviation—as both are perceived by the people exposed to wind: c¼

¯ i þ si U , ¯ ref þ sref U

Fig. 6. Selected measurement points.

(1)

¯ i is the average wind velocity at a point i, si is the where U ¯ ref is the average wind standard deviation at a point i, U velocity at the reference point and sref is the standard deviation at the reference point. The index c expresses the standard deviation and the average wind speed at a given point of the spectators’ terrace related to those of the reference wind. The latter is measured by a Pitot pipe placed 200 m high (full scale) in front of the stadium, at a free area where the airflow is undisturbed by the surrounding objects.

The data—in order to be more easily interpretable— have been transferred and recalculated to a reference point at a height of 2 m. In this case, the scale is not that of the building, but that of the human body. The ensemble of the data measured at the selected points of the spectators’ terrace represents a statistical sample. This implies that the localisation of the points corresponding to the highest and the lowest c values are not known. In consequence, the ‘‘extreme’’ conditions concerning the airflow on the spectators’ terrace have been featured by some characteristic c values obtained from the wind tunnel data:

  

the arithmetic average c value of the data population; c10% , the first decile of the data population; c90% , the last decile of the data population.

The arithmetic average of the c values describes the average airflow conditions on the entire spectators’ terrace. Instead of or with the arithmetic average, the median value can also be used. The arithmetic average is the sum of the observations (c values of all measurement point) divided by their number. The median represents the middle value of the data population, i.e. it divides the data so that half of all the data items are greater than the median and the other half of them are less than the median. The ‘‘extreme’’ c values, namely the first (c10%) and the last deciles (c90%) give information about the less and the most intensively ventilated zones of the spectators’ terrace. Fig. 8 shows a characteristic cumulated frequency curve of the c values for one incidence belonging to a stadium configuration. On the curve, the first and the last deciles are marked, the first corresponding to a probability of occurrence (P) of 10%, the last to 90%. The decile is an abscissa value corresponding to an ordinate value of the distribution curve occurring by a given probability (P). The first decile, the c10%, is the value that is exceeded by the 90% of the

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SECTION/ DETAIL

PHOTO

DESCRIPTION

° continuous spectators' terrace; ° continuous horizontal roof; ° horizontal opening between the

spectators' terrace and the roof running all around the stadium façade.

° continuous spectators' terrace; ° continuous roof sloping towards the pitch;

° horizontal opening between the

spectators' terrace and the roof running all around the stadium façade.

° continuous spectators' terrace; ° continuous roof rising towards the pitch;

° horizontal opening between the

spectators' terrace and the roof running all around the stadium façade.

_

 =

Ui +  i _

U ref +  ref

° continuous spectators' terrace; ° continuous horizontal roof with an overhang;

° horizontal opening between the

spectators' terrace and the roof running all around the stadium façade.

° continuous spectators' terrace; ° continuous horizontal roof; ° no opening between the spectators'

terrace and the roof running all around the stadium façade.

° continuous spectators' terrace divided

into sectors by uniformly distributed vertical openings; ° continuous horizontal roof; ° horizontal opening between the spectators' terrace and the roof running all around the stadium façade.

° continuous spectators' terrace divided

into sectors by continuous horizontal openings; ° continuous horizontal roof; ° horizontal opening between the spectators' terrace and the roof running all around the stadium façade.

Fig. 7. Selected examples of tested configurations.

data population. The last decile, the c90%, is the value that is exceeded by the 10% of the data population. The measurement points are placed in a rectangular mesh providing a relatively dense grid that facilitates creating iso-c lines. The latter are the elements of the iso-c charts—shown in Fig. 9—representing the airflow at the spectators’ terrace at spectators’ height for the three wind incidences. On the iso-c charts, the dark zones mark more intensive flow corresponding to c values equal and higher than the c90%, while the light ones, representing c values equal and lower than c10%, indicate calm areas. This graphic method is informative and facilitates the representation of the air velocity fields indicating their

location and size. The differences between the iso-c charts of the investigated architectural configurations are easily perceptible. However, in themselves they do not allow a simple, unambiguous and fast comparison of the different design options. That is why they are to be treated together with the characteristic c values. The ratios of the c10% and the c90% to the mean value describe the homogeneity of the flow. The closer these ratios to 1, the more homogeneous the airflow. Fig. 10 shows that identical ‘‘extreme’’ c values (c10% and c90%) can belong to different probability density curves. That is why when selecting a configuration, the simple comparison of the characteristic c values is not sufficient; the ratio of

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A higher c average value C2 , with a lower standard deviation is preferred in warm climates, while a lower c average value, C1 or C3 , is more advantageous in cold or temperate climatic regions. The latter are determined in function of the climatic characteristics or meteorological data of the given location during the predetermined period of use. Furthermore, a more appropriate analysis can be executed containing the concerned areas by the extreme c values. Indices expressing the homogeneity consisting of the weighted average have been established integrating the extreme c values and the concerned bowl areas proportionated to the average c values and the entire bowl area:

P P=90%

P=10% 



10%

90%



IH  A90% ¼

C90% AC90% , CA

(2)

IH  A10% ¼

C10% AC10% , CA

(3)

Fig. 8. Cumulated frequency curve of c values.

where c90% and c10% are the last and the first deciles of the data population; AC90% and AC10% are the concerned ¯ is the average c value; and A is the total bowl areas; C area. Besides the extent, the location of the areas concerned by the first and last deciles is of prevailing importance. It changes in function of the wind incidence within a given configuration. Fig. 11 shows the shift of the c10% and c90% zones by the wind direction for a configuration . The c10% zones can be found on the leeward side of the terrace on the lower lateral terrace for all investigated wind incidences. While the most intensively ventilated zones occur at the higher spectators’ terrace part on the windward side. This representation indicates the extent, the location and in addition the shift, in other words the sensitivity for the wind direction of the most and the less intensively ventilated zones. A less wind incidence-sensitive configuration should be chosen at a site where the wind occurs from

Fig. 9. Iso-c values for fac- ade porosity of about 30%.

F(%)

F Ψ90% F Ψ10%

 Ψ10%

Ψ3 Ψ2

Ψ1

Ψ90%

Fig. 10. Identical c10% and c90% values originating from curves characterising different distributions.

the extreme values to the average c value has to be studied as well. A more accentuated wind protection implies lower c values, while again more homogeneous ventilation signifies a ratio of the average c and extreme c values closer to 1, i.e. narrower extent of the data population.

Fig. 11. Zones of c10% and c90% values for all three investigated wind incidences. The arrows indicating the incidence are of the same line type as the c10% and c90% zone boundaries.

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5.2. Airflow modifying effect of architectural parameters The results of the wind tunnel tests show that each investigated architectural parameter modifies the air flow characteristics. First, the effect of the canopy slope has been studied. The roof is often inclined due to structural, functional or aesthetic reasons. The tested configurations included horizontal roof, inclined roof towards the pitch by 151 and rising roof by 151 and 301. The airflow on the spectators’ terrace is relatively homogeneous for all three wind directions with a horizontal roof; however, some more intensively ventilated zones occur at the highest seating tiers all around the spectators’ terrace. This is due to the mutual position of the spectators’ terrace and the roof that results in a reduced available flow cross section along the upper edge of the bowl [13]. A rising roof by +151 results globally the same kind of flow paths as a horizontal one. Nevertheless, its protecting capacity on the windward spectators’ terrace side is more accentuated than that of a horizontal roof. At the same time, intensively ventilated areas occur on the lateral upper rears. That indicates a more heterogeneous flow, i.e. the spread of the measured data is wider. Further increase of the roof slope (+301) has a flow intensifying effect especially for diagonal wind incidence. The surface of the most intensively ventilated zone has increased to a great extent; moreover, the highest c values exceed those of the previous case. A sloping roof towards the pitch accelerates the airflow globally on the spectators’ terrace for all three wind directions in comparison to a horizontal roof. The average c values are higher by 25–30% and also the c10% and average c ratios exceed those calculated for a horizontal canopy. The last indicates that the c values in the less intensively ventilated zones are closer to the average c values. In other words, the flow is more intensive and homogeneous. The surface of the most intensively ventilated zones has been tripled for diagonal wind incidence. The global ventilation of the stadium can be intensified by altering the roof position. A roof rising towards the pitch has less accentuated air flow intensifying effect than a sloping one. Comparing the average c values for all three wind directions, Fig. 12 shows that the horizontal roof seems to provide the most effective wind protection at the spectators’ terrace as the average c values are the lowest in this case. At the same time, the roof rising towards the pitch protects as effectively as the horizontal roof for longitudinal and transversal incidences. In case of diagonal

Average Ψ values in function of the canopy inclination 0.8 0.7 Average Ψ values

many directions, i.e. several dominant wind directions can be defined. However, in case of a well defined dominant wind(s), a direction-sensitive configuration can also be selected providing that care has been taken to its orientation.

0.6 0.5 0.4 0.3

diagonal

0.2

longitudinal transversal

0.1 0.0 -20

-10

0

10

20

30

40

Canopy inclination (°) Fig. 12. Influence of canopy inclination on the airflow. Air flow homogeneity in function of the canopy inclination 2.50 IH10% and IH90%

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diagonal

2.00

longitudinal

1.50

transversal diagonal

1.00

longitudinal

0.50

transversal

0.00 -20

-10

0

10

20

30

40

Canopy inclination

Fig. 13. Influence of canopy inclination on homogeneity. The points of higher values show the index IH90%, while the lower ones, the index IH10%.

wind incidence, the spectators’ terraces are globally more intensively ventilated with a roof rising towards the pitch. Besides the protecting character, the flow homogenising character of the stadium has to be taken into account when assessing its aerodynamic quality. The average c values do not provide sufficient information in themselves; they have to be completed by the homogeneity indices obtained as the ratio of the c90% or c10% and the average c values. In this way, the effect of extreme c values are also taken into consideration. For the first approach, the ratios of the extreme c values and the average c values of the investigated stadium configurations have been chosen, as indices of homogeneity. Fig. 13 illustrates the indices expressing the weighted average of the characteristic c values. The indices containing the last deciles (c90%) of the populations show a greater dispersion than those containing the first deciles (c10%). It indicates that the roof inclination has less important modifying effect on the homogeneity of airflow in less intensively ventilated zones than in the more intensively ventilated ones. Concerning the indices containing the last deciles (I90%), the flow is the most homogeneous in case of a horizontal

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Evolution of the average Ψ values in function of the façade porosity 1.2

Average Ψ

1.0 0.8 0.6 0.4 0.2 0.0 0%

10%

20%

30%

40%

50%

Façade porosity (%) Fig. 14. Average c values for the investigated wind incidences.

Ψ 90%values weighted by the concerned areas

25

diagonal 20

longitudinal transversal

15 I90%

roof and a roof sloping towards the pitch by 151. The values belonging to a roof rising towards the pitch by 301 are slightly higher than those of the horizontal roof. In addition, they show practically no sensitivity to wind incidence. However, a risen roof by 151 towards the pitch shows the greatest dispersion among the above presented configurations of the I90% values according to the wind incidence. It reaches its maximum for diagonal wind. The exterior roof overhang acts as a deflector that channels the airflow through the horizontal opening between the roof and the bowl. The overhang has a ventilation intensifying effect all over the spectators’ terrace which is even more intensive than that of the roof sloping towards the pitch, in particularly along the contour line of the stadium [14]. The surface of the less intensively ventilated zones has been significantly reduced. A roof with reduced central opening (oculus) has a significant protecting effect: similar to the roof rising by 151 it increases the area of the less intensively ventilated zones but to a greater extent. At the same time, some accelerated areas occur particularly on the upper rears of the corners. Both a roof with an overhang and a reduced oculus calls forth an increase in the roof surface. They both perturbate the homogeneity of the flow compared to the roof with the identical layout as the spectators’ terrace. The fac- ade porosity, in other words the ratio of the openings on the fac- ade, and the total fac- ade surface modify significantly the characteristics of the airflow in the spectators’ terrace. Two types of fac- ade opening have been investigated:

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10

5

0

 

concentrated or non-uniformly distributed (openings of great size, such as completely opened corners) and uniformly distributed (e.g. horizontal openings on the bowl running all-around the fac- ade, without interruption, or uniformly distributed vertical openings dividing the spectators’ terrace into vertical sections).

The investigated uniformly distributed openings are either covered by a porous wind screen or completely open. The fac- ade porosity and the average air flow characteristics (described by the average c values) show a good correlation for the three studied wind directions. The intensity of the airflow increases with the porosity—the tendency is well illustrated in Fig. 14. It is supposed that beyond a porosity rate between 50% and 100%, the curve has an inflexion point. Leaving the inflexion point, the curve begins to near to an asymptote whose theoretic c value represents that obtained for a stadium with an infinitesimally thin roof and a fac- ade of 100% porosity (absence of fac- ade). The theoretic c value equals to that obtained without the presence of the stadium [15]. Characterising the homogeneity with the index expressing the weighted average of the last decile of the c values (I90%), a definite maximum occurs at around 20% of

0%

20% 40% Façade porosity (%)

60%

Fig. 15. I90% in function of fac- ade porosity.

porosity due to the large associated spectators’ terrace surface (Fig. 15). The I90% homogeneity index has a high value if the concerned area is large, or if the c90% is high. Both could be the measure of discomfort. If the concerned area is small, local treatment (e.g. windbreak) could be employed; however, if the concerned surface is large, it is preferable to chose another variant. The areas concerned by the extreme c values and related to the entire bowl area are featured by the average c value. The effect on the airflow of the concentrated openings has also been tested on two variants. The first represents an ‘‘English stadium’’ [16] a completely open-cornered stadium (Geoffroy-Guichard Stadium in St. Etienne France). The second has narrow openings on the corners that serve as pitch-level entrances (e.g. Auguste-Delaune Stadium in Reims France). In both cases, the mutual position of the different spectators’ terrace parts induce anomaly like local effects, similarly to urban areas where narrow passageways, corners of high-rise buildings too close to each other

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call forth gusty accelerations [17] due to Venturi effect. Some extremely accelerated zones at the corners appear in the investigated open-cornered configurations. The airflow modifying effects of different architectural parameters cannot be considered favourable or less favourable from spectators’ aerothermal comfort point of view till the location of the stadium is not known. The adequate configuration has to be chosen considering the nature of the climate. 6. Transposition of the results 6.1. Meteorological data During the parametric wind tunnel tests, the effects of the characteristic architectural parameters on the airflow at the spectators’ terraces have been investigated in order to determine the prevailing airflow features. For the spectators’ comfort assessment, to be carried out at the design phase of the project, besides the wind tunnel results, the following geographic, topologic and climatic data of the given stadium location also have to be used: (1) Geographic data, namely the longitude and latitude are needed in order to define the localisation of the stadium site. (2) The localisation of the meteorological station relative to the stadium site and the roughness of the close surroundings have to be defined. It has to be checked that no topographic element influences the character of the wind at the meteorological station. It has to be also checked that the roughness around the meteorological station is smooth and corresponds to a farmland-type wind (Class II according to the Eurocode). No neighbouring parasite construction should interfere with the meteorological sensors [1]. (3) Hourly wind data of the stadium site classified by sector are needed for the predefined period of use, including the length of the period with no wind. The hourly distribution of the outdoor temperature data and monthly values of the predefined period of use, the last completed by the monthly solar radiation values, are needed for the aerothermal comfort assessment. The monthly solar radiation value signifies the daily sum of the global irradiation falling on a horizontal plane for a representative day of the month. In order to treat, at a qualitative level, the effect of the relative humidity on thermal comfort, some data of informing character are needed.

affected by the airflow characteristics at the spectators’ terraces, as the wind is one of the environmental factors that can be altered to a great extent by the stadium morphology. After the selection of the appropriate stadium configurations from functional, security and structural points of view, their air flow characteristics have to be examined and compared to each other. In principle, those architectural configurations are preferred that provide more accentuated wind protection at the spectators’ terrace and are more homogeneously ventilated. A configuration presenting unfavourable airflow characteristic for one wind direction should not be excluded from the analysis, as for a given location the occurrence frequency of the wind from the ‘‘critical’’ direction might be low, moreover negligible. However, this criterion can only be dealt with when the orientation of the building has been defined. ‘‘The underlying principle of the stadium orientation is that runners in athletics and sportsmen in ball games should never have the late afternoon sun in their eyes.’’ [18] In consequence, the ideal orientation of a multifunctional stadium is a longitudinal axis running north–south. Fig. 16 indicates the acceptable range of axis orientation for different sports. After having determined the orientation of the stadium based on the functional, security and structural aspects, the stadium axes (longitudinal, diagonal and transversal) have to be collated with the compass directions as shown in Fig. 17. (The directions are marked in Fig. 17. by their first letters, D signifies diagonal, L longitudinal and TR transversal wind direction.) The wind data are usually presented by sectors of 201, which means altogether 18 sectors. In this manner, some sectors contain the stadium axes, while others not. However, the regression functions facilitate the interpolation of the characteristic c values for each wind sector. The sectors containing an axis inherit the characteristic c values belonging to the different stadium axes, while the characteristic c values of those with no axes are calculated using the data of the neighbouring sectors and the regression functions. The wind rose overlapped by

6.2. Preliminary choice of configurations for a given site The adequate stadium configuration(s) for a given geographical location are preselected, based on the function of the stadium, the security aspects (e.g. evacuation, separation of the fans supporting the opposing teams) and structural aspects. Beside these features, the choice is

Fig. 16. Recommended pitch orientations. A—best common axis for most of the sports; B—range acceptable for rugby and football; C—best range for track, field and pitch games [18].

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2237

Fig. 17. Wind rose and stadium plan overlapped.

the stadium plan is an expressive presentation of the dominant wind directions. When analysing the aerothermal and comfort in case of low temperatures, it is sufficient to treat the sectors where strong winds occur with a great frequency. However, in case of a complete aerothermal comfort analysis, all sectors have to be investigated, even that of the light winds; furthermore the periods of calm have to be dealt with. The periods with no wind are as significant as those with wind, particularly in warm climatic regions or in summer period in moderate climates, as the absence of air movement might lead to aerothermal discomfort and even to heat disorders by the spectators. This is well illustrated by the computer simulations predicting human thermal responses, carried out on the Stadium Australia with semi-transparent and opaque roofs [19]. The results have shown that under a semi-transparent roof, the spectators could be exposed to severe thermal conditions, more precisely to very high air temperature—especially those on the upper seating tiers where the high temperatures are coupled with restricted air movement due to the relative position of the roof and the bowl. 6.3. Wind data treatment The wind tunnel tests’ results are represented in the form of the coefficient c that expresses the average wind speed and the relative standard deviation at a point i on the spectators’ terrace, related to those of the reference point. The latter is situated at 2 m height, at a free area in front of the stadium where the airflow is not disturbed by the surrounding objects. The data of the closest meteorological station have to be transferred and recalculated to the stadium site in order to

facilitate the comfort calculations. In that case when no representative meteorological station exists, a prolonged in situ meteorological study has to be carried out [1]. At the meteorological station, the average wind speed is measured at 10 m height ðU 10 meteo Þ, and simultaneously at a z1 chosen height ðUz1 meteo Þ. The z1 has to be placed high enough to be independent from the roughness of the site. The simultaneous measure of the wind speeds at the heights facilitates to define experimentally the transfer coefficient, K1, between the average wind speed at the height z1 and 10 m: Uz1 meteo ¼ U z1 site ¼ K 1 U 10 meteo .

(4)

However, in practice, the average wind speed on the site at the height z1 is more often calculated using the wind profiles defined by the Eurocode: U z1 ¼ k ln

z1 U 10meteo , z0

(5)

where k is the roughness parameter and z0 is the roughness height. In this way, the c coefficient can be expressed in function of the wind speed at the meteorological station and in function of the site characteristics where the stadium is to be built: Wi ¼

U i þ si , U ref þ sref

(6)

where Ui is the average wind velocity at a point i, si is the standard deviation at a point i, Uref is the average wind velocity at the reference point and sref is the standard deviation at the reference point.

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The c coefficient can be defined by using the turbulence intensity: sref I ref ¼ , (7) U ref where Iref is the turbulence intensity at the reference point; U i þ si U i þ si ¼ U ref þ sref U ref ð1 þ I ref Þ U i þ si U i þ si ¼ ¼ . zref k ln z0 U 10 meteo ð1 þ I ref Þ U 10 meteo C

Wi ¼

ð8Þ

When assessing the aerothermal comfort of spectators, the characteristic c values together with different wind speed intervals on the meteorological station are related to different air velocities (sum of average velocity plus standard deviation) on the spectators’ terrace. In other words, the c values are relative to the reference wind, they are meaningless until a wind speed, a direction and site category is associated with them. Using the average c value, the average airflow conditions those the spectators’ are exposed to can be calculated for different wind directions and wind speed intervals at the meteorological station. Taking the ‘‘extreme’’ c values, the characteristic average wind speed and standard deviation sums encountered in the most and the least intensively ventilated zones of the spectators’ terrace can be calculated. The wind tunnel data can be recalculated for any terrain category (Fig. 18). 6.4. Comfort–discomfort frequency calculation During the comfort–discomfort frequency analysis, both the climatic data of the site and the building specific airflow data—in form of characteristic c values—are treated. A table containing these data illustrated by Fig. 19 has been established in order to facilitate the spectators’ aerothermal comfort assessment. On one hand, it contains the temperature (DBT) and the wind data measured at the meteorological station at 10 m height ðU 10 meteo Þ, and furthermore, the coefficient C (Eq. (8)) that links the latter to the reference wind ðU ref þ sref Þ. On the other hand, the table integrates the stadium-specific airflow data. The latter

consist of the characteristic c values, the area concerned by these values and the encountered average characteristic airflow conditions of the concerned zone of the spectators’ terrace in form of the average wind speed and its standard deviation sum ðU þ sÞ. The table combines i outdoor temperature intervals with k wind speed intervals belonging to j wind sectors. It has j+1 sectors; the +1 signifies the dead calm, in other words the time period with no wind. Each climatic element of this table can be characterised by an occurrence frequency during the defined period of use: the probability of the concomitance of a Pk wind speed interval frequency from a given sector of Pj frequency by a Pi outdoor temperature interval frequency is represented by their product: PiPjPk. In this way, each cell of the table corresponds to the frequency of the coinciding occurrence of given temperature, a given wind speed and a given sector. The extent and the number of the temperature and the wind speed intervals are of informing character, they can be varied in function of the precision of the analysis. If there is no correlation between the wind characteristics and the outdoor temperature, the occurrence frequency of each temperature interval is equal within each wind speed interval for each sector. In this manner, the temperature occurrence frequency (Pi) values are identical within each temperature column of the table. This signifies that the occurrence frequencies of the different wind speed intervals within different sectors are identical within all temperature intervals belonging the same wind sector and a wind speed interval, i.e. the PjPk products are equal in all cells within a wind speed interval for each sector. If the wind data represent some regular characteristics due to mezo- or microclimatic phenomenon (sea–continent, mountain–valley, etc.), they have to be confronted to the period of use. It cannot be excluded that the occurrence frequency of the different outdoor temperature intervals and those of the wind are correlated, i.e. they are not independent random variables. In this case, the correlation has to be taken into account, thus the PiPjPk products are to be adjusted to the wind-temperature coincidence frequency. A table of that kind is to be prepared for each characteristic c value, namely for the average c value

Fig. 18. Wind data transfer from the meteorological station to the stadium site.

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2239

Number and name of the stadium configuration : C value : Wind sector (°)

j=1 0 20

j=2 20 40

j=3 40 60

j=19 No wind

Wind speed at meteorological Characteristic  value e. g. station: Ψ U 10 méteo (m/s) k=1 0 2 k=2 2 4 k=3 Ψ1 4 6 etc. k=1 0 2 k=2 2 4 Ψ2 k=3 4 6 etc. k=1 0 2 k=2 2 4 Ψ3 k=3 4 6 etc. k=0 0 0

Characteristic wind conditions on the spectators' terrace (m/s): U + 

Concerned spectators' terrace area (%): A

Dry Bulb Temperature (°C) (-8) (-4) i=1

(-4) 0 i=2

0 4 i=3

4 8 i=4

( U +  )11

Pi1 · Pj1 · Pk1

Pi2 · Pj1 · Pk1

Pi3 · Pj1 · Pk1

Pi4 · Pj1 · Pk1

( U +  )12

Pi1 · Pj1 · Pk2

Pi2 · Pj1 · Pk2

Pi3 ·Pj1 · Pk2

Pi4 ·Pj1 · Pk2

Pi1 · Pj1 · Pk3

Pi2 · Pj1 · Pk3

Pi3 · Pj1 · Pk3

Pi4 · Pj1 · Pk3

Pi1 · Pj2 · Pk1'

Pi2 · Pj2 · Pk1'

Pi3 · Pj2 · Pk1'

Pi4 · Pj2 · Pk1'

Pi1 · Pj2 · Pk2'

Pi2 · Pj2 · Pk2'

Pi3 · Pj2 · Pk2'

Pi4 · Pj2 · Pk2'

Pi1 · Pj2 · Pk3'

Pi2 · Pj2 · Pk3'

Pi3 · Pj2 · Pk3'

Pi4 · Pj2 · Pk3'

Pi1 · Pj3 · Pk1''

Pi2 · Pj3 · Pk1''

Pi3 · Pj3 · Pk1''

Pi4 · Pj3 · Pk1''

Pi1 · Pj3 · Pk2''

Pi2 · Pj3 · Pk2''

Pi3 · Pj3 · Pk2''

Pi4 · Pj3 · Pk2''

Pi1 · Pj3 · Pk3''

Pi2 · Pj3 · Pk3''

Pi3 · Pj3 · Pk3''

Pi4 · Pj3 · Pk3''

Pi1 · Pj19 · Pk0

Pi2 · Pj19 · Pk0

Pi3 · Pj19 · Pk0

Pi4 · Pj19 · Pk0

( U +  )13

A1

( U +  )21 ( U +  )22 ( U +  )23

A2

( U +  )31 ( U +  )32 ( U +  )33

0

A3

A19

Comfortable period relative to the length of the total predefined period of use (%):

Fig. 19. Comfort–discomfort calculation table for independent random variable DBT and wind data.

ðCÞ, the first (c10%) and the last (c90%) deciles. Each characteristic c value varies in function of the wind direction. Their value is defined by interpolation using the interrelations obtained through the wind tunnel tests. At the same time, the encountered average characteristic airflow conditions on the spectators’ terrace featured by the average wind speed and its standard deviation sum, belonging to identical characteristic c values, are different for each wind speed intervals. This calculation carried out for the average c value facilitates the characterisation of the average airflow conditions on the entire spectators’ terrace, while repeating for the first and the last deciles, it allows to check and ponder the expected ‘‘extreme’’ airflow conditions, i.e. intensively ventilated or very calm zones. The concerned area of the spectators’ terrace, similarly to the characteristic c value, varies in function of the wind direction, too. For each cell, the relevant air movement is calculated as a function of the incidence and the velocity of the reference wind, using the relevant c value. The air movement and the coinciding temperature are compared with the borders of the comfort zone. It can be stated whether the parameters of the given cell meet the aerothermal comfort conditions or not. With regard to the discomfort, the following sub-categories are distinguished:



wind discomfort due to mechanical effect (excess of the mechanical threshold, 3.6 m/s) with no thermal discomfort (zone A in Fig. 20);

v

C 3.6

"Minimal" threshold

"Maximal" threshold

(m /s)

A

E

B "Mechanical" threshold

3.0 2.0

COMFORT ZONE 1.0 D

4

26

33

DBT (°C)

Fig. 20. Comfort zone outlined by different sorts of discomfort areas (marked by A, B, C, D and E).

   

thermal discomfort due to the coincidence of low temperature and intensive air movement without exceeding the mechanical threshold (zone B in Fig. 20); simultaneous thermal and wind discomfort due to the coincidence of low temperature and intensive air movement (zone C in Fig. 20); thermal discomfort due to high temperature and lack of satisfactory air movement (zone D in Fig. 20); simultaneous thermal and wind discomfort due to the coincidence of high temperature and intensive air movement beyond about 37 1C or wind speed exceeding

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3.6 m/s but below the minimum velocity required to maintain the thermal equilibrium (zone E in Fig. 20). Repeating the comparison for each cell and summarising the occurrence frequencies of cells of comfort and discomfort, the lengths of periods of comfort and those of different types of discomfort can be determined for the season. The aerothermal quality of a stadium configuration is judged more favourable, if the length of the calculated comfortable period relative to the total predefined period of use is longer. 7. Conclusion The estimation of the spectators’ comfort in early stage of design is of prevailing importance. Remedy of faults, incorrect decisions is expensive and rarely can be complete. The proposed method facilitates to characterise a stadium configuration with a few dimensionless numeral values. The series of parametric wind tunnel tests proved that regression functions can be obtained between the geometric parameters, the incidence of wind and the aerothermal comfort characteristics. Using these functions, other but measured configurations can be evaluated, too, interpolating or—up to a limit—extrapolating the measured data. Based on these results as a first step, adequate stadium configurations can be preselected, pondering on one hand the protecting and homogenising character of different morphologic solutions, and on the other hand the climatic features of a given building site. The preselected configurations can be further evaluated transposing the wind tunnel tests’ results to the building site. For the spectators’ terrace and its ‘‘extreme’’ zones, the occurrence frequency of the periods of comfort and discomfort can be calculated. These numeral values provide a safe background to the final decision of the designers and builders in the conceptual stage of the project. References [1] Gandemer J, Guyot A. Inte´gration du phe´nome`ne vent dans la conception du milieu bati—Guide me´thodologique et conseils pratiques. La Documentation Francaise, 1976.

[2] Arens E, Gonzalez R, Berglund L. Thermal comfort under an extended range of environmental conditions. ASHRAE Transactions 1986;92(Part 1):18–26. [3] Auliciems A, Steven Vajk Szokolay SV. Thermal comfort. PLEA Note, Queensland, published by Passive and Low Energy Architecture International in association with Department of Architecture, The University of Queensland, 1997, 61pp. ISBN:0-86776-729-4. [4] Steven Vajk Szokolay SV. Environmental handbook. London: The Construction Press; 1980. [5] Moreau S. Caracte´risation et de´veloppements ae´rodynamiques de l’espace interme´diaire en climat tropical humide: conception d’une architecture de confort adapte´e a` la contrainte cyclonique. PhD thesis, 1999. [6] Deval J-C, Berger X. Le confort en climat chaud. Se´minaire REXCOOP ‘‘habitats climatiques’’, Laboratoire d’Ecothermie Solaire CNRS, 1983 28pp. [7] Spagnolo J, de Dear R. A field study of thermal comfort in outdoor and semi-outdoor environments in subtropical Sydney Australia. Building and Environment 2003;38:721–38. [8] Szucs A. Stadia in the environment—environment in stadia. In: Proceedings of the 21st international conference of passive and low energy architecture (PLEA), Eindhoven, The Netherlands, 2004. p. 169–74. ISBN:90-386-1636-8. [9] Blackmore AP. Review of the physiological and psychological effects on the outdooe climate on pedestrian comfort. In: Proceedings of the workshop impact of wind and storm on city life and built environment, Nantes, France, 2002 p. 60–9. [10] Weber W, Lachal B, Drexler H, Gallinelli P, Gonzalez D, Butera F. Pascool Electronic Metahandbook (P.E.M), Passive cooling design guidelines for Mediterranean climate & countries, Version 1.2. 1995. [12] Gandemer J, Guyot A. La protection contre le vent. Ae´rodynamique des brise-vents et conseils pratiques. 1981 132pp. [13] Szucs A., Allard F., Moreau S. Influence of canopy on airflow in stadia. In: European and African wind engineering conference, Prague, Czech Republic, 2005. [14] Szucs A, Allard F, Moreau S. Effect of roof on thermal and visual comfort of spectators in stadia. COTEDI—IV Congreso Latinoamericano Sobre Confort y Comportamiento Termico de las Edificiones, Mexico City, 2005. [15] Szucs A, Allard F, Moreau S. Effect of fac- ade porosity on air flow in stadia. In: Seventh Nordic building physics symposium, Reykjavik, Iceland, 2005. [16] Vigneau F. Les espaces du sport. Que sais-je, Presses Universitaires de France, 1998. ISBN: 2-13-049432-3. [17] Allard F. Natural ventilation in buildings. A design handbook. James & James (Science Publishers) Ltd.; 1998 352pp. [18] John G, Rod S. Stadia. A design and development guide. Oxford: Architectural Press; 2000 267pp. [19] Fiala D, Lomas KJ. Application of a computer model predicting human thermal responses to the design of sports stadia. In: CIBSE ‘99, Harrogate UK, Conference Proceedings, 1999 p. 492.