the impact of multi storey car parks on wind …€¦ · the impact of multi storey car parks on...

International Journal on Architectural Science, Volume 3, Number 1, p.30-42, 2002

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THE IMPACT OF MULTI STOREY CAR PARKS ON WIND PRESSURE DISTRIBUTION AND AIR CHANGE RATES OF SURROUNDING HIGH RISE RESIDENTIAL BUILDINGS IN SINGAPORE N.H. Wong, H. Feriadi, K.W. Tham, C. Sekhar, K.W. Cheong and K.Y. O Department of Building, School of Design and Environment, National University of Singapore Singapore 117592 (Received 30 November 2001; Accepted 6 February 2002) ABSTRACT This study investigates the impacts of three different types of car park namely open (surface), multi-storey and integrated car park on the wind pressure distribution as well as the air change rates of the surrounding high rise residential buildings in Singapore. 1:200 scaled models were used in wind tunnel to gather pressure distribution data on the exterior surface of the surrounding housing blocks. Utilizing the pressure coefficient (Cp) values obtained, the air change rates (ACH) in the selected units at different heights were calculated using CONTAM96. The study shows that the air change rates of the units are governed not only by the spacing of the residential blocks but other influential factors such as the block layout relative to the prevailing wind directions, architectural form of the housing block as well as the window orientation. 1. INTRODUCTION About 86% of the population in Singapore is staying in high-rise public housing, which utilize natural ventilation. Rapid development has changed the condition of their living environment. With the higher income per-capita and better standard of living, the number of vehicles has been increased significantly. Multi-storey car parks have been adopted in the past ten years to provide more parking area [1]. It is believed that the presence of new car park buildings has affected the natural ventilation performance of the surrounding housing units. In the 60’s, Housing Development Board (HDB) first started to allocate space for Surface Car Parks (SCP) adjacent to the residential blocks. But with the increasing number of personal vehicles, HDB had to further expand their parking facilities. In the 90’s, HDB introduced Multi-Storey Car Parks (MSCP) to meet the increasing demand. This is usually detached from residential blocks but within the same development boundary. Recently, HDB has adopted the idea of Integrated Car Parks (ICP) to achieve a higher Gross Plot Ratio (GPR) and thus responds to the land saving policy of the government. Under this ICP concept, car park and residential blocks have been closely integrated (no setback is required) and spacing between blocks are reduced (see Fig. 1). Undoubtedly, the concept of ICP could save the land, but on the contrary, it may lead to some

impacts on environmental quality of the surrounding residential blocks especially for their natural ventilation performance and indoor air quality (IAQ). The objectives of this study are as follows: • To investigate the impacts of three types of car

parks (SCP, MSCP and ICP) on wind pressure distribution and air change rates of the surrounding high rise residential buildings.

• To analyse the design factors that affect the air change rates of the surrounding residential buildings.

2. METHODOLOGY The wind pressure distribution on a building’s envelope is usually represented by a dimensionless pressure coefficient (Cp). It is the ratio of the surface dynamic pressure to the dynamic pressure in the undistributed flow pattern measured at a reference point. It is a parameter empirically derived to account for the variations in the wind-induced pressures caused by the influence of surrounding obstructions on the prevailing characteristics [2-4]. Thus Cp values are useful for evaluating the impact of the surrounding obstructions on the wind pressure distribution on the façade of the buildings. They can also be utilized to compute the air change rates (ACH) of the selected units using a typical multi-zone network model such as CONTAM96 software [5].

International Journal on Architectural Science

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In this study, an atmospheric boundary layer wind tunnel located at the Wind Tunnel laboratory in National University of Singapore (NUS) was used [6]. The wind tunnel has a dimension of 1.75 m high, 3.75 m wide and 17.00 m long. This type of wind tunnel is an open circuit and short type consisting of ten 750 mm diameter fans at the bell-mouth shaped inlet to generate airflow into the wind tunnel. The arrangement of roughness elements includes honeycomb layer flow dispersion, eight tall spires, tripping fence and wood blocks roughness elements spaced equally apart on all sides. All roughness elements were 100 x 100 x 50 mm. They create a long, rough upwind fetch to generate a turbulent boundary layer. It provides a wind velocity with a flow generated from a rough boundary to simulate the atmospheric wind flow in a built up urban area. Fig. 2 shows a sectional view of the wind tunnel in NUS. Utilization of wind tunnel provides data at several levels. Not only two but also three-dimensional impressions of airflow can be obtained. In addition wind tunnel studies are sensitive to minor details in buildings. The greatest advantage of wind tunnels is that the model responds quickly to multiple variations and numerous factors and can be easily modified. 2.1 Models Used For this study, three sites with ICP, MSCP and SCP, that consist of similar building layout and height with 17 storey residential blocks were selected. The schematic layouts of all the three sites are shown in Fig. 1. The wind tunnel test for MSCP and SCP shared the same scaled-model with the only variation of a car park building in the middle. Model ICP is relatively different from the other

two models since it has closer block spacing and different architectural form for the housing blocks. All the models used for pressure measurement were completely sealed and equipped with pressure taps. These correspond to the scale of 1:200 based on the exterior dimensions. The sizes of the models were selected to be large enough to maximize the internal Reynolds number but small enough to limit the maximum wind tunnel blockage and allow adequate modeling of the boundary layer. A SETRA type 239-pressure transducer was the primary means for generating pressure data. Stainless steel tubings with gross diameter of approximately 1.6 mm were incorporated in the models. The pressure port tubing was connected to the pressure transducer by means of vinyl tubings. Each pressure tap fixed to the sealed models was connected via a 0.6 m (approximately) long, 1.6 mm diameter vinyl tube to a rotary valve that allowed each tap to be subsequently connected to a single pressure transducer. The pressure at each tap of the building model was measured simultaneously with the reference dynamic pressure at the Pitot tube measured at the roof-top of a residential block. 48 channels were scanned with a sample size of 25 in order to capture the best average Cp value. A high wind speed of approximately 6 ms-1 for all pressure tests was used due to the sensitivity of the 239-pressure transducer (see Fig. 3). The models were tested for four different wind directions, i.e. North, South, East and West. The pressure tap distribution was selected so that the surface pressure could be measured over areas corresponding to the window openings on the models. Figs. 4 and 5 show the positions of the 48 pressure taps on each Model.

Fig. 2: Sectional view of wind tunnel in National University of Singapore

Honeycomb screen to straighten flow

Fan

Tripping fence

Roughness elements of wooden blocks Circular turntable

testing area

Eight spires to increase boundary height

2.80 4.70

WIND

9.50


33

Fig. 3: Equipment set up for coefficient of pressure (Cp) measurement

Fig. 4: Locations of pressure taps, points 1-48 (Block A, B, C, D, E and F) for ICP 3. RESULTS AND DISCUSSION 3.1 Effects of Wind Direction on Cp Values Fig. 6 shows the Cp values obtained for ICP for the four wind directions. The result shows that for the prevailing North and South wind directions, the highest Cp values obtained from the South wind are from points 21 to 24 (Block B) and points 13 to16 (Block C) with readings ranging from 0.58 to 0.77. For the North wind, the highest Cp values are obtained from Blocks A and E with readings ranging from 0.54 to 0.66. As the windows of the

surrounding housing blocks are mainly facing East and West (see Fig. 4), high Cp values are observed at Blocks C and D for the East wind direction and Block F for the West wind direction. Eventhough Blocks A and B are having windows facing North-South directions, they are enjoying high Cp values from the East and West wind. This is primarily due to the channel effect created in the spacing between the housing blocks. On the other hand, for Block E, it has low Cp values for West wind due to the obstruction of the wind caused by the projection of the block.

Reference point Computer

Data acquisition

Pressure transducer

Rotary scanivalve

Sealed Model

A

B C D

E F

1- 4

5 - 8

9 - 12

13 - 1617 - 20 21 - 24

25 - 28 29 - 32 37 - 40

41 - 44

45 - 48

ICP33 - 36

A

B C D

E F


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Section A-A shows the section of one of the typical blocks and the positioning of the pressure taps

Fig. 5: Locations of the pressure taps, points 1 to 48 for MSCP/SCP

MSCP

A

A

Living / Dining

Kitchen

Main Bed room

Bed room Bed

room

Balcony

Plan View of a typical unit

Car Park

2

8

3 7

6

4

5 1


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Figs. 7 and 8 show the Cp values obtained for the four different wind directions for MSCP and SCP respectively. The results show that the Cp values and distribution for the two car parks are very similar. In the prevailing North-South wind directions, the highest Cp values are points 37 to 40 (Block F) ranging from 0.84 to 0.96 in the South wind. For the North wind direction, the highest Cp values are points 13 to 15 (Block C) ranging from

0.52 to 0.62 and points 5 to 8 (Block D) ranging from 0.38 to 0.5. Eventhough the windows for all the units are facing the North and South directions (see Fig. 5), most of the blocks do not enjoy high Cp values. For example, Blocks A, B and E have low Cp values due to the blockages caused by the surrounding blocks as well as the projection of the housing blocks.

Cp Values of ICP

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1 5 9 13 17 21 25 29 33 37 41 45

Location

Cp

valu

es

North South East West

D C B A F E

Fig. 6: Cp values of ICP for the four wind directions

Cp values of MSCP

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1 5 9 13 17 21 25 29 33 37 41 45

Location

Cp

valu

es

West East South North

D C B A F E

Fig. 7: Cp values of MSCP for the four wind directions


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Cp values of SCP

-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1 5 9 13 17 21 25 29 33 37 41 45

Location

Cp

valu

es

West East South North

D C B A F E

Fig. 8: Cp values of SCP for the four wind directions 3.2 Evaluation of the Air Change Rates

(ACH) using CONTAM96 Since Cp values alone will not give a good indication of the natural ventilation performance of the units in the housing blocks, in order to study the effects of Cp values on the air change rates (ACH) of the surrounding residential units, a multi-zone model CONTAM96 was used. Cp values obtained from the wind tunnel experiment were converted to wind pressure using the power law equation. The wind pressure, in Pascals (Pa) is approximated by [7]:

2.C.

P2

pw

νρ= (1)

where ρ is the density (in kgm-3) and v represents the local wind velocity at a specified reference height (in ms-1). 3.2.1 Wind profile in Singapore

Singapore is located at approximately 1o 21” North, 103o 54” East. The proximity of the island to the equator, in addition to maritime influences, somewhat alters the characteristics of the monsoon climate experienced, making the wet and dry periods not so distinct. The temperature is relatively uniformed, with high humidity of up to between 50% to 90% and mean monthly temperature variation not more than 1.1oC from the mean annual temperature of 26.6oC and the average diurnal variation is 7oC. The mean daily maximum and minimum temperature are 30.7o C and 23.7o C respectively. The average wind speed during the Northeast and Southwest monsoon periods range from 0 to 3 ms-1 and 0 to 2.5 ms-1 respectively.

During the changeover months of April and October, winds are light and variable in direction. The wind speeds for the four different wind directions were computed using the Singapore weather data [8]. Figs. 9 and 10 show the mean wind speed for the eight different wind directions and their respective percentage frequency. The reference wind velocity at the roof top level for each wind direction was then calculated using the following equation [9]: u / um = Kza (2) where u is the wind velocity at roof top of the building, um is the wind velocity measured at a height of 10 m at the weather station, K and a are constants depend upon the terrain and are given in Table 1.

Wind Direction and Mean Speed (average annual data for 18 years)

0.00

1.00

2.00

3.00

4.00N OR T H

N OR T H EAST

EAST

SOU TH EAST

SOU T H

SOU T H WEST

WEST

N OR T H WEST

m ean speed (m /s )

Fig. 9: Average wind speed for the eight wind directions in Singapore


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Wind Percentage Frequency (average annual data for 18 years)

0

5

10

15

20NORTH

NORTH EAST

EAST

SOUTH EAST

SOUTH

SOUTH WEST

WEST

NORTH WEST

Percentage Frequency

Fig. 10: Wind percentage frequency for the

eight wind directions in Singapore Table 1: Factors for determining mean wind

speed at different heights and for different types of terrain

Terrain K a

Open flat country 0.68 0.17

Country with scattered wind breaks 0.52 0.20

Urban 0.35 0.25

City 0.21 0.33 An urban terrain was selected for the computation. In the CONTAM96 simulations, each window opening was taken to be 2 m2 and was assumed to be fully opened. 3.2.2 Effects of wind directions on air change

rates

Figs. 11 to 14 show the comparison of the air change rates at different floor levels for the three different car parks. In general, it is observed that ICP has the highest air change rates in the East wind direction. This is primarily due to the fact that most of the units have windows facing East-West directions. For the South wind direction, Blocks B and C of ICP also show consistently higher air change rates than the other two car parks. For the North wind direction, Block A of ICP also shows consistently higher air change rates due to the North-South facing windows. Block E also shows high air change rates due to the projection of the block that captures the wind from the North. For MSCP and SCP, though the windows of the units are facing the North-South directions, their air change rates are relatively low due to the blockage caused by the projections of the blocks (Fig. 15). For the North wind direction, only Blocks C and D of MSCP and SCP show higher air change rates than ICP. For the West wind direction, Blocks D and E show higher air change rates than that of ICP due to the U-shaped layout with opening facing the West wind direction.

Fig. 11: Comparison of air change rates at lower

floors for three types of car parks

North - 3rd/ 4th floor

0

20

40

60

80

LA LB LC LD LE LFBlock

Air

Cha

nge

Rat

e (A

CH

)

ICP MSCP SCP

South - 3rd / 4th floor

0

20

40

60

80

100

120

LA LB LC LD LE LF

BlockA

ir C

hang

e R

ate

(AC

H)

ICP MSCP SCP

West - 3rd/ 4th floor

0

10

20

30

40

50

60

70

LA LB LC LD LE LFBlock

Air

Cha

nge

Rat

e (A

CH

)

ICP MSCP SCP

East - 3rd/ 4th floors

0

20

40

60

80

100

LA LB LC LD LE LF

Block

Air

Cha

nge

Rat

e (A

CH

)

ICP MSCP SCP


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Fig. 12: Comparison of air change rates at 7th floor for three types of car parks

Fig. 13: Comparison of air change rates at 11th

floor for three types of car parks

South - 7 th floor

0

20

40

60

80

100

120

MA MB MC MD ME MFlocation

Air

Cha

nge

Rat

e (A

CH

)

ICP MSCP SCP

East - 7 th floor

0

20

40

60

80

100


Air

Cha

nge

Rat

e(A

CH

)

ICP MSCP SCP

East - 7 th floor

0

20

40

60

80

100


Air

Cha

nge

Rat

e(A

CH

)

ICP MSCP SCP

North - 11 th floor

0

20

40

60

80


Air

Cha

nge

Rat

e(A

CH

)

ICP M SCP SCP

South - 11 th floor

0

20

40

60

80

100

120


Air

Cha

nge

Rat

e(A

CH

)

ICP MSCP SCP

East - 11th floor

0

20

40

60

80

100

120


Air

Cha

nge

Rat

e(A

CH

)

ICP M SCP SCP

West - 11 th floor

0

20

40

60

80

MA MB MC MD ME MF

location

Air

Cha

nge

Rat

e(A

CH

)

ICP M SCP SCP

Air

Cha

nge

Rat

e (A

CH

)

A

ir C

hang

e R

ate

(AC

H)

Air

Cha

nge

Rat

e (A

CH

)

North -7 th floor

0

20

40

60

80

MA MB MC MD ME MF

location

Air

Cha

nge

Rat

e (A

CH

)

ICP MSCP SCP


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Fig. 14: Comparison of air change rates at higher floor for three types of car parks

Table 2 shows the comparison of the air change rates for lower floors between MSCP and SCP. From Table 2, it can be seen that for the West wind direction, MSCP has lower air change rates in most

of the blocks as compared to SCP. In general, an increase of 4% - 29% occurs when the car park building is removed to form SCP. When wind is directed to the car park, the larger space from the SCP allows a more uniform distribution of air to the units around the car parks, thus ensuring a better distribution of air to all units in the lower floors. On the other hand, MSCP has higher air change rates than SCP for the North wind direction. The channel effect created between the buildings and MSCP increased the ventilation rate of the units on the lower floors (see Fig. 16). 3.2.3 Effects of different heights on air change

rates

In order to have a better understanding of the effects of the car parks on the air change rates at different heights of the residential blocks, the average value of the air change rates at different heights are computed (see Table 3). For ICP, it is observed that for the North wind direction, the highest air change rate occurs at 7th floor. The presence of the ICP has diverted the wind upwards causing the air change rate at 7th floor to increase. For the South and East directions, the air change rates decrease at 7th floor followed by an increase as the floor height increases. For the West direction, the air change rates increase as the floor height increases. For MSCP, in general it is observed that the lower floor has the highest air change rates especially for the North, South and East wind directions. This is primarily due to the channel effect that occurred between the car parks and the residential blocks. However, for the West direction, the air change rate is lower than 7th floor since the presence of the MSCP has reduced the wind speed drastically, thus causing lower air change rate at the lower floor. For SCP, the air change rate is the highest at the lower floor for all the four wind directions. The absence of the multi-storey car park and the presence of void decks have caused the air change rate to be high. 3.2.4 Comparison among three models

Table 4 summarizes the comparison of the natural ventilation performance of the surrounding residential blocks at different heights for the different wind directions for the three types of car parks. From the results, it is obvious that ICP has the best performance. By forming a U-shaped layout with the opening facing the prevailing North-South wind directions, it forms a wind scoop to capture the wind. The orientation of the window openings on the East-West directions also results in high air change rates for the East wind directions.

South - higher floor

0

20

40

60

80

100

120

HA HB HC HD HE HFlocation

Air

Cha

nge

Rat

e(A

CH

)

ICP MSCP SCP

East - higher floor

0

20

40

60

80

100

120


Air

Cha

nge

Rat

e(A

CH

)

ICP MSCP SCP

West - higher floors

0

20

40

60

80


Air

Cha

nge

Rat

e(A

CH

)

ICP MSCP SCP

Nor th - h ighe r floor s

0

20

40

60

80

HA HB HC HD HE HFl o c a t i o n

IC P M SC P SC P


40

Fig. 15: Reduction of the air change rate in unit caused by the projection of the block

Fig. 16: Channel effect in the space between housing block and multi-storey car park

Table 2: Comparison of the air change rates for lower floors between MSCP and SCP

Locations A B C D E F MSCP (N) 37.01 30.60 63.70 62.64 30.21 29.66 SCP (N) 36.72 14.70 63.38 54.98 26.89 21.20

%100MSCP

SCPMSCP×

−

0.8%

52.0%

0.50%

12.2%

11.0%

28.5%

MSCP (W) 14.02 38.88 21.87 46.31 49.63 32.21 SCP (W) 14.67 54.92 26.68 38.92 55.30 34.31

%100SCP

MSCPSCP×

−

4.4%

29.2%

18.0%

-19.0%

10.3%

6.12%

Table 3: Average air change rates for the three car parks at different heights

Average air change rates (ACH) North South East West

ICP MSCP SCP ICP MSCP SCP ICP MSCP SCP ICP MSCP SCP

3rd 48.4 42.3 36.3 40.1 49.5 42.3 65.4 52.3 52.8 34.8 33.8 37.5

7th 49.5 36.6 31.5 39.9 47.8 42.0 50.9 43.5 45.0 36.6 35.0 36.9

11th 42.7 32.6 39.0 42.4 38.0 38.2 64.2 49.6 46.3 36.7 33.9 37.8

15th 41.8 30.2 25.0 47.0 39.1 39.7 65.0 34.7 37.2 38.7 28.3 29.4

WIND


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For the SCP, due to the formation of the U-shaped layout with the opening facing the West direction as well as the absence of the multi-storey car park, it has the best performance for the West wind direction. Comparing MSCP and SCP, the absence of the multi-storey car park has resulted in SCP having high air change rates for both East and West wind directions. However, for the North and South wind directions, the presence of the multi-storey car park has in fact resulted in higher air change rates for MSCP especially for low and middle floors due to the channel effect created between the residential blocks and the multi-storey car park. Fig. 17 shows the plot of the air change rates with height for the three types of car parks. The air change rates are computed by averaging the air change rates for the four wind directions, taking

into consideration the percentage frequency for each wind direction. The figure shows that ICP has the best air change rates for the different heights. As explained earlier, this is attributed to the proper arrangement of the building blocks forming a U-shaped layout with the opening facing the prevailing North-South wind direction. On the other hand, for the MSCP and SCP, the opening is facing the West direction and thus unable to optimise the prevailing North-South wind direction. Comparing the MSCP and SCP, it is interesting to see that at lower levels of the building blocks (up to 9 storey high), the air change rates for the MSCP is higher than SCP. This is primarily due to the channel effects created between the building blocks and the multi-storey car park. However, at higher floors (above 9 storey), the air change rates of SCP are higher.

Table 4: Comparison of natural ventilation performance of residential blocks of ICP, MSCP and SCP

ICP MSCP SCP Levels/Site N S E W N S E W N S E W W3rd/4th floor (low)

7th floor (middle)

11th floor (middle)

15th floor (high)

Legend:

Perform the best among the 3.

Comparing MSCP and SCP only.

30

32

34

36

38

40

42

44

46

48

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Height (Storey)

Air

Cha

nge

Rat

es (m

3/s)

ICP MSCP SCP

Fig. 17: Comparison of the air change rates for three types of car parks

Air

Cha

nge

Rat

e (m

3 s-1)


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4. CONCLUSION The impacts of multi-storey car park on the natural ventilation performance of the surrounding residential blocks were investigated using a combination of the wind tunnel testing and a multi-zone network model CONTAM96. The results show that the impacts of the car parks are determined not only by the distance between the residential blocks and the multi-storey car parks but also other influential factors such as the block layout relatively to wind direction, the architectural forms of the housing blocks as well as the window orientation in each units etc. In this study, the results have shown that ICP has overall better natural ventilation performance compared to MSCP and SCP despite the narrow spacing between the surrounding residential blocks and the multi-storey car park. Its proper block layout and window orientation as well as the simple architectural form configuration have contributed significantly in achieving higher ventilation performance of its units. ACKNOWLEDGEMENT The authors wish to acknowledge the research funding from NUS – R-292-000-033-112 and a sincere support of Mr. ABM M Rahman and Mr. Komari Bin Tubi throughout the test in wind tunnel laboratory. REFERENCES 1. Housing Development Board Annual Report,

Singapore (2001).

2. M. Grosso, “Wind pressure distribution around buildings: a parametrical model”, Energy and Buildings, Vol. 18, pp. 101-131 (1992).

3. S. Murakami and A. Mochida, “3-dimensional numerical simulation of turbulent flow around buildings using the k-ε turbulent model”, Building Environment, Vol. 24, No. 1, pp. 51-64 (1989).

4. D.A. Paterson and C.J. Apelt, “Computation of wind flows over 3D buildings”, Building Environment, Vol. 24, No. 1, pp. 39-50 (1989).

5. G. Walton, Contam96 user manual, NIST, USA (1997).

6. American Society of Civil Engineers (ASCE), Wind tunnel studies of buildings and structures, ASCE Manuals no 67. ASCE publishers. Virginia-USA (1999).

7. M.V. Swami and S. Chandra, “Correlations for pressure distribution on buildings and calculation of

natural ventilation airflow”, ASHRAE Transactions, Vol. 94, Part 1, pp. 243-266 (1988).

8. Singapore Meteorological Service (SMS) Annual Report on Singapore Weather 1982-2000, SMS Office, Singapore (2000).

9. B.S. 5925: Code of Practice for design of Buildings: Ventilation principles and designing for natural ventilation, British Standards Institution, London (1980).

the impact of multi storey car parks on wind …€¦ · the impact of multi storey car parks on...

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