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Climatic aspects in urban design—a case study
(n vi vt modal ver) Climatic aspects in urban design—a case study
Isaac G. Capeluto, , A. Yezioro and E. Shaviv
Faculty of Architecture and Town Planning, Technion—Israel Institute of Technology, 32000, Haifa, Israel
Received 11 December 2001; revised 13 February 2002; accepted 21 February 2002.; Available online 4 April 2002.
We present a case study of a design of a new business district in Tel Aviv city. In this work climatic aspects were taken into consideration in the very early design stages. For that purpose, two models SustArc (Proceedings of the ISES 1997 Solar World Congress, Taejon, Korea, 1997, p. 148) and FLUENT 5.0.2 (Fluent's User's Guide, Fluent Inc., NH, USA, 1999) were applied in order to achieve solar and wind rights. The new business district was designed as a high-density urban area and is located near an old low-rise residential quarter. SustArc was used as a design tool to create the solar envelope that shows the maximum available volume in which it is possible to build without violating the solar rights of existing residential neighborhood, the main avenues and the pedestrian sidewalks. FLUENT, on the other hand, was implemented as an evaluative tool, in a trial and error method, until a design solution could be achieved, in which the wind rights of the residential neighborhood were preserved, while ensuring tolerable winds inside the business district. The paper presents the process of sun and wind controlled planning, as well as the recommendations.
Author Keywords: Solar rights; Wind rights; Urban design; Design tools
2. Sun, winds and urban design
3. Planning control for sun and winds in a new business district
4. Planning control for sun access
5. Planning control for wind access and protection
6. Summary and conclusions
During the conceptual design phase of urban districts, the designer deals with different geometrical characteristics related to the building's height and width, in relation to the open spaces and the pedestrian sidewalks. New buildings may create a different microclimate, like changing the wind regime and shading of existing neighborhoods, as well as in the new district. To protect solar rights, as well as wind rights, is a complex task. Moreover, tolerable winds should be achieved along the pedestrian sidewalks. The determination of a preferable design solution becomes specially complicated due to mutual influences. On the other hand, ignoring the solar rights at the stage of the preparation of the master plan may cause discomfort conditions around the buildings beyond repair.
Different design tools for solar insolation conscious design were developed. We can classify these tools into generation tools and evaluation tools. The generative design tools aid to define the proper geometry. Some examples are [1, 2, 3, 4, 5 and 6] for determining solar rights. These tools generate nomograms that present all possible solutions to a given problem. These nomograms are called “Solar Envelopes”. The evaluation tools, on the other hand, analyze the performance of a given design. Some examples are Kroner and Abrey , Yezioro and Shaviv [8 and 9] and Capeluto and Shaviv  for evaluating solar rights for buildings and in open spaces among them. Heliodons are also used to evaluate the proposed design, namely a scaled down 3D physical model examined in the laboratory.
For microclimate and wind rights conscious design, there are today only evaluative design tools. These are either wind tunnel studies, or computational fluid dynamics (CFD) simulation tools. The CFD models are very powerful, require heavy calculations, but provide detailed results that can show clearly the defects in suggested designs. As a result, new design alternatives may be thought of and re-evaluated, until a good and satisfactory design is achieved.
In the design of the new business district in Tel Aviv (Fig. 1), we have used SustArc, as the design tool to evaluate the proposed design (Fig. 2). We have also used SustArc to create the solar envelope that shows the maximum available volume in which it is possible to build while keeping the solar rights of the existing neighborhood (Fig. 5). We used FLUENT to evaluate the existing situation, the proposed solution and the mitigation design, in which the wind rights to the residential neighborhood were preserved, while ensuring tolerable winds along the pedestrian sidewalks ( Fig. 8 and Fig. 9). The paper presents the design process along with the different design tools implemented to create the solutions and to simulate and evaluate the proposed design. Using these tools we could develop rules and design guidelines that ensure proper insolation and ventilation in the existing residential neighborhood as well as creating good microclimatic conditions inside the new business district.
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Fig. 1. An aerial view of the business district.
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Fig. 2. Sun-view presenting the shading of the main green avenue (the area adjacent to the tall buildings from the right) and the residential neighborhood (the long square on the right).
2. Sun, winds and urban design
There are many places in which urban design take into consideration solar rights and winds protection. Let us mention a few: New York, Boston, Chicago, Philadelphia, Pittsburgh and San Francisco, in the USA, Calgary, Edmonton, Halifax, London, Montreal, Ottawa and Toronto, in Canada . Many tall buildings were built during the past in all of the above cities. These tall buildings caused different problems, like shading, loss of daylight, and creation of strong winds around the tall buildings on one hand, and at the same time avoid good ventilation by creating wind stagnation at some parts around them. From the accumulating experience, the city leaders and designers recognized the need to control the changes in the microclimatic conditions created by a proposed design. In many cities, large projects, including tall buildings, require wind studies, as well as shading evaluation. Nevertheless, in most places, the planning control for wind protection and solar insolation are not mandatory and are not imposed by standards, but rather open to negotiations with the developers.
Defining urban standards can be carried on along three different approaches :
Prescriptive and descriptive standards, in which the exact physical solution is given. For example, the specific maximum height of buildings in the inner city neighborhood of San Francisco is dictated along with the angle of the slope of a plan that cut the upper floors further from the street.
Performance standards, in which the expected performance of the design is given. For example, Boston zoning ordinance dictates for some downtown areas, that “No net increase in shadow is permitted between 8 a.m. and 2.30 p.m.”. In San Francisco for example a performance standard is set for the maximum allowed wind velocity so that “a building form which causes wind speeds to exceed 11 miles/h in areas where people walk and 7 miles/h in areas where people sit, should not be used”.
Discretionary review, in which a comprehensive study is required as part of the environmental impact study (EIS) process. For example, in New York, sponsors of large development projects are required to conduct wind studies. The expected wind velocities in new or existing open spaces shall not exceed the mean wind velocity in existing comparable open spaces.
There are not yet urban standards and legislation in Israel about how much a building can shade neighboring buildings, open spaces, or what is the maximum allowed wind speed. As for solar rights, we were contracted to develop legislation  and we hope that our future recommendations will be imposed. However, in many cities, the Israeli ministry of the environment demands from every developer, who intends to build high-rise buildings, a discretionary review for winds and shading, as part of the EIS. As there are not yet urban standards, the results of the study are not always imposed on the project.
The shading study is usually an evaluation process, carried by different computer codes, while the wind assessment is in most cases a wind tunnel study. In the following chapter we shall demonstrate the approach to deal with planning control for sun access and wind access and protection  that was carried out in the design of a new business district in Tel Aviv.
3. Planning control for sun and winds in a new business district
A new business district is being planned in the heart of Tel Aviv on an area of 250,000 m2. The urban density was changed from 200% to 450%. As a result, the developers wish to build in the area many high-rise buildings 40 stories and above. Existing low-rise residential buildings that surround this new business district will be affected by the high-rise buildings (see Fig. 1).
The new master plan of this district was not approved yet, and the residents of neighboring communities can submit objections to the new plan, which they did. The designers of the Tel Aviv City planning department produced a 3D model of the site, in which they assumed that all developers would build the maximum allowed (a likely outcome). The model allows the visualization of the spatial drawbacks in the preliminary design. In particular, it was found that the new buildings create a high wall that would deprive the sun and winds (coming mainly from west) from the existing buildings (Fig. 2). Therefore, the designers of Tel Aviv City planning department decided to adopt certain rules for the design of this new business district so as to ensure sun and winds in the existing residential neighborhood.
The Tel Aviv climate is hot and humid, and the sea breeze helps in summer to bring about thermal comfort in open spaces, as well as indoors. Therefore, the proposed high-rise buildings should not block the sea breeze. Moreover, tall buildings can create strong winds at the foot of the buildings. This fact complicates the situation, as near tall buildings the wind velocity may change very fast from extremely strong wind to no breeze at all.
The sun in Tel Aviv is undesirable in summer but it can cause any open space and parks to be a very pleasant and enjoyable place to stay in winter. Therefore, permanent shading, even if needed in summer, compromises winter sun. A dynamic solution, like shading open spaces and sidewalks in summer by deciduous trees that supply winter insolation, is preferred. In general, at least one pedestrian sidewalk should be exposed to winter sun to provide thermal comfort in winter. The other sidewalk, which is shaded by the building in winter, can be protected from the summer sun by permanent shading devices, or by evergreen trees. On top of it, in Israel there is a requirement by law, for every residential unit, to have solar panels for hot water. It is mandatory, therefore, that these panels will be exposed to the sun the year around.
4. Planning control for sun access
“Solar Rights Envelope” defines the space of all possible solutions for the determination of a design that does not violate the solar rights of existing buildings and open spaces during a given period of the year (See Fig. 3). The model SustArc creates such an envelope [5 and 6].
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Fig. 3. The solar rights envelope.
In the design of the new business district, the use of solar envelopes was recommended to protect the solar rights. The requirement was to achieve solar access during the entire winter, between 8 a.m. and 3.00 p.m., in the residential neighborhood, as well as in the main avenue that is the only existing green open area. The solar envelope that fulfills the above requirement, as well as the obtained shape of the buildings under this envelope, are presented in Fig. 4.
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Fig. 4. The solar envelope that ensure solar rights in the existing residential neighborhood as well as in the main green open space.
Although the requirements were only to ensure solar access to the residential neighborhood, we added the demand that the main two avenues from west to east will be exposed to the sun during the same period. This is in order to ensure that the morning and afternoon walk from the railway station to work, is in the sun (see Fig. 6). On top of it, we required that the main inner street parallel to the main green avenue would have solar access during lunchtime from 12.00 to 13.00. These requirements will allow the people to enjoy walking in the sun to the two avenues (A and B in Fig. 6) that lead them to the main green avenue, to have lunch in the garden, or in the planned restaurants along the green avenue.
Fig. 5 presents the solar envelope that was accepted as design guidelines for the relocation or reshaping of the tall buildings in the business district. All buildings higher than this envelope (these are the buildings that can be seen above the net of the envelope) must be displaced to another location, or should be reshaped (see Fig. 6). This is a descriptive approach, in which all possible consistent solutions are given in advance. However, we mixed the descriptive approach with the performance one, by allowing some exceptions, as long as the shading caused by these buildings is not above a given standard. But, till such standard will exist, a discretionary review approach might be necessary.
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Fig. 5. The solar envelope that ensure solar rights in the existing residential neighborhoods as well as in the main avenues and streets.
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Fig. 6. The maximum allowed floors for each building, keeping solar rights in the existing residential neighborhoods as well as in the main avenues and streets.
Based on these design guidelines a new scheme was suggested by the city planners, that follows the solar envelope (see Fig. 7). Until now, few tall buildings have already been relocated and reshaped, so that they will not stick out from the given solar envelope.
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Fig. 7. Design guidelines on building mass as proposed by the city planners. View from south–east.
5. Planning control for wind access and protection
Contrary to other cities, where the requirements were only to protect from high wind velocity around tall buildings, the demand in our case was to ensure good ventilation to the residential neighborhood located east to the business center. The situation today is, that in the business district, most of the buildings are seven floors high and are in a very bad physical condition. Therefore, the majority of the buildings should be demolished and replaced by new ones. The only exception is the first row of buildings near the freeway that are seven floors and new. Today, one can feel in the neighboring residential quarter the good breeze coming from the west. The proposed new tall buildings, thirty floors and above, may block the breeze. As a result, the residents of this quarter objected to the new design on the ground of wind rights. The question that was raised, therefore, was what should be the ventilation corridors inside the business district tissue, so that good natural ventilation will remain in the residential neighborhood, as well as in the new business district. This fact complicated the situation, as in many design alternatives the very solution for ensuring the breeze in the residential quarter, may cause excessive winds in the business district (see Fig. 9 and Fig. 10).
There are not yet design tools that can create the envelope of all possible solutions that satisfy wind requirements, or wind protection. Therefore, performance approach and evaluation technique were applied by using a CFD simulation model FLUENT 5.0.2 . We required the following: wind velocity in the main avenues and streets should be in summer at least 2 m/s in walking areas, and 1 m/s in sitting areas. In winter, wind velocity in the main green avenue should not be higher than 5 m/s in walking areas and 3.5 m/s in sitting areas. In other streets, where people move fast, it can reach up to 9 m/s . In the residential neighborhood, the breeze should be similar to what exists today.
FLUENT is a very powerful tool. It requires heavy calculations, but gives detailed results that can show the wind pattern in any plan or cross-section (see Fig. 9 and Fig. 10). We used FLUENT with the k– turbulence model, to evaluate the existing situation. We compared it with the proposed design (Fig. 9) and with the design based on the solar envelope ( Fig. 10). Many different design alternatives were proposed and evaluated, until a design solution was found, in which the wind rights of the residential neighborhood are preserved while ensuring tolerable winds inside the business district.
As the buildings are not yet designed, and for the master plan only general information about the mass of the buildings is required, we assumed simple shapes, and conducted parametric evaluation, in order to find the influence of each design option on the wind pattern. Also, as the simulations are CPU time intensive, we shorten the evaluation procedure by presenting in the same plan different widths for the ventilation corridors, as well as different widths for the north–south streets. In this way we could learn from the same run, what is the preferred width of the ventilation corridors.
Fig. 8 presents the standard meteorological wind measurements on site, 10 m above the ground. From this figure one learns that the desired winds in the hot seasons come mainly from the west and northwest and in general the wind velocity is about 3.5 m/s. To ensure wind rights, we carried simulations for these two directions. In all simulations we assumed a wind profile appropriate for the urban roughness and wind velocity of 3.5 m/s at the entrance.
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Fig. 8. The wind rose as measured in site.
Fig. 9 and Fig. 10 show few of the simulations results and only for winds coming from the west. In general, the wind coming from northwest gave better results than what is shown here. Fig. 9 and Fig. 10 top left present the existing situation; i.e. all buildings are seven floors high. The widths of the north–south streets are 16 and 36 m according to the existing situation and widths of the east–west streets are 36, 24 and 12 m. The latter are the ventilation corridors that should allow the sea breeze to reach the residential neighborhood. According to the wind pattern obtained, we required that the north–south streets should be at least 36 m in order not to have wind stagnation in the street. The ventilation corridors should be at least 24 m, preferable 36 m. For the 12 m wide ventilation corridor, the wind velocity in the residential neighborhood is too low, even in the existing situation of seven floors high buildings. Therefore, we defined the wind pattern examined area as the area east to the 24 and 36 m ventilation corridors only. Based on these recommendations ventilation corridors were designed and required by the city planners (see Fig. 11).
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Fig. 9. Simulating winds in the business district by using FLUENT: Top left—existing situation. Top right—proposed design. Bottom—parametric analysis.
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Fig. 10. Simulating winds in the business district by using FLUENT: Top left—existing situation. Top right—proposed design according to the Solar envelope section. Bottom left—parametric analysis: Seven floors buildings along the pathway. Bottom right—parametric analysis: Adding trees at the entrance of the ventilation corridor.
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Fig. 11. Ventilation corridors as requested by the city planners.
Fig. 9 presents the simulation results for the proposed design (top right) and parametric analysis for mitigation (bottom left and right). We can see that cutting the building in 45ฐ at the exit of the business district, improves the ventilation in both the main green avenue and the residential neighborhood (bottom left). On the other hand, changing the plan of the middle tower from square to round deteriorates the microclimate conditions.
Fig. 10 presents the simulation results for the proposed design according to the solar envelope section (top right) and parametric analysis for mitigation (bottom left and right). We can see that the design according the solar envelope preserves also the wind rights. However, the wind velocity along the east–west pathway is too high, and should be reduced. Adding seven floors high buildings along the 36 m wide pathway, reduces the high velocity wind speed in this pathway, but also reduces a little bit the wind velocity in the residential neighborhood. Adding trees at the entrance to the ventilation corridors reduces the wind velocity inside the ventilation corridors, and also in the residential neighborhood. However, the wind pattern obtained is quite satisfactory (bottom right).
6. Summary and conclusions
This work presents a case study in which, for the first time, the solar rights envelope was used in Israel for the design of a new business district in Tel Aviv, keeping solar rights in a high-density urban area. Using this solar envelope we could determine the maximum allowed heights of the buildings that ensure proper insolation in the existing residential neighborhood as well as in the new business district and the main green avenue. The requirement to build under the solar envelope is a prescriptive/descriptive approach.
To protect both, the solar and wind rights, the solar envelope was created first, this envelope was then evaluated using a CFD technique to ensure the wind rights and tolerable winds inside the business district. From the many simulations performed (only few of them shown here) we found that it is not easy to use the prescriptive/descriptive approach, as was done in the solar rights requirement. Only for the determination of the geometry of the ventilation corridors to ensure wind rights, such an approach can be applied. However, for the wind control, a standard performance approach should be applied. This is because the winds pattern depends on the exact geometry of all buildings around. Changing the geometry of one building can influence the wind pattern around other buildings. Therefore, performance standards should be established, and the wind pattern around the building should be evaluated against these standards, by certified tools and users.
This research was supported by the fund for promotion of research at the Technion. Research Number 022.732, and 022.751.
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Corresponding author. Tel.: +972-4-829-4013; fax: +972-4-829-4617
Building and Environment
Volume 38, Issue 6, June 2003, Pages 827-835
Result from Foreign Dictionaries (1 entries found)
From WordNet (r) 3.0 (2006) [wn]:
n 1: a careful study of some social unit (as a corporation or
division within a corporation) that attempts to determine
what factors led to its success or failure
2: a detailed analysis of a person or group from a social or
psychological or medical point of view
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