Architectural Elements

2. The wind factor in air movement

The architectural design can ensure natural air movement through two principles.

• In the first, differences in wind velocity produce a pressure differential which results in air flowing from the higher to the lower air pressure region.
• In the second, air is warmed, causing convection, with the warm air rising and being replaced by cooler air. A cool draft is created in the space between the warm area and the cool-air intake opening. The rate of airflow caused by convection in buildings is determined by the difference in the level of openings, with greater airflow resulting from a greater difference in the heights of the openings. It is most important when the outside air is still and yet the interior requires ventilation to achieve comfort.

Both these principles have been used in architectural design and town planning in many ways using several innovations.

Air movement by pressure differential
An important concept in understanding how wind-generated pressure differentials produce air movement is "Venturi action," which is based on the Bernoulli effect. From Bernoulli's theorem, the pressure of a moving fluid decreases as its velocity increases. This concept can be used in a variety of ways to provide steady streams of air through buildings.
For indoor air movement caused by a pressure differential, the airflow is steadier in cases that depend more on the suction resulting from low air pressure than on the high air pressure caused by wind force. Obviously, a window or an opening will not create the desired air movement in a room unless an air outlet of some sort is also provided. Experience has shown that air movement is faster and steadier when the area of the openings on the leeward side of a structure is larger than the inlets on the windward side.
Vents also can be used as outlets for hot air. The triangular vents are positioned on the wall just under the roof to evacuate hot air collected in the higher parts of the room by convection. The air passing through these outlet vents is then replaced with air drawn from cooler parts of the building.

The Claustrum

Often a multitude of small vents is preferable to a few large openings for purposes of privacy, security, uniform distribution of air flow, blocking of direct solar rays, and decoration. Large openings, used mainly for ventilation and lighting and set at specific places in the building, can then be filled with lattice work, in the form of a pierced screen wall. These lattices, called claustra, were originally used in large openings at high levels in the Roman baths.

In vernacular architecture, they generally are made in different decorative patterns of carved plaster plates, unlike the mashrabiya which are wooden. Claustra are mainly used to evacuate the hot air collected in the higher parts of the room, or in parapet walls, the low walls around roof edges, to produce drafts over people sleeping on the roofs in summer.

In modern architecture, claustra are sometimes used inappropriately over the entire facade of a building to serve as a brise-soleil. In fact, the claustrum is a screen to be set in an opening of proper size and should not be used as a bearing wall. In extending it beyond its frame and scale to cover an entire facade, the structural scale and aesthetic rules of architecture are disturbed.

Furthermore, when claustra are set at eye level, they annoy the eye with dazzling contrasts of light and shade, resulting from the inappropriate relative and absolute sizes of the solid and void lattice components and the lack of graduation caused by the rectangularity of the bars. When a claustrum is used as a brise-soleil, it shares with the latter many defects which are overcome by the mashrabiya.

However, the claustrum is effective at eye level in infrequently used indoor spaces, such as in a staircase wall, or in outdoor spaces, like courtyards or roofs, where the play of light and shade does not dazzle the eye when looking outward.

The wind-escape

The technique of using the suction caused by low air-pressure zones to generate steady air movement indoors is used in the design of the windescape. The funnel and side tube used to illustrate the Bernoulli effect or Venturi action are transposed into the structural elements of an architectural design in order to accelerate air movement and to create drafts in places with no exposure to the outside, such as basements .

This concept can be applied more advantageously in designs for use above ground. The wind-escape can accelerate effective ventilation and air circulation when used with other devices for air movement such as windows, doors, and the malqaf or wind-catch.

The malgaf

In hot arid zones, a difficulty is found in combining the three functions of the ordinary window: light, ventilation, and view. If windows are used to provide for air movement indoors, they must be very small, which reduces room lighting. Increasing the size to permit sufficient lighting and an outside view lets in hot air as well as strong offensive glare. Therefore, it is necessary to satisfy the three functions ascribed to the window separately.

To satisfy the need for ventilation alone, the malqaf or wind-catch was invented. This device is a shaft rising high above the building with an opening facing the prevailing wind. It traps the wind from high above the building where it is cooler and stronger, and channels it down into the interior of the building. The malqaf thus dispenses with the need for ordinary windows to ensure ventilation and air movement. The malqaf is also useful in reducing the sand and dust so prevalent in the winds of hot arid regions. The wind it captures above the building contains less solid material than the wind at lower heights, and much of the sand which does enter is dumped at the bottom of the shaft.

The value of the malqaf is even more obvious in dense cities in warm humid climates, where thermal comfort depends mostly on air movement. Since masses of buildings reduce the wind velocity at street level and screen each other from the wind, the ordinary window is inadequate for ventilation. This situation can be corrected by using the malqaf.The malqaf is much smaller than the building facade and therefore offers less surface area to screen the malqaf of buildings downwind.

The Bãdgir

In Iran and the countries of the Gulf, a specific type of malqaf called the bãdgir was developed. It has a shaft with the top opening on four sides (occasionally only two), and with two partitions placed diagonally across each other down the length of the shaft to catch breezes from any direction. This shaft extends down to a level that allows the breeze to reach a seated or sleeping person directly.

A great advantage of the malqaf and the bãdgir is that they solve the problem of screening resulting from the blocking of buildings in an ordinary town plan. Several research centers have been working to develop the best configuration for the implantation of blocks of buildings, while avoiding screening of blocks by those upwind. But after six or seven blocks, no configuration will solve the problem of screening. The malqaf and the bãdgir, however, being smaller in size than the buildings themselves, do provide an effective solution.

When designing the malqaf and the bãdgir, it is important to determine the airflow pattern around the house, following the principles of aerodynamics, and to orient the inlet appropriately in the airflow. Generally, a building placed in the wind will create a zone of compression to the windward side and a low-pressure zone to the leeward side. This low-pressure zone continues a certain distance beyond the building, depending on the wind velocity, the faster the wind velocity, the shorter the low-pressure zone extends, because of eddies created on the leeward side which disrupt the smooth airflow pattern. For normal wind velocities, the length of the low-pressure zone can be taken to be five times the height of the building.

Architectural Elements

2. The wind factor in air movement

The architectural design can ensure natural air movement through two principles.

• In the first, differences in wind velocity produce a pressure differential which results in air flowing from the higher to the lower air pressure region.
• In the second, air is warmed, causing convection, with the warm air rising and being replaced by cooler air. A cool draft is created in the space between the warm area and the cool-air intake opening. The rate of airflow caused by convection in buildings is determined by the difference in the level of openings, with greater airflow resulting from a greater difference in the heights of the openings. It is most important when the outside air is still and yet the interior requires ventilation to achieve comfort.

Both these principles have been used in architectural design and town planning in many ways using several innovations.

Air movement by pressure differential
An important concept in understanding how wind-generated pressure differentials produce air movement is "Venturi action," which is based on the Bernoulli effect. From Bernoulli's theorem, the pressure of a moving fluid decreases as its velocity increases. This concept can be used in a variety of ways to provide steady streams of air through buildings.
For indoor air movement caused by a pressure differential, the airflow is steadier in cases that depend more on the suction resulting from low air pressure than on the high air pressure caused by wind force. Obviously, a window or an opening will not create the desired air movement in a room unless an air outlet of some sort is also provided. Experience has shown that air movement is faster and steadier when the area of the openings on the leeward side of a structure is larger than the inlets on the windward side.
Vents also can be used as outlets for hot air. The triangular vents are positioned on the wall just under the roof to evacuate hot air collected in the higher parts of the room by convection. The air passing through these outlet vents is then replaced with air drawn from cooler parts of the building.

The Claustrum

Often a multitude of small vents is preferable to a few large openings for purposes of privacy, security, uniform distribution of air flow, blocking of direct solar rays, and decoration. Large openings, used mainly for ventilation and lighting and set at specific places in the building, can then be filled with lattice work, in the form of a pierced screen wall. These lattices, called claustra, were originally used in large openings at high levels in the Roman baths.

In vernacular architecture, they generally are made in different decorative patterns of carved plaster plates, unlike the mashrabiya which are wooden. Claustra are mainly used to evacuate the hot air collected in the higher parts of the room, or in parapet walls, the low walls around roof edges, to produce drafts over people sleeping on the roofs in summer.

In modern architecture, claustra are sometimes used inappropriately over the entire facade of a building to serve as a brise-soleil. In fact, the claustrum is a screen to be set in an opening of proper size and should not be used as a bearing wall. In extending it beyond its frame and scale to cover an entire facade, the structural scale and aesthetic rules of architecture are disturbed.

Furthermore, when claustra are set at eye level, they annoy the eye with dazzling contrasts of light and shade, resulting from the inappropriate relative and absolute sizes of the solid and void lattice components and the lack of graduation caused by the rectangularity of the bars. When a claustrum is used as a brise-soleil, it shares with the latter many defects which are overcome by the mashrabiya.

However, the claustrum is effective at eye level in infrequently used indoor spaces, such as in a staircase wall, or in outdoor spaces, like courtyards or roofs, where the play of light and shade does not dazzle the eye when looking outward.

The wind-escape

The technique of using the suction caused by low air-pressure zones to generate steady air movement indoors is used in the design of the windescape. The funnel and side tube used to illustrate the Bernoulli effect or Venturi action are transposed into the structural elements of an architectural design in order to accelerate air movement and to create drafts in places with no exposure to the outside, such as basements .

This concept can be applied more advantageously in designs for use above ground. The wind-escape can accelerate effective ventilation and air circulation when used with other devices for air movement such as windows, doors, and the malqaf or wind-catch.

The malgaf

In hot arid zones, a difficulty is found in combining the three functions of the ordinary window: light, ventilation, and view. If windows are used to provide for air movement indoors, they must be very small, which reduces room lighting. Increasing the size to permit sufficient lighting and an outside view lets in hot air as well as strong offensive glare. Therefore, it is necessary to satisfy the three functions ascribed to the window separately.

To satisfy the need for ventilation alone, the malqaf or wind-catch was invented. This device is a shaft rising high above the building with an opening facing the prevailing wind. It traps the wind from high above the building where it is cooler and stronger, and channels it down into the interior of the building. The malqaf thus dispenses with the need for ordinary windows to ensure ventilation and air movement. The malqaf is also useful in reducing the sand and dust so prevalent in the winds of hot arid regions. The wind it captures above the building contains less solid material than the wind at lower heights, and much of the sand which does enter is dumped at the bottom of the shaft.

The value of the malqaf is even more obvious in dense cities in warm humid climates, where thermal comfort depends mostly on air movement. Since masses of buildings reduce the wind velocity at street level and screen each other from the wind, the ordinary window is inadequate for ventilation. This situation can be corrected by using the malqaf.The malqaf is much smaller than the building facade and therefore offers less surface area to screen the malqaf of buildings downwind.

Architectural Elements

Introduction

In designing and planning for the hot arid and warm humid zones, two of the main problems confronting the architect are to:

(i)ensure protection against heat ; and

(ii)provide adequate cooling.

The built environment produces changes in the microclimate. The configuration of buildings, their orientations, and their arrangement in space create a specific microclimate for each site. To this must be added the building materials, surface textures and colors of exposed surfaces of the buildings, and the design of open spaces, such as streets, courtyards, gardens, and squares.

These man-made elements interact with the natural microclimate to determine the factors affecting comfort in the built environment: light, heat, wind, and humidity.There is no doubt that certain configurations create better microclimates than others. For each site, there is an optimum arrangement in space that the designer should seek and use as a standard of reference in the process of deciding upon a certain design.

Where it can be avoided, it is inappropriate and irresponsible to implement a design that adds even one degree of temperature or reduces air movement by one centimeter per second, if this would negatively affect thermal comfort. This obviously includes defective designs which require energy intensive mechanical means for their rectification.

1. The sun factor

Orientation
In hot climates, the sun is the major source of heat. To plan any site, the position of the sun must be determined for all hours of the day at all seasons as well as the direction of the prevailing winds, especially during the hot season.

For an ensemble of buildings forming a sector, there will be reflection from adjacent buildings and wind screening by clusters of buildings, which contribute to a specific microclimate for each location in the sector. Wind movement and humidity also are important and should be considered simultaneously with the direct and indirect effects of the sun.
The main objective is to establish the optimum orientation with regard to the sun and the prevailing wind. The problem is complex, and it is useful to begin by considering the simple case of a block consisting of a single row of buildings. On the basis of this, more complex cases can be understood.

Shading
Although the optimal orientation for single buildings and blocks of row houses is with the long side aligned from east to west, for many reasons this cannot always be applied so simply over the entire plan of a city or sector. Some single buildings or row houses must face streets and squares that may be oriented at any angle from the north, with each case requiring an appropriate means of shading, depending on its orientation.
Generally, a building with a facade opening to the west is the worst case encountered, owing to the heat gain of the surrounding environment during the day and the angle of altitude, which allows the sun's rays to penetrate into the interior.

However, for a sector with the long facade facing west and east, blocks of buildings can themselves shade one another. To ensure this, the height of the blocks must be designed according to the width of the street and the angle of altitude of the sun.

Facades

Northern Facade
This facade is least exposed to the sun. In fact, exposure occurs only in the early and late hours of summer days when the angle of altitude is low and the angle of declination is such that the sun's rays are almost tangential to the surface of the wall. An advantage to rooms opening on this facade is that their illumination is always evenly distributed, making them ideal for hospital operating rooms and for school classrooms.

Southern Facade
With regard to the sun factor, an advantage of southern exposure in the Tropics and Subtropics is that the sun is high over the horizon in summer and can be shaded using a relatively small overhang.

In winter it is low, allowing the sunshine to penetrate when it is most desirable. However, with regard to the wind factor, a disadvantage of the southern exposure is that it receives no wind, since the cool prevailing winds generally blow from a northerly direction in the Northern Hemisphere.

Eastern and Western Facades
The eastern facade is exposed to the sun's rays only from sunrise to noon. The walls cool down considerably by evening, making this exposure more suitable for bedrooms than the western exposure.
Shading of the facades of buildings can be achieved by covering the streets, as is often found in older cities and oasis villages of West Asia and North Africa.

For a single building, shade can be provided by architectural elements such as balconies, covered loggias or open galleries, and verandas to shield the facade, or by introducing special devices such as the Venetian blind, the brise-soleil, and the mashrabiya to shield the openings.

Openings

Window openings normally serve three functions:

(i) to let in direct and indirect sunlight,

(ii) to let in air; and

(iii)to provide a view.

In hot arid climates it is rarely possible or desirable to combine these three functions in a single architectural solution, several solutions were developed which concentrate on each function separately.

The Venetian Blind

• One device which can be added directly to the window is the venetian blind.
• This blind is made of small slats, about 4-5 cm (1.6-2 inches) wide, closely set in a wooden frame at an angle that will intercept the sun's rays.
• The slats are often movable so the angle can be changed. This feature of adjustability renders venetian blinds very useful in regulating solar radiation and wind flow into rooms.
• The sun's rays can be blocked out without obstructing the breeze, which generally blows from the northwest in most hot arid areas, including Egypt, Iraq, and North Africa.
• When the blinds are drawn, they completely obstruct the view to the outside as well as considerably dim the light reaching the interior.
• However, sometimes the venetians blind is not a satisfactory solution to the problem of adjusting radiation and airflow.
• In summer, the blind can be adjusted to deflect the wind downward onto the occupants
• Also, if the slats are made of metal, they then absorb some incoming radiation and reradiate it into the room as heat.

The Brise-soleil

• The brise-soleil or sun-breaker is a new shading device that requires a special sophisticated support.
• It is generally used to shield entire facades of glass-wall and concrete or steel frame buildings.
• A brise-soleil properly designed to intercept the sun's rays reduces the heat gain to at most one-third, which although an improvement is still inadequate.
• There is the additional disadvantage of using the brise-soleil with regard to the view to the outside.
• The brise-soleil is a transposition of the venetians blind, with the slat width increased from 4 to about 40 cm (1.6 to about 16 inches) to suit the scale of the entire facade instead of just the window opening in a solid wall.
• When the angles of altitude and declination for screening direct sunlight are calculated, the required space between the slats is much larger than for the venetians blind. The result is a view slashed by large dark stripes interspersed by offensive glare.
• It may be used advantageously in some cases of modern architecture if comprehensively articulated in the facade with due regard for reduction of physical glare and for aesthetics.

The Mashrabiya

• The name mashrabiya is derived from the Arabic word "drink" and originally meant "a drinking place."
• This was a cantilevered space with a lattice opening, where small water jars were placed to be cooled by the evaporation effect as air moved through the opening.
• Now the name is used for an opening with a wooden lattice screen composed of small wooden balusters that are circular in section and arranged at specific regular intervals, often in a decorative and intricate geometric pattern.
• The mashrabiya has five functions. Different patterns have been developed to satisfy a variety of conditions that require emphasis on one or more these functions.
• These functions involve: (1) controlling the passage of light, (2) controlling the air flow, (3) reducing the temperature of the air current, (4) increasing the humidity of the air current, and (5) ensuring privacy. Each mashrabiya design is selected to fulfill several or all of these functions. In the design, it is the sizes of the interstices (spaces between adjacent balusters) and the diameter of the balusters that are adjusted. Different names identify certain of these patterns.

The roof
If the outdoor air temperature is higher than the indoor temperature, the outer surface of the roof exposed to the sun is heated as it absorbs radiation, and, being in contact with the outside hot air, also is heated by conduction.

The roof then transmits this heat to the inner surface, where it raises the temperature of the air in contact with it by conduction. At the same time, it radiates heat that is absorbed by people and objects indoors, thereby affecting thermal comfort.Therefore, the reflectivity of the outer surface of the roof and the thermal resistivity of its materials are of primary importance.

Shade can be achieved by using a double roof with a layer of air between or by covering the roof surface with hollow bricks. Insulating materials such as fiberglass, styrofoam, and lightweight blocks are often used. This solution, however, requires special commercial materials and increases the cost of the building beyond the means of most inhabitants in hot arid zones.
A useful idea is to shade the roof more naturally by designing it to suit popular traditions. In hot arid countries, since the air temperature drops considerably during the night, the inhabitants have arranged the roof architecturally into loggias or open galleries and lightweight roof covers. These loggias and roof covers have the double function of shading the roof during the day and providing physiologically comfortable living and sleeping spaces at night.

The shape of the roof is also of considerable importance in a sunny climate.

• A flat roof receives solar radiation continuously throughout the day, at a rate that increases in the early morning and decreases in the late afternoon due to changes in both solar intensity and angle of the sun.
• Pitching or arching the roof has several advantages over a flat structure :

(i) the height of part of the interior is increased, thereby providing a space far above the heads of the inhabitants for warm air that rises or is transmitted through the roof.

(ii) the total surface area of the roof is increased with the result that the intensity of solar radiation is spread over a larger area and the average heat increase of the roof and heat transmission to the interior are reduced. (iii) for most of the day, part of the roof is shaded from the sun, at which time it can act as a radiator, absorbing heat from the sunlit part of the roof and the internal air, and transmitting it to the cooler outside air in the roof's shade.

• This latter effect is particularly effective for roofs vaulted in the form of a half-cylinder and those domed in the form of a hemisphere since at least part of the roof is always shaded except at noon when the sun is directly overhead.
• Domed and vaulted roofs also increase the speed of any air flowing over their curved surfaces due to the Bernoulli Effect rendering cooling winds more effective at reducing the temperature of such roofs.

Hot and Dry Climate

b) Climatic Features

(i) Temperature :

Here, maximum temperatures of 40 to 45°C are common, although during colder periods of the year, night-time temperatures can drop to freezing or below due to the exceptional radiation loss under the clear skies.

Heat and cold produces the least observable effect in sandy desert. In contrast, their impact is much greater in rocky deserts.
In cold desert, rainfall is frozen at night in winter. When water goes into the tiny cervices and expands, the rocks are forced to split up.
In hot deserts, the fragmenting force of temperature is slower. Rock surfaces reach 70oC or 80oC at midday and cool down to freezing point at midnight. Expansion under the sun and contraction at night weaken the surface layers and cause flaking.

(ii) Rainfall :

Rain rarely falls in desert. When it comes, it comes in the form of thunderstorm. The rainfall in desert areas is less than 250 mm or 10 inches per year, and some years may experience no rainfall at all.

In sandy desert, the rain usually drains away promptly and only change the landscapecomparatively slightly. In contrast, the torrential downpour in rocky deserts drains into wadis (rocky watercourses that is dry except after heavy rain). This deepen the dry valleys. Heavy downpour can build up into flash flood, carrying sand, gravel and then large rocks and boulders. Thus, at the end of most wadis, there is an enormous bank of sand and stone( known as "alluvial fan" ).The surplus sediment from the flash flood forms muddy lakes of different size and duration.

(iii) Winds :

Wind dehydrates soils and living things. Sand and dust particles are moved by desert winds. Desert winds also remove organic debris that makes the soil fertile. Since plants are scarce in deserts, wind erosion occurs more easily. Take Prairie States of North America as an example, a productive area was reduced in the 1930s to desert by desert wind (devastating tornado winds from desert), over-cropping and over-cultivating. The fine dust can be carried to kilometers away and thousands of meters up. Large amount of the fine duct rest in more temperate or moister regions and from the basis of loess (a fertile soil).

Hot and Dry Climate

a) Regional Distribution

The adjoining world desert map represents the worldwide distribution of hot and temperate deserts.

A desert despite being a vast land area that is extremely dry with little or no vegetation, nevertheless comprises one of the major ecosystems on this planet and supports a range of plant and animal species that are attuned to survive in the harsh conditions.

Although some definitions of desert also includes areas which are too cold to support any vegetation such as frigid deserts, the world desert map however, shows the distribution of only those areas that are conventionally termed hot and temperate deserts.

Some of the most important deserts in the world are as follows:

Sahara Desert : The largest desert in the world, the Sahara occupies an area of 8,600,000 square km and comprises almost the whole of North Africa. The Sahara is bounded on the west by the Atlantic Ocean; on the north by the Atlas range of mountains and the Mediterranean; on the east by the Red Sea; and on the south by a vast zone of unmoving sand dunes.

Arabian Desert : Occupying almost the whole of the Arabian Peninsula and covering an area of about 2, 300, 000 square km, most of the area of this desert is comprised by the Kingdom of Saudi Arabia. To its southwest is Yemen; Oman on its eastern edge; Jordan in the northwest; and the United Arab Emirates and Qatar forms its northern limit along the southern coastline of the Persian Gulf.

Great Sandy Desert : A vast wasteland in northern Western Australia, the Great Sandy desert extends from Eighty Mile Beach on the Indian Ocean eastward into Northern Territory and from Kimberly Downs southward to the Tropic of Capricorn and the Gibson desert.

Thar Desert : Occupying about 200,000 square km, the Thar Desert is a vast tract of sand hills covering the India state of Rajasthan and parts of Pakistan. The barrenness of the area is mainly attributed to the dryness of the seasonal monsoon winds which do not bring enough rain to make the region moist and wet.

Simpson Desert : Occupying about 143,000 square km, the Simpson Desert in a largely uninhabited barren area in Central Australia on the southeastern corner of the Northern Territory and including parts of Queensland and South Australia. The desert is famous for its national parks and protected areas that are home to some of Australia's rare desert wildlife.

Takla Makan Desert : A great desert of Central Asia and one of the world's largest sandy deserts, the Takla Makan covers an area of about 320,000 square km in China's Xinjiang province.

Gobi Desert : Another of Central Asia's great deserts, the Gobi is actually not much a sandy desert but a vast stretch of arid land comprising mostly bare rocks that occupy about 1,300,000 square km in Mongolia and China.

Sonoran Desert : An arid region in North America, the Sonoran Desert occupies an area of 310, 800 square km and comprises of southwestern Arizona and southeastern California in the United States; the Mexican state of Baja California; and the western part of the Mexican state of Sonora.

Kalahari Desert : Occupying about 930,000 square km, the Kalahari comprises the whole of Botswana; the northern part of Northern Cape Province in South Africa; the eastern part of Namibia; and in the southwest merges with the Namib Desert.

Atacama Desert : The Atacama Desert is a 1,000 to 1,100 km long cool and arid coastal strip in northern Chile, which extends southward to border on Peru.