| BASIN- News No. 9 - January 1995: Appropriate Building Technology |
|WAS - Wall building advisory services|
GTZ GATE, Section 4130, Dag-Hammarskjold- Weg 1, D-6236 Eschborn 1, Germany
Spherical domes with relatively thin cross-section of brickwork compared to their span can lead to failure due to the tensile ring forces, which mud bricks cannot withstand. This is because the forces, resulting from the weight of the bricks (acting vertically) and their horizontal thrust, run along a line that does not coincide with their cross-section (Figure 1). In an optimized construction, the line of forces runs along the centre line of the brickwork (Figure 2), which can be relatively thin, thus avoiding tensile stress, and saving a great deal of building material.
In the case of a Nubian vault built symmetrically around a straight axis, the geometry of the curvature is comparatively easy to determine, namely by hanging a chain freely between two points (which are the same distance apart as the two parallel walls the vault rests on) and inverting the curve (called catenary). The corresponding ideal cross-section of a dome can only be worked out graphically or mathematically, by determining the vectors of each successive layer. Since this is a tedious method, a special computer programme was developed at the Building Research Laboratory of the University of Kassel in Germany to determine the ideal curvature fora given radius and height of the dome. The table on the next page gives the co-ordinates for seven common dome shapes, that is, from h = 0.8 r to 1.4 r.
Knowing the structurally optimized curve of a dome is an important step, but without a practical method of constructing the dome, it is of little value. At the Building Research Laboratory, a special construction technique was developed in 1987, and successfully applied at many projects in Germany, India and Egypt.
The rotation guide
The essential device to construct the optimized dome, is a guide, which rotates around a vertical shaft in the center of the dome. The key component of the guide is a curved T-bar, which has been shaped according to the required curvature of the optimized section of the dome. A small steel angle slides along the T-bar, and can be fixed at any point along it. The adjustable guide angle enables the mason to place the bricks at the exact position and inclination, row by row.
The curved T-bar is fixed to the central shaft by horizontal arms and additional bracing elements. Figure 3 gives an example of a guide, which was used for a dome of 7 m free span and 5 m height. The number of horizontal arms, the distance between them and the design of the bracing depends on the stiffness of the T-bar, and the kind of platforms used by the masons laying the bricks. Above all, it must be ensured that the guide can be rotated freely, without being hindered by the scaffolding, or bricks and mortar containers placed on it. In Figure 3, the different heights of the stands are marked with dotted lines. The lower part of the guide must be dismantled when the second platform is installed.
With domes of larger spans it is preferable to counterbalance the arms, as seen in Figure 3, so thinner profiles can be used for the guide.
The rotation guide should preferably be constructed in such a way that it can be used for domes of varying sizes and shapes. This means that the central shaft, tripod and adjustable guide angle are the standard parts, while the curved T-bar and parts of the horizontal arm and bracing are interchangeable.
Prior to laying the bricks, it must be ensured that the central shaft of the guide is exactly vertical, so that each brick course is laid horizontally. Before the first course is laid, the guide is taken around the base of the dome to detect any unevenness, which has to be corrected with a layer of mortar.
Nearly all kinds of fired or unburnt bricks (mud bricks) can be used for the construction of structurally optirnized domes. The bricks are positioned and inclined with the help of the guide angle, leaving a gap of 0. 5 cm between the bricks and the guide angle, so that it can rotate freely back and forth. The bricks are placed with their long axis perpendicular to the curvature of the dome (Figure 4). Usually the horizontal mortar joint is spread over a distance enough to lay 10 to 15 bricks. Then the bricks are placed accurately according to the position of the guide angle, and finally the vertical joints are filled. After completing each brick course, the guide angle is readjusted higher up, to start the next layer.
Figures 5 and 6 show domes constructed with the rotation guide in a slightly different manner. For each brick layer, the guide angle is turned down by 0.5 to I degree more than the previous layer, so that each course must be laid with a slight cantilever inwards (corbelled), in order to maintain the predefined curvature. This is done to avoid slippage of the bricks in the topmost layers, where the perpendicular to the curve becomes too steep. So when the last course is laid, the inclination of the bricks is 10 to 20 degrees less than the perpendicular.
This corbelling is not only aesthetically appealing, but also gives a better sound distribution, that is. it reduces the focussing effect of sound waves.
This article is an excerpt from a forthcoming GATE publication by Gemot Minke and Kiran Mukerji: Structurally Optimized Domes: A manual of design and construction, Aus der Arbeit von GATE, Eschbom, 1995. A new video film of the BASIN series "Sustainable Building Technologies", entitled Vaults and Domes from Earth (1994), is also available from the BASIN Wall Building Advisory Service.
The Vertical Shaft Brick Kiln (VSBK) is classed as a continuous updraft kiln, which combines the simplicity and low cost of updraft firing with very impressive fuel economy, plus the benefits of continuous operation. In China, where it was developed in the late 1960s, it is used by small scale seasonal brickmaking entrepreneurs, wherever there is an abundance of good brickmaking clay and a reliable supply of coal fines. Its operation is very similar to that of a vertical shaft lime kiln, with coal and bricks being loaded at the top and fired bricks combined with a small amount of ash being unloaded at the bottom.
Within a roofed and buttressed rectangular support building is a well insulated firebrick lined firing shaft, open at the top and bottom. This shaft is approximately 6.5 metres in height, with the central 4 metres being lined with a single layer of firebrick. At the base of the shaft is an arched unloading tunnel running through the centre of the kiln. This tunnel allows access to both sides of the base of the firing shaft and contains the brick support and unloading equipment. There are two versions of the kiln currently in use: a single shafted model with a 1.5 x 1 metre rectangular shaft, and double shafted model where the shafts are 1 metre square.
In the single wider shaft kiln, the shaft is loaded at the top with bricks and coal a batch at a time, the coal fines being sprinkled among the green bricks one layer at a time. Each batch of bricks is made up of four layers, making a total of 320 bricks. As the shaft holds twelve batches, it gives a firing capacity of 3840 bricks every 12 hrs. The bottom layer of bricks in each batch, which is the first loaded, consists of 68 bricks arranged in 7 rows to provide six open channels running across the firing shaft. These channels are to allow the placement of the steel beams that support the bricks in the kiln. The second layer of 84 bricks is placed at right angles to the first layer bridging the channels. The third and fourth layers each of 84 bricks are added at alternating right angles to make up a complete batch. The bricks in each layer are carefully spaced apart to provide a gap of 1.5 to 2 cm around each brick. Coal is spread evenly on top of each layer of bricks except the first layer with the cross channels. It is then brushed off the top surface of the bricks into the gaps around the sides, before the next layer of bricks is added. A fresh batch of bricks is added each time a batch of fired bricks is removed from the base of the kiln,
With the double smaller shafted kiln, the shafts are loaded and unloaded alternately with 188 bricks in each batch, 12 batches per shaft making a total firing capacity of 4512 bricks every 12 hrs. Ideally a batch of bricks is unloaded every 45 minutes to 1 hour, but the usual practice is to unload and load 3 or 4 batches at a time. The number of batches and the time of unloading is decided by the fire master, who judges this by the colour and position of the firing zone in the shaft. The quantity and quality of the coal fines and condition of the green bricks will determine the speed of firing.
The kiln is started by lighting a fire with wood among the bricks at the bottom of the shaft. The fire moves up the bricks and coal in the shaft, until it reaches the middle. At this point a batch of fired bricks is removed from the base of the shaft and a fresh batch of green bricks and coal loaded at the top.
The position of the fire in the shaft is maintained by the rate of removing the fired bricks and reloading with green bricks. The firing temperature is controlled by the amount of coal fines added to each batch of bricks loaded. In China 33kg of coal fines are added to each batch of bricks, 0.103kg per brick. The calorific value of the coal was 26.6 MJ/kg resulting in a specific energy consumtion of 975 KJ per kg fired brick. In Pakistan, a two shafted kiln had specific energy consumption of 1287 KJ/kg fired brick. When this is compared with the figures for coal firing Hoffmann kilns 2800 KJ/kg fired brick or Bull’s Trench at 3116 KJ/kg fired brick, the efficiency of the VSBKs is very impressive.
Brick wastage is very low when compared with the Hoffmann and Bull's Trench kiln, being 2 to 5% in China and 5 to 7% in Pakistan. This is mainly due to the fact that the kiln fires so evenly with no over or under fired bricks. Any wastage results from careless brick handling and low quality green bricks being loaded.
Bricks are removed a batch at a time, by raising an unloading trolley until it just takes the weight of all the bricks in the shaft. This allows the six brick supporting steel bars to be removed from the channels running through the bottom batch of bricks. With the weight of the whole shaft of bricks on the trolley, it is slowly lowered by the depth of one batch, when the six support bars are replaced in the six channels in the next batch. By continuing to lower the trolley, the weight of the remaining batches in the shaft is taken up again by the bars, leaving the trolley with one batch of bricks to be lowered to the ground. The trolley of fired bricks is uncoupled from the lifting gear to be pulled out from the unloading tunnel at the base of the kiln along two steel rails.
The trolley is raised and lowered by being attached to either a 5 tonne chain pulley system or a screw jack arrangement. The pulley system is less expensive to buy and install, and can be quickly replaced if a fault occurs. It is easier to operate, though potentially slower than the screw jack system. The screw jack is more substantial and will last longer, if made from good quality steel. The kiln is straightforward and not too expensive to construct. The single shaft kiln is easier and less expensive to build, as only one set of unloading gear is needed, though its output per 24 hour operating period is lower than the two shafted model. The unloading equipment is the most expensive part of the kiln, representing 40% of the total construction costs. With a pair of chain pulley blocks, instead of the screw jack unloading system, the overall cost of the kiln is further reduced, as the chain blocks can be purchased off the shelf at a quarter of the cost of the screw jack unloading system.
Kiln maintenance is low, because the only machinery involved is the basic unloading system.
Despite there being no controllable fireboxes and the kiln working on the normally least efficient updraft system, it is extremely fuel efficient. This is because the firing shaft creates just enough natural draft for complete and efficient combustion of the fuel spread evenly throughout the firing zone. The packing of the bricks restricts the draft enough to limit the amount of excess air not required for combustion. The cooling bricks below the firing zone heat up the air for combustion. The hot exhaust gases from the firing zone preheat the green bricks and fuel prior to combustion. The firing shaft is very well insulated on all four sides, so heat loss is minimised. Once the kiln is up to temperature, all the heat from the coal fines goes into the firing of the bricks. Very little heat is lost in the exhaust gases or in steady state heat loss through the fabric of the kiln.
There is a need for more research into the possibility of the Vertical Shaft Kiln being used with bricks where the fuel is combined with the clay during brick manufacture. Combining agricultural waste with the coal amongst the bricks is also a possibility.
Advantages of the Vertical Shaft Brick Kiln
- The VSBK represents a very energy efficient low cost method of firing bricks on low grade coal fines. Its efficiency is twice that of Hoffmann or Bull's Trench kilns running on coal. The fired quality of the bricks is high with a low wastage.
- The kiln is cheap and straightforward to build, and requires very little maintenance once constructed. It does not require an external power source, except for electric lighting during night operation.
- The kiln is very compact, not requiring a large area of land and can be built nearto the clay source. It is small and straightforward enough to be considered for rural brick production, where coal fines are available to replace the less efficient coal and wood burning brick clamps used in many countries. In China, the kilns are easily taken apart and rebuilt with the original materials, when the clay source in one location is worked out, so it has the flexibility, in this respect, to be used in a similar manner to brick clamps.
- The construction cost is low and several shafts can be linked together in a row to cope with larger capacity brick works.
- The kiln is not effected by variations in the weather, as long as dry green bricks are available.
- Because of the highly efficient combustion of coal in the kiln, localized air pollution is minimal. There is little or no visible smoke while the kiln's running. Steam and combustion gases are quickly dispersed from the top of the firing shaft, which is well ventilated.
- More than one shaft allows for firing flexibility to cope with seasonal and economic variations in brick production and demand.
- Labour requirements are low, requiring one man to load and two men to unload, during an 8 or 12 hour shift. The labourers are not working continuously, as there are 3 or 4 hours between each loading/unloading session.
Disadvantages of the Vertical Shaft Brick Kiln
- It is a new type of kiln and method of firing bricks that has only recently been introduced to countries outside China. Despite there being thousands of this type of kiln operating in China, the technology has not yet been adopted anywhere else in any quantity.
- The kiln requires good quality green bricks, because they have to be able to withstand being stacked 5 metres high in the firing shaft. This does not exclude hand moulded bricks, as has been demonstrated in Pakistan, where all the bricks were slop moulded.
Author: Tim Jones,
Appropriate Development Consultants.
"Cannon Hey", Storrs Park,
Cumbria LA23 3LD, United Kingdom