Projections

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Projections are often useful in presenting a proposed building to someone who is not familiar with the presentation in plan, section and elevation drawings.

However, the rural population, in particular illiterates, may understand pictures and illustrations in a different way than intended or not at all. Even the idea that a message can be contained in a picture and that something can be learned from it can be new. This is mainly because they do not see many pictures and have not learned to understand the symbolic language often used in illustrations. Some of the most common difficulties in comprehension of illustrations involve close- up illustrations where a part, e.g., a person's hands or head, is used to represent the whole. While too much detail, particularly in the bac kground may be confusing, outlined or stick figures contain too little detail, and are not as recognizable as toned-in line drawings. Perspectives, where objects in the distance are drawn smaller can present difficulties as can pictures of small items, e.g. insects, drawn to a much larger size than the actual. Best understood are pictures containing a single message and portraying a culture, e.g., persons or clothing that the viewer can identify with.

Isometric or oblique projections are useful in presenting a pictorial, although slightly distorted, view of a structure and are particularly suitable for free-hand sketching. The axonometric projection is best suited to show the interior of rooms with its furniture, equipment or machinery. The two point perspective is a bit more complicated to construct, but gives a true pictorial view of a building as it will appear if standing at about the same level as the building and at some distance.

All types of projections can be constructed to scale, but they become really useful to the building designer once the technique is so familiar that most of the details in the drawing and eventually even the major contours of the picture may be drawn freehand.

Figure 1.11 Time schedule.

Isometric Projection

With isometric projection, horizontal lines of both front view and side view of the building are drawn to 30 from the horizontal using dimensions to scale. Vertical lines remain vertical and the same scale is used. Curved and slanted lines are developed by working within lightly sketched squares or rectangles, which are erased after use.

Oblique Projection

An oblique projection starts with a front view of the building. The horizontal lines in the adjacent side are then draw to an angle, usually 30 or 45, from the horizontal. The dimensions on the adjacent side are made equal to 0.8 of the full size if 30 is used or 0.5 if 45 is used. Curved and slanted lines are constructed in the same manner as in isometric projections.

Figure 1.12a Isometric

Figure 1 .1 2b Oblique projections.

Figure 1. 13 Axonometric projection.

Axonometric Projection

In axonometric projection the plan view of the building is placed on the drawing table with its side inclined from the horizontal at any angle. Usually 30, 45 or 60 is chosen since those are the angles of the set squares. All vertical lines of the building remain vertical and are drawn to the scale of the plan view.

Perspective

The different technical terms used in perspective drawing can be explained if you imagine yourself standing in front of a window looking out at a building at some angle so that two sides of the building are visible. Trace on the window pane the outline of the building as you see it through the glass. You have then just made a perspective drawing of the building and if the glass could'be removed and laid on the drafting table the drawing would look like any perspective drawing made on paper.

Station point is the viewing point, supposedly occupied by the eye of the observer. The viewing point is also determined by the eye level, usually assumed to be 1.7m above ground level. Looking across a large body of water or a plain, the sky and water/ground appears to meet in the distance - the horizon line. This must always be considered present even when hidden by intervening objects. The horizon line is at eye level.

When standing and looking down a straight road, the edges of the road appear to meet at a point - the vanishing point, which is on the horizon line and therefore also at eye level.

Similarly parallel horizontal lines of a building appear to meet at vanishing points, one for each visual side.

The outline of the building was brought to the window by your vision - vision rays. The picture was traced on the window pane, which therefore can be called picture plane.

Since the technique with a window pane obviously can not be used for a proposed but still non-existing building, the perspective has to be constructed from available documentation. A perspective drawing of a building can be constructed using the plan view or, if several buildings are to be included, the site plan would be more suitable. In addition one would require elevations of all visual sides of the building(s) i.e., in the case of one building the front elevation and one end elevation.

Construction of a Perspective

Step 1 Locate a suitable station point (SP).

The distance between the station point and the object represents the true distance from the viewer to the building to the scale of the drawing. Accordingly, the longer the distance the smaller the building will appear in the picture.

Next draw a centre line of vision i.e., a line from the station point to the building. Fix the drawing on the drawing board with the centre line of vision (CLV) in a vertical position and cover with a transparent paper. Check that the building is falling within a 60 cone of vision, since parts of it falling outside this cone will appear distorted when looking at the picture.

Step 2 Locate the picture plane (PP) and vanishing points (VP).

The picture plane is a line drawn at 90 to the centre of vision line i.e., horizontal on the drawing board. The distance between the station point and the picture plane will directly influence the size of the perspective picture. Think again of the situation where the outline of a building was traced on a window pane. If the window pane was moved closer, the outline picture would be smaller. Thus, if the reader of a perspective drawing is to get an image of the true size of the illustrated building, he will have to look at the perspective from the same distance as the distance between the station point and the picture plane when it was constructed. Therefore this distance is normally taken to be 400 to 600mm. The vanishing points are then located by drawing lines from the station point to the picture plane parallel to the visual sides of the building.

Figure 1.14a Construction of a perspective drawing.

Step 3 Locate the horizon line (Hz) and the ground line (GL).

The horizon line can be located anywhere on the paper as long as it is parallel to the picture plane, but leaving enough empty space to allow the perspective picture to be constructed around it. The ground line is then drawn parallel to the horizon line at a distance corresponding to the eye level to the scale of the drawing. The horizon line will always be above the ground line if the view point is above ground level. The vanishing points are then vertically transferred to the horizon line. It is helpful to put needles in the vanishing points on the horizon line to guide the ruler in further construction of the perspective.

Step 4 Locate a height line (HL) and mark the heights on this line.

True heights of the building can only be scaled on a height line in the perspective picture. Start by locating a height line on which heights concerning the front wall can be scaled. This is a vertical line from the point on the picture plane where it is crossed by a line extended from the front wall in the plan view. The point where the height line crosses the ground line will represent ground level and all heights in the front wall can now be scaled from this point to the scale of the plan view. Top and bottom lines for the front wall can now be drawn from the vanishing point through the marks on the height line.

Step 5 Visual rays (VR) to locate points in the perspective view.

Visual rays are drawn to locate the exact position of the corners of the front wall in the perspective. The rays are drawn from the station point through the point to be located in the perspective to the picture plane. From the picture plane the line is continued vertically to the intersection with the top and bottom lines. With further visual rays the outline of the visual walls can be drawn in the perspective.

Step 6 Further height lines and visual rays.

To find the top line for a double pitched roof a new height line must be constructed since that height is at a plane behind that of the front wall. Visual rays are then used to find the ends of the ridge. Doors and windows in the front wall are constructed with the height line for the front wall and further visual rays to find points in the perspective.

Step 7 Completing the perspective view.

When the major outline of the building and principal objects in the visual sides, such as doors and windows, have been constructed in the perspective view, the drawing tends to be quite crowded with lines. Further details are therefore usually more easily constructed freehand.

Finish the perspective by drawing vegetation and miscellaneous objects which will appear in the surroundings of the building. People in the picture will always be drawn with their eyes on the horizon line. The size will then determine the distance to the viewer. Finally cover the perspective drawing with tracing paper and redraw the picture leaving out all the construction lines.

Figure 1.14b

Model buildings

Any person, including those who have had a good basic education, will need considerable experience to be able to visualise fully a building from a set of drawings. The farm building engineer will therefore soon learn that the average farmer not only finds it very difficult to understand simple plan view and section drawings, but may even find it hard to interpret fully rendered perspectives. However, the fact that a model, unlike drawings, is three-dimensional and thus can be viewed from all sides brings more realism to the presentation and usually results in communication and transfer of ideas.

Figure 1.14c

Figure 1.14d

Figure 1.14e

Figure 1.14f

There are three types of models in common use for presentation of farm building projects:

Three-dimensional maps or site plans are used to present development plans for large areas or the addition of a new building on an old site with already existing structures. These models have contours to show the topography while structures are carried out in simple blockform with cardboard or solid wood, usually without any attempt to show detail.

Basic study models are used for examination of relationships and forms of rooms and spaces in proposed buildings. They are often built in cardboard, and there is usually little attempt to show details, although furnishings and equipment may be indicated. Windows and door openings are shown with dark coloured areas or left open. Contours are shown only if they are of importance for the building layout.

Fully developed models may be used in extension campaigns, for public exhibition, etc. These models show details to scale and have close representation of actual materials and colours. Part of the roof is left out or made removable in models aiming to show the interior of a building.

A sturdy base for the model, made of either plywood or particle board, not only facilitates handling but also adds to the protection of the model. For models to be displayed in public it is advisable to have well finished borders, preferably in hardwood and, although expensive, an acrylic plastic (Plexiglass) cover. During transport a plywood box, without bottom, fixed to the base of the model with screws, will give sufficient protection if handled with care.

The size of the model is determined by the scale at which it is made and the size of the actual project. While detail is easier to include in a model made to a large scale, too much detail may distract from the main outlines and essential features; and if too large, the model will be more costly and difficult to transport. Basic study models are often made to a scale of 1:50 or 1:100 to allow for coordination with the drawings, while fully developed models of small structures may be made to a scale of 1:20 or even larger. Whatever scale is used for the model, it is desirable to include some familiar objects, such as people or cars, to the same scale as the model to give the observer an idea of the size of the actual structure.

The construction of contours and elevations requires access to a map or a site plan with contour lines to the same scale to be used in the model. One way of showing contours is to build up with layers of cardboard or styrofoam sheets having a thickness equal to the scale of the real difference in height between contour lines. Employing one cardboard piece for each contour line, trace the line on to the cardboard using carbon paper, cut out the contour, place it on the model and secure it with glue. The contours can be either left as they are, giving sharp, distinct lines or be smoothed to a morenatural roll, using sandpaper or putty. For more elaborate models the landscaping may be represented by painting. Trees and bushes can be made from pieces of sponge or steel wool on twigs or toothpicks. Coloured sawdust can be used for grass and fine sand for gravel. If available, model railroad supplies and other hobby materials can be useful.

Although the same or close simulations of the materials employed in the actual building are used for the most elaborate models, cardboard, or for models made to a large scale, plywood, is usually easier to work with and can be finished by painting to represent most types of materials. Cardboard or plywood of the right scale thickness for use as walls are often not available, but it will make no difference as long as the overall scale dimensions of the building are maintained. Round wooden posts commonly used in farm buildings for post and beam or pole construction are conveniently made from twigs or hardwood sticks. Any finish on the walls to represent openings or materials should be applied before the model is put together. Neat, clean-cut lines are easier to accomplish in this way. While a plain cardboard roof is adequate for most purposes, corrugated paper painted to a suitable colour may be used to represent corrugated roofing materials and thin grass glued on to the cardboard can be used to represent thatch.

Models can be increased in strength and rigidity by bracing the walls with square pieces of cardboard in positions where they will not show in the finished model. Bracing is particularly important in models which are going to be painted as paint will tend to warp cardboard and sheet wood if applied over large areas. Regardless of the material being represented, colours should be subdued and have a flat, not glossy, finish. Distemper or water colour is best for use on cardboard and unsealed wood, but care must be taken to remove excess glue as this will seal the surface and cause the colour to peel off.

A photograph of the model may be used in cases where it is not feasible to transport the model or when photos need to be included in information material and the actual building has not yet been completed. Models often appear more realistic when photographed, particularly in black and white, because of better contrast, but adequate lighting from a direction which produces a plausible pattern or sun and shadow on the building must be assured. Outdoor photography allows for a sky or terrain bac kground to be incorporated into the photograph of the model.

Figure 1.15 Basic study model.

Further reading

Bellis H.F., Schmidt W.A., Architectural Drafting, New York, McGraw - Hill Book Co., 1971.

McBean G., Kaggwa N. and Bugembe J., Illustrations for Development, Nairobi, Afrolit Society, 1980.

Styles K., Working Drawings Handbook, London, Architectural Press, 1982.

Taylor R., Model Building for Architects and Engineers, New York, McGraw-Hill Book Co., 1971.

V. Winden J., de Keijzer M., Pforte W., Hohnerlein F., Rural Building - Drawing Book, Maastricht, Netherland, Stickting Kongregatie F.I.C.


Chapter 2 Surveying

A simple survey of a building site provides accurate information needed to locate a building in relation to other structures or natural features. Data from the survey is then used for drawing a map of the site including contours and drainage lines if needed. Once located, the building foundation must be squared and leveled. This chapter will cover the several procedures involved.

Distances

Steel tapes or surveyor's chains are used for measuring distances when stations are far apart and the tape or chain must be dragged repeatedly. Linen or fiberglass tapes are more suitable for measuring shorter distances such as offsets when making a chain survey, or in laying out a foundation. To obtain accurate results a chaining crew must first practice tensioning the chain or tape so that the tension will be equal on each measurement.

Range poles are 2 to 3 metre metal or wooden poles painted with red and white stripes, and used for sighting along the line to be measured.

Land arrows come in sets of 10 and are set out by the lead man in a chaining crew and picked up by the following man. The number picked up will be a check on the number of lengths chained.

A field book is used in which to draw sketches and record measurements.

When measuring for maps or site plans, horizontal distances are required. Thus when chaining on sloping land, stepping will be necessary. This procedure allows the tape or chain to be kept level, as checked with a hand or line level, while the point on the ground under the high end of the tape is located with a plumb bob as shown in Figure 2.1.

Angles

There are several types of tripod-mounted levels available, some of which are equipped with horizontal rings allowing them to be used for measuring or setting out horizontal angles. Theodilites are designed to measure or set out both horizontal and vertical angles. Although these surveying instruments provide the most accurate means of measuring angles, they are expensive and rather delicate. Fortunately much of the surveying of rural building sites involves only distances, 90 angles and contours which may be measured or set out with rather simple equipment.

One of the simplest, yet accurate means of setting out the 90 corners of a building foundation makes use of Pythagora's theorem or the 3, 4, 5 rule (or any multiple of the same). Starting at the corner of the foundation site, a line is stretched representing one side of the foundation.

A distance of 4m along the line is marked. Then another line is stretched from the corner at approximately 90 and 3m is measured along this line. When using the tape between the 4m and the 3m marks, the second line is swung slightly until exactly 5m is measured between the marks. The first two lines then form a 90 angle.

Figure 2.2 illustrates this procedure as well as the method of swinging an arc to erect a perpendicular.

Two simple instruments for setting out right angles are the cross stave and the optical square (Figure 2.3). Either one is mounted at eye level on a range rod at the corner where the angle is to be set out. In either case the instrument is turned carefully until one line of the right angle can be sighted. Then the second line can be swung slightly until it can also be sighted.

Figure 2.1 Stepping on sloping ground.

Figure 2.3

Figure 2.4 Plumb bobs.

Figure 2.5 Builder 's level and line level.

Vertical alignment

A surveyor's plumb line consists of a sturdy cord, a distance bar and a conically shaped plumb bob with a hardened steel point. It is used for positioning surveying instruments or when stepping with a tape or chain. It may also be used to check the vertical alignment of foundations, walls and posts. A simple plumb line for these latter jobs can be made from string and a stone.

Levelling

Just as in the case of angle measurement, there is a wide variety of surveying instruments used for levelling. Most are designed for accuracy and are rather expensive. Although built for use in the field or on a building site, like any precision instrument, they require careful handling and regular attention to ensure good service.

Fortunately, there are several rather simple devices that may be used for levelling foundations, running contours or aiding in step-chaining.

Builder's levels are made of wood, plastic or aluminium and are available in several lengths, one metre being a convenient size. The bubble tubes are graded for sensitivity to suit the work. Most are now made of plastic and filled with fluorescent liquid, an aid in poor light.

Line levels are designed to hang on a tightly stretched line. Both of these types are useful in foundation construction work.

Hand levels and Abney levels are both hand-held instruments incorporating a spirit bubble tube and a split image mirror. Thus, when they are held to the eye and the bubble centered, one is looking at a point at exactly eye level. They are useful for keeping a chain or tape horizontal when stepping and for doing simple contouring. The accuracy of work with either of these levels may be improved somewhat by placing the level on a rod of known length, still keeping the instrument at approximately eye level. As they have either a low-power scope or no telescope, they are only suitable for distances of up to approximately 30 metres.

For leveling the lines used in laying out a foundation, a builder's water level is a simple, inexpensive device that provides a satisfactory degree of accuracy. It consists of a length of rubber or plastic tubing at each end of which there is a transparent sight tube of glass or plastic. It works well over a distance of about 30m and is particularly useful for transferring levels around corners, from outside a building to inside, or around obstacles where the two leveling points are not intervisible. It is also a useful tool for obtaining the slope in pipe runs. Note Figure 2.6 for the method of use.

Figure 2.6 Setting out corner profiles.

Figure 2.7a Field book sketch of the site with stations and main survey lines.

Figure 2.7b Field book recordings of offsets along chain line A-D.

Chain surveying

In a chain survey, the area to be surveyed is enclosed by one or more triangles whose sides are measured and recorded. Then the perpendicular distance from the side of a triangle to each point of detail such as trees, buildings, boundaries, etc. is measured. From this information a detailed plan of the site can be drawn to scale. A proposed structure may then be superimposed on the plan and its location transferred to the actual land site.

The following step-by-step procedure is used in a chain survey:

1 Make a preliminary survey by walking around the site, deciding where to put stations and where the main survey lines should be arranged. Stations should be selected so that they are intervisible and the lines laid out so that obstacles are avoided. Make a sketch of the site in the field book (Figure 2.7a).

2 Set the range poles, chain the triangle sides and record the distances.

3 Measure the perpendicular offsets from the chain lines to the details of the site. This will be easier to do if the chain lines have been arranged so that offsets can be kept as short as possible. Record the measurements in the field book (Figure 2.7b). Each page should record offsets along one chain line. Entries start from the bottom of the page and details are entered to the left or right of the center column where distances along the chain line are noted.

Not all details are measured by perpendicular offsets. Sometimes it is more accurate and convenient to use pairs of inclined offsets which together with a portion of the chain form acute-angled triangles. Note the top corner of the house in Figure. 2.7b.

4 If contour lines need to be included on the map or site plan, the next step will be to measure levels with a levelling instrument and a staff.

The grid method is most commonly used in connection with construction projects provided the ground does not slope too steeply. The grid is pegged out on the site in the position considered most suitable and levels are taken at points where lines intersect. Sides of squares may be 5 to 30m, according to the degree of accuracy required. If the area is reasonably small, staff readings may be recorded near to each point on a sketch or drawing similar to that shown in Figure 2.7c. Alternatively staff readings may be recorded in a field book. Each point has a reference letter and number.

If all points on the site will be within range of the levelling instrument, and providing the staff at each point can be seen through the telescope, the instrument should preferably be set up near the middle of the site, so that all readings can be taken from one position. The first staff reading is made on an Ordinance Bench Mark, (O.B.M.) if one is available in the near vicinity, or alternatively on a site datum which may be assumed to be at a reduced level of 10.0m, or any other convenient height.

It is normal practice to leave a number of selected and carefully driven pegs in position on the site to assist in the work of setting out when development work commences.

From the spot levels obtained by this grid method, the contours can be drawn, the volume of earth to be excavated can be calculated and the average level of the grid can be determined.

5 Map or site plan. Start by making a scale drawing showing the main surveying lines. Then plot the offsets to buildings and other features in the same order as they were recorded in the fieldbook.

If contour lines are to be included start by drawing the grid to the scale of the drawing. The contour lines may then be indicated by interpolation. Contour points are plotted on each line between each pair of spot levels in the grid assuming the ground has a fairly constant slope. A smooth curve is then drawn to link up points of the same height. Note that contour lines cannot cross, but only come close at points where the gradient of the ground surface is steep.

To produce the final map or site plan, cover the preliminary drawing with tracing paper and draw the final plan leaving out the survey lines, offset lines and the grid.

Figure 2.7c Site plan made up to scale from field book recordings.

Figure 2.8 Builder's square.

Setting out building work

Before a decision about the final siting of a building can be made, a number of factors have to be taken into account. Consideration must be given to local authority and planning regulations, to functional requirements, orientation, view, prevailing wind, noise, shelter, water supply, access, slope of ground, privacy, and the type of soil on which to build.

Orientation can be important. Perhaps the best position for comfort is an east-west alignment. This arrangement eliminates much glare by confining the sun's rays to the end walls only. It also allows cross ventilation - very necessary when the humidity is high.

To set out a building it is necessary to have a base line (one side of the building) and a fixed point on the line, usually one corner of the building. At this point, as at all other corners, a peg is first driven and then a nail is driven in the top of the peg to mark the exact position of the corner.

The distance from one peg to the next is carefully measured with a steel tape and the peg and nail firmly driven. Depending on the size and nature of the building, the correct position of all other lines and pegs in relation to the base line and each other may be obtained by means of:

Having obtained the direction of all lines, measured all distances and driven pegs and nails at the points, a check on the accuracy of the setting out may be made by measuring the overall horizontal distances in both directions. Pairs of lines should be exactly equal.

Check again on the accuracy of the setting out by measuring the diagonals of the rectangle. For buildings having sides from 5 to 20m long, the length of the diagonals A and B in Figure 2.9 should not differ more than 0.5%. If adjustments are necessary following this check, it is advisable to keep the two longest parallel sides fixed and to make the required adjustments on the short sides.

Finally check the drawing with the setting out to ensure that lines and corners are in their correct positions and that dimensions are correct.

Figure 2.9 Corner profiles and checking for accuracy.

When the setting out and checking have been done timber profiles are erected. Profiles consist of horizontal rails supported by vertical pegs set up clear of the excavation. Inside and outside faces of the wall and the width of the foundation are marked on the horizontal rail by means of fine nails or sawcuts. Strings are later stretched between these nails or sawcuts on opposite rails to guide the workers during trench excavation and footing and foundation wall construction.

Ideally, profiles should be set up for all corners and internal walls. The profile shown at A in Figure 2.10 should be located at A1, if the foundation area is to be excavated.

Figure 2.10 Plan of walls and profiles.

Excavation depth control

When any building work is to be done, it is usually necessary to excavate at least a foundation trench. Frequently, if concrete is to be used, some excavation is required in order to make the floor finish at the level required. In addition, it may be necessary to finish a surface such as a roadway or ditch-bottom to an even gradient. In all these cases it is necessary to control the depth of the excavation to ensure that the correct amount of soil is removed.

Sight Rails

Sight rails are made either across the line of an excavation such as a trench as shown in Figure 2. 11, or alongside an area such as a roadway or floor. If the excavation is to be level, then the tops of the crosspieces must all be at the same height. If there is a gradient to the excavation, however, the tops of the sight rails should be at heights such that they fall on the same gradient (Figure 2.12).

On a small building site it may be possible to use a long straightedge with a spirit level to get the sight rails level. However, with longer excavations or where a gradient is required, it may be necessary to use a tape and level to get the appropriate fall from one sight rail to another.

Traveller

A traveller, also known as a "boning rod", is 'T'shaped and normally wooden. The overall length is the same as the distance from the sight rail down to the excavation depth required, as shown in Figure 2.11. It can be an advantage therefore to set the sight rails up at a known height above the excavation. For example, a level excavation will normally be specified as having a minimum depth. If a trench is required with a minimum depth of say 0.5m and the ground rises along the length of the trench by 0.7m, then the first profile must be set high enough for the second to be above the ground, and a traveller of 1.5m may be used. The first profile will then be 1 m above the ground. See Figure 2.13.

As the excavation progresses, the depth can be checked by looking across from the top of one profile to another. As long as the traveller crosspiece can be seen, the excavation is not deep enough and should be continued until the crosspiece is just invisible.


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