Arches

Openings in walls such as doors and windows have to be closed at the top in such a way that the structure above them is supported. Before the use of reinforced concrete became common, smaller openings were bridged with wooden lintels, solid stone lintels and arches (see Lintels).

Wider openings were generally spanned by arches, or by a series of arches carried on piers or columns: the so-called arcade. in this book we discuss only one type of arch, the "segmental arch" where the arch is formed like a segment of a circle. Many other forms are possible with arches, but they are not important for Rural Building.

Before we continue with this chapter, it is essential to understand the technical terms that are used in connection with arches. The terms below are indicated in Fig. 1.

TECHNICAL TERMS

ABUTMENT: The word "abut" means to meet at one end or to border on something. An abutment (Fig. 1, a) is that portion of solid masonry that carries the weight of the structure of an arch. in Rural Building, the minimum width of an abutment at the end of a wall should be 3/4 of the span of the arch. This means that the distance from the arch to the end of the wall should be at least 3/4 of the span of the arch.

CENTRE: This is the midpoint of the circle which describes the curve of the arch (b).

CROWN: This is the portion of the arch which forms the top of the curve (c).

EXTRADOS: The outer curved line of an arch, or the upper surface of the archstones(d).

HAUNCH: The flanks of an arch, the sides of the curve (e).

INTRADOS: The under surface or soffit of an arch (f).

KEYSTONE: The central wedge-shaped archstone at the crown of an arch (g), which is the last stone to be put in place.

RADIUS: The straight line (h) from the centre of an arch to any point on its in-trados (the shorter radius, r); or to any point on its extrados (the longer radius, R).

SPRINGING LINE: The line across the arch which would connect the springing points (j).

RISE: The height of an arch measured perpendicularly from the springing line to the highest point of the intrados (k).

SPRINGER: The first stone laid in an arch on either side (1).

SKEWBACK: That portion of the abutment which directly supports the springers (m). It is so called because the surface slopes towards the opening.

SPAN: The horizontal distance between the springing points (n); the length of the springing line.

CAMBER: This is the space between the springing line and the intrados (o).

ARCHSTONES: These are any of the stones or blocks which form the arch itself.

 

STABILITY OF ARCHES

All arches, regardless of their shape or dimensions, function as wedges. An arch distributes the load imposed on it to the abutments on both sides; these have to be strong enough to withstand the pressures (Fig. 1, a). This is the reason why the abutment which is at the corner or end of a wall must be at least as wide as 3/4 of the span of the arch (Fig. 1, b). Between two arches, thinner supports such as columns or piers can be used (Fig. 1, c).

It is always better to build two or three courses of blocks above an arch because this weight will improve its stability. The preferred material for constructing an arch is sandcrete blocks, because they can be cast to any shape required and can withstand high pressures. Of course landcrete or mud blocks can also be used, but these must be well protected against moisture penetration.

DISADVANTAGES OF ARCHES:

• All arches, especially those which have unusual shapes, require more space than a lintel (Fig. 1, d).
• Frames must be shaped on their top according to the curve of the intrados (Fig. 1, e); or if a square frame is to be fitted, the remaining space above it must be closed with plywood etc.
• A temporary wooden support, the centring or turning piece, is needed during the construction; this piece has to be exactly the correct shape.
• Masonry arches cannot be prefabricated.
• Specially made archstones are often required.

ADVANTAGES:

• Arches are more economical because scarce and expensive building materials like reinforcement iron are not needed.
• Locally available building materials such as mud blocks can be used to build arches.
• In contrast to cast-in-situ lintels, the walling above an arch can continue immediately after it is completed, because the arch does not have to set.
• Traditional building styles can be maintained and developed in a simple and cheap way.
• Arches, if well constructed, give the building a more attractive appearance and demonstrate the skill of the builder.

THE SEGMENTAL ARCH

The segmental arch is the most important one in Rural Building. This is mainly because:

The construction depth measured from the springing line to the crown is comparatively small; with the result that the building costs are low.

The construction is not too difficult and specially made tapered blocks are not necessarily required.

The time spent on the construction of this kind of arch can be shorter than the time spent for constructing a reinforced concrete lintel.

TYPES OF CENTRING

"Centring" is the term generally applied for the curved wooden piece which temporarily supports arches or domes during their construction. It is comparable to the strutting (not the shuttering) for concrete members.

The actual shaping part of the centring is called the "turning piece". There are three main types of centring:

A solid wooden board, about 5 cm by 7, 5 cm, placed edgewise. Its upper surface is carefully and evenly shaped according to the curve of the intrados (Fig. 1). Its use is restricted to short-span openings; not exceeding about 100 cm wide. The disadvantage of this type is that the archstones rest on only a small area, which might lead to the blocks tilting during the laying.

Two wooden planks or sheets of plywood are connected by spacers, so that the archstones are supported on their outer edges (Fig. 2). Both turning pieces must be precisely cut and planed to the same shape, and must match with the curve of the intrados.

Arches spanning wider openings and those which have a rise of more than about 1/10th of the span need to be supported by a stronger centring, with either open or closed lagging (Fig. 3).

The three types of centring may be combined with the frame or may form part of the frame; the head. in this case no strutting is needed, and the centring piece simultaneously acts as a permanent seal of the space between the springing line and the intrados (Fig. 4). The lagging must then be fixed in between the turning pieces so it is flush with the upper curved edge and is not seen (Fig. 4).

SETTING OUT THE TURNING PIECE

To set out the turning piece means to mark the curve of the intrados on a suitable piece of board or plywood, so that after it is cut and planed it can serve as a temporary support and guide for the arch construction. The positions of the archstones are also marked on the turning piece so that no mistakes can be made during laying.

As far as Rural Building is concerned, the radius of the circle which describes the curve of the intrados will always be three times the span of the opening. For example: if the span of the opening is 80 cm (the clear width), the radius must be 3 x 80 cm = 240 cm; if the span is 180 cm, the radius is 540 cm, etc.

The curve obtained from this rule will result in an arch which is flat enough so that not much space is wasted, but also curved enough to ensure stability.

- SEQUENCE OF OPERATIONS:

You need a large, flat and level space for the setting out.

Mark the span of the opening on a suitable board (Fig. 1, a).

Determine the springing points (at least 2 cm from the edge of the board) and drive in short nails to mark these points. Leave the nails projecting, and make sure that they are both the same distance from the edge of the board (Fig. 1, A & B).

Place the board on the ground and secure it with nails or pegs as shown, so that it cannot shift to any side.

Fix your mason line on nail A, and measure off from there three times the span of the opening along the line. Use this length to describe a short section of a circle on the ground (Fig. 1, b\.

Repeat this procedure with the line fixed on nail B (Fig. 1, c). Both circle sections will intersect at point C, which is the centre of the arch. Secure the centre point with a nail or peg.

fasten the mason line on peg C and tie a pencil at the distance of three times the span of the opening.

Keep the line taut and describe a circle section, while marking the section on the board. This curved line should intersect with the springing points, A and B, and the curve resembles the curve of the intrados.

To cneck the accuracy of the setting out, measure the rise of the arch perpendicularly from the springing line to the highest point of the curve. This distance must be 4.2% of the span (0, 042 x span (in cm) equals the rise (in cm).

POSITIONS OF THE ARCHSTONES

Regardless of which type of arch is constructed and what kind of archstones are used, the total number of blocks or bricks must add up to an odd number: the keystone in the middle is flanked by an even number of archstones (Fig. 1).

Therefore the position of the keystone is usually marked first, followed by the archstones to the left and right. Cross joints are 1 cm wide at the intrados (Fig. 1, a), and wider at the extrados, because common blocks are used which results in wedge-shaped joints.

The marking continues until the springing points are either just reached or a little overlapped (Fig. 1, b). If it happens that there is a distance of more than 1 cm between the lower outside corner of the springers and the springing points, then one more block must be added to each side (Fig. 2). These two blocks will then become the springers, although they overlap by far the springing points.

There are two ways to avoid this:

One can reverse the marking by starting from the springing points and marking by turns from the left and right up towards the crown. The remaining opening is then filled with either a specially made keystone, or with concrete (Fig. 3). This is preferred if the springers would overlap the springing points by too much.

The most perfect solution would be to use specially made archstones which are wedge-shaped and fit exactly in the arch (Fig. 4). This requires some detailed and accurate planning, and more time and materials.

SETTING UP THE CENTRING

The board on which we marked the curve of the intrados and the positions of the archstones is now prepared for assembly and erection.

The first step is to cut off both ends of the board squarely at the marks which indicate the span. These cuts must pass through the springing points. Then the intrados is roughly shaped with the saw and then exactly planed to the mark, to make a smooth and evenly curved edge.

If the centring will be a part of the frame, it is now fitted to the frame head as in Figs. 1 and 2, and set into its place together with the frame. in this case it is best to make two identical turning pieces to seal the camber on both sides of the wall. They are fixed outside and inside flush with the frame. The hollow part in between the pieces is filled with a mixture of sand and ash; both to keep insects out and if there is no lagging to give more support to the archstones which otherwise would have to rest on the turning pieces only.

Note that the marks of the springing points are cut off when the turning piece is combined with the frame. This is because it is cut flush with the outside of the frame. This means that the thickness of the joint between the frame and the wall has to be taken into account when marking the slope of the skewback.

If the centring piece is to be removed after the arch is finished, it must be set in a way so that it can easily be taken out. This is done by placing it on wedges as in Fig. 3, (a). It might be necessary to cut the turning pieces slightly shorter on one end so that the centring fits easily into the opening. in that case the end which has not been cut rests against the wall while the gap on the other side is closed tight with wedges to prevent shifting (Fig. 3, b).

Try to avoid making the springing points in line with a bed joint. The sideways force from the arch can possibly cause cracks along the bed joint. The best solution will be to distribute the pressure over two courses (Fig. 3).

When you set the centring piece in position, do not forget to level the soffit, which must be exactly horizontal and parallel to the springing line.

SKEWBACK TEMPLATE

Before you can start laying the archstones, the abutments with their skewbacks must be prepared so that the springers can be laid at the correct angle. To be sure that the angle of the skewback is correct, the Rural Builder can use the template described below (Fig. 1).

This skewback template can only be used for a segmental arch where the radius of the arch is 3 times the span of the opening. Provided the rule of 'r = 3 x span" is observed strictly, the same template can be used for any segmental arch, regardless of the length of the span. in other words the width of the opening and the size of the arch make no difference in the angle of the skewback for arches made according to this rule; the angle will always be the same.

MAKING THE TEMPLATE:

The template is made from wooden battens. It is simply a triangle with the outside dimensions as follows: side "a" = 33 cm, side "b" = 50 cm, and side "c" = 55 cm (Fig. 1). The angle at C is the angle we need to mark the skewback.

Use three battens that are a few centimetres longer than the measurements required. This allowance can be cut off when the template is assembled. Plane one edge of each until it is exactly straight and mark the length on this edge. Nail the battens together as in Fig. 1; or make accurate halving joints and join the pieces together with wing nuts instead of nails, so that they can be taken apart and put back together when they are needed in the future. The corners should be marked on both sides with the letters A, B and C, so that the ends are not accidentally mixed up when it is reassembled.

USE OF THE TEMPLATE:

The outside edge measuring 50 cm (side b) is held along the springing line, while the tip of the corner C is at the springing point (Fig. 2). The direction of the shortest side (side a) extended upwards with a straight edge gives the slope of the skewback. To mark the skewback on the other side, the template is simply turned around and the procedure is repeated.

- Note that corner B must always point downwards during the marking.

LAYING THE ARCHBLOCKS

The four blocks which have the skewbacks are marked and cut before they are laid in mortar. They are set temporarily in place on wooden battens which have the same thickness as the bed joint (Figs. 1 & 2, right sides). Mark the slope of the skewback with the template, and remove the blocks to cut them to shape. Then they can be laid in mortar and the arch can be continued.

Fig. 1 shows a segmental arch built with landcrete blocks. For this purpose, 1/2 blocks have to be cut, or preferably made specially in the Tek block press. This is easily done by pitting a 1 cm thick board (for the 1 cm joint) edgewise in the middle of the mould box. The arch is built up by laying the blocks by turns on alternate sides, as indicated in Fig. 1 by the numbers.

The axis of each block should point towards the centre of the arch, with the result that ideally only the middle part of each block should rest on the turning piece, and the lower corners of the blocks do not contact the turning piece; there should be small wedge-shaped gaps. This is because the block faces are straight but the top edge of the turning piece is rounded.

The wedge-shaped joints between the archblocks need special attention. They must be properly filled with a rather wet mortar. If there is a gap left in the centre of the arch it is closed with concrete, which functions as a keystone (Fig. 1).

Segmental arches over a span exceeding approximately 120 cm should be constructed with sandcrete blocks. The skewback blocks which abut the springers should also be sandcrete, so as to better distribute the pressure (Fig. 2).

To meet this requirement specially made blocks are needed. The skewback blocks must be 1 cm smaller in height than normal sandcrete blocks, so that they fit in with the landcrete courses. This is achieved by inserting a 1 cm thick piece of wood into the sandcrete block machine.

The archstones consist of 1/4 blocks which can also be made in the machine by inserting three 2 cm thick boards at a distance of 10 cm apart. The keystone is formed with a normal block of 15 cm thickness, cut to match the thickness of the wall.

If the centring is not combined with the frame, it is slightly lowered on completion of the arch, and removed after four days.

NOTE: Try to set the centring so that the crown of the arch will be level with the top of the upper abutment course (Figs. 1 & 2) and not somewhere in the middle. The distance between the springing line and crown can be calculated as follows: 0,042 x span, plus height of archblocks.