Details of arches

Details of arches their types and material used are explained in detail.

1. Sanjivani Rural Education Society's Sanjivani College of Engineering, Kopargaon -Department of Civil Engineering- Course Title: (BUILDING TECHNOLOGY AND MATERIALS) Course Introduction By Mr. Sameer shaikh (Assistant Professor) Mail ID- [email protected]

2. ARCH :- INTRODUCTION An arch may be defined as mechanical arrangement of wedge-shaped blocks of stones or bricks mutually supporting each other and supported at end by piers or abutments. Arch is a structure that spans a space and supports structure and weight below it.

3. Function of an Arch:- The function of an arch is to carry weight of the structure above the opening. Because of their shape, the blocks support each other by mutual pressure of their own weight. Hence the structure remains in the position by the resistance from the support.

4. Elements of Arches

5. Technical Terms: The various technical terms used in arches are as follows:- In tra dos: This is an inner curve of the arch. Extrados: It is the outer curve of an arch. Voussoirs: The voussoirs or arch stone are the wedge shaped units forming the arch. Crown: It is the highest part of the extrados. Keystone: This is the highest central wedge-shaped block of an arch. Haunch: It is the lower half of the arch between the crown and skewback. Springing points: These are the points from which the curve of the arch springs.

6. Spandril: This is the curved triangular space formed between the extrados and the horizontal line through the crown. Rise: It is the clear vertical distance between the highest point on the intrados and springing line. Abutment: This is the end support of an arch. Pier: This is an intermediate support of an arcade . Springing line: It is an imaginary line joining the springing points of either end. Skewback: It is the inclined or splayed surface on the abutment which is so prepared to receive the arch and from which the arch springs. Span: This is the clear horizontal distance between the two supports.

7. A) Classification of Arches Based on Shape: Flat arch:- In this type of arch the intrados is apparently flat and it acts as a base of equilateral triangle which was formed by the horizontal angle of 60° by skewbacks. These flat arches are generally used for light loads, and for spans up to 1.5m.

8. Segmental Arch:- This is the basic type of arch used for buildings in which centre of arch lies below the springing line. In segmental arch, the thrust transferred to the abutment is in inclined direction.

9. Semi-circular Arch:- The shape of arch curve looks like semi-circle and the thrust transferred to the abutment is perfectly in vertical direction since skewback is horizontal. In this type of arch, the centre lies exactly on the springing line.

10. Horse Shoe Arch:- Horse shoe arch is in the shape of horse shoe which curves more than semi-circle. This type of arch is provided mainly for architectural consideration.

11. Pointed Arch:- In this type of arch two arcs of circles are met at the apex hence triangle is formed. This may be either isosceles or equilateral. It is also known as Gothic arch.

12. Venetian Arch:- This type of arch is another form of pointed arch. Venetian arch has deeper depth at crown than springings. It contains four centre's , all located on the springing line.

13. B) Classification of arches based on number of centres: One Centred Arches:- Segmental, semi-circular, flat, horse-shoe arches and silted arches are one centred arches. In some cases, perfectly circular arch is provided for circular windows which is called as bull’s eye arch also comes under these category.

14. Two Centred Arches :- Pointed or gothic or lancet arches comes under this category. Three Centred Arches:- Semi-elliptical arch and Florentine arches comes under this category as they have three centres .

15. Four Centred Arches:- Venetian arch is a typical example for four centred arch. Five Centred Arches:- A good semi-elliptical shape arch Comes under this category.

16. 2. Brick arch – ( a) Rough arch ( b) Axed or rough cut arch ( c) Gauged arch ( d) Purpose made brick arch C) Classification of arches based on Material and Workmanship 1. Stone arch – (a) Rubble arches (b) Ashlar arches 3. Concrete arch – ( a) Precast concrete arch ( b) Monolithic arch

17. (1) Stone Arch (1) Rubble Arches: Rubble arches are very weak and used only for inferior work, these are used for spans up to 1m. These are made of rubble stones which are hammer dressed, roughly to shape and size and fixed in cement mortar. Sometimes these are also used as relieving arches up to a depth of 37.5cm, but these are constructed in one ring. If the depth is more, we can go for two rings in alternate course of headers and stretchers.

18. (2) Ashlar Arches: In this type of arch the stones are cut to proper shape of voussoirs (a wedge-shaped or tapered stone used to construct an arch) and fully dressed, joined with cement mortar. Ashlar stones are also used to make flat arches.

19. (ii) Brick Arches (1) Rough Brick arches: This type of arch is constructed with ordinary bricks without cutting these to the shape of voussoirs. The arch curve is provided by forming wedge shaped joints with greater thickness at extrados and smaller thickness at intrados. So, it looks unattractive. That's why it is not recommended for exposed brick works.

20. (2) Axed Brick arches: The bricks are cut into wedge shape with the help of brick axe. So, these are roughly dressed in shape and size. Arch formed by these axed bricks is not very pleasant.

21. (3) Gauged Brick arches: This type of arch is constructed of bricks which are prepared to exact size and shape of voussoirs cutting it by means of wire saw. The surface of bricks are fine dressed with the help of a file. F or gauged brick arches only soft bricks are used. The joints formed in gauged brick arch are fine, thin (1 to 1.5 mm) and truly radial. Lime putty is used for jointing.

22. (4) Purpose Made Brick Arch: These arch are made with special bricks which are manufactured in exact shape and size of voussoirs. This is very fine workmanship. Lime putty is used as binding material.

23. (iii ) Concrete Arches Precast Concrete Block Arches: Such arches are made from precast concrete blocks, each block being cast in the mould to the exact shape and size of voussoirs. For keystone and skewbacks special moulds are prepared. Because of exact shape and size of blocks, good appearance of the arch is achieved also joints made of cement mortar are quite thin. However, casting of blocks is costly and such work is economical only when the number of arches is quite large. Cement concrete of 1:2:4 mix is usually used.

24. (2) Monolithic Concrete Arches: Monolithic concrete arches are cast-in-situ concrete, either plain or reinforced depending upon the span and magnitude of loading. These arches are quite suitable for larger span. The arch thickness is 15cm for arches up to 3m span. Formwork is used for casting the arch, and is removed only when concrete has sufficiently hardened and gained strength. The curing is done for 2 to 4 weeks.

25. Construction of Arches The construction of arches, of all types of materials ( i.e. bricks, stones, concrete) is carried out in three steps: (1) Installation of centring or formwork, (2) Laying or casting the actual arch and (3) Striking or removal of centring as formwork.

26. Step 1 – Installation of Centring : Centring is the temporary structure required to support brick, stone or concrete arch during its construction till it has gained sufficient strength. The centring is installed in such a way that its upper surface corresponds with the intrados of the arch. For minor works centring may be made of mud masonry constructed to match with the inner soffit of the arch and then plastered. This masonry is dismantled later when the arch has been constructed and cured. The usual centring is made of timber or steel. Wooden centring is simplest and cheapest one which is used for moderate spans.

27. It is easy to construct and easy to dismantle and it can be used several times. The centring made of wooden piece is known as turning piece. They are supported by props and a pair of folding wedges is provided at the top of the props under the centring to tighten or loosen the centring. Centring for wide soffits and bigger spans

28. Step 2 – Laying of Arch: After the erection or installation of centring, skewbacks are first prepared and placed very carefully in the correct angles. Voussoirs are then arranged in proper and required forms, starting from skewbacks and proceeding towards the crown from both ends. Finally, keystone is inserted so that all voussoirs are locked in position. It is important that the masonry above the skewback at the haunches should be loaded by filling up the spandrels upto the level of the crown of the arch or at least 2/3 of the height of the arch to counteract the thrust, this should be done on both the sides

29. Before the mortar hardens the centring is slightly slackened by means of easing the wedges on the top of props by only 2 to 3 mm to permit the voussoirs to sit on their beds properly. Such a process is an essential requirement in stone arches, since it permits the voussoirs to settle upon their beds properly.

30. Step 3 – Removal of Centring : When the arch has developed sufficient strength, the centring can be removed. No load should be placed on the arch unless the centring has been removed. For small spans, removal of centring is done by loosening the folding wedges. When the span is more than 7 m, sand box method can be used for loosening, so that shocks are avoided. A sand box as shown in fig is placed below the prop. Sand is filled in box with a plugged hole at its bottom.

31. Prop rests on the steel plate placed on the top of sand. In order to lower the centring, plug is taken out due to which the sand flows out and lowers the prop gradually. Sand Box Method