ARCHES, LINTELS & BALCONY

ARCHES

An arch is a structure constructed of wedge shaped units (brick or stone), jointed together

with mortar and spanning an opening to support the weight of the wall above it with other

super imposed loads.

TERMS USED:

1. Intrados: This is the inner curve of an arch.

2. Soffit: It is the inner surface of an arch.

3. Extrados: This is the outer curve of an arch.

4. Voussoirs: The wedge shaped units of masonry which constitute an arch.

5. Crown: The highest part of Extrados.

6. Key: The wedge shaped unit fixed at the crown of the arch.

7. Spandril: The curved triangular space formed between extrados and horizontal line through

the crown.

8. Skew back: The inclined surface on the abutment from which the arch springs.

9. Springing Points: These are the points from which the arch springs.

10. Springing Line: It is an imaginary line joining the springing points of either end.

11. Springer: It is the first voussoir at springing level or immediately adjacent to the skewback.

12. Abutment: This is the end support of an arch.

13. Pier: This is an intermediate support between 2 arches or an arcade.

14. Arcade: It is a row of arches in continuation.

15. Haunch: The lower half of the arch between the crown and skewback.

16. Ring: The circular course forming an arch.

17. Center or Striking point: The geometrical center from which the extrados, arch rings and

intrados are described.

18. Span: It is the clear horizontal distance between the supports.

19. Rise: It is the clear vertical distance between highest point on intrados to the springing line.

20. Depth or Height: Perpendicular distance between intrados and extrados.

21. Thickness: It is the breadth of the soffit or the horizontal distance measured perpendicular

to the front and back faces of an arch.

CLASSIFICATION OF ARCHES

BASED ON SHAPE:

1. Flat Arch: A flat arch has usually the angle formed by skewbacks as 60º with horizontal

thus forming an equilateral triangle with intrados as the base.

The intrados is flat but given a slight rise of about 10-15mm per meter width to

accommodate small settlements. The extrados is horizontal and flat. They are used for

light loads, and for spans up to 1.5m.

2. Segmental Arch: This is the common type of arch used in buildings.

The center of arch lies below the springing line. The thrust transferred to the abutment

is in inclined direction.

3. Semi-circular Arch: This is a modification of the segmental arch in which the center lies

on the springing line. The shape of the arch curve is a semi-circle.

The thrust transmitted to the abutments is perfectly in vertical direction since the

skewback is horizontal.

4. Horse-Shoe Arch: The arch has a shape of an horse shoe, incorporating more than a

semi-circle.

5. Pointed Arch: It consists of two arches of a circle meeting at the apex. The triangle

formed in the inner portion may be equilateral or isosceles. They are known as Gothic

arch and lancet arch respectively.

6. Relieving Arch: The arch is constructed either on a flat arch or a wooden support or

lintel to provide greater strength. This type of arch enables replacement of decayed

lintel without disturbing the stability of the structure.

7. Stilted Arch: It consists of a semi-circular arch with 2 vertical portions at the springing.

The center of arch lies on the horizontal line through the tops of vertical portions.

BASED ON NUMBER OF CENTERS

1. One Centered Arch: Segmental arches, semicircular arches, flat arches etc all come

under this category. Sometimes a perfectly circular arch, known as bull’s eye arch is

provided for circular windows.

2. Two-Centered Arch: Pointed arches come under this category

3. Three-Centered Arch: Elliptical Arches come under this category.

4. Four-Centered Arch: It has 4 centers. Tudor arch is a good example.

5. Five-Centered Arch: These type of arches having 5 centers give a good semi-elliptical

shape.

BASED ON MATERIAL AND WORKMANSHIP

1. Stone Arches: Depending upon workmanship, stone arches are of two types

(i)RUBBLE ARCHESs: These arches are comparatively weak and used for inferior

work. These are made of rubble stones which are hammer dressed, roughly to

shape and glued together using cement as the mortar. They are used for spans of

1m. They are also used as relieving arches over wooden lintels.

(ii)ASHLAR ARCHES: In this the stones are cut to proper shape of the voussoirs

and are fully dressed, set in lime or cement mortar.

This method can be used also to make flat arches, in which the joints are

either joggled or rebated as shown.

2. Brick Arches: Brick arches can be classified as follows depending upon

workmanship and the quality of bricks used

(i)ROUGH BRICK ARCHES: This type of arch is constructed with ordinary bricks

without cutting them to the shape of voussoirs. To provide the wedge shape to the

arch the joints are made wedge shaped. Due to this the beauty of the arch is spoiled,

so these are not used when the brick work is exposed.

(ii) AXED BRICK ARCHES: In this the bricks are cut wedge shaped with the help of

brick axe. Due to this the joint thickness is uniform along the radial line. However

appearance is not very pleasant as bricks cut to wedge shape are not properly

dressed.

(iii) GUAGED BRICK ARCHES: In this the bricks are prepared to exact size and shape

of voussoir by cutting it by means of wire saw. The surface of the bricks is finely

dressed with a file. Only soft bricks called rubber bricks are used. The joints formed

are fine, think and truly radial. Lime Putty is used as the mortar.

3. Concrete Arches: Concrete Arches of two types

(i)PRECAST CONCRETE BLOCK ARCHES: Such arches are made from precast

concrete blocks, each block being cast in the mould to exact shape and size of the

voussoirs. Joints are made of cement mortar and are thin. However casting the

blocks is costly and hence it becomes economical only when number of arches is

large.

(ii) MONOLITHIC CONCRETE ARCHES: These arches are constructed from cast-in-

situ concrete, either plain or reinforced as per requirement. These are ssuitable

for larger span. Form work is used for casting the arch, and is removed only when

concrete has sufficiently hardened and gained strength. Curing is done for 2 to 4

weeks.

STABILITY OF AN ARCH An arch transmits the super-imposed loads to the side of the walls through friction between

the surfaces of voussoirs and the cohesion of mortar. Every element of the arch remains in

compression. An arch may therefore fail in the following ways:

• Crushing of the masonary: Crushing will occur when compressive stresses in the arch

exceed the compressive strength of the brick masonry. If compressive stresses are too

large, the arch must be redesigned with a shorter span or a greater arch depth.

• Sliding of voussoir: Sliding of the voussoirs will happen depending on the angle of

skewback (measured from horizontal) and the vertical load carried by the arch. This

wont happen if the courses of stone are laid perfectly parallel to the abutments, which

in a regular arch causes them to lie perpendicular to its faces. It can also be prevented

by providing voussoirs of variable heights i.e less height near crown and more near the

abutments

• Uneven Settlement of abutment/pier: The uneven settlement of the abutment may

cause secondary stresses in the arch. Hence the abutment, which has to ultimately bear

all the loads transferred to it through the arch, should be strong enough. Also the arch

should be symmetrical so that unequal settlements of the two abutments is minimized

• Rotation of some joint about an edge: Rotation occurs when tension develops in the

arch. Tension can be reduced by increasing the depth or rise of the arch. It can also be

prevented by keeping the line of resistance is kept between the extrados and the

intrados. Also the line of thrust should be made to cross the joint away from the edge to

prevent the formation of hinges.