WOODEN LINTELS
In hilly areas, where timber suitable for the construction of lintel is
easily available, wooden lintels are commonly used. In plains, the use of
wooden lintel is very rare on account of high cost of the material. Timber if
not properly ventilated, is liable to decay. Wood being combustible, wooden
lintels cannot be recommended for fire proof construction. The minimum depth of
the lintel should be 7.6 cm. Sometimes, a rough arch is constructed above a
lintel to relieve the latter of the weight above. When used for large spans,
the lintel should be properly designed as a timber beam simply supported at its
ends.
STONE LINTELS
The use of stone lintels is very common in stone districts. Its use is
usually confined to stone masonry structures or buildings faced with stones. In
general, stone lintel, on account of its high cost and its inability to
withstand excessive transverse stresses is rarely used. The minimum thickness
of stone lintel in kept 8 cm. and as a thumb rule, the thickness is taken as 4
cm. per 30 cm. length of span.
BRICK LINTELS
They are constructed with hard, well burnt, first class bricks laid on
end or on edge. Depending upon the span, their depth varies from 10 cm. to 20
cm. They are considered suitable for small openings with light loading conditions.
The frogs, when filled with mortar, form joggles which increase the shear
resistance of the end joints and ultimately result in increased strength of
lintel.
REINFORCED BRICK LINTELS
When brick lintels are required to be provided over large spans, they are
reinforced with steel reinforced. The depth of the lintel is limited to 10 cm.
or a multiple of one brick thickness. Bricks are so arranged that 1.9 cm. to
3.8 cm wide space is left length wise for the insertion of reinforcements. The
gap is then filled with rich cement mortar or cement concrete.
REINFORCED CONCRETE LINTELS
On account of their durability, strength and fire resisting properties,
reinforced concrete lintels are very common in use. They are economical and
easy in construction. They can be used for bigger spans and heavy loading
conditions.
R.C.C. lintels are constructed with 1:2:4 cement concrete reinforce with
steel bars. They may be pre cast or cast-in-situ. For smaller spans, say up to
1.2 m pre cast lintels are preferred.
Pre-cast lintels are manufactured and cured well in advance and as such
prove economical since the same mould can be used a number of times. The use of
pre cast, R.C.C. lintels expedites the construction. While laying a pre cast
lintel special care should be taken to see that the marked face of the lintel
is kept at top. If the lintel is placed in reverse position the marked face
pointing downward, disastrous result may take place. For larger spans, lintels
are cast-in-situ, as pre cast lintels will have to be provided with additional
reinforcements to guard against the lifting stress. For lintels over large
spans subjected to heavy loadings, the depth of the lintel and the area of the
reinforcement should be accurately computed.
Depth of lintel for ordinary loading:
Up to 1.2 m length of span, a 15 cm. depth lintel is found to be safe. As
general rule, add 2.5 mm. for every additional 30 cm. span.
Reinforcement for lintels for
ordinary loading:
The number of main bars for a lintel depends upon the load from wall to
be carried above and the span of the opening. As a general rule, for each 10
cm. thickness of wall, one main bar should be provided. The diameter of the bar
varies with the span. Up to 1.2 m span 10 mm dia bar; from 1.2 m. to 2 m.
span-12 mm. dia bar and from 2 to 3 m. span-16 mm. dia bar will be found
adequate.
STEEL LINTELS
When a large opening carrying heavy wall load is to be spanned by a
lintel an R.C. lintel will work out to be very deep. If due to architectural
reasons, the depth of the lintel is to be restricted, it is common to use steel
lintel. Steel lintel consists of section of rolled steel joists, used singly or
in a combination of two or three. Built up lintel consists of three R.S.J’s
spaced together and connected to each other by bolts passing them at intervals.
This system is embedded in concrete and is cured like R.C. lintel.
ARCH
An arch may be defined as mechanical arrangement of wedge-shaped blocks
of stones or bricks mutually supporting each other and supported at the end by
piers or abutments. In common with lintels, the function of an arch is to carry
the weight of the structure above the opening. Because of their shape, the
blocks support other by the mutual pressure of their own weight and the
structure remains in position by the resistance from the support.
TYPES OF ARCHES
Arches derive their names from the shape of curve formed of their soffit
or intrados. This type of arches commonly adopted in construction work are:
1.Flat Arch
2. Semi-circular Arch
3. Segmental Arch.
FLAT ARCH:
When used over opening of door or window, the flat arch acts similar to a lintel. Flat arch also known as straight or camber arch, is laid with its bed joints radiating to centre. The skewback is made to rest in an inclined position so as to make an angle with the springing line. The extrados of the arch is horizontal but the intrados has a straight camber or upward curvature. The camber is usually 2 mm. for every 30 cm. of span and is necessary to avoid optical illusion of sagging. Flat arches are not strong as compared with other them.
SEMI-CIRCULAR ARCH:
The arch derives its name from the shape of the curve given to the arch soffit
SEGMENTAL ARCH:
This type of arch is segmental in shape and is commonly provided for openings of various sizes. The bed joints of segmental arch radiate from a common point, which lies below the springing line and is equidistant from all pints on the arch curve. When used over a lintel, segmental arch is termed as “relieving arch”.
MATERIALS OF CONSTRUCTION
Arches may be constructed with bricks, stones or concrete. Classification
of arches by material is given below:
BRICK ARCHES:
Depending upon the nature of workmanship and the quality of bricks used,
brick arches may be classified as rough arches, axed brick arches, gauged brick
arch and so on.
Rough arches: These arches are built with ordinary bricks which are not
cut to wedge shape. In order that all the bed joints may be normal to the curve
of arch, the joints are made wedge shaped.
Thus the joints at the extrados are wider than those at the intrados. The
wedge-shaped joints spoil the appearance of the arch and as such rough arches
are not recommended for exposed brick work.
These arches are generally used over wooden lintels in order to relieve
the latter of the weight of the masonry above. Hence they are sometimes termed
as “relieving arches”.
Axed brick arches: In this type of construction, the bricks are cut to
wedge shape. Thus the joints of the arch are of uniform thickness. Since the
wedge-shaped units are not finely dressed, the arch does not present an
attractive appearance. It is therefore common to finish the brick work by
plastering.
Gauged brick arches: As the name suggests, gauged bricks are those which
have been accurately prepared to a wedge shape for the arch construction. Hard
bricks cannot be used in this type of construction as it is difficult to cut
them to true wedge shape accurately. Hence, specially made bricks known as
“rubber bricks” are used in the arch construction. The bed joints are very thin
usually 1.6mm. thick. Cement mortar is considered too coarse for such joints;
hence lime putty is used for binding blocks. The thin white joints add to the
appearance of the finished work.
Bonds in brick arches: For ordinary work, bricks in concentric rings of
half bricks are laid with non-continuous radial joints. Each ring is fully
completed before constructing the ring immediately above it. For spans above
1.6 m. the concentric rings of brick work have a tendency to separate and there
is a continuous danger of the whole pressure being thrown to one ring, which
might get crushed and thus collapse the entire arch structure. The danger is
avoided either by laying bricks alternately as headers and stretchers in
section with continuous radial joints or by providing key stones at intervals.
STONE ARCHES
Considering the nature of workmanship and dressing of the material, stone
arches may be divided in the following two categories:
Rubble arches:
They are constructed with rubble stones which have been hammer dressed,
roughly to required shape and size of the voussoirs. In arch ring up to 38 cm.
in thickness, all stones should be of the full depth of the arch. In arches
above 38 cm. in thickness, the stones are laid in two rings in alternate course
of headers and stretchers. This type of construction is weak in strength and is
not attractive in appearance and is as such provided for small openings only.
Ashlar arches:
They consist of arch stones which have been dressed full and true to
their proper shapes. In all the cases where the depth of the arch is 60 cm. the
voussoirs are made of full thickness of the arch and are set in fine mortar
with their radial joints.
The stones are laid as headers and stretchers alternately, all headers
being of full depth of the ring and not more than two stretchers going to make
the full depth of the arch ring. In case where the arch thickness has to be
more than 90 cm. only the quoins and the key stones are made of the full
thickness of the arch ring. In order that the voussoirs may be dressed to wedge
shape, a full size arch I first set out on a level platform. The sizes of
voussoirs and that of the keystone are carefully marked on the platform after
leaving gap for the mortar joints between them. This method of setting out
requires slight adjustment and thereafter the required shape of the voussoirs
and key stones is cut with the help of arch set out on the platform. Finally,
the stones are cut true to the shape of the template and the voussoirs thus
formed satisfy specification of the ashlar arch construction.
CONCRETE ARCHES
Concrete block arches:
For small opening in a building concrete block arches can be provided
with advantage. The voussoirs, in the form of cement concrete blocks, are
prepared in specially made moulds for the purpose. The moulds of concrete block
for the key or shew backs are of slightly different dimensions. The concrete
mix usually adopted for block construction consists of one part of cement, two
parts of sand and four parts of aggregates. After the concrete in the mould is
fully set, the blocks of concrete are removed for curing in a water pond,
constructed for the purpose. Generally, the concrete blocks are used without
reinforcement. The arch construction with concrete block is similar to that for
stone or brick arch construction.
Monolithic concrete arches:
Monolithic concrete arching is frequently used for roofing of buildings,
culverts and bridges. The construction for smaller span with ordinary loading
conditions requires no reinforcements. The arching can be done in lime concrete
as well as in cement concrete. For roofing arches, a rise of 5 cm. for every 30
cm. of span is usually allowed when lime concrete is used. The minimum
thickness of arch should not be less than 15 cm. for spans up to 3 m. As a
general rule, add 38 mm. for each additional 30 cm. of span. For larger spans,
arches have to be reinforced.
Method of Construction:
Centering:
The usual centering for arch construction consists of a horizontal frame
of timber known as centre or turning piece. The centering differs according to
the shape of arch curve, the material used in arch construction and the span of
the opening. The centering is very rare due to high cost of material and is
usually preferred in large construction works where the same centering is to be
used a number of times. Wooden centering in its simplest form consists of a
pair of solid ribs, cut from a plank of thickness required for centering. The
upper surface of the ribs is given the shape of the soffit of the arch to be
constructed. Lagging or cross battens are nailed across the ribs at lose
intervals for supporting the voussoirs at top. The spacing of laggings may be
increased in case of stone arch. The distance between the two ribs depends upon
the thickness of the wall to be carried by the arch. Bearing pieces are nailed
at the bottom of the ribs so as to enable them to be supported on the vertical
props. For large spans, built up ribs are used instead of solid ribs. To
facilitate easing or lowering of the centering, a pair of wedges, is provided
between the supports and the bearing piece.
Procedure of construction:
After the centering is properly cerated in position, skewback are first
prepared and voussoirs are arranged in the form of arch curves starting from
the skewbacks and proceeding towards the crown. Keystone is finally inserted to
lock all the voussoirs in position. The voussoirs must be properly bedded to
ensure strength and stability of the arch. After the key has been placed in
position, the centre should be eased by slackening the wedges. During the
process of easing, great care should be taken to see that the centre is lowered
only by a small height say 3 mm. the lowering of centre is necessary in stone
arch construction as it allows the voussoirs to settle upon their beds nicly.
No load should be super imposed upon the arch work before it has fully set. The
centre is removed before the construction of masonry above the arch.
RELIEVING ARCH
Where a wooden lintel is required to span over wider opening, brick
relieving arch is usually constructed above the lintel. The arch thus
constructed relieves the lintel of the load from the masonry above and hence
the size of the lintel can be appreciably reduced. The relieving arch should
have its abutments clear of the ends of the lintel so that the skewback slopes
outward and away from the top edges of lintel ends. Thus the span of a
relieving arch will be equal to the clear span of the opening plus twice the
bearing of the existing wooden lintel. He advantages in this type of
construction is that the wooden lintel when decayed can be replaced without
affecting the stability of the structure.
DUTCH OR FRENCH ARCH
This type of arch is similar to a flat arch in design but differs in
shape and method of construction. This form of arch is not so sound and as such
it is only suitable to be provided for small openings.
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