Lintel and Arches

58
LINTEL and ARCH Lecture 6

description

Civll Engineering

Transcript of Lintel and Arches

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LINTEL and ARCH

Lecture 6

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LINTEL• Definition: A horizontal structural member

which is fixed over the openings.– For example: doors, windows etc

• Purpose: to support the structure over the openings.

It is generally rectangular in shapeMaterials- Timber, Stone, Brick, Reinforced

brickwork, RCC or Rolled steel sections

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• Best materials is RCC• Timber is liable to decay• Stone lintels are costly and also difficult to

obtain the stone of desired length without flaws

• Rolled steels are better than timber or stone, but due to high rate of expansions it becomes loose when exposed to direct sunlight.

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General rules of Constructions• Adequate bearing should be provided– End bearing should be 100 mm or 40 mm for every

300 mm span, whichever is higher– For a long span, bearing should be at least equal to its

depth• Depth: 1/12 th of the span or 150 mm whichever

is higher– Rounded off to the multiples of thickness of masonry

layers• Width of the lintel is equal to the width of the

structure above it• Design load on lintel

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Types of Lintels• Wooden lintels:– Oldest type of lintel– These are commonly used in hilly areas– These are susceptible to catch fire, attacked by

termites or liable to decay – Either a single piece for small span, or multiple

pieces bolted together can be used for longer span– It should be made from hard and sound timber.

Sal, Teak or rose wood etc. can be used as lintel– To reduce the load, arches are build

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Stone Lintel• USE: It can be used in the places where stone

is easily available and the structure is made of stone masonry

• The use of stone lintel is generally not recommended because of the following reasons:– Stone being poor in tensile strength, cannot

withstand the transverse stresses. Hence it should not be used for more than 1 m wide opening

– It is generally costly and difficult to obtain the piece of sufficient length and thickness

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Example of stone lintel

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Brick Lintel

• Use: for small openings (about 1 m) with light loading

• Bricks are normally laid on its end and occasionally on it edges

• Constructed over temporary wooden support (Turning piece)

• Bricks with frogs are more suitable as they form the bond

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Salient features of brick lintels

• Brick should be well burnt, free from defects, cracks etc. with sharp and square edges

• Cement morter (1:3) should be used• Brick lintels can not be used for heavy loads

and longer spans

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Steel Lintel

• Made up with RSJ (Rolled Steel Joist)• May be embeded in concrete• May be used over large openings• Can support heavy loads• Sections are: angles, I-sections, T- sections etc.• But very costly for longer openings

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RCC Lintel

• Advantage of RCC lintel over others:– RCC is fire proof, durable, strong, economical and

easy to construct– Provision of arches are eliminated– Can be used for any span and loading– Speed of construction can be increased if precast

lintels are used

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• Types of RCC lintels and their uses:– Precast and Cast in Situ– Precast is more useful for multi-storied buildings

with similar doors and window openings up to 2 meters

– Cast – in – situ are useful when openings differs– Care should be taken while placing the precast

RCC lintels; top mark should be kept at top– Time of construction and expenses can be saved in

precast lintels as same mould can be used for multiple lintels

– Curing time can be saved- so speed of construction increases

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RCC Lintel

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Pre-cast RCC lintel

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ARCHES• Arch- A mechanical arrangement of wedge

shaped block of stone/bricks which mutually support each other and in turn, the entire arch is supported at the ends by piers or abutments

• The wedge shaped units are so arranged together along a curved line that they balance their own weight by the mutual pressure and exert only vertical pressure which can be sustained safely by the supports below

• Arches can also be made of RCC or Steel as a single unit

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Terminology related to arch• Abutments- These are the outermost supports

of an arch, from which the arch springs. The abutments are designed to resists the inclined thrust from one or more arches, so they are made of ample size.

• Piers- these are the intermediate supports of a series of arches.

• Arcade- This is a series or row of arches supporting a wall above and being supported by piers

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• Skew-backs- these are the inclined or splayed surfaces of the abutments which are prepared to receive the arches and from this the arches springs. In some types of arches skew-backs are not required.

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Voussoirs – these are the wedge shaped or tapered units of stone, brick or concrete forming the archRing/Rim/Ring course- this is the circular course forming the archExtrados/back- this is the upper or external curve of the arch

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• Intrados- this is an inner curve of the arch• Soffit or bottom – this is the inner or under

surface of the arch• Crown- This is the highest point of the

extrados on a arch• Key – This is the uppermost or central voussoir

of an arch. It is sometimes made prominent by making it larger and projecting it above and below the outline of an arch.

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• Springing point- These are the points of the intersection between the skew-backs and the intrados and from these points only the curve of an arch springs

• Springing line – This is the horizontal imaginary line joining the two springing point

• Springer- These are the extreme or lowest voussoirs of an arch which are placed at springing level on either side immediately adjacent to the skew-backs

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• Haunch- this is the lower half portions of the arch between the crown and skew-back or springer

• Span- This is the clear horizontal distance between the support or springing points

• Rise – this is the vertical distance between the springing line and the highest point of the intrados

• Center/striking point – this is the geometrical center point from which the arch forming the extrados, arch rings and intrados are described or struck

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• Depth or height – this is the perpendicular distance between the intrados and extrados

• Thickness (width of the soffit) – this is the horizontal distance between and at right angles to the front and back faces of an arch

• Spandril- this is the triangular space formed between the extrados and the horizontal line drawn through the crown

• Jambs- These are the sides of the abutments or piers below the springing line

• Impost – this is the projecting course(s) at the upper part of the pier or abutment to stress the springing line.

• Bed joints- these are the joints between the voussoirs which radiate from the center

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Stability of an arch

• It depends on the friction between surface of wedge shaped blocks and the cohesion of the morter.

• Stability may be endangered by:– Crushing of the arch material– Sliding of the wedge shaped block– Rotation or overturning about any edge– Differential settlement of the supports

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Important points about stability

• To prevent crushing of the arch material:– The size of the block should be adequate to

withstand the thrust– For small spans, the thickness of the arch ring

is kept uniform from crown to the springing– The thickness may be taken as 1/12 th of the

span or 20 cm (upto 1.5 m), 30 cm (1.5 – 4 m), 40 cm (4 - 7.5 cm)

– For larger spans (> 7.5 m) thickness of the arch ring may be increased at springing by 20% to the thickness at the crown

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Important points about stability (contd..)

• To safegaurd against sliding of one voussoir over another the following points should be considered:–All the bed joints should be perpendicular

to the line of least resistance– The depth of the voussoirs should be

adequate to resists the tendency of the joints to open and to slide upon one another

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To prevent rotation or overturning about edges

– The line of resistance or thrust at any section should be within middle third of the arch– The thickness of the arch and its curves are so

designed that the line of resistance falls within the section and crosses each joint away from the edge

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To safegaurd against the differential settlement of the support:– The abutments should be sufficiently strong to

resist the thrust of the arch due to self-weight and superimposed loads. Semicircular arches are the strongest and exert no horizontal thrust to the abutments or piers

– Foundation of the abutment/pier is most important and take necessary precaution

– Whatever may be the shape of the arch, it should be symmetrical to avoid differential settlement

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Types of arches

• Classification according to the number of centers

• Classification according to the shape formed by the intrados

• Classification according to the materials and workmanship employed in the construction

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Classification according to the number of centers

• One centered arch –– Semi-circular arch, Horse – shoe arch, Stilled arch,Bull’s eye/circular arch (used for circular window)

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Semi circular arch

Circular/Bull’s eye arch

Circular archNo force is coming from the top

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Two centered arch – Equilateral arch (gothic arch)– Acute arch (lancet arch)– Obtuse arch (drop arch)

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Three centered arch

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According to the shape of intrados

• Flat arch –• Semi-circular arch-• Segmental arch –• Relieving arch –• Others – gothic, elliptical, horse-shoe, acute,

corbel, cusped arch

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Flat arch

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French/Dutch arch

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Relieving arch

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According to the materials

• Stone arch– Rubble– Ashler

• Brick arch– Rough brick arches– Gauged brick arches

• Concrete arch– Concrete block arches– Monolithic arches

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Concrete arch

• Concrete block arch- it is constructed same as other arches

• Monolithic concrete arches- It is constructed cast in situ concrete preferably with reinforcements

• For smaller span and loads it can be constructed without reinforcements

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Method of construction (Arch)

• Basically three steps:– Installation of centering or formwork for arches– Laying of arch-work or courses– Striking or removal of centering

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Centering for arches

• Construction of arches starts from the spring point and brought up uniformly up to the crown

• Hence support is required (called as centering)• Most common centering – timber/mild steel

truss• It depends upon- span, shape of the arch,

thickness of soffit , materials for construction

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Timber centering

• Simple form (shorter span) - turning piece

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smaller spans (thicker soffit) - Laggings

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Very long span – Steel Truss

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Important points about centering

• Centering type should be selected with due consideration to the width of soffit and span of the opening– For half-brick or small spans: single turning piece– For full brick or more and small span: ribbed centering– Larger span: steel struss

• Top of the centering should be the same shape that of arch soffit

• The length of the laggings and the spacing of ribs depends on the thickness of the wall

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Details of Lintels