Spanning of Opennings:A Power Point Presentation Prepared By Vaishali Anagal of Dr.B N College of Architecture,Pune.

spanning of openings Lintels and Arches

Prepared by,

v a i s h a l i a n a g a l

Introduction to spanning of openings

• Openings for doors , windows and access openings must be bridged over by some means to support the masonry above.

• Arches and lintels used to bridge the openings.

• Lintels act like a beam and transfers the load vertically downwards.

•The wall over the lintel will tend to create an arch over the opening, leaving only the area under the arch to be carried by the lintel beam. •Valid for running bond, but not stack bond. •Loads will "arch" around the load triangle, as shown in figure.

Lintels

• Openings in masonry walls are spanned by horizontal members known as lintels.

• The ht. of lintel is kept in multiple of brick course ht. i.e. 65mm.

• The depth of Lintels is not usually less than 150mm.

The lintels may be constructed of: • Timber• Stone• Brick.• Precast or cast-in-situ concrete • Structural steel • Reinforced concrete masonry.

Timber lintel

• Timber –good in compression

• Used in ancient structures.

• Not being used today because of unavailability of material, cost.

• Likely to get damaged in case of fire.

• Likely to rot causing wall over it to collapse.

Stone lintels

• Used where stone is widely available.

• High cost• Inability to withstand

transverse stresses.• Min. thk. Of stone lintel 8cm.

Brick lintels

• Hard, well burnt, first class bricks are laid on edge- soldier bricks.

• Depending upon span, depth varies from 100mm to 200mm.

• Suitable for small openings up to 900mm with light loading conditions.

• Bricks having frogs are more suitable for lintel construction.

•Frogs filled with mortar increase shear resistance of brick lintel.•For larger spans, brick lintels are to be provided with steel reinforcement.•Depth limited to 100mm or multiples of brick courses.

Concrete lintel

• Very common in use today.• Usually reinforced with steel.• Used for small and large spans, heavy loading

conditions.• Concrete- relatively cheap can be molded in

desired shape & size when in plastic condition, good strength in compression- good strength against crushing.

• Concrete lacks strength against tensile forces.

• When lintel is subjected to load, it will slightly bend; particles in upper part exert compressive forces i.e. particles are pushed against each-other, particles in lower part exert tensile forces i.e. they are pulled apart and particles in middle part are neither in compression and nor in

tension i.e. they are in neutral axis

Concrete lintel

• To give concrete required strength to resist tensile forces, steel is added at bottom of lintel with cover of concrete.

• Steel has good strength in tension.

• Cover of concrete protects steel from corrosion. In case of fire, steel rod if exposed would expand and come away from concrete and lintel would collapse.

• 1:2:4 cement mortar is used.

Reinforcing steel• 10 or 12 dia. steel

reinforcement.• Bent up of hooked at

end.• Purpose of bent up bar-

when the lintel does bend, the rod do not lose their adhesion to the concrete around them.

Casting lintel• Pre-cast lintel: lintels are casted into mould and

have been hardened before it is built into wall. • For smaller spans up to 1.2m precast lintels are

used.• Marking is done to know the location of

reinforcement in the lintel. Letter ‘T’ is marked to know position of top surface of lintel.

• Additional reinforcement has to be provided to guard against lifting stress.

• Advantage of precast lintels – brickwork can be raised on lintel immediately after it is placed in position.

• Cast in situ lintel: lintel is cast in position inside formwork over the opening in walls.

• Need formwork for casting.• Need to be cured and hardened before the masonry

is being constructed above.• For cantilever of chajja additional reinforcement

has to provided at top of cantilever portion to take care to tensile forces that develop.

Pre-stressed concrete

lintels • Used particularly over internal openings. • Concrete is casted around high tensile steel wire

which are anchored into the concrete. • Concrete is compressed by tension in the steel

wires• Under the load, the compression of concrete due

to pre-stressed wires has to overcome before the lintel bends.

Steel lintels

• For large spans and heavy loading conditions, RCC lintels may prove uneconomical because of increasing depth.

• Steel lintels- section of rolled steel joists used singly or in combination of two or three.

• Connected to each other by bolts passing through them at intervals.

• System is embedded in concrete and cured like RCC lintel.

• Less depth than RCC lintels.

Introduction- arches

•‘A curved structure for spanning an opening, designed to support a vertical load primarily by compression’

•Example of form following function.

•The invoiced stresses are principally compressive.

•Because brick masonry has greater resistance to compression than tension.

• Masonry arch is frequently the most efficient structural element to span openings.

LOAD

Arch Forms - Components

Springer

Voussoir

Keystone

Arch Forms - Terminology

AbutmentJamb

Springer

Voussoir

Key Stone

Extrados

Intrados

Soffit

Span

Centre

Springing line

Arch Forms - Terminology

Forms of Arch• The first civilization to make extensive use of

arches: Romans• Shape of Roman arches:

semicircular• why?• Circle - the easiest way

to set out

Semi Circular Arch

•An arch whose intrados is a semicircle (half circle).

•Exactly half a circle, centre is on the Springing Line

•All joints radiate from the centre of the circle

•All these types of arch have an odd number of arch stones or voussoirs

•The centre voussoir is called the Keystone, the final stone to be fixed…

Forces acting in semicircular arch

• A linear semicircular arch: loaded by uniform radial pressure

Segmental Arch•An arch whose intrados is circular but less than a semicircle.

•Shallow arch with the centre below the springing line

•Lateral movement of the abutment is due to the horizontal thrust of the arch.

•This thrust develops in all arches.

•The flatter the arch, the greater the horizontal thrust.

•This thrust must be sufficiently restrained so that lateral movement of the abutment does not cause cracking in the arch or its collapse.

Jack arch/Flat Arch

• A flat arch with zero or little rise.

• The relatively small rise of a jack arch is called camber which is provided to correct illusion of sagging.

• Bricks are placed in ‘soldier’ manner adjacent to each other.

• Lesser strength than semi-circular and segmental arch.

• For spans over 1.0m need additional support such as a metal flat or angle.

• Metal cramps can be inserted at vertical joints to resist tensile stresses.

• Suitable for small spans.

Bull’s eye arch

• An arch whose intrados is a full circle.

• Also known as a

Circular arch.

CORBEL ARCH

• Do not requirestaging or form work

• Simple to construct

Gothic Lancet Arch

•Pointed arch

•Tall and narrow opening

•Centres are located on the springing line outside the clear opening or width of the arch

•Taller than width

•Can have a keystone or a joint at the point…

Forces acting in Gothic arch

Gothic Drop Arch

•Pointed arch

•Centres on the springing line inside the clear opening

•Wider than height

•Can have a keystone or joint at the point…

Gothic Equilateral Arch

•Based on an equilateral triangle

•The centres are on the springing line exactly at the clear opening.

•Can have a keystone or in this case a joint at the point…

Three Centred Arch

•A geometrically formed arch

•Has three centres two on the springing line and one in the centre below

•The bedding layers can be clearly seen in this example…

Tudor Four Centred Arch

•Uses 4 curves and centres to make up the arch…

•A pointed, four-centered arch of medium rise-to-span ratio whose four centers are all beneath the extrados of the arch.

Elliptical Arch

An arch with two centers and continually changing radii.

•Probably one of the most complex arch shapes to produce in stone

•Arch is circular in plan and circular in elevation

•Requires complex geometry to set out to produce templets…

Circle on Circle Arch

Horseshoe Arch

•Arch extends below the centre springing line

•In this case it is a redundant railway tunnel arch…

Venetian

•An arch formed by a combination of jack arch at the ends and semicircular arch at the middle. •Also known as a Queen Anne arch

Rough Arches• Constructed from ordinary

uncut bricks.• Give rise to edge shaped

joints.• To reduce too wide

mortar joints at extrados, arches are constructed in header course.

• Used for low cost construction or where appearance has little importance than cost.

failure of an archThere are three failure modes of an un-reinforced brick arch:

• Rotation of the arch about the abutment-Rotation occurs when tension develops in the arch. Tension can be reduced by increasing the depth or rise of the arch. If tension develops in the arch, reinforcement can be added to resist the tensile forces.

• Sliding of the arch at the skewback, Sliding of the arch will depend on the angle of skewback (measured from horizontal) and the vertical load carried by the arch. Reinforcement can be added to avoid sliding at the skewback, as the reinforcement acts as a shear key.

• Crushing of the masonry. 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. Compression failure seldom occurs.

failure of an arch

A stone arch (no strength in tension) will fail when the thrust lie reaches the extrados and intrados in four points, becoming amechanism

Arches: design

• Avoid cracking (tensile stresses) under loads • Keep the thrust line within the middle third of

the arch cross-section

Arches: design

• Thrusts at springing (reactions at supports) are inclined:

– vertical component– horizontal

component• Horizontal reactions

tend to spread the supports apart.

• Buttresses can be used, especially for arches/vaults on high walls

Arches: buttresses

C/L

Arch Forms – Setting Out

Semi-Circular

Set out Springing LineSet out Centre LineSet out arch using a rad of 120mm

Set out Face of 20mmProduce Joint line from centre

Formwork for arches

Formwork for arches

•Temporary support to be given until the brick joints have set & arch gains sufficient strength to support itself and carry the loads over opening.•Formwork has less width than the soffit of an arch to allow the setting a plumb to check alignment.•Type of formwork will depend upon-

-The wt. to be supported-Span-The width of soffit.

Formwork for arches

The brickwork is built over the frame.

Formwork for arches

Frame must be removed after the mortar joints are set and arch has received suffient strength to take self load and load of masonry above.

Formwork for small span arches

Formwork for spans up to 1.5m

Formwork for spans up to 4.0 m

Jack Arch Construction Sequence

Jack Arch Construction Sequence •Templates of hardboard or plywood are made to cut voussoirs.

•Wedge shaped bricks cut with axe and rubbed with abrasive stone to get smooth & sharp arris.

Jack Arch Construction Sequence

Jack Arch Construction Sequence

Jack Arch Construction Sequence

Jack Arch Construction Sequence

Jack Arch Construction Sequence

Jack Arch Construction Sequence

END OF THE LESSON!