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E-STRESSED CONCRETE

wbacks of RCC:

akness of Concrete in Tension:

crete in Tension Zone Cracks

imit crack width in concrete in Tension in steel and

ce stress in steel is kept low.

in in steel = stress in steel / Es

= Youngs modulus of Elasticity in steel)

n RCC Beam

cking of concrete in Tension zone may cause corrosion

eel of reinforcement.

cking in concrete makes concrete in Tension zone

fective to take stresses. Hence Section in an RCC

m is not fully utilized

nce NOT fully efficient.

prestressed concrete beam hollow tube is replaced by

crete beam. Steel rod is replaced by steel wires or steel

es. Nuts are replaced by steel wedges and endhorages.

tressing is the Technique of introducing in a concrete

mber compressive force of permanent nature, so that it

es compressive stresses in that zone of member,

re tensile stresses will be caused by external loads.

ciple of Prestressing: Principle of applying a

pressive force of permanent nature to a prestressed

crete member may be explained by analogy of

ying compressive force to a hollow steel tube by end

l plates by tensioning a threaded a steel rod.

entric Prestressing force applied eccentrically below

Neutral Axis will induce compressive stresses below.

N.A. and tensile stresses above the N.A. Thishnique is called ECCENTRIC Prestressing. Stresses

uced by the eccentric prestressing will be of nature

osite to those induced by the external loading.

ESTRESSED CONCRETE - ELABORATE DEF.

oncrete in which there have been introduced Internal

sses of such magnitude and distribution that the

sses resulting from given external loading are

nteracted to a desired degree.

tressing ( Eccentric P.S ) will induce compressive

sses in those zones where Tension is induced by

ernal loading and Tensile stresses in those zones where

p. stresses are induced by External loading.

Magnitude and eccentricity of the Prestressing force are so

adjusted so that final stresses are always compressive and

no tension develops at any stage.

MATERIALS USED IN PSC

CONCRETE:

Min Grade:

Pre Tensioned PSC: M 30

Post Tensioned PSC: M 35

Concrete strength gradesCommonly used in PSC: M 35 to M 60

Steel:

High strength steel with yield strength of 1200 to 1800

N/mm2 in the shape of (i) wires 4 to 8 mm dia

(ii). Strands in the shape of a rope with 8 to 12 wires.

(iii). Cables with 8 to 20 strands.

METHODS OF PRESTRESSING

Pre tensioning: (def :) A method of Prestressing concrete

in which tendons (Prestressing steel wires or rods)are

tensioned (stressed) before concreting. Prestressing steel

wires are tensioned before casting concrete in moulds.

Post tensioning: (def :) a method of prestressing concrete

in which prestressing steel is tensioned against hardened

concrete. Prestressing steel tensioned against hardenedconcrete.

PRE TENSIONING:

Prestressing wires temporarily anchored against strong

abutments.

Transfer of prestress to concrete is by bond.

Transfer of prestress after concrete has set.

Axial prestressing generally adopted.

Bending of wires difficult and hence generally not

adopted.

Small sized prestressing wires (4 mm to 8 mm) generally

used.

Pre tensioning generally used for small pre cast factory

produced elements.

Applications of Pretensioning:For precast small factory produced building elements like

precast beam elements, precast slab or wall elements.

Precast Railway sleepers.

Precast electric poles.

Other similar factory produced precast conc. elements

which may also be pretensioned to reduce dead weight.

Pretensioned precast slab units

pretensioned precast tee beam ( building flement )

Long Line Method:

A method used for manufacture of pre tensioned

identical elements (such as wall panels, slab, pane

Electric poles or railway sleepers), in which a long

permanent bed with strong abutments at each end

for casting a number of elements simultaneously

Steam Curing:

A process of accelerated curing normally adopte

factory produced pre tensioned pre cast concrete e

by long line method, in which the pre cast elemen

cured by passing saturated steam (circulating satu

steam around the pre cast elements). Heat and hum

provided by the saturated steam accelerates curing

hours steam curing is considered adequate for rem

pre cast elements from casting beds. Steam curing

curing beds be used quickly.

Post Tensioning (def.):

A method of Prestressin

concrete in which Prestressing steel is tensioned a

hardened concrete.

ADVANTAGES OF POST TENSIONING:

As post tensioning is norma

adopted for In situ casting, longer span

feasible.

Cables can be bent, hence m

efficient.

Less loss of prestress comp

pre tensioning.

Stage prestressing possible

Demerits of Post Tensioning :

Post Tensioning needs sheathing, end anchorages,Plates etc. and is therefore costlier and more tedio

Pretensioning.

BENDING OF CABLES IN POST-TENSIONED

PRESTRESSED CONCRETE BEAMS : Cables a

such that the effective eccentricity at any section i

proportional to external moment at that section so

tension develops at any section.

Steps For Post Tensioning :

To place prestressing tendons, a duct is formed in

concrete member at the time of casting.

The concrete member is cast and allowed to harde

achieve adequate strength.

Steel tendons are inserted and placed loose in the

after the conc. member has hardened with end stee

and end anchorages devices like steel wedges.

The tendons are stretched and anchored at each en

means of steel wedges.

Tendons are anchored at each end. Prestressing is

transferred to concrete through bearing plates plac

each end.

Equipments Needed For Post-Tensioning :

Concrete Beam (To Be Post-Tensioned

High Strength Steel Cables.


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Sheathing To House Cables.

End Anchorages.

Steel Bearing Plates With Holes For Cables.

Steel Wedges (To Anchor Cables To Plates)

Special Reinf. To Prevent Splitting Of

crete Due To Concrete Due To Heavy Concentrated

sses.

Electric or hydraulic jacks for tensioning

es.Grouting equipment.

Grouting tank

Grouting pump

Grouting material

Spl. Grade high strength cement.

Water

Super plasticisers.

plications of Post Tensioning

For large span and heavily loaded beams for

ctures like bridges, flyovers, and similar structures.

For Ground Anchorages.

plications

Large span and heavily

loaded beams in buildings, bridges

and flyovers.

Transfer girders in buildings.

Large span cantilevers.

Large span folded plates and

shells.

Large diameter and large

sized tanks(fluid container).

Containment shells in

nuclear reactors.

Industrial structures, large

sized cooling towers.

Ground anchors.

Diaphragm walls, etc.

und Anchorages

Technique of Post Tensioning is used to provide

horage and support for deep retaining walls, basementrs and floors of large underground water tanks with

h water table.

ction Ground Anchors

rits of Pre-Stressed Concrete over

C: Cracking of concrete in Tension zone

y cause corrosion of steel of reinforcement

ch occurs due to the forces created in the

tion below the neutral axis in an RCC beam

ch does not happen in pre-stressed

crete as no tension is created.

Since no corrosion of steel occurs so we can

actually use a higher grade of steel and hence

achieve a more effective section.

Similarly as no cracks are developed in

concrete so a higher grade of concrete can be

used.


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ar Walls: A structural wall which has high in planeness by virtue of its form.

hear Wall may consist of:

solid wall.

perforated wall

closed loop or a core.her such form.

d of Shear Wall in a building:

ost of a building Increases a lot with increase in the

mber of storeys. Also the magnitude of Hor. Loads

ond 12 15 storeys is large so a structural systeme efficient than FRAMES are needed to resist hor.

s more efficiently. Shear walls resist horizontal loads

e efficiently than frames and are accordingly provided

uildings beyond 12 15 storeys in height.

hese are named so because they are used to resistzontal shear forces in a structure.

verse Effects of Large Horizontal Deflections in a

lding:

tress to Glazing.tress to partitions.

tress to cladding.

tress to service pipes & Installations.

ychological effects.

eling of insecurity and danger.

me of the locations to place a shear wall in a

ding:

me desirable features for planning and design of

ar walls:

ation -These act as Functional walls also.

not interfere with Architecture of the building.

mples:

nclosures around lift wellsaircase walls

xternal walls

ome of partition walls can be made to

t as shear walls

ng Both Axes

horizontal force due to wind or EQ may act from either

ction, shear walls should be provided along both axes

rovide resistance along both axes.

Symmetric layout about both axes

-Shear walls must be placed symmetrically about both

axes to avoid torsion.

-Eccentrically placed shear walls may induce large

stresses due to torsion and may damage the bldg.

Adopt Avoid

This Aspect Is Very Imp. In Planning The Layout Of

Shear Walls In A Bldg.

Well Distributed Along Both Axes: -Avoid Narrow Core

At Centre Of Building.

Adopt Avoid

To Receive Sufficient Vertical Load To Achieve InDesign:

Shear wall design is most economical if tension isavoided. This can be achieved by adding sufficient vertical

load on the shear wall.

Continuity:

Shear walls, when provided, should be continued upto

foundations. These should not be discontinued in thelower storeys.

Ductile Detailing

Shear walls

should be detailed forductility and brittle failure should be avoided.

Olive VewHospital Sanfernado

Shear walls should be continued upto the fobecause shear walls act as vertical cantilevers

ground and if discontinued on the ground floor

distress in the columns at the time of earthquake.

Types of Shear Wall construction

Shear Walls- frame interaction-Shear walls- fr

interaction is a combination for shear walls and

(cols).

Basic principle of Shear wall action :

-Hor. Load in a bldg. is resisted by various

vertical elements in a bldg. (columns andwalls) in proportion to their stiffness.-More Stiff elements carry resist more

horizontal load compared to less stiff

elements.

-Shear walls have very large stiffness

compared to stiffness of all cols (50 to 100times or even more )in a bldg. Hence shear

walls resist most of hor. Loads relieving

columns to resist primarily vertical loads

only.

-This method is used commonly these daysbecause it helps in resisting loads very

efficiently also reducing the cost.

Advantages:

-Provide flexibility of planning.

-Feasibility of providing large spans.-Ideal for buildings from 15 to 40 storey ht.Most common form for medium height to

high rise buildings,15 to 4o storeyes ht. to be

constructed in India in near future.

Cross Wall Construction: A no. of cross and lon

acting both as load bearing walls and shear walls tworkloads. Examples:

-Masonry Buildings:

Ht. Range limited to 3-4 storeyes due to weamasonry in tension.

-RCC Solid Wall Construction

Very stiff & very efficient.

Advantages:

-Very stiff system.

-Walls can be made to act both functional walls astructural walls.

Disadvantages:

-Walls must be permanent.

-Large openings cannot be provided.Suitability:

-Residential bldgs. Which have large no. of

cross walls which can be made to act as shea

Example of cross wall construction: 3 4

load bearing brick masonry bldgs.

29 storeyes HAMILTON COURT Apartment DLF Gurgaon. This bldg. has RCC walls acting

wall.


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SE1 STUDIES

14 storeyed Delhi Administration office Bldg.

at I.T.O. New Delhi.

Vikas Minar, New Delhi; A 21 storeyed DDA

Head quarter office Bldg.

General Office Complex at Nizam Palace,

Kolkata. A 20 stpreyed office Bldg. at KOLKATTA.

S.T.C. Bldg. at Janpath, New Delhi.

Engineers India office Bldg. at Bhikaji Cama

Place, New Delhi.

Videsh Sanchar Bhawan, Baba Kharagh Singh

Marg, New Dehi.

AM AND SLAB

its:

Can be adopted for any shape and

size of building

Usually most economical system.

Large openings e g.for staircase and lift shafts

can be easily formed.

Can conveniently support partition walls and

concentrated loads.

Light weight and long span system.

Good seismic resistance.

Easy to analyse and design.

merits

Larger spans may need deeper beams (eg for

span = 12 m d = 1000 mm)

Sometimes architectural and aesthetic need may

decide floor system other than beam and slab

system.

s system is the most common system because ofconstruction cost and less cost of shuttering, less

erial used and structural stability.

main beam is the primary beam which spans

ween the 2 columns and a secondary beam is the

m which spans between the main beams.

nverted beam is the one which is not below but is

t above the slab, used mostly in a sunken slab in

ets and for terrace gardens.

Concealed beams are those in which the beams are

broad and thin such that they are accommodated in

the slab, also called as flat plates or flat slabs

SHELLS:-Curved surfaces with thickness

small compared to other dimensions.Their shape is so curved that these carry primarly

direct stresses. Bending stresses are eliminated orkept

to minimum.THICKNESS :Small precast shells : 30 -50 mm

Cast in Situ concrete shells: 50 150 mmAverage : 100 mmCLASSIFICATION OF SHELLSSINGLY CURVEDDOUBLY CURVED SHELLSSHELLS OF REVOLUTIONSHELL OF RULED SURFACESHELLS OF TRANSLATION

Spherical shell roof

Cylindrical shell roof

SINGLY CURVED SHELLSGauss curvature = 0Examples:- Cylindrical shellsDOUBLY CURVED SHELLS:Gauss curvature is NOT zero. It may be +ve,partially ve or partially +v

ELLIPTICAL PARABOLOID

HYPERBOLIC PARABOLOID

SHELLS OF REVOLUTIONA Shell surface generated by revolving a curvan axis.Example : A DomeThe curve of revolution may be an arc of a cirellipse, parabola, hyperbola, an incline straigsegment orb any such curve.

CONOID : Conoid is the shell surface geby a moving a straight line with one of its eplane curve and other end on a straight line.APLLICATIONS OF CONOIDS :Industrial buildinds The raised portions of conmay used as North light glazing

APPLICAIONS OF DOMES :1. Large Halls ( Examples are Parliam2. Indoor Sports Stadia ( Examples arTalkatora Indoor Stadium)3. Over Head and Under ground Wate4. Sanitary structures5. Industrial StructuresStorage Bins

Slab 1/20-25, 1/25-30 and 1/25-30 and 1/30-35

Beam 1/10-15 and 1/10-8 , width d/2 or 2d/3

Column 4- 1/15 and 6 1/15-12 and 8-1/12-10 andmore -1/10-8


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ATE PLATE/FLATE SLAB SYSTEMSnsist of solid RCC slabs and columns (no beams)

AT PLATE SYSTEMS :tems with solid RCC slabs (with or without dropels) and RCC cols (without col.heads)

nsist of only slabs & columns beams beingmmitted.

FLATESLAB

STEMS:tems with solid RCC slabs (with or without dropels) and RCC cols with col.heads

ckness of slab = 125 to 250 mmcing of columns = 4 to 8 m centresheads & drop panels provide resistance againstching shear.

RITSeasing clear flat ceiling.an reduce storey height to accommodate moreof storeyes.mple and less form work.asy and quick construction.

MERITS OF FLAT PLATE/ FLAT SLABSTEMS :cker slabs needed to resist punching shear.avier weight and costlier system compared to beam

slab system (25 to 40 % costlier).aker under workloads (due to EQ/ wind) Provisionhear walls needed for m.s. buildings above 6 to 8ies with these floor systems.

Need more or (les) regular square layout of panels.Difficult to analyses for panels with irregular layoutsDifficult to provide large openings e.g. for stairs and

lift wells. These may need framing.

APPLICATIONS :Parking BuildingsLibraries etc.

EXAMPLE:NDMC parking at cannaught place New Delhi.

MCD parking in front of LNGP Hospital New Delhi.National museum building, Janpath New Delhi.Central sectt. Library at Shastri Bhavan New Delhi.

FLOOR SYSTEMS COMPARATATIVECOSTSBeam and slab systems : 100Waffle slab system : 125Flat slab system : 140

CONCLUSION :Adopt beam and slab floor system unless there is

justification for adopting any other system due tofunctional,ArchitecturalandAestheticreasons.

WAFFLESLABFLOORSYSTEMSConsist ofribs at

closer intervals which act as T beams. (beam t span /20-25 and col- span/15)

MERITS :

Architecturally more pleasingReduced overall depth (D ~ span/ 20 to span/ 25,

More head room, aesthically appealing for largerspans. Comparatively light weight system.Rib can be architecturally treated.Electric lights and filings can be aesthetically

concealed within ribs.Larger span feasible without excessive beamprojections.RIBBED (Waffle ) SLAb System :

Consist of RIBS at closer intervals along both axiswhich actas T beams.Depth of ribs = span/20 to span/25SPAN RANGE 6M- 10MTypes of Waffle slabs with wide shallow beamsaround peripheryGenerally the depth of wide shallow beams is equal todepth of ribs.Such system are analyzed as two way slabs it spacingof ribs < 12x thickness of slab (This condition issatisfied in most of practical cases)Such systems are easier to analyses and morecommonly adopted.ADVANTAGES OF WAFFLE SLAB SYSTEMS :

(i) Reduced dead wt. due to form action of rHence comparatively light wt. system.(ii) Architecturally, More pleasing.(iii) Electric lights and fittings can be aestheticconcealed within ribs.(iv) Ribs can be Architecturally treated.(v) This system can be used for longer spans10m or so) without excessive beam projectio

TRUSSES: A trusses consists of an assemblbut elastic members jointed in the form of tria

act as a beam. The safe working tensile stressteel is about 20 times that of structural timbesteel trusses work out to be economical, espebigger spans. Out of the various shapes of stsections, angles are considered most suitableroof truss. This is on account of the fact that acan resist both compressive and tensile streseffectively. In additional angles can be produceconomically and can be jointed easily.

Advantages of steel roof truss over timberare given below:1. Steel trusses are stronger than timber truss2. Steel section forming the truss are light in wand can be fabricated in any desired pattern tarchitectural requirement.3. There is no danger of the material being ea

away by white and or other insects.4. Steel trusses are more fire-resisting than titrusses.5. They do not have span restrictions and as steel trusses in be used for industrial buildinother such structures where large areas are rto be covered without obstructions due to col6. The sections forming a steel truss are easytransportation.7. The sections can be obtained in any desirelength to suite the requirements and there is nwastage of the material in cutting etc.8. On account of their easy erection techniquprogress of roofing work with steel trusses is

GRID FLOORSGrid floor systems are:-

System consists of beams spaced intervals in prependicular directions.

The beams are monolithic with a toslab.

Each RIB (beam) acts as a T-beam

SHAP

E

HSHAPE

RECTANGULAR GRID


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DIFFERENT COMPONENTS:-

1. Slab (65-100)2. Ribs(width- 150-250, dep- span/20-25,spac- 2-3)3. Columns4. beam

RITS OF GRID FLOORused to span large spaces upto 30m withoutany central columnsmore aesthetical due to beams at regular

intervals.Less depth can take the amount of load

MERITS OF GRID FLOORExpensive systemNot very large spans i.e. more than 30m can bespanned

PLICATIONS

Grid floors are adopted for large halls where flat

RCC roof is desired. Examples are Auditoriums,Conference Halls, Multiplex halls etc.(hall oftechnology, pragati maidan)

FERENT TYPES OF GRID FLOOR:-

Rectangular grid

Square grid

Rectangular dia grid

Circular grid

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