8/3/2019 structres finaley
1/7
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.
8/3/2019 structres finaley
2/7
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.
8/3/2019 structres finaley
3/7
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.
8/3/2019 structres finaley
4/7
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
8/3/2019 structres finaley
5/7
8/3/2019 structres finaley
6/7
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
8/3/2019 structres finaley
7/7
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