prestressed concrete&metrorail 22-11-12
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PRESTRESSED CONCRETE
Prestressed concrete is basically concrete in which the stresses of suitable magnitude and distribution are introduced to counteract the stresses due to dead and live loads to the desired degree.
Thus, the concrete in the entire section will be compressive and no tensile stresses are permitted
Prestressing in vogue centuries back
i. The earliest example of wooden barrel construction by force fitting of metal band
ii. Shrink fitting of metal tyres of wooden wheels
iii. The forks of cycle
Cement concrete is a homogeneous material and strong in compression and weak in tension
A concrete of M20 will take an ultimate compressive stress of 20 N/mm2 but its tensile strength is hardly 3.1 N/mm2 (0.7 fck)
Steel is used to reinforce the concrete in tension zone. Reinforced concrete is a composite section
In a composite section, strains of different materials are same. The sum of loads taken by individual materials equal to total load
Reinforced concrete has the following disadvantages
i. Concrete takes tension along with steel though steel is designed to take entire tension. In this process, minute cracks develop in the concrete due to concrete unable to take strains along with steel
ii. In long beams, to limit the diagonal tension within limits, deeper sections are required
iii. Reinforced concrete member develops cracks even due to shrinkage
These disadvantages are overcome in prestressed concrete.Advantages of prestressed concrete
i. Cross section more efficiently utilisedii. Members posses improved resistance to shearing
forcesiii. Flexural members are stiffer under working loadsiv. More economicalv. Free from cracks during working loads and have
more durabilityvi. Absorbs energy efficiently due to impact loadsvii. Beams from 10 m to 30 m span are more
economical
Fundamentals of Prestressing
Materials used in prestressing
• Concrete – High strength concrete more than M35, with low shrinkage
• Steel – 5 mm to 7 mm diameter. High tensile steel with proof stress 1000 – 2000 N/mm2 (against 540 N/mm2 for tor steel)
• Steel Tendons – Steel made as single wire or group of wires. Group wires is called tendons.
Loss of prestressingPrestressing force is lost upto 10%
i. Loss due to friction : Loss due to friction between materials
ii. Loss due to curvature : Tendons are not straight but in a curve
iii. Loss due to slip of Anchorages : Wires are stretched and locked – in that process
iv. Loss after prestressing : a. Due to shrinkage of concreteb. Creep of concretec. Elastic shorteningd. Creep in steel
Prestressed concrete is used in
i. Beams
ii. Tanks
iii. Sleepers
TYPES OF PRESTRESSING
i. Linearii. Circular
In Linear prestressing the prestressing wires are in lines. This is used in beams and slabs
In Circular prestressing cables take circular path. This is used in tanks, pipes.
Methods of Prestressingi. Pre tensioningii. Post tensioningIn pre-tensioning, the tendons are tensioned even before casting the concreteOne end of tendon is secured to abutment. The other end is pulled with jacks.
In post tensioning, the beam is cast first leaving ducts for placing the tendonsDepending upon forces, there may be number of ducts
In post tensioning, not a solid beam but a series of blocksCables are inserted and will be prestressed
Post Tensioning in Blocks
End BlockWhatever may be the shape of beam, the end block is a rectangular section. The entire prestressing will be transferred by the end block
Anchorages will be embedded in end blocks
Systems of prestressing
It is the process of tensioning of tendons. Secures firmly to concrete till the lift of member. Many systems are in practice.
i. Freyssinet system
ii. Magnel Blaton system
iii.Gifford Udall system
i. The Freyssinet system : a. High tensile wires 12 No.b. Arranged to form a group into cables with a
spiral spring inside to give clearance between the wires
c. They will be inserted in a metal sheeting cablesd. The cable will be free to move initially and after
prestressing it will be grouted with cement mortar
e. The anchorages consists of a cylinder with central conical hole
Placement of ducts along with reinforcement in a box girder bridge
Freyssinet System
Fluted male cone
Female anchorage with steel spirals
Duct former
Steel wedge
Sandwich Plate
Distribution Plate
Magnel Blaton System
Magnel Blaton System – where 8 wires can be prestressed individually
Stages of prestressing
i. Ducts for tendons (strands) are placed along with reinforcements before casting of concrete
ii. Then it will be concreted
iii. Care should be taken that concrete (cement slurry) should not go into cable
iv. After concreting the PSC wires are moved to and fro so that the concrete if it goes inside cable should not set
v. After concrete set, according to design tendons are stressed in 7 days, 28 days and before live load is allowed
vi. After prestressing and locking in each tendon is grouted with cement slurry so that the concrete and tendon are well bonded.
FIRE RESISTANCE
• Concrete is non combustible
• Failure of concrete members usually due to progressive loss of strength of reinforcing steel or tendons
• Concrete has greater fire resistance than steel
• Reduction in strength of high tensile steel is less at high temperatures compared to ordinary steel
• Greater cover to tendons, PSC will be more fire resistant
• Application of prestressing
i. Floor slabs, columns, beams
ii. Bridges, water tanks
iii. Piles, wall panels, frames, window mullions, fence posts
iv. Railway sleepers
P.S.C. BRIDGES
PSC girders with part of deck as flange are castPrestressing in stages – first stage to take dead loads and erection loads
Girders are launched on piers and aligned
Launching will be by cranes or with the help of launching girderThe gap of alignment will be filled with cement concrete to get continuous road way and wearing coat laidThen second stage prestressing to take live loads
Instead of a single girder, the entire cross section of bridge will be sliced of one meter width and cast so that its weight is hardly one ton or soIf the bridge is of length 30 m, there will be 30 piecesAll these pieces will be lifted and put at proper place on the shuttering providedPSC tendons will be inserted and prestressedThe wearing coat will be laid
Bending moment – zero at supports and max at centre
where
w l2
Max B.M. =8
w l2
Max B.M. =8
MBending stress (σ) = ±
Z
MBending stress (σ) = ±
Z
1Z = bd2
6
1Z = bd2
6
It gives a direct load P and a moment P.e at top and at bottom
Total stress at top = Max. permissible stress in concrete
At bottom = Near to 2000
P P.e P P.eStress = ± i.e. –
A Z A Z
P P.e P P.eStress = ± i.e. –
A Z A Z
P P.e P P.eStress = ± i.e. –
A Z A Z
P P.e+
A Z
P P.e+
A Z
P P.e+
A Z
P P.e+
A Z
P P.e+
A Z
P P.e+
A Z
P P.e+
A Z
P P.e+
A Z
P P.e– σ + + = 0
A Z
P P.e– σ + + = 0
A Z
P P.e– σ + + = 0
A Z
P P.e– σ + + = 0
A Z
P P.e+
A Z
P P.e+
A Z
M P P.e= – + +
Z A Z
M P P.e= – + +
Z A Z
M P P.e= + + –
Z A Z
M P P.e= + + –
Z A Z
METRO RAIL SYSTEM
Mass Rapid Transit System (MRTS)
High capacity and frequency
Grade separation from other traffic
Located in underground tunnels (or) elevated viaducts (or) few grade separated tracks.
Growing cities, growing population and growing traffic – require shift from private mode to public
MRTS is successful with 70% of total transport is public one
London underground railway system – 1863 is first metro system
Technology quickly spread to Europe and USA
Recently the largest growth is in Asia with driverless system
New York city subway in 1904 largest track of 1335 kms of RTS
Shanghai metro – largest length of passenger lines
Tokyo subway, Seoul metropolitan subway and Moscow metro – busiest metro
London Underground Railway systemNew York City Subway
Tokyo Subway
By 1940 – 19 metro rail system 1984 – Swelled to 66 2013 – About 170 metro systems
India like other developing countries lagging behind
First metro rail – Kolkata – commenced in 1984 – Route length 97.5 Kms.
Available land for road transport is insufficient hence underground route
Delhi first phase – 65.11 Km in 2002 Second phase – 125 Km recently completed Mostly elevated i.e. viaduct
First one recognised by UN – Saves power – Regenerative waste by using green house gas
Chennai MRTS – Elevated one – completed in 2007
System normal electrical multiple units and not modern (without automatic doors)
Poor maintenance, lack of security, so not popular
Hyderabad metro – elevated one – 71 Kms in the first phase
First two track elevated transit system
Commenced on 2011 – expected to be completed by 2015
Bengaluru – MRT – Elevated and underground with double line corridors
Total length 33 Kms – First phase expected to be completed by 2013
MRTS – consumes less energy
Echo friendly – Less sound
Averts number of accidents
Efficient in terms of space, occupancy, provides comfort – ultra modern coaches
Modern system – Automatic ticketing
Advanced signalling system – Automatic train protection system
Integrated security
Maximum speed 80 kms/hr. Average 34 km/hr
Peak work hour capacity – more than 3 lakhs passengers
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