post tension slabs in residential building
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Transcript of post tension slabs in residential building
WHAT IS POST TENSIONING SYSTEM?
In post-tensioning, the concrete units are first cast by incorporating ducts or grooves to house the tendons .when the concrete attains sufficient strength, the high-tensile wires are tensioned by means of jack bearing on the end face of the member and anchored by wedges or nuts.
PRINCIPLES OF POST TENSIONING
Wedge action producing a frictional grip on the wires. Direct bearing from rivet or bolt heads formed at the
end of the wires. Looping the wires around the concrete.
BONDED SYSTEM
• For bonded system the post-tensioned strands are installed in galvanized steel or plastic ducts.
• The duct can be either circular-or oval-shaped.• The anchorage transfers the force from the stressing
jack into the concrete.• The duct and the strands contained within are
collectively called a tendon.• Due to the concentrated arrangement of the strands
within the ducts a high force can be applied to a small concrete section.
UN –BONDED SYSTEM
• In an unbonded system the individual steel strands are encapsulated in a polyurethane sheath & the voids between the sheath & the strand are filled with rust-inhibiting grease
• The tendon can be prefabricated off site.• The force loss due to friction is lower than for bonded tendons due to the
action of the grease. • A broken tendon cause’s prestress to be lost for the full length of that
tendon.• Careful attention is required in design to ensure against progressive
collapse.
DIFFERENT POST-TENSIONING SYSTEMS
FREYSSINET SYSTEM
GIFFORD-UDALL
LEE-MC CALL SYSTEM
MAGNEL- BLATON SYSTEM
BBRV PT SYSTEM
PRESCON SYSTEM
BAUR-LEONHARDT SYSTEM
DYWIDAG SINGLE-BAR SYSTEM
FREYSSINET SYSTEM
Widely used in Europe and India It consists of cylinder with a conical interior Large no of wires or strands can be
simultaneously tensioned using the double acting hydraulic jack.
GIFFORD-UDALL SYSTEM
The system developed in UK It consists of steel spilt-cone and
cylindrical female cone anchorages. In this system each wire is tensioned
separately
LEE-MC CALL SYSTEM
The tendons comprise high –tensile bars of diameter varying from 12-40 mm which are thread at ends.
After tensioning each bar is anchored by screwing a nut & washer tightly against the end plates
It has a disadvantage in that curved tendons cannot be used
MAGNEL-BLATON PT SYSTEM
Adopt metallic sandwich plates, flat wedges and a distribution plate for anchoring the wires
Sandwich plate can house up to four pairs of wires
The no. of wires in the magnel cable varies from 2-64
BBRV SYSTEM
Developed in1949 by four Swiss engineers-Birkenmeier,Brandestini,Ros&Vogt.
This system well suited for transmitting large forces
At present tendons capable of developing forces up to 12000KN are available under this system
ONE WAY & TWO WAY SLAB
• One way floors carry the applied loading primarily in one direction and are treated as beams or plane frames.
• Two-way spanning floors have the ability to sustain the applied loading in two directions.
MATRIALS & EQUIPMENT
MATERILS
• FORM WORK
• CONCRETE
• STRANDS
• TENDONS
• DUCTS
• ANCHORAGE
• WEDGES
FORM WORK
• The formwork for a PT floor is similar to that for a reinforced concrete floor.
Tendons arrangement inside formwork prior of casting
STRAND
• Strand is made from seven cold drawn high carbon steel wires.
• Seven individuals wires, with six wires helically wound to a long pitch around a centre wire.
• All strands should be grade 1860 Mpa• Strand is mostly available in two nominal size
12.7mm&15.7mm diameter
TENDONSTendons protection
Unbonded tendons
• unbonded tendons are protected by a layer of grease inside a plastic sheath.
UNBONDED TENDONS
• Single strand coated with corrosion inhibiting grease and encased in polyethylene sheathing• PT force is transferred to the concrete by
the anchors provided at the ends
BONDED TENDONS
ADVANTAGES
• Higher flexural capacity.
• Good flexural crack distribution.
• Good corrosion protection.
• Flexibility for later cutting of penetrations.
• Easier demolition.
DUCTS
• The duct is the sheath for the PT strand or bar.• The duct must be accurately fabricated, placed
and secured.• Duct materials are required to be high density
polyethylene(HDPE),polypropylene or schedule 40galvanized steel pipes.
ANCHORAGES
• The anchorage points apply additional horizontal force and vertical forces to the existing structure.
The factors that need to be considered in the design of anchorages are:• Attached to soffit.• Through a beam or column.
EQUIPMENTS1.Stressing jack2.Swaging jack3.Strand threading
machine4.Strand cutters or
shears5.Grout
mixer&pump
Monojack
Prestressing jack. STRESSING JACKS
ADVANTAGES OF PT
1.Longer span2.Overall structural
cost3.Reduced floor to
floor height.4.Deflection &cracks
control5.Waterproof slabs6.Early formwork
stripping7.Materials handling8.Fast construction
ESSENTIAL ADVANTAGES OF PRESTRESSING:
� Reduce the Quantity of Concrete;� Reduce Quantity of Reinforcement;� Simplify Reinforcement Works;
This will lead to:� Decrease cost of material for slab;
DISADVANTAGES
Since there are a number of tendons and wires spread inside the post tension slab, it can result in corrosion.
Complexity of work.
Poor workmanship can lead to accidents.
APPLICATIONS OF PT It used to fabricate large members, such as long
–span bridge decks of the box – girder type by prestressing together a number of smaller precast units
The chief merit of post –tensioning is that it allows the use of curved and stopped-off cables
Post-tensioning is variably used for strengthening concrete dams, circular prestressing of large concrete tanks & biological shields of nuclear reactors