hello
PSC
For long span beams, prestressed concrete is used cos PS prevents occurrence of cracking. This is by applying an initial compressive load or ‘PS’ by high-strength steel tendons in a concrete member to enable it to counteract or neutralise the tensile stresses arisen during its service period.
PS-C +
1. PSC section remains uncracked under service load; reduced risk of corrosion to steel and hence improved durability; whole section is uncracked > higher stiffness & less deflection (i.e., improved serviceability) -increase in shear capacity (due to pre-compression)
2. Higher span to depth ratio ; Larger span is possible with PS > larger column free spacing
in building & bridges.
PSC Limitations
1. $$ for normal structures as special site operation.
2. Loss of PS due to anchorage slip, concrete shrinkage and creep, & steel
relaxation > reduce usable prestressing force.
3. Design & construction more complex reinforced concrete.
Examples
Wooden barrels – metal bands tighten under tensile stress > induce a state of initial hoop
compression between the staves allowing them to counteract the hoop tensile pressure
created by filling of liquid in the barrels.
Stack of books – can be held together & moved as a single elongated object from
one place to another if pressed together from both sides hard enough.
Common PSC Terms
Tendon: It comprises of a group of wires or strands, or one or more hot-rolled bars
wound together.
Wire: A single unit made from hot-rolled rod that is cold-drawn through dies to
increase its strength. nominal diameter (5 or 7mm)
Strand: High-strength steel wires wound helically around a centre wire, usually in a
7-wire arrangement.
Hot-rolled bar: A long, slender structural element, usually made from threaded
high-strength steel. A tendon can be made of a single steel bar.
Unbonded tendon: A tendon in which the tensioned element (strand, wire or bar) is
unbonded from and permanently free to move relative to the surrounded concrete.
Bonded tendon: A tendon which is permanently connected to the surrounding
concrete along the full length between its end anchorages. Such connection can be
direct, or through internal grouting of the tendon’s ducting.
Duct: Encapsulating tube-like component, usually constructed from galvanised steel
or polythene, which creates a void space within the concrete and surrounds the
strands, wires or bar of a post-tensioned tendon. can be circular or rectangular with round corners.
Anchorage: A rigid component, commonly constructed of cast iron, located at the
end of a tendon and which directly transfers a tendon’s force to the surrounding
concrete.
Dead end anchorage: A “passive” end anchorage of a prestressing tendon which
does not have any jacking operations undertaken at that end.
Live end or stressing end anchorage: A “active end anchorage of a prestressing
tendon which has jacking operations undertaken at that end.
Fully PSC: In full prestressing condition, concrete member would
be crack-free as flexural tensile stresses are not allowed under full working load.
Partially PSC: In partial prestressing condition, tensile stresses
could be permitted and hence partially prestressed concrete could be regarded similar
to conventional reinforced concrete in which cracking is allowed in the flexural
tensile zone
3.5 Basic Methods of PSC
(1) pretensioning and (2) posttensioning.
(1) tendons are tensioned, one at a time, BEFORE the concrete
is placed. each tendon is anchored at the dead end buttress or
abutment using a barrel-and wedge anchorage device and is tensioned at the live
(stressing) end with a proper prestressing jack using another anchorage device. After
the required tension is attained, concrete is cast into the formwork following the
proper compaction procedure. Once the concrete has hardened (achieved its design
strength), the tendons are severed. This action will make the steel under tension to go
back to its original position, which will be prevented by the concrete. Concrete will
resist the steel tension as compression; in other words, the steel with tendency to
shrink its length, will transfer its tension onto the concrete. This method allows for
more than one beam to be manufactured in a single operation and is mainly used in
factory situations (precast beams).
(2) tendons are tensioned AFTER the concrete is placed. initially, steel or plastic ducts are placed at the desired or
predetermined profile. After the concrete is casted, properly compacted in the
formwork and hardened to the required strength, the tendons are inserted into the
ducts and tensioned using an appropriate prestressing jack by reacting against the
concrete. Grout may be injected to the duct later to bind the tendon with the duct and
the rest of the beam. can be done to prestress either a precast beam or a
cast-in-place beam
2- AS3600 tables will not be provided. You can bring whatever you feel important in the 4 x A4 sheets (8 pages). These pages will be submitted with your exam booklet.
3- If you think that you miss any data in the exam, you ‘can assume a suitable number and state this assumption clearly’, the calculations will be marked according to that assumption. Each one may get a different answer – but we will take care and mark appropriately.
The system is tuned to mark what you know the best for the question and not as punitive tool. Have faith in the system and do your best.
hhhhhhhhh