20-08-2012, 04:24 PM
POST-TENSIONED SLABS
POST-TENSIONED.pdf (Size: 3.68 MB / Downloads: 355)
Introduction
General
Post-tensioned construction has for many
years occupied a very important position,
especially in the construction of bridges and
storage tanks. The reason for this lies in its
decisive technical and economical
advantages.
The most important advantages offered by
post-tensioning may be briefly recalled here:
- By comparison with reinforced concrete, a
considerable saving in concrete and steel
since, due to the working of the entire
concrete cross-section more slender
designs are possible.
- Smaller deflections than with steel and
reinforced concrete.
- Good crack behaviour and therefore
permanent protection of the steel against
corrosion.
- Almost unchanged serviceability even
after considerable overload, since
temporary cracks close again after the
overload has disappeared.
- High fatigue strength, since the amplitude
of the stress changes in the prestressing
steel under alternating loads are quite
small.
Historical review
Although some post-tensioned slab
structures had been constructed in Europe
quite early on, the real development took
place in the USA and Australia. The first posttensioned
slabs were erected in the USA In
1955, already using unbonded posttensioning.
In the succeeding years
numerous post-tensioned slabs were
designed and constructed in connection with
the lift slab method. Post-tensionmg enabled
the lifting weight to be reduced and the
deflection and cracking performance to be
improved. Attempts were made to improve
knowledge In depth by theoretical studies and
experiments on post-tensioned plates (see
Chapter 2.2). Joint efforts by researchers,
design engineers and prestressing firms
resulted in corresponding standards and
recommendations and assisted in promoting
the widespread use of this form of
construction in the USA and Australia. To
date, in the USA alone, more than 50 million
m2 of slabs have been post tensioned.
Punching shear
General
Punching shear has a position of special
importance in the design of flat slabs. Slabs, which
are practically always under-reinforced against
flexure, exhibit pronounced ductile bending failure.
In beams, due to the usually present shear
reinforcement, a ductile failure is usually assured in
shear also. Since slabs, by contrast, are provided
with punching shear reinforcement only in very
exceptional cases,because such reinforcement is
avoided if at all possible for practical reasons,
punching shear is associated with a brittle failure of
the concrete.
This report cannot attempt to provide generally valid
solutions for the punching problem. Instead, one
possibile solution will be illustrated. In particular we
shall discuss how the prestress can be taken into
account in the existing design specifications, which
have usually been developed for ordinarily
reinforced flat slabs.
POST-TENSIONED.pdf (Size: 3.68 MB / Downloads: 355)
Introduction
General
Post-tensioned construction has for many
years occupied a very important position,
especially in the construction of bridges and
storage tanks. The reason for this lies in its
decisive technical and economical
advantages.
The most important advantages offered by
post-tensioning may be briefly recalled here:
- By comparison with reinforced concrete, a
considerable saving in concrete and steel
since, due to the working of the entire
concrete cross-section more slender
designs are possible.
- Smaller deflections than with steel and
reinforced concrete.
- Good crack behaviour and therefore
permanent protection of the steel against
corrosion.
- Almost unchanged serviceability even
after considerable overload, since
temporary cracks close again after the
overload has disappeared.
- High fatigue strength, since the amplitude
of the stress changes in the prestressing
steel under alternating loads are quite
small.
Historical review
Although some post-tensioned slab
structures had been constructed in Europe
quite early on, the real development took
place in the USA and Australia. The first posttensioned
slabs were erected in the USA In
1955, already using unbonded posttensioning.
In the succeeding years
numerous post-tensioned slabs were
designed and constructed in connection with
the lift slab method. Post-tensionmg enabled
the lifting weight to be reduced and the
deflection and cracking performance to be
improved. Attempts were made to improve
knowledge In depth by theoretical studies and
experiments on post-tensioned plates (see
Chapter 2.2). Joint efforts by researchers,
design engineers and prestressing firms
resulted in corresponding standards and
recommendations and assisted in promoting
the widespread use of this form of
construction in the USA and Australia. To
date, in the USA alone, more than 50 million
m2 of slabs have been post tensioned.
Punching shear
General
Punching shear has a position of special
importance in the design of flat slabs. Slabs, which
are practically always under-reinforced against
flexure, exhibit pronounced ductile bending failure.
In beams, due to the usually present shear
reinforcement, a ductile failure is usually assured in
shear also. Since slabs, by contrast, are provided
with punching shear reinforcement only in very
exceptional cases,because such reinforcement is
avoided if at all possible for practical reasons,
punching shear is associated with a brittle failure of
the concrete.
This report cannot attempt to provide generally valid
solutions for the punching problem. Instead, one
possibile solution will be illustrated. In particular we
shall discuss how the prestress can be taken into
account in the existing design specifications, which
have usually been developed for ordinarily
reinforced flat slabs.