05-09-2012, 02:29 PM
Prestressed Concrete Bridge Design Basic Principles
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Reinforced Concrete
Recall Reinforced Concrete knowledge:
Concrete is strong in compression but weak in
tension
Steel is strong in tension (as well as compression)
Reinforced concrete uses concrete to resist
compression and to hold the steel bars in place, and
uses steel to resist all of the tension
Tensile strength of concrete is neglected (i.e. zero)
RC beam always crack under service load
Principle of Prestressing
Prestressing is a method in which compression force is
applied to the reinforced concrete section.
The effect of prestressing is to reduce the tensile stress
in the section to the point that the tensile stress is below
the cracking stress. Thus, the concrete does not crack!
It is then possible to treat concrete as an elastic material
The concrete can be visualized to have 2 force systems
Internal Prestressing Forces
External Forces (from DL, LL, etc…)
These 2 force systems must counteract each other
Classification and Types
Pretensioning vs. Posttensioning
In Pretension, the tendons are tensioned against some
abutments before the concrete is place. After the
concrete hardened, the tension force is released. The
tendon tries to shrink back to the initial length but the
concrete resists it through the bond between them, thus,
compression force is induced in concrete. Pretension is
usually done with precast members.
Lump Sum Prestress Loss
Pretress losses can be very complicate to
estimate since it depends on so many factors
In typical constructions, a lump sum estimation of
prestress loss is enough. This may be expressed
in terms of:
Total stress loss (in unit of stress)
Percentage of initial prestress