List of Prestressed Concrete Design Standards: [Show]
There are two types of deck using prestressed concrete :
Pre-tensioned beams with insitu concrete.
Post-tensioned concrete.
The term pre-tensioning is used to describe a method
of prestressing in which the tendons are tensioned before the concrete
is placed, and the prestress is transferred to the concrete when a suitable
cube strength is reached.
Post-tensioning is a method of prestressing in which the tendon is tensioned
after the concrete has reached a suitable strength. The tendons are anchored
against the hardened concrete immediately after prestressing.
There are three concepts involved in the design of prestressed concrete :
Prestressing transforms concrete into an elastic
material.
By applying this concept concrete may be regarded as an elastic material,
and may be treated as such for design at normal working loads. From
this concept the criterion of no tensile stresses in the concrete
was evolved.
In an economically designed simply supported beam, at the critical
section, the bottom fibre stress under dead load and prestress should
ideally be the maximum allowable stress; and under dead load, live
load and prestress the stress should be the minimum allowable stress.
Therefore under dead load and prestress, as the dead load moment reduces
towards the support, then the prestress moment will have to reduce
accordingly to avoid exceeding the permissible stresses. In post-tensioned
structures this may be achieved by curving the tendons, or in pre-tensioned
structures some of the prestressing strands may be deflected or de-bonded
near the support.
Prestressed concrete is to be considered as a
combination of steel and concrete with the steel taking tension and
concrete compression so that the two materials form a resisting couple
against the external moment. (Analogous to reinforced concrete concepts).
This concept is utilized to determine the ultimate strength of prestressed
beams.
Prestressing is used to achieve load balancing.
It is possible to arrange the tendons to produce an upward load which
balances the downward load due to say, dead load, in which case the
concrete would be in uniform compression.
Types of beams in common use are inverted T-beams, M-beams and Y beams. Inverted T-beams are generally used for spans between 7 and 16 metres and the voids between the beams are filled with insitu concrete thus forming a solid deck. M-Beams are used for spans between 14 and 30 metres and have a thin slab cast insitu spanning between the top flanges with the aim of forming a voided slab type deck. The top face of the bottom flange of M-Beams cannot be readily inspected, also the limited access makes bearing replacement difficult. As a consequence of these restrictions the Y-beam was introduced in 1990 to replace the M-beam. This lead to the production of an SY-beam which is used for spans between 32 and 40 metres. The U-beam is used for spans between 14 and 34 metres and is usually chosen where torsional strength is required.
Post-tensioned bridge decks are generally composed of insitu concrete in which ducts have been cast in the required positions.
When the concrete has acquired sufficient strength, the
tendons are threaded through the ducts and tensioned by hydraulic jacks
acting against the ends of the member. The ends of the tendons are then
anchored.
Tendons are then bonded to the concrete by injecting grout into the ducts
after the stressing has been completed.
It is possible to use pre-cast concrete units which are post-tensioned
together on site to form the bridge deck.
Generally it is more economical to use post-tensioned construction for
continuous structures rather than insitu reinforced concrete at spans
greater than 20 metres. For simply supported spans it may be economic
to use a post-tensioned deck at spans greater than 20 metres.
Bridge Components | Choice of Deck