List of Prestressed Concrete Design Standards: [Show]
There are two types of deck using prestressed concrete :
Pre-tensioned beams with insitu 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
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.