Civil Engineering Reference
In-Depth Information
b) Deck end arrangements and bearings
Usually, slab decks are carried by laminated rubber bearings. The spacing of these
bearings should preferably not exceed fi ve time the thickness of the slab. The need to
anchor the last compression strut of the equivalent truss must not be forgotten (
3.10.2
and
5.21
). Thus a prestress anchor with a factored ultimate strength equal to the
ultimate reaction carried by the bearing should be located adjacent to each bearing and
as low as possible in the slab. If this is not possible, this anchor force must be provided
by passive reinforcement, acting either alone or in combination with the tendon.
The prestressing tendon anchorages should be distributed as evenly as possible along
the end of the deck to minimise the equilibrium forces due to the prestress compression
as it fans out from the anchorages to uniformity in the slab. Either additional transverse
reinforcement or prestress must be provided to resist these forces.
The end zone of a prestressed slab can become congested due to the addition of
the reinforcement necessary for the crossbeam actions, equilibrium steel, primary
prestress anchorage steel, bars to anchor the expansion joint and longitudinal bars.
Careful planning and detailing is necessary.
11.3.5 Construction of slab bridges
Where a multi-span deck is built span-by-span, there is a risk that each new deck
cast will suffer heat of hydration cracking adjacent to the construction joint with the
previous deck (
3.6.4
and
9.3.8
).
As is usual where such cracking is likely, transverse reinforcement should be
provided to limit the opening of such cracks to a nominal 0.2 mm. If it is desired
to eliminate such cracking entirely, a light transverse prestress should be applied to
the older concrete, adjacent to the construction joint, just as the new concrete starts
to cool from its peak heat of hydration temperature. The aim should be to impose
a transverse compressive stress at the centre of the deck and at the location of the
construction joint of approximately 1 MPa.
Slab bridges are rarely so long that mechanised falsework is justifi ed. Usually, general
scaffolding is adopted.
11.3.6 Conclusion
When compared with reinforced concrete slabs, prestressed slabs offer:
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•
reduced tonnage of materials to handle;
increased speed of construction due to the early striking of forms, allowing a rapid
turnaround of falsework;
predictable self-weight defl ections;
the most slender of decks where this matters;
fl exibility to cope with relative settlement;
reduced approach embankment heights due to the slenderness of the deck;
good durability as the deck will be crack free under permanent loads;
for skew bridges, or decks carried on irregularly placed columns, the cancelling
out of transverse moments and torques under permanent loads.
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•
•
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