Civil Engineering Reference
In-Depth Information
defl ections can give problems for the design of integral abutments unless the side spans
are short.
It is diffi cult to calculate accurately the self-weight defl ection of reinforced concrete
bridge decks. Whereas the nominal
E
-value of mature concrete of a specifi ed strength
may be known with reasonable accuracy, the
E
-value at seven days, when the span
is likely to be struck, is much more variable. Furthermore, the effective moment of
inertia at zones of higher stress will be reduced by concrete cracking.
As an example, consider a 750 mm deep bridge deck with spans of 15 m being built
span by span. The instantaneous self-weight defl ection of the new span calculated
with
E
= 20,000 MPa will be approximately 100 mm. However, if the
E
were to be
15,000 MPa and the cracked inertia 75 per cent of the uncracked value, the calculated
defl ection would increase by nearly 80 per cent, to 180 mm. As creep will increase this
defl ection by a factor of two to three, the diffi culty in predicting fi nal defl ections and
in providing suitable pre-cambers should be clear.
For decks built span-by-span, creep gives rise to uncertainty on the fi nal distribution
of bending moments between span and support sections (see
6.21
). The uncertainty
is much greater for reinforced than for prestressed sections. Although this uncertainty
may not affect the true ultimate strength of the deck it certainly does affect the cracking
behaviour at the SLS.
In conclusion, reinforced concrete solid slab bridges are best used in the following
conditions:
•
one-off short-span cast-in-situ bridges where the contractor does not wish to
mobilise prestressing expertise;
spans below about 15 m, with span/depth ratio not shallower than 20 for
continuous structures and 17 for statically determinate structures;
preferably where the full bridge length may be cast in one operation.
•
•
11.3 Prestressed concrete slab bridges
11.3.1 General arrangement
For the designer, the main difference between prestressed and reinforced concrete solid-
slab bridge decks is that the self-weight defl ections of the former are entirely cancelled
by the effect of the prestress; fully prestressed slabs defl ect upwards slightly under
long-term permanent loads. Thus the only limits on span/depth ratio are those defi ned
by acceptable bending stresses, and of course economy since the cost of the structure
rises with excessive slenderness. A span/depth ratio of 30 is probably optimal for a
continuous deck (although 33 is quite practical, and 35 feasible), allowing a 600 mm
thick slab to span economically up to about 18 m, and if necessary up to 21 m.
Prestressed concrete slabs are very valuable for bridge decks where the supports are
skew or for irregularly shaped decks where the columns do not defi ne organised spans.
As the prestress opposes the defl ections due to permanent loads, the transverse bending
moments and torques due to these loads are also eliminated or reversed (
12.5
).
An additional advantage of slender prestressed continuous slabs is their great
fl exibility. In areas where settlement of supports is to be feared, either through mining
subsidence, variable ground or due to settlement of the abutments under the weight of
