Civil Engineering Reference
In-Depth Information
thin black layer, such as a waterproof membrane. In general, it is adequate to ensure
that there is an appropriate mesh of anti-crack steel on both surfaces of the web.
The temperature gradients cause signifi cant defl ections and beam end rotations, and
this is likely to have an effect on the dimensioning of bearings and expansion joints.
When building decks in balanced cantilever, the free end of the cantilever will move
up and down signifi cantly on a daily cycle, affecting the surveys required to control the
shape of the bridge, and the procedures for casting the mid-span stitch.
b) Effect of temperature gradients on the design of continuous beams
In typical continuous beams the longitudinal restraint has been released, while the
rotational restraint has only been released at the end supports. Consequently the
temperature gradients cause bending moments in the beam, Figure 6.13. At any section
of the continuous beam there will be a bending moment together with the internally
balanced stresses. At the SLS, these bending moments should be added to the table of
moments due to loads, and the prestress designed in consequence. As the moments
are due to locked-in strains, they should not be added to the moments due to external
loads and prestress at the ULS.
In the author's opinion, the internally balanced stresses on the extreme fi bres are
harmless for the reasons explained in (a) above, and should be ignored in the sizing of
the prestress. An additional reason for this is the disproportionate effect the inclusion
of these shallow surface tensile stresses would have on the amount of prestress required.
For instance, in a deck with an average prestress
P/A
of 5 MPa, which is typical of
bridge decks, the need to provide typically an additional compressive stress of 1.5 MPa
on both extreme fi bres would increase the prestress force required by 30 per cent.
This is not only uneconomical, but increases the permanent compressive stress in the
concrete, the congestion of the cross section for concreting, the movement at the
expansion joints and bearings, and in general deteriorates the quality of the design.
The best prestress scheme is that which needs the least pre-compression in order to
fulfi l the design objectives!
Consequently, in the author's opinion, the prestress for continuous bridges subject
to non-linear temperature gradients should be designed to accommodate the bending
moments caused by the gradients, but not the internally balanced stresses. These are
adequately catered for by ensuring that the top and bottom surfaces of the bridge
deck are reinforced with a mesh of bars as described above. Where the deck is made
of precast segments, clearly the tensile stresses are released harmlessly at the segment
joints.
Some authorities do insist that these internally balanced stresses should be
considered for the design of the prestress, although this position would appear to be
inconsistent with ignoring these same effects in statically determinate beams. Under
Figure 6.13
Bending moments caused by temperature gradients
