Civil Engineering Reference
In-Depth Information
If the same beams had been cast in timber shutters using internal vibrators, it would
have been necessary to increase the thickness of the webs to allow poker vibrators to
pass between the prestress ducts and the web lacers. It would under these circumstances
be sensible to put the lacers outside the shear steel. With an 87 mm duct, this would
give rise to a minimum web thickness of about 270 mm leaving a chimney of at least
60 mm for a 50 mm poker vibrator, Figure 9.24 (b). Clearly this minimum thickness
will need to be adapted for different duct and reinforcement diameters and different
concrete cover. It should be noted that external vibrators do not work properly on
timber shutters; timber shutters shake under the effect of vibrators, which tends to
help the concrete fl ow, but they do not transmit the correct vibration frequency to
compact it.
Generally, the parabolic shape of the cable profi le for statically determinate beams
effectively counteracts the shear force, and it is not necessary to thicken the webs
towards the supports. However, at the extreme ends of the beams, for a length usually
approximately equal to their height, it is necessary to thicken the webs to house the
prestress anchors and contain their bursting forces. For instance, the 2.7 m high Sungai
Kelantan beams were thickened to 500 mm, for a distance of 2500 mm, including a
500 mm chamfer.
b) Width of webs near the mid-span of continuous box girders
In continuous beams the tendon profi les do not balance the shear forces as effi ciently
as in statically determinate beams, leading to heavier shear steel. Also, the webs of
box decks are subjected to signifi cant transverse bending moments, further increasing
the density of the reinforcement. There is frequently the addition of equilibrium
reinforcement related to face or blister prestress anchors. Consequently, the webs of
box girder bridges are more highly reinforced than those of precast beams. This more
dense reinforcement makes it more diffi cult to compact the concrete. Furthermore,
whereas precast beams have a compact heel that is relatively easy to fi ll with concrete,
box girders have a wide bottom slab that is always troublesome to cast.
The concrete must fl ow from the web into the bottom slab of the box. Near mid-
span the bottom corner of the box usually houses prestressing ducts that are typically
parked in the bottom of the web and in the bottom slab haunch. In some sections as the
tendons change position to move up the web this bottom corner may be signifi cantly
congested with tendons in addition to the reinforcement linking the bottom slab to
the web. Prestress anchors may be housed in blisters situated at the junction of the
web and the bottom slab requiring a dense grillage of bars crossing the web which
is superimposed on the tendons and reinforcement described above, Figure 9.25 (a).
The consequence of this inevitable congestion of tendons and reinforcement is that,
in some critical positions in the span, it is often diffi cult to coax the concrete to fl ow
from the web into the blisters and the bottom slab, Figure 9.25 (b). These diffi culties
in casting and compacting the concrete are made more severe if the webs of the box
are sloping.
All these reasons lead to a more conservative approach to sizing the webs of box
girders. However, they are all under the control of the designer, who can make a good
job of ensuring that his design is buildable, or on the contrary can create the most
intractable problems for the contractor. It is reasonable to affi rm that the majority of
failures of compaction in bridge decks are due to poorly detailed webs, Figure 9.26.
