Civil Engineering Reference
In-Depth Information
statically determinate, the external loads only apply positive bending moments, and
consequently the heel is progressively unloaded by the application of:
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addition of the cast-in-situ portion of the top slab;
loss of prestress;
deck fi nishes;
live loads.
Thus, the essence of the design of these beams is fi rstly to minimise the number of
beams in the cross section, and then to size the heel such that its stresses vary from
the safe limit under the initial prestress combined with its self weight, to the allowable
tensile stress under long-term working loads.
The very rare accidents that have occurred during the construction of these beams
have generally been due to the overstressing of poorly compacted concrete in the heel
when the prestress is applied.
The strength of the heel is sometimes increased by using compression reinforcement.
This is not usually recommended, as it is more economical to increase the size of the
heel. In exceptional circumstances, where for instance, a few beams in a series are called
on to carry heavier live loads due to a local widening of the deck or to a single longer
span, this is an expedient that allows the standard formwork to be maintained.
Three different types of top fl ange are used. The web may be stopped below the
slab with a thickening serving to stabilise the beam during handling, Figure 10.10 (a).
This arrangement minimises the weight of the prefabricated beams, making handling
in the casting yard and launching more economical. There is a possibility of torsional
instability when a beam with a small top fl ange is lifted.
The connection between beam and slab is simply created by stirrups projecting out
of the top of the webs. The shutter for the top fl ange has to be propped off the heels
of the beams, which may be relatively inaccessible. Alternatively, precast formwork
such as Omnia planks may be used, which are relatively expensive. This type of beam
uses more prestress than those described below, as the neutral axis of the beam alone is
lower, giving a smaller prestress eccentricity. A further disadvantage is that the narrow
top fl ange does not give a safe working platform for installing the slab formwork.
The other extreme is the beam shown in Figure 10.10 (b). Here, the bottom lift of
the top slab over its full width is incorporated into the precast beams. This 'pre-slab'
must be profi led to the crossfall of the road. Once the beams have been erected, there
is a full-width working platform on which the reinforcement of the top slab may
be assembled and the remainder of the top slab poured, with no need for any other
signifi cant falsework. The 'pre-slab' must be designed to carry the weight of the wet
concrete and the other construction loads such as men and compressors. This requires
some careful defi nition of the most onerous conditions that are likely to be met. For
instance, if the concrete is discharged from hoppers, the possibility of a substantial
thickness being deposited locally on the end of a prefabricated cantilever must be
considered.
The principal disadvantages of this arrangement are the increased weight and size of
the beams to be handled and launched, the additional weight of reinforcement due to
the need for top steel in the precast pre-slab, the diffi culty in providing economically
for sagging reinforcement at the centre of the slabs, and the fact that the beam spacing
should be restricted to about 3 m, increasing the number of beams required. For a
