Civil Engineering Reference
In-Depth Information
fi ll, these slabs may be designed to accept large movements without the need to resort
to hinged structures.
Slabs may be 'creased' to follow a crossfall that is variable across the width without
the unnecessary expense of varying the slab thickness. For normal crossfalls up to at
least 5 per cent any 'folded plate' action may safely be ignored.
11.3.2 Transverse bending
Slabs decks are subjected to transverse bending moments and shears under live loading,
and under localised dead loads such as footpaths, central reservations and parapets.
These moments may be resisted by transverse reinforcement, or by light transverse
prestress. The transverse bending under the effects of unequally distributed loads
across the slab may also be controlled by varying the intensity of the longitudinal
prestress across the width of the slab.
11.3.3 Crossbeams
For regular slabs with piers consisting of rows of supports, reinforced or prestressed
crossbeams may be incorporated beams, within the thickness of the slab, or downstand
beams. Supports should normally be arranged so that the span/depth ratio of the
crossbeam does not exceed about 5.
For a reinforced concrete incorporated beam designed at the ULS, it is conventional
to consider it as a Tee beam, with a web width, for calculation of shear strength, equal
to the width of the bearing plus the thickness of the deck, and a slab width equal to the
web width plus 1/10 of the distance between points of infl ection for the span of the
crossbeam beam either side of the web, Figure 11.1
.
This slab is at the top for sagging
moments and at the bottom for hogging moments. The depth of this virtual slab for
calculation at the ULS should not exceed 40 per cent of the thickness of the slab, to
maintain a ductile section.
For prestressed incorporated beams designed at the SLS, the section properties of
the crossbeam should be calculated with widths of the web and of the top and bottom
slabs defi ned as for reinforced concrete. The thickness of the top and bottom virtual
slabs at the SLS may be assumed to be 30 per cent of the depth of the deck. It must not
be forgotten that the prestress force will be dissipated in the deck at an angle of 35°
Figure 11.1
Incorporated crossbeams
