Civil Engineering Reference
In-Depth Information
(
5.26
and
9.6.2
), so only a proportion of the compression due to prestress (P/A) will be
useful for the crossbeam, although the moment effect (
Pe
) will be entirely effective.
Generally crossbeams may be designed on the assumption that dead loads and UDL
live loads are uniformly distributed along the beam, while concentrated live loads are
applied directly to the beam. This is correct for downstand beams but conservative
for incorporated beams where the slab itself will span transversally to concentrate the
loads over the points of support. If the scale of the project justifi es it, or if support
points are widely spaced, a more accurate calculation of incorporated beams should be
carried out using a carefully defi ned grillage or a fi nite element analysis.
Where the columns supporting the slab are spaced widely, drops or mushroom
heads to the columns may be used to control the local bending moments and shears,
Figure 11.2. The roof slab of Canada Water underground station on the Jubilee Line
Extension in London, designed by the Benaim-WORKS JV, carried highway loads, and
was supported by mushroom heads on the columns, Figure 11.3.
11.3.4 Prestress arrangements and typical details
a) Prestress layouts
The main cables are draped to carry part of the shear forces, although stirrups are
still required for the ULS. These stirrups also act as chairs to carry the tendons. If the
slab is designed to Class 1 prestressing (
4.2.3-4.2.5
), the only passive longitudinal
reinforcement required, other than a nominal mesh to control heat of hydration
effects, is the support bars for the stirrups. The reinforcement content for decks fully
Figure 11.2
Drops and mushroom heads
