Civil Engineering Reference
In-Depth Information
Figure 8.3
Alternative deck for the Sungai Kelantan Bridge
loading the effective total live load bending moment and shear force carried by the
deck is governed by the capacity of the deck to re-centre eccentric loads.
For instance, consider initially a deck consisting of two torsionless beams carrying
a top slab. As a point load is placed on each beam in turn, it defl ects and rotates, and
no transverse distribution of the load takes place. As each beam must be designed to
carry 100 per cent of the load, it is carried twice. For a span
L
and a load
W
the total
moment carried by the deck is 2×
PL
/4. Joining the beams with diaphragms makes no
difference to this result. Figure 8.4 shows such a deck that was designed as part of a
jetty, carrying heavy dump trucks.
This conclusion may be extended to a multi-beam deck such as the notional
torsionless three-beam system with a hinged slab shown in Figure 8.5
(a); no load
is distributed from the edge beam to the centre beam, as illustrated by the defl ection
diagram. As the live load may be placed over each beam in turn, it is effectively carried
three times. Therefore, the logic of dimensioning beam type decks is to minimise the
number of beams.
If the deck is given a fi nite transverse bending stiffness, loads will be distributed
transversally to some degree. This may be achieved by using diaphragms, Figure 8.5 (b),
or more simply by giving the deck slab an adequate transverse bending stiffness,
Figure 8.5 (c). Analysis and experience have shown that intermediate diaphragms are
costly and unnecessary (see
9.6
), as an appropriately dimensioned slab provides adequate
transverse strength. Real, virtually torsionless systems without intermediate diaphragms
are described in Chapter 10, and typical examples are shown in Figure 10.14 and
Figure 10.17. A concentrated load on one beam will now be shared to a small degree
by adjacent beams, due to the transverse bending strength of the slab. A consideration
of the defl ected shapes shown in Figure 8.5 helps to clarify this behaviour.
