Civil Engineering Reference
In-Depth Information
Figure 9.40
Diaphragm action for side-sway
Bridge decks other than boxes must of course also resist external lateral forces.
Stocky sections, such as twin rib decks, can carry the lateral loads in portal action, and
in general do not need diaphragms (see Chapter 12), while slender structures such as
precast beam decks (see Chapter 10) do need diaphragms at the piers.
9.6.4 Trapezoidal boxes
Boxes with inclined webs apply a concentrated transverse compression force to the
deck soffi t at each pier. This may best be understood by considering each web of the
box as an inclined truss, Figure 9.41. A uniformly distributed load on the deck is
applied to the truss as a point load on the nodes of the top boom. Thus at each such
node the vertical component of the force in the diagonal compression strut is greater
than the vertical component of the force in the upright tension member. As the web is
inclined, this out of balance creates a transverse tension in the top fl ange of the deck at
each node,
h
. This is a relatively small effect, as it is caused by the difference in force
in the uprights and the diagonals, but it may require additional reinforcement in the
top slab. However, at a support, the last diagonal compression strut, which is carrying
the whole of the shear in the web, applies a substantial concentrated transverse force,
H
, which is the sum of all the forces
h
in the half span, and which could overstress the
