Civil Engineering Reference
In-Depth Information
9.4.7 Resal effect
The title to this section celebrates the brilliant French designer of bridges (including
the Alexandre III Bridge over the Seine in Paris) who, in the nineteenth century,
allegedly fi rst documented this important aspect of the behaviour of bridge decks. The
compressive force in the inclined bottom slab of a deck of variable depth that is subject
to a hogging moment, carries a signifi cant part of the shear force, Figure 9.23 (a).
In fact, the inclined fi bres of the web below the neutral axis also carry some of the
shear force, but this calculation is more subtle and can frequently be conveniently
neglected.
Comparison with an equivalent truss, Figure 9.23 (b), makes it clear that there
exists an inclination of the bottom slab that would carry the entire shear; the force in
the last tension diagonal would be zero. If the applied moment is
M
, the shear force
S
,
the lever arm
l
a
and the inclination of the bottom fl ange with respect to the horizontal
is
α
, the tension in the top member, and the horizontal component of the compression
Figure 9.23
Resal effect
