Civil Engineering Reference
In-Depth Information
disadvantage, as it resists the length changes of the deck. If the pier is divided into
two fl exible leaves, the bending stiffness can be retained, while the shear stiffness is
greatly diminished. This may best be understood by considering a pier consisting of
an 'I' beam, Figure 7.19. The bending strength of the beam is provided by the two
fl anges, while the shear strength is provided by the web. If the web is removed, the
bending strength remains, but the shear strength is destroyed. Thus the benefi ts to
the deck bending moments are retained, while the length changes of the deck are less
restrained, allowing a longer length of the deck to be built-in in this way.
The spacing of the two leaves has a marked infl uence on the design. Live load on
one span will cause uplift on the remote column of the pair. If the leaves are close
together, the uplift will overcome the dead load reaction on the remote column and
put it into tension. The degree of moment fi xity of the deck gradually reduces as
the columns are brought closer together. This is due to the shortening of the near
column and the extension of the remote column, which allows the node between
Figure 7.19
Development of twin leaf piers
