Bridge
Weight
Ice
Concrete
Load
Pier
Scour
Bending
Shear
Reaction
Reaction
Timber
Pier
e
(a)
(b)
(c)
Figure 5. Simplified failure scenario: a) as built, b) reduction in embedded pile length caused by scour, and
c) lateral displacement of the pile, making the bridge weight an eccentric load.
depends on the developed lateral and bending
more flexible. The increases in stress (i.e. bend-
stiffness of the piles. Both aspects are a function
ing and shear) ultimately resulted in the piles
of the pile's embedded length (Fig. 5a). As built,
snapping at the change in cross section. With the
the piers had sufficient stiffness to resist any lat-
limited lateral support, the piles could easily be
deflected or move sideways by the ice loads. The
eral movement of the foundation.
combined shear and bending stresses could eas-
Removing gravel, as during a scour event, re-
ily have exceeded the bending resistance at the
duces the pile's embedded length and compro-
transition between the pile and concrete, causing
mises the structural integrity of the pier. As the
the pier to buckle. This scenario is supported by
length of the exposed pile increased, the rela-
the snapped timber pile left in the riverbed and
tively slender piles are more prone to flexing. In
in the concrete. The concrete portions of the piers
the extreme case with short penetration, the pile
were found on the bottom of the river adjacent to
will rotate as a unit within the soil (Fig. 5b). In
the line of timber piles (Fig. 6).
this condition most of the bridge weight is sup-
ported by the point bearing at the tip of the piles.
Basically, 10 ft of scour would leave the pier as a
block of concrete supported by nine 16-ft stilts.
With the relatively low stage during the 1990
breakup, the ice impacted the pier close to the
bed, requiring the foundation to take a larger
portion of the load. Because of the scour the pier
would be more flexible, allowing the pier to de-
flect sideways as it experiences side impacts. The
lateral deflection causes the weight of the bridge
to be an eccentric load (Fig. 5c), increasing the
bending stresses. The abrupt transition from the
timber piles to the piles encased in concrete fur-
ther aggravates the stress conditions. There is a
considerable difference in the stiffness between
the two sections, with the wooden piles being
Figure 6. Piers on the river bottom.
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