Civil Engineering Reference
In-Depth Information
12.3.2 Pier diaphragms
The purpose of diaphragms at piers for a ribbed slab deck is to stabilise the deck under
the effect of transverse wind and traffi c loads on the deck, and of seismic loads applied
to the deck through the piers. Pier diaphragms may also be used to offset the bearings
from the rib centreline if required for any reason. Figure 12.8 shows such a use of
diaphragms in the Tai Ho Viaduct in Hong Kong, designed by the author at Arup with
the architectural assistance of Humphrey Wood, where the crosshead was shortened
for visual effect.
By impeding any twisting of the ribs about a longitudinal axis, diaphragms act as
torsion sumps increasing both the dead (
12.4.2
) and live load torque in the ribs. If
the deck is sharply curved, the presence of diaphragms will also increase both the
dead and live load torque that is due to the curvature. The torque generated in the
ribs adjacent to the pier diaphragms, when combined with the shear force, is likely
to defi ne the width of the rib, and in all cases greatly increases the rib reinforcement
when compared with a deck without pier diaphragms. Consequently pier diaphragms
should only be used when there is no better alternative. The fi rst option for increasing
the strength or stiffness of the slab should be to increase its thickness.
Figure 12.8
Contract 308 of the HKMTR, Tai Ho Viaduct: twin rib deck with pier diaphragms
(Photo: Robert Benaim)
