Shake Table Studies of Bridge Columns with Double Interlocking Spirals

The current seismic design philosophy for reinforced
concrete structures relies on confinement of concrete to
provide the necessary ductility and energy dissipation
capacity of structural members. Confinement is mainly
provided by the transverse reinforcement, which in columns
usually consists of spirals in members with circular or square
shape and ties in those with square or rectangular cross
sections. Spirals confine concrete more effectively than
rectilinear ties because they counteract the dilation of
concrete through hoop action instead of a combination of
bending and hoop action that takes place in rectilinear ties.
As a result, to provide the same level of confinement, the
amount of tie reinforcement is greater than that provided by
spirals. Another advantage of spirals is that they are generally
easier to construct. The circular shape of spirals makes them
suitable for circular and square columns. To use the benefits
of spirals in rectangular columns, two or more sets of
interlocking spirals are used.
The Caltrans Bridge Design Specifications (BDS) and
Seismic Design Criteria (SDC)
2
are currently the only codes
in the U.S. that include provisions for the design of columns
with interlocking spirals. Because the amount of research on
interlocking spirals has been limited, the design provisions
are driven mainly by research on single spirals. Studies
3-5
were conducted on the effect of several design parameters,
including a comparison between interlocking spirals and
ties, horizontal distance between centers of the spirals,
quantity of transverse reinforcement, variation of the axial
load ratios, appropriate size and spacing of longitudinal bars
in the interlocking region, and cross section shape. These
studies generally concluded that flexural and shear
capacities of columns with interlocking spirals can be
conservatively estimated using current procedures. Conflicting
80% of the gross area (SDC).
2 A shear stress index was
defined as the average shear stress divided by 0.083√f ′
c
(MPa) (√f ′
c [psi]). This index represents the level of shear in
the column. In this study, two levels of shear were selected:
low index equal to 3 and high index equal to 7. These indexes
represent column shear stresses in real bridges. Actual
bridge columns are designed to be ductile and the load
capacity is controlled by flexure, although shear damage is
expected to increase as the shear index increases.
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