Nonlinear Material Behavior of the Bridge Components - Finite Element Analysis and Design of Steel and Steel-Concrete Composite Bridges - page 51

compatible with the chemistry of weathering steel grades and are required
for use in unpainted applications where both the bolts and the base metal can
develop rust in service.
Cables and wires used in bridges in the United States are either strands,
which are covered by ASTM A586 [ 2.7 ] , or ropes, which are covered by
ASTM A603 [ 2.8 ]. Cables and wires are constructed from individual
cold-drawn wires that are spirally wound around a wire core. The com-
monly used nominal diameters are between 1/2 (12.7 mm) and 4 in.
(101.6 mm) depending on the intended application. The capacities of the
cables and wires are defined as the minimum breaking strength that depends
on the nominal diameter of the cables or wires. Cables and wires are used as
tension members in bridges. Because relative deformation between the indi-
vidual wires will affect elongation, strands and ropes are preloaded to about
55% of the breaking strength after manufacturing to “seat” the wires and
stabilize the deformation response. Following preloading, the axial deforma-
tion becomes linear and predictable based on an effective modulus for the
wire bundles. A bridge rope has an elastic modulus of 20,000 ksi
(138,000 MPa). The elastic modulus of a bridge strand is 24,000 ksi
(165,000 MPa). Seven-wire steel strands (tendon) are commonly used for
prestressed concrete bridge decks. They are also used as cable stays, hangers,
and posttensioning members. They consist of seven individual cold-drawn
round wires spirally wounded to form a strand with nominal diameters
between 0.25. (6.4 mm) and 0.60 in. (15.2 mm). Two grades are available
(250 and 270) where the grade indicates the tensile strength of the wires
( f pu ). The net cross-sectional area of the seven-wire strand (area of the indi-
vidual wires) should be used in all calculations, and prestress losses should be
accounted for, either by measurements or based on specified values in cur-
rent codes of practice. Mechanical properties of seven-wire strands are mea-
sured from tensile coupon tests. The tensile strength is calculated by dividing
the breaking load by the net cross-sectional area of the seven-wire strand.
Compared to structural steels, strands do not exhibit a yield plateau, and
there is a gradual rounding of the stress-strain curve beyond the proportional
limit. The yield stress in this case may be calculated as the stress at the 0.1%
strain offset line ( f 0.1 ). Strands are loaded provided that they do not reach the
yield stress. AASHTO [1.23, 1.24] defines the yield strength as f py ¼ 0.90 f pu .
The ASTM A370 [ 2.9 ] and the ASTM E8 [ 2.10 ] specifications cover
tensile coupon testing procedures for determining the material properties
of steel products. The main properties measured from a tensile coupon test
are the yield strength ( f y ), tensile strength ( f u ), Young's modulus ( E s ),