Civil Engineering Reference
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Stress
f
f
u
f
p
0.2 %
offset
line
E
s
e
y
e
u
e
final
Strain
Figure 2.1 Engineering stress-strain curve for structural steels without a defined yield
plateau.
ultimate strain at failure (
e
u
), and full nonlinear stress-strain curve. The full
nonlinear stress-strain curve is known as the engineering stress-strain curve,
which can be measured by recording the load and elongation of an exten-
someter during the tensile coupon test. Young's modulus for steel can be
determined by predicting the slope of the elastic initial portion of the
stress-strain curve as shown in
Figure 2.1
.
In the absence of the measured
engineering stress-strain curve, Young's modulus for steel can be conserva-
tively taken as
E
s
ΒΌ
29,000-30,000 ksi (200,000-207,000 MPa) for structural
calculations for all structural steels used in bridge construction. The yield
strength of steel is determined by the 0.2% offset method. A line is plotted
parallel to the elastic part of the stress-strain curve below the proportional
limit with an x-axis offset of 0.2% (0.002) strain. The intersection of the off-
set line with the stress-strain curve defines the yield strength.
Figures 2.1
and
2.2
show the 0.2% offset method applied to steels without a definite
yield plateau and to steels that exhibit a yield plateau, respectively. For
the steels that exhibit a yield plateau, there is an upper yield point that is
greater than the yield strength. When yielding first occurs, there is typically
a slight drop in load before the steel plastically deforms along the yield
(483 MPa) undergo plastic deformation at a relatively constant load level
defining the yield plateau. The length of this plateau varies for different
steels, but approximately,
e
st
is around 10
e
y
. Strain hardening begins at
the end of the plateau and continues until the maximum load is achieved
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