Page 19 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch01.qxd 9/29/05 4:59 PM Page 1.17

PROPERTIES OF STRUCTURAL STEELS AND EFFECTS OF STEELMAKING AND FABRICATION

STRUCTURAL STEELS, STEELMAKING, AND FABRICATION 1.17

FIGURE 1.7 Effects of strain rate on yield and tensile strengths of structural steels at low, normal,
and elevated temperatures. (From R. L. Brockenbrough and B. G. Johnston, USS Steel Design Manual,
R. L. Brockenbrough & Associates, Inc., Pittsburgh, Pa., with permission.)

has a relatively small influence on the yield strength. But a faster strain rate causes a slight decrease
in the tensile strength of most of the steels.
Ductility of structural steels, as measured by elongation or reduction of area, tends to decrease
with strain rate. Other tests have shown that modulus of elasticity and Poisson’s ratio do not vary sig-
nificantly with strain rate.

1.11 EFFECT OF ELEVATED TEMPERATURES
ON TENSILE PROPERTIES

The behavior of structural steels subjected to short-time loadings at elevated temperatures is usually
determined from short-time tension tests. In general, the stress-strain curve becomes more rounded
and the yield strength and tensile strength are reduced as temperatures are increased. The ratios of
the elevated-temperature value to room-temperature value of yield and tensile strengths typical for
structural steels are shown in Fig. 1.8a.
Modulus of elasticity decreases with increasing temperature, as shown in Fig. 1.8b. The relationship
shown is typical for structural steels. The variation in shear modulus with temperature is similar to that
shown for the modulus of elasticity. But Poisson’s ratio does not vary over this temperature range.
Ductility of structural steels, as indicated by elongation and reduction-of-area values, decreases
with increasing temperature until a minimum value is reached. Thereafter, ductility increases to a
value much greater than that at room temperature. The exact effect depends on the type and thick-
ness of steel. The initial decrease in ductility is caused by strain aging and is most pronounced in the
temperature range of 300 to 700°F. Strain aging also causes an increase in tensile strength in this
temperature range shown for some steels.

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