Building Design and Construction Handbook - Merritt - Ventech!

STRESS, KSI
120
100
80
60
40
20
0.005
BUILDING MATERIALS 4.59
A514 STEEL
0.2% OFFSET YIELD STRENGTH
0.5% E.U.L. YIELD STRENGTH
UPPER YIELD LIMIT
�
LOWER YIELD LIMIT st
HEAT-TREATED, HIGH-STRENGTH
CARBON STEEL
A36 STEEL
SLOPE = E st
PLASTIC RANGE STRAIN-HARDENING RANGE
INELASTIC RANGE
ELASTIC RANGE
SLOPE = E
0
0 0.005 0.010 0.015 0.020 0.025 0.030
0.002 STRAIN, IN. PER IN.
HIGH-STRENGTH, LOW-ALLOY STEEL
FIGURE 4.4 Magnification of the initial portions of
the stress-strain curves for structural steels.
elasticity E, which is given by the slope of the curves, is nearly a constant 29,000
ksi for all the steels. For carbon and high-strength, low-alloy steels, the inelastic
range, where strains exceed those in the elastic range, consists of two parts: Initially,
a plastic range occurs in which the steels yield; that is, strain increases with no
increase in stress. Then follows a strain-hardening range in which increase in strain
is accompanied by a significant increase in stress.
The curves in Fig. 4.4 also show an upper and lower yield point for the carbon
and high-strength, low-alloy steels. The upper yield point is the one specified in
standard specifications for the steels. In contrast, the curves do not indicate a yield
point for the heat-treated steels. For these steels, ASTM 370, ‘‘Mechanical Testing
of Steel Products,’’ recognizes two ways of indicating the stress at which there is
a significant deviation from the proportionality of stress to strain. One way, applicable
to steels with a specified yield point of 80 ksi or less, is to define the yield
point as the stress at which a test specimen reaches a 0.5% extension under load
(0.5% EUL). The second way is to define the yield strength as the stress at which
a test specimen reaches a strain (offset) 0.2% greater than that for elastic behavior.
Yield point and yield strength are often referred to as yield stress.
Ductility is measured in tension tests by percent elongation over a given gage
length—usually 2 or 8 in—or percent reduction of cross-sectional area. Ductility
is an important property because it permits redistribution of stresses in continuous
members and at points of high local stresses.
Poisson’s ratio, the ratio of transverse to axial strain, also is measured in tension
tests. It may be taken as 0.30 in the elastic range and 0.50 in the plastic range for
structural steels.
Cold working of structural steels, that is, forming plates or structural shapes
into other shapes at room temperature, changes several properties of the steels. The
resulting strains are in the strain-hardening range. Yield strength increases but ductility
decreases. (Some steels are cold rolled to obtain higher strengths.) If a steel