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Since axially loaded tension members are subjected to uniform tensile stress,
their load deformation behaviour (Fig.4.3) is similar to the corresponding basic material
stress strain behaviour. Mild steel members (IS: 2062 & IS: 226) exhibit an elastic range
(a-b) ending at yielding (b). This is followed by yield plateau (b-c). In the Yield Plateau
the load remains constant as the elongation increases to nearly ten times the yield
strain. Under further stretching the material shows a smaller increase in tension with
elongation (c-d), compared to the elastic range. This range is referred to as the strain
hardening range. After reaching the ultimate load (d), the loading decreases as the
elongation increases (d-e) until rupture (e). High strength steel tension members do not
exhibit a well-defined yield point and a yield plateau (Fig.4.3). The 0.2% offset load, T,
as shown in Fig.4.3 is usually taken as the yield point in such cases.
Design strength due to yielding of gross section
Although steel tension members can sustain loads up to the ultimate load without
failure, the elongation of the members at this load would be nearly 10-15% of the
original length and the structure supported by the member would become
unserviceable. Hence, in the design of tension members, the yield load is usually taken
as the limiting load. The corresponding design strength in member under axial tension is
given by (C1.62)
Where, fy is the yield strength of the material (in MPa), A is the gross area of
cross section in mm2
and γmO is the partial safety factor for failure in tension by
yielding. The value of γmO according to IS: 800 is 1.10.
4.2.2 Design strength due to rupture of critical section
Frequently plates under tension have bolt holes. The tensile stress in a plate at
the cross section of a hole is not uniformly distributed in the Tension Member:
Behaviour of Tension Members elastic range, but exhibits stress concentration adjacent
to the hole [Fig 4.4(a)]. The ratio of the maximum elastic stress adjacent to the hole to
the average stress on the net cross section is referred to as the Stress Concentration
Factor. This factor is in the range of 2 to 3, depending upon the ratio of the diameter of
the hole to the width of the plate normal to the direction of stress.