It is related to the guaranteed minimum yield point of steel with an appropriate factor of safety. Most of the codes assume a factor of safety of 1.67.
The direct stress in axial tension or the effective net area should not exceed given by the equation,
Where, = minimum yield stress of steel in (MPa).
Indian Standard IS: 226
It stipulates the following permissible stress in axial tension for steel,
|(i)||Plates, angles, tees, I-beams, channels and flats|
|upto and including 20mm thickness||150 MPa|
|above 20mm thickness to 40mm||144 MPa|
|Over 40 mm thickness||138 MPa|
|(ii)||Bars (round, square and hexagonal)|
|Upto and including 20mm diameter||150 MPa|
|Over 20mm diameter||144 MPa|
Permissible Combined Stress
When tension members are subjected to both axial loading and bending moment, then the permissible stress is governed by the formula,
Where, = axial tensile stress,
= permissible axial stress in tension,
= calculated bending stress in tension in the extreme fibre, and
= permissible bending stress in tension.