Permissible Stresses in Steel:
The permissible (or allowable) stress in steel is determined by dividing the yield stress (σy) by a suitable factor of safety. The value of the factor of safety is decided on the basis of the following considerations:
(a) Variations of the strength of any individual specimen from the average strength (this depends upon the manufacturing processes and workmanship).
(b) Uncertainty within the design loads, if estimated on insufficient data.
(c) Method of structural analysis adopted and it's precision.
(d) Uncertainty at the site conditions, proper supervision and inspection.
(e) Variations in environmental conditions, e.g. temperature, support settlement, weather, corrosion, etc.
(f) Significance of the structure and extent of damage or losses caused by its failure.