For a given frame and loading if re exists any distribution of bending moments throughout frame which is both safe and statically admissible with set of load P, n value of load P must be less than or equal to collapse load.

When length of column is relatively small and its component elements are prevented from local buckling, n column will be able to attain its full strength or squash load (squash load = yield stress x area of cross section).

Cantilevers and over hanging beams generally collapse as single-bar mechanisms with a single plastic hinge at one of supports.

Load factor is ratio of collapse load to working load. It is represented by, F = W_c / W.

The design moment capacity M_d of each of four classes of cross sectioncan be calculated as i) Plastic ? M_d = Z_pf_y, ii) Compact ? M_d = Z_pf_y, iii) Semi-Compact ? M_d = Z_ef_y, iv) Slender ? Md < Z_ef_y, where Z_p = plastic modulus, Z_e = elastic modulus.

When full plasticity is produced, stress in web will be equal to f_w, f_w =?(f_y² -3?_w² ) = f_y?[1-(_w/?_y)² ] where ?_y is shear strengths when yield is in pure shear.

Plastic or class I sections are cross sections which can develop plastic hinges and have a rotation capacity required for failure of structure by formation of plastic mechanism.

The difference between plastic design and elastic design is that plastic design takes into account favourable redistribution of bending moment which takes place in indeterminate structure after first hinge forms at point of maximum bending moment.

Both elastic and plastic methods neglect influence of stability, which may significantly affect load carrying capacity of structures or elements which are slender and subjected to compressive stresses.

The lowest plastic limit load is obtained when all conditions of equilibrium, mechanism and plasticity of plastic analysis are satisfied.