Single-span beams generally collapse as two-bar mechanisms with a hinge at each support and plastic hinge within span.

In kinematic method, a mechanism is assumed and virtual work equations are used to determine collapse load.

The following are advantages of plastic design methods: (i) realisation of uniform and realistic factor of safety for all parts of structure , (ii)simplified analytical procedure and rapidity of obtaining moments since no need to satisfy elastic strain compatibility conditions, (iii) no effect due to temperature changes, settlement of support, etc, (iv) saving of material over elastic methods resulting in lighter structures.

For simply supported rectangular beam with uniformly distributed load, hinged length is equal to span/?3.

When every fibre of section has a strain equal to or greater than ?_y=f_y/E_s , nominal moment strength is referred as plastic moment and is given by M_p = Z_pf_y , where Z_p = ?ydA is plastic section modulus and f_y = yield stress.

The effective width of hot-rolled and welded plates is given by b_e/b = ? ?(f_cr/f_y), where ? is parameter that indicates inclusions of influence of initial curvatures and residual stress. Generally ?=0.65.

For a thin plate simply supported along both transverse edged and one longitudinal edge and free along or longitudinal edge, buckling coefficient can be approximated by k = 0.425 + (b/a)².

When a member is subjected to uniaxial tensile or compressive stress in presence of shear stress ?, yield occurs when f_y² = f² + 3 ?².

When gravity load governs design, a good estimate of required section may be obtained by using following formulae :
For pin based frames : M_p =?L(wL²/8)[1+k+(1+k)^0.5] For fixed=-base frame : M_p =?L(wL²/8)[1/[1+0.5k+(1+k)^0.5]], where ?_L = 1.7 global load factor, k = h₂/h₁.

Shape factor, v = Z_p/Z_e = M_p/M_y , where Z_p and Z_e are plastic and elastic section modulus respectively, M_p and M_y are plastic and elastic moments respectively.