A prismatic beam (as shown below) has plastic moment capacity

A prismatic beam (as shown below) has plastic moment capacity of Mp, then the collapse load P of the beam is

Degree of static indeterminacy, D_s = 0
For mechanical, no. of plastic hinges required = D_s + 1 = 1

From principle of virtual work

- M_P θ - M_P θ + P.	L	θ -	P	×	L	θ = 0
	2		2		3

- M_P θ +	PL	θ -	PL	θ = 0
	2		6

2M_P =	PL	×	PL
	2		6

=	(3 - 1)PL
	6

⇒ 2M_P =	1	PL
	3

∴ P =	6M_P
	L

	4M_P
	L

	8M_P
	L