TOS ~ Moment Distribution Method
Moment Distribution Method
Introduction of Moment Distribution Method
- Suggested by Prof. Hardy Cross in the early 1930s.
- The method is widely used for the analysis of indeterminate structures.
- Ideally suited for a fairly higher degree of indeterminate structures
Indeterminate structures of Moment Distribution Method
- These are the structures that cannot be fully analysed by the condition of equilibrium.
- The equations of statical equilibrium which are based on Sir Isaac Newton’s law governing the motion of bodies 1687 are
- The summation of all forces in any axis is zero.
- The summation of all moments in any axis is zero.
If summation is taken about the x, y and z axes, symbolically this becomes Moment Distribution Method.
- ∑Fx = 0; ∑Fy = 0; and ∑Fz = 0;
- ∑Mx = 0; ∑My = 0; and ∑Mz = 0;
For Planar structures and forces acting in the same plane, the equation reduces to
- ∑Fx = 0, ∑Fy = 0, and ∑Mz = 0,
Types of Supports in Moment Distribution Method
- Simple support
- Roller support
- Hinge or Pin support
- Fixed support
Calculation of Indeterminacy by Formulae
- Degree of Indeterminacy of Frame: i=(3m +r) – (3j +c)
- Or statically determinate if 3j = 3m +r
- Degree of Indeterminacy of Plane Truss: i=(2j -3) – m
- Or statically determinate if m=2j -3
Terminology on Moment Distribution Method
- When a moment is applied at one end of a member allowing rotation of that end and fixing the far end, some moment develops at the far end also, this moment is called the carryover moment
- The ratio of the carryover moment to the applied moment is called the carryover factor.
- Carryover factor = M’/M
- The moment required to rotate an end by the unit angle 1 radian, when rotation is permitted at that end, is called the stiffness of the beam. Thus in the beam above, if θA is the rotation at end A,
- Stiffness of the beam AB = k (M/ θA)
- When a structural member of a uniform section is subjected to a moment at one end only, then the moment required to rotate that end to produce a unit slope is called the stiffness of the member.
Distribution factor of Moment Distribution Method
- If a moment is connected to a rigid joint where several members are meeting, the connected moment is shared with the branches meeting about that joint.
- The ratio of the moment shared by a member to the applied moment at the joint is called the distribution factor of that member.
- Thus, if MoA is the moment shared by member OA when moment M is applied at joint O, then the distribution factor for member OA is dOA = MOA/M
Application of Moment Distribution Method to Continuous Beams with Fixed Ends
- Assuming all ends are fixed, find the fixed end moments developed.
- Calculate distribution factors for all members meeting at a joint.
- Balance a joint by distributing balance moment to various members meeting at the joint proportional to their distribution factors. Do a similar exercise for all joints.
- Carryover half the distributed moment to the far ends of the members. This upsets the balance of the joints.
- Repeat steps 3 and 4 till distributed moment are negligible.
- Sum up all the moments at a particular end of the member to get a final moment.