Support Reactions  Equilibrium
Static equilibrium is achieved when the resultant force and resultant moment equals to zero.
Equilibrium of a body requires both a balance of forces to prevent the body from translating or having accelerated motion along a straight or curved path  and a balance of moments to prevent the body from rotating.
Any static force system will be in equilibrium if the resultant force and resultant moment both are equal to zero.
Static equilibrium in a three dimensional system can be expressed as
ΣF = ΣF_{x} = ΣF_{y} = ΣF_{z} = 0 (1)
ΣM = ΣM_{x} = ΣM_{y} = ΣM_{z} = 0 (2)
where
F = force (N, lb)
M = moment (Nm, ft lb)
x, y, z = orthogonal axes
Often the loading of a body can be simplified to a two dimensional system with coplanar forces in the xy plane. Eq. 1 and 2 can be reduced to
ΣF = ΣF_{x} = ΣF_{y} = 0 (3)
ΣM = ΣM_{z} = 0 (4)
The best way to account for all forces acting on a body is to draw the body's freebody diagram. A freebody diagram shows the relative magnitude and direction of all forces acting upon an object in a given situation.
Freebody diagram example  gravity and friction forces acting on a body on an inclined plane
Example  Support Reactions on a Beam with Eccentric Load
A beam with length 6 m has an eccentric load of 9000 N 4 m from support 1. Applying the equations of equilibrium we have
F_{x} = R_{1x} = R_{2x} = 0 (5)
F_{y}  (R_{1y} + R_{2y}) = 0 (6)
M_{1} = F_{y} a  R_{2y} (a + b) = 0 (7)
Rearranging (7) to express R_{2y }
R_{2y} = F a / (a + b) (7b)
Eq. (7b) with values
R_{2} = (9000 N) (4 m) / ((4 m) + (2 m))
= 6000 N
= 6 kN
Rearranging (6) for R_{1y}
R_{1y} = F_{y}  R_{2y} (6b)
Eq. (6b) with values
R_{1y} = (9000 N)  (6000 N)
= 3000 N
= 3 kN
Example  Reaction Forces on a Structural Frame
A weight F (1000 N) is hanging in a structural frame as shown in the figure above. A structural analyses can be done with the following equations:
∑M_{A} = R_{By} (230 cm)  (1000 N) (100 cm) = 0 (1)
∑M_{B} = R_{Ay} (230 cm)  (1000 N) (130 cm) = 0 (2)
∑M_{CB} = R_{By} (130 cm)  R_{Bx} h = 0 (3)
F = R_{Ay} + R_{By} (4)
R_{Ax} = R_{Bx} (5)
Eq. 1 can be rearranged to
R_{By} = (1000 N) (100 cm) / (230 cm)
= 435 N
Eq. 2 can be rearranged to
R_{Ay} = (1000 N) (130 cm) / (230 cm)
= 565 N
The height h in eq. 3 can be calculated as
h = (153^{2}  130^{2})^{1/2}
= 80.7 cm
Eq. 3 can then be rearranged to
R_{Bx} = (435 N) (130 cm) / (80.7 cm)
= 700 N
From eq. 5
R_{Ax} = R_{Bx}
= 700 N
Related Topics

Mechanics
Forces, acceleration, displacement, vectors, motion, momentum, energy of objects and more. 
Statics
Loads  forces and torque, beams and columns.
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