Engineering ToolBox - Resources, Tools and Basic Information for Engineering and Design of Technical Applications!

This is an AMP page - Open full page! for all features.

• the most efficient way to navigate the Engineering ToolBox!

# Support Reactions - Equilibrium

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

Σ F = Σ F x = Σ F y = Σ F z = 0                          (1)

Σ M = Σ M x = Σ M y = Σ M 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 co-planar forces in the x-y plane.  Eq. 1 and 2 can  be reduced to

Σ F = Σ F x = Σ F y = 0                          (3)

Σ M = Σ M z = 0                          (4)

The best way to account for all forces acting on a body is to draw the body's free-body diagram . A free-body diagram shows the relative magnitude and direction of all forces acting upon an object in a given situation.

Free-body 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)

M1 = 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

R2= (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 C-B = 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 = (1532- 1302) 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

The relationships between forces, acceleration, displacement, vectors, motion, momentum, energy of objects and more.

### • Statics

Forces acting on bodies at rest under equilibrium conditions - loads, forces and torque, beams and columns.

## Related Documents

### Acceleration of Gravity and Newton's Second Law

Acceleration of gravity and Newton's Second Law - SI and Imperial units.

### Bodies Moving on Inclined Planes - Acting Forces

Required forces to move bodies up inclined planes.

### Force

Newton's third law - force vs. mass and acceleration.

### Impulse and Impulse Force

Forces acting a very short time are called impulse forces.

### Mass Moment of Inertia

The Mass Moment of Inertia vs. mass of object, it's shape and relative point of rotation - the Radius of Gyration.

### Mass vs. Weight

Mass vs. weight - the Gravity Force.

### Torque or Moment of Force - Online Converter

Torque or moment - the tendency of a force to rotate an object.

## Search Engineering ToolBox

• the most efficient way to navigate the Engineering ToolBox!

## SketchUp Extension - Online 3D modeling!

Add standard and customized parametric components - like flange beams, lumbers, piping, stairs and more - to your Sketchup model with the Engineering ToolBox - SketchUp Extension - enabled for use with the amazing, fun and free SketchUp Make and SketchUp Pro . Add the Engineering ToolBox extension to your SketchUp from the Sketchup Extension Warehouse!

## Privacy

We don't collect information from our users. Only emails and answers are saved in our archive. Cookies are only used in the browser to improve user experience.

Some of our calculators and applications let you save application data to your local computer. These applications will - due to browser restrictions - send data between your browser and our server. We don't save this data.