Presentation on free body diagram 10.01.03.119

Course name: PRE-STRESS LABName: Safa Shafiq RahmanID : 10.01.03.119Year: 4th Semester: 2nd Section: C

FREE BODY DIAGRAM:EXTERNAL & INTERNAL FORCE

CONTENTS:

1 Definition of free body diagram

2 Purpose of free body diagram

3 Rules of construction of free body diagram

4 External & internal forces

5 Examples of external forces

6 Application of internal forces

FREE BODY DIAGRAM

A free body diagram, sometimes called a force diagram, is a pictorial device, often a rough working sketch, used by engineers and physicists to analyze the forces and moments acting on a body.

Block on a ramp (top) and corresponding free body diagram of just the block (bottom). For equilibrium, the line of action of the three force arrows must intersect at a common point.

PURPOSE OF FREE BODY DIAGRAM:

• Drawing a free body diagram can help to determine the unknown forces on, moments applied to, and equations of motion of the body and thus help to analyze a problem in statics or dynamics.

• In analysis of structures, free body diagrams for a component of a structure or, part are used in determining shear forces and bending moments

Rules to construct a valid free-body diagram (FBD):

Rule 1. Select the appropriate body or body segment, keeping in mind the unknowns you want to compute. That is, make sure the free-body includes at least one of the unknowns.

Rule 2. Draw all known forces or moments of force at their respective points of application (e.g., weight vector, measured external forces, etc.). Note that the weight vector is placed at the center of gravity of the free-body.

Rule 3. Draw all unknown forces and moments that directly contact the free-body. Wherever the free-body is separated from other parts of the body, replace the excluded parts of the body with an unknown force (two components) and a moment of force.

Rule 4.Do NOT include Internal forces which originate and terminate within the free-body (e.g. joint forces of internal joints).

EXTERNAL & INTERNAL FORCES:

The loads and forces which are acted on the beam are called the external forces. The reactions developed are also external forces. The shear force, bending moment, axial force, torsional moment, etc. which are introduced in the beam due to the effect of external forces are internal forces. The internal forces must balance the external forces to satisfy the equilibrium condition of the structure.

forces in beam

EXTERNAL & INTERNAL FORCES:

Internal loads in a structural member are the result of externally applied loads. The external loads are transmitted to different parts of the structure through these internal loads. The internal loads can be determined by the method of sections. The structural action of a beam is represented by internal forces called bending moments and shear forces. A beam is subjected to two sets of external forces. These are the loads applied to the beam and reactions to the loads from the supports. The beam transfers the external load set to the external reaction set by a system of internal forces.

external and internal forces in beam

EXAMPLE OF EXTERNAL FORCES:

APPLICATION OF INTERNAL FORCES:

A member is cut at the point of interest, and the internal loads are revealed as equivalent external loads. Since the member was in equilibrium before being cut, these equivalent external loads must keep the member in equilibrium.For the common case, there are two forces, an axial force and a shear force, plus one moment. These forces and moments are shown in the diagram below.

Unknown Internal Loads V, A, and M Replace. Again, F & reactions at the supports Ay,By & Bx are external loads.Deleted 2D Structure Section to Maintain Equilibrium

As one can calculate the forces and moments transmitted through joints between members, one can also calculate the internal forces which one part of a member exerts on another. To calculate these internal forces, simply draw a free-body diagram of only part of the member, cutting through the member at the point you are interested in knowing the forces and moments. For example, consider the following member

If you are interested in knowing the forces and moments that are transmitted through the member at point D, you can draw the free-body-diagram of the portion to the left of D to get -

THANK YOU