Sign convention of shear force and moment on beam

In summary, the concept of positive and negative moments in a beam depends on an arbitrary convention chosen by the author to maintain consistency in calculations. This convention can be different for different types of forces, as long as it is consistently applied. The orientation of the moment (clockwise or anticlockwise) does not determine its sign, rather it is the effect of the moment (sagging or hogging) that determines its sign in a bending moment diagram.
  • #1
chetzread
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1

Homework Statement


I don't understand why the sign of moment of beam at left (clockwise) and moment at the right of beam (anticlockwise) are considered as positive...
Why we can't assume the clockwise moment as positive and anticlockwise moment as negative?
yS8bRDE.png

Homework Equations

The Attempt at a Solution

 
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  • #2
chetzread said:

Homework Statement


I don't understand why the sign of moment of beam at left (clockwise) and moment at the right of beam (anticlockwise) are considered as positive...
Why we can't assume the clockwise moment as positive and anticlockwise moment as negative?
yS8bRDE.png

Homework Equations

The Attempt at a Solution

It's just an arbitrary convention assumed by the author to keep calculations consistent.

You can assume a different convention for shear forces and bending moments, as long as you maintain consistency.
 
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  • #3
SteamKing said:
It's just an arbitrary convention assumed by the author to keep calculations consistent.

You can assume a different convention for shear forces and bending moments, as long as you maintain consistency.
look at the diagram carefully, can i assume moment of beam at left (clockwise) as positive and moment at the right of beam (anticlockwise) are considered as neagtive..?
in OP, the author assume moment of beam at left (clockwise) and moment at the right of beam (anticlockwise) are considered as positive..,which different orientation of moment has same sign...
 
  • #4
This is similar to another post of yours, to which I replied: What definition of Moment are you using? There are two possible definitions, each of which can be derived from the other. The one I prefer is that the bending moment at a section is the algebraic sum of the moments on ONE SIDE of the section. You have to have a sign convention that should yield the same moment, whether you take it to the left of the section, or to the right.The significant drawings for sign are shown in the lowest line of the table you quote. Sagging moments are positive and are produced by a combination of a clockwise and an anticlockwise moment. Although in some situations it can help to ascribe a sign to a clockwise or anticlockwise moment (determining the overall equilibrium of a body - for example), when drawing bending moment diagrams, you need to consider whther the cause of the bending moment is having a sagging (+) or a hogging (-) effect. Nothing to do with CW or ACW .
 
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Related to Sign convention of shear force and moment on beam

1. What is the sign convention for shear force and moment on a beam?

The sign convention for shear force and moment on a beam is based on the assumption that the beam is loaded from left to right. This convention states that upward forces and clockwise moments are considered positive, while downward forces and counterclockwise moments are considered negative.

2. Why is the sign convention for shear force and moment important?

The sign convention for shear force and moment is important because it helps us determine the direction and magnitude of these forces and moments at any point along the beam. This information is crucial in analyzing the structural integrity and stability of the beam.

3. Can the sign convention for shear force and moment be applied to any type of beam?

Yes, the sign convention for shear force and moment can be applied to any type of beam, including cantilever, simply supported, and overhanging beams. The convention remains the same regardless of the beam's shape or support conditions.

4. How do we determine the sign of shear force and moment at a specific point on a beam?

To determine the sign of shear force and moment at a specific point on a beam, we first draw a free-body diagram of the portion of the beam to the left or right of the point. Then, we apply the sign convention to the forces and moments acting on that portion of the beam. The resulting sign will be the sign of the shear force and moment at the specified point.

5. Are there any exceptions to the sign convention for shear force and moment on a beam?

There are a few exceptions to the sign convention for shear force and moment on a beam. For example, in some cases, the convention may need to be reversed if the beam is loaded from right to left. Additionally, the convention may differ for curved or inclined beams. It is important to carefully consider the loading and shape of the beam when applying the sign convention.

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