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Torque & Moment in Engineering

Mark K said:
A torque is a moment about the axis of the member.
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Ok, that helps. So, a torque is a specific type of moment. Basically, all torques are moments, but not all are torque.

Therefore:

The force applied to a bolt being tightened would be a "moment" of torque, but the wind pressure on a building or beam being loaded at a point would be a moment, but not a torque.
 
I think I know why I always had such confusion. Let me put it into words based on my perception and correct me if I am wrong.

When a beam is under load, let's say a point load, the top of the beam under load is now in compression and the bottom is under tension. Somewhere between those two is a neutral axis. This is where I had my issue. When I see the word axis, I think of rotation, but that may not be fitting for a beam under load, correct?

So the fact that there is a bending moment in the beam, essentially a cause and effect, there really is no rotational torque around the neutral axis.

Am I thinking too much or is this about right?
 
There are different types of axis's. There are global axis's that are different from member axis's.. The neutral axis is perpendicular to the longitudinal member axis but also located at the point where the tension and compression stresses associated with bending are equal to zero.

A textbook on strength of materials should help to clarify this issue.

Torque is associated with twisting.
 
jar546 said:
I think I know why I always had such confusion. Let me put it into words based on my perception and correct me if I am wrong.

When a beam is under load, let's say a point load, the top of the beam under load is now in compression and the bottom is under tension. Somewhere between those two is a neutral axis. This is where I had my issue. When I see the word axis, I think of rotation, but that may not be fitting for a beam under load, correct?

So the fact that there is a bending moment in the beam, essentially a cause and effect, there really is no rotational torque around the neutral axis.

Am I thinking too much or is this about right?
Click to expand...

No. There's more than one kind of axis. Torque deals with rotation, and rotation has an axis of rotation.

In a beam regardless of it's shape, there is a linear axis (actually, there are two -- one in the horizontal plane and one in the vertical plane) called the neutral axis. In the case of a simple beam (supported at each end, with a uniform load along the length) everything above the neutral axis is in compression, and everything below the neutral axis is in tension.

This link shows the neutral axes for some common steel shapes. X is the horizontal neutral axis, and Y is the vertical neutral axis.


But you are correct that there is no rotation about the longitudinal neutral axis.
 
A beam under bending rotates about the neutral axis

Rotational torque is understood to be about the longitudinal member axis. A member can be subject to both bending about the neutral axis and torque. at the same time
 
Questions on this topic:

1) Is this torque vs moment distinction important? Will any problems arise from calling a moment a torque or a torque a moment?

2) Is the following a fair statement: Unbalanced moments will yield a net non-zero torque that will cause a rotation.

Mark K said:
A beam under bending rotates about the neutral axis
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That doesn't sound right. If you have a cantilevered beam with fixity at one end, the bending is going to cause points on the beam to rotate about an axis through the fixed end.

Cheers, Wayne
 
jar546 said:
I agree with Mark because the top is under compression and the bottom is under tension which are rotating in opposite directions.
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That statement alone is insufficient, and the axis about which it would rotate is not the neutral axis.

If the beam section has balanced compression forces on each side of the top of the section, and balanced tension forces on each side of the bottom of the section, then the section is not subject to any net rotation. But when the shear force at the section is non-zero, as is typical, the axial bending stresses will differ on each side of the section, resulting in a next moment around a horizontal axis perpendicular to the neutral axis.

Cheers ,Wayne
 
jar546 said:
I agree with Mark because the top is under compression and the bottom is under tension which are rotating in opposite directions.
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Rotating around what? The only rotation in a simple beam is possibly at the end connections, and it occurs only as the load increases or decreases.

Bending is not rotation.
 
A beam under bending rotates. This is a necessary consequence of the varying compression and tension stresses produced by the bending moments.

This discussion is an example of why I say the building official should be a licensed architect or engineer. If that is not possible the department should have an architect or engineer on staff.

Many of the building code provisions were written by engineers and it is common for engineers to submit calculations documenting code compliance. As a result in order to understand the code provisions and the submissions by the engineers you will typically need to see things the way an engineer sees them.
 
"Bending is not rotation"

Bending causes incremental rotations.

While it may be easy to see rotation at the support this is just the accumulation of the rotation that occurs along the length of the member
 
Mark K said:
A beam under bending rotates. This is a necessary consequence of the varying compression and tension stresses produced by the bending moments.

This discussion is an example of why I say the building official should be a licensed architect or engineer. If that is not possible the department should have an architect or engineer on staff.

Many of the building code provisions were written by engineers and it is common for engineers to submit calculations documenting code compliance. As a result in order to understand the code provisions and the submissions by the engineers you will typically need to see things the way an engineer sees them.
Click to expand...

Rotates around what?

I am a licensed architect. I have a Master's degree in Architecture and I took post-graduate level structures classes. There's no rotation involved in a beam bending -- what occurs is shear -- the material wanting to slide internally as the distance from the neutral axis increases and the forces increase proportionally.
 
I am a licensed Structural Engineer with a masters in structural engineering.

If we were to draw vertical lines on the sides of the undeflected beam and then loaded the beam so that the beam was subject to bending moments we would see that the lines had rotated.

While the deflected beam would be related to the shear stresses we are talking about the deflected shape of the beam.

The shears are the first derivative of the moments. On the other hand the rotations are proportional to the integral of the bending moments.

My strength of materials textbook talks about slope instead of rotation but these two terms are related. A larger slope is related to a larger rotation.
 
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