Nuts and bolts of moment and torque: What you need to know for ARE 5.0
PDD — Section 1, Objective 1.3
Here's an illustration to help you understand the similarity between moment and torque.
Bending moment & torque are measured as force ✕ distance
The force acting on a beam/column connection is similar to that on a nut/bolt connection when it's tightened by a wrench. They are dimensionally equivalent, i.e., the product of the force and its distance from the connection.
In architecture and structural engineering, moment and torque are closely related concepts that play a crucial role in the design and analysis of structural connections. Moment in a column-beam connection refers to the force acting on the connection multiplied by the distance from the point of rotation. Similarly, torque in a nut-bolt connection refers to the rotational force acting on the connection, which is the product of the force and the distance from the axis of rotation.
A column-beam connection in a rigid-frame structure must be designed to resist the moment forces acting on the structure. Likewise, in a bolted connection, the rotational force applied to tighten the nut and bolt, torque, must be carefully measured and controlled in order to ensure the stability and longevity of the connection.
While similar, moment and torque are unique concepts. In broad terms, moment is for static cases, i.e., when a force acts from a distance but no movement occurs. However, torque is dynamic in nature when there is motion, i.e., measuring the turning force of a rotating object.
For our purposes, ignore the semantics of "foot-pounds" vs. "pound-feet." That's a discussion for another time, and is beyond the scope of the ARE. Instead, consider this; 1 foot-pound is a unit of energy equal to the energy transferred by applying one pound of force through a linear displacement (a rotational force) of one foot.
BTW, using a "torque wrench" is one of the four different methods (or tools) used to ensure that a high-strength bolt connection is secure (recognized by AISC).
- Turn-of-nut method (after initial tightening, position marks on the nut and bolt indicate the amount of additional nut rotation required, e.g., 1/3 turn, 1/2 turn, etc.)
- Calibrated wrench method (a torque wrench indicates when the required amount of torque is applied to the nut)
- Twist-off bolt/tension control bolt method (a specialized tool is used to tighten the nut until the end of the bolt snaps off)
- Direct tension indicating method, DTI (special washers indicate full tension, e.g., hollow bumps flatten out or silicone is squeezed out as the nut is tightened)
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