If a point in a body is fixed, so that the body cannot move out of its place, but may rotate about that point; a force which acts at any other point, but in a direction that does not pass through the fixed point, will produce rotation.
In applying this principle to the effect of wind on the scaffold pole, the ground level will be the fixed point about which the standard may rotate.
The wind acting upon the exposed surface of the pole may be likened to a series of parallel forces that, not acting through the fixed point, tend to produce rotation.
An advantage is gained if, instead of taking the wind as a series of parallel forces, it is considered as a resultant force of proportionate magnitude exerting a pressure upon the centre of the exposed surface. In practice it will be sufficiently correct to take the centre of surface of the pole at a point at half its height.
The tendency of a force to produce rotation about a fixed point is termed its moment about that point. It is measured by multiplying the units of force exerted by the units of the distance between its point of application and the fixed point.
Example: If at the centre of surface of the pole under consideration, that is at 15 feet above the fixed point, the resultant force of the wind is equal to a pressure of 100 lbs., the moment about the fixed point will be 15 by 100 = 1500.
In like manner the moment of resistance due to the weight of earth packed round that portion of the pole below ground can be estimated.
For the pole to remain in equilibrium it will be necessary for the moment of resistance to equal the moment of the overturning force, assuming that the fixed point is stable.
No practical good can result by pursuing this calculation further. It may be taken for granted that as the wind occasionally exerts a pressure of over 50 lbs. per square foot, and a pressure of 40 lbs. per square foot is the least for which calculations should be made, it will always be necessary when scaffolding to any height to adopt special measures to preserve stability.
When the ledgers are added to the standards they have some effect upon the equilibrium of the erection, and this must now be considered.