This resistance to movement is to some extent due to the resistance of the timber to crushing.
The larger parallelograms formed by any two legs, the trussed beam and the ground level, have joints that can only be destroyed by very great force.
As the highest pressures noted in this country have equalled 80 lbs. per square foot, and therefore have to be guarded against, it is wise to triangulate the sides as shown on Frontispiece.
The Force of Weight or Gravity.—The weight of a body equals the force with which that body is drawn towards the earth’s centre.
The weight of a scaffold may thus be considered as a force acting vertically downwards. The point at which the force acts is known as the centre of weight or the centre of gravity.
As a first example, the effect of the forces of gravity may be considered when acting upon an independent unloaded scaffold in its simplest form, consisting of four standards, erected square on plan, with ledgers and braces on each side.
This form of scaffold having regular sides, and its weight being equally distributed throughout, may be considered as a single, evenly disposed, rigid, rectangular body. The centre of such a body will coincide with its geometrical centre, and may be found thus:—
Draw a diagonal from A to B and from C to D (see [fig. 127]). The point of intersection will represent the centre of gravity required.
Fig. 127