Fig. 129
When boards are laid on a scaffold, they may be treated as a load. If, however, they are evenly distributed over the entire top of a scaffold, their comparatively light weight may be usually neglected, as, their centre of gravity occurring immediately above the centre of gravity of the scaffold itself, would have no other effect than to slightly raise the centre of gravity of the structure.
To find the Centre of Gravity of Scaffold Boards laid to form a Platform.—A scaffolding platform, being of a slight depth in comparison with its length and breadth, may be treated as a surface usually rectangular.
Fig. 130
The centre of gravity of a rectangular surface is the point of intersection of its diagonals ([fig. 130]).
To find the Centre of Gravity of a Dependent Scaffold and the Effect of Loads upon it.—A dependent scaffold, having only one frame of standards and ledgers, to which are attached the putlogs, cannot be considered as an evenly disposed regular body. Nevertheless, the rule that the scaffold will not be in equilibrium unless a line from the centre of gravity fall within the base still holds good. In the case under consideration, as the wall of the building to which the scaffold is securely attached by the putlogs and ties, carries its share of the weight of the loads and putlogs, it must be taken as forming an integral portion of the scaffold itself.
Therefore the centre of gravity of a dependent scaffold will be the resultant centre of gravity of the outer frame, the putlogs, and the wall, considered as separate bodies.
The centre of gravity of the frame may be found by taking it as a rectangular surface. If necessary, the boards may be treated in like manner.
The system of putlogs, if they are regularly placed, may be treated as a regular body, and the centre of gravity found by the method already given; but in practice their weight would have no effect towards loss of equilibrium.