The stability of these scaffolds depends upon fixing at least two sets of putlogs, connected by means of stays as shown in [fig. 32]. Bracing is unnecessary if the putlogs and bolts tightly grip the building. When these scaffolds are used on circular chimneys, chucks have to be fitted on the inside of the putlogs to prevent them being drawn by the bolts to a curve. The chucks ([fig. 35]) can be fastened to the putlogs before they are fixed, if the curve of the building is accurately known. When this is not the case, the putlogs are fixed by a holdfast at their centre. The chucks are then placed in position, and clamped to the putlogs as shown in [fig. 36].
Additional holdfasts are then driven into the wall immediately under the chucks, so that the putlogs are kept level.
Fig. 34
Fig. 35
The putlogs are fixed on edge, and when not exceeding 16 feet in length are 7 in. by 3 in. Above that length they are 9 in. by 3 in. The stays should be 4 in. by 2 in., and connected to the putlogs by 5⁄8-inch iron bolts. The platform is usually of three boards 11 in. by 2 in.
Fig. 36
