critical is that the bottom timbers be placed so they rest evenly and firmly on the ground.

Cribbing is desirable when lifting loads by jacking stages. This procedure requires blocking to be placed under the jacks, lifting the load to the maximum height the jacks can safely accommodate, placing the cribbing under the load in alternating tiers, with no personnel under the load, and then lowering the load onto the cribbing.

When cribbing is not high enough or at the correct height, build up the blocking under the jacks until the jacks can bear against the load while in their lowered position. Raise the jacks again to their maximum safe height and lower onto the added cribbing. This procedure can be repeated as many times as necessary to build up the cribbing to the desired height.

SCAFFOLDS

The term scaffold refers to a temporary elevated platform used to support personnel and materials, for immediate usage, throughout the course of construction work. You will use scaffolds in performing various jobs which cannot be done safely from securely placed ladders. We will take a brief look at a few of the different types of scaffolds which you will need from time to time on the job.

Planking and Runway Scaffold

A planking and runway scaffold shown in figure 6-38 consists of single scaffold planks laid across beams of upper floors or roofs. It is frequently used to provide working areas or runways. Each plank should extend from beam to beam, and not more than a few inches of the planks should extend beyond the end supporting beam. A short overhang is essential to safe practice to prevent personnel from stepping on an unsupported plank and falling from the scaffold. Planks should be thick enough to support the load safely and applied without excessive sagging. When the planking is laid continuously, as in a runway, make sure the planks are laid so that their ends overlap. Single plank runs may be staggered with each plank being offset with reference to the next plank in the run.

Swinging Platform Scaffold

The most commonly used type of swinging scaffolding is the platform scaffold shown in figure 6-39. The swinging platform scaffold consists of a frame with a deck of wood slats. The platform is

Figure 6-38. - A planking and runway scaffold.

supported near each end by iron rods, called stirrups, which have the lower blocks of fiber line fall attached to them. This tackle arrangement permits the platform to be raised or lowered as required. The tackle and platform are supported by hooks and anchors on the roof of the structure. The fall line of the tackle must be secured to a part of the platform position to prevent it from falling. when in final

Figure 6-39. - A platform scaffold.