As the working level of a structure rises above the reach of crew members on the ground or deck, temporary elevated platforms, called scaffolding, are erected to support the crew members, their tools, and materials,
There are two types of scaffolding in use today-wood and prefabricated. The wood types include the swinging scaffold, which is suspended from above, and the pole scaffold, which is supported on the ground or deck. The prefabricated type is made of metal and is put together in sections, as needed.
SWINGING SCAFFOLD CONSTRUCTION
The simplest type of a swinging scaffold consists of an unspliced plank that is made from 2-by-8-inch (minimum) lumber. Hangers should be placed between 6 and 18 inches from the ends of the plank. The span between hangers should not exceed 10 feet. Make sure that the hangers are secured to the plank to stop them from slipping off. Figure 4-42 shows the construction of a hanger with a guardrail. The guardrail should be made of 2-by-4-inch material between 36- and 42-inches high. A midrail, if required, should be constructed of 1-by-4 lumber.
Figure 4-42.Typical hanger to use with plank scaffold.
Swing scaffolds should be suspended by wire or fiber line secured to the outrigger beams, A minimum safety factor of 6 is required for suspension ropes, The blocks for fiber ropes should be the standard 6-inch size consisting of at least one double block and one single block. The sheaves of all blocks should fit the size of rope used.
The outrigger beams should be spaced no more than the hanger spacing and should be constructed of no less than 2-by-10 lumber. The beam should not extend more than 6 feet beyond the face of the building. The inboard side should be 9 feet beyond the edge of the building and should be securely fastened to the building.
Figure 4-43 shows a swinging scaffold that can be used for heavy work with block and tackle.
Figure 4-43.Swinging scaffold.
POLE SCAFFOLD CONSTRUCTION
The lower ends of poles must not bear directly on a natural earth surface. If the surface is earth, a board footing 2-inches thick and 6- to 12-inches wide (depending on the softness of the earth) must be placed under the poles.
If poles must be spliced, splice plates must not be less than 4-feet long, not less than the width of the pole wide, and each pair of plates must have a combined thickness not less than the thickness of the pole. Adjacent poles must not be spliced at the same level.
A ledger must be long enough to extend over two pole spaces, and it must overlap the poles at the ends by at least 4 inches. Ledgers must be spliced by overlapping and nailing at polesnever between poles. If platform planks are raised as work progresses upward, the ledgers and logs on which the planks previously rested must be left in place to brace and stiffen the poles. For a heavy-duty scaffold, ledgers must be supported by cleats, nailed or bolted to the poles, as well as by being nailed themselves to the poles.
A single log must be set with the longer section dimension vertical, and logs must be long enough to overlap the poles by at least 3 inches. They should be both face nailed to the poles and toenailed to the ledgers. When the inner end of the log butts against the wall (as it does in a single-pole scaffold), it must be supported by a 2-by-6-inch bearing block, not less than 12 inches long, notched out the width of the log and securely nailed to the wall. The inner end of the log should be nailed to both the bearing block and the wall. If the inner end of a log is located in a window opening, it must be supported on a stout plank nailed across the opening. If the inner end of a log is nailed to a building stud, it must be supported on a cleat, the same thickness as the log, and nailed to the stud.
A platform plank must never be less than 2-inches thick. Edges of planks should be close enough together to prevent tools or materials from falling through the opening. A plank must be long enough to extend over three logs, with an overlap of at least 6 inches, but not more than 12 inches.
PREFABRICATED SCAFFOLD ERECTION
Several types of scaffolding are available for simple and rapid erection, one of which is shown in figure 4-44. The scaffold uprights are braced with diagonal members, and the working level is covered with a platform of planks. All bracing must form triangles, and the base of each column requires adequate footing plates for bearing area on the ground or deck. The steel scaffolding is usually erected by placing the two uprights on the ground or deck and inserting the diagonal members. The diagonal members have end fittings that permit rapid locking in position. In tiered scaffolding, figure 4-45, the first tier is set on steel bases on the ground, and a second tier is placed in the same manner on the first tier with the bottom of each upright locked to the top of the lower tier. A third and fourth upright can be placed on the ground level and locked to the first set with diagonal bracing. The scaffolding can be built as high as desired, but high scaffolding should be tied to the main structure. Where necessary, scaffolding can be mounted on casters for easy movement.
Figure 4-44.Assembling prefabricated independent-pole scaffolding.
Figure 4-45.Tiered scaffolding.
Prefabricated scaffolding comes in three categories: light, medium, and heavy duty. Light duty has nominal 2-inch-outside-diameter steel-tubing bearers. Posts are spaced no more than 6- to 10-feet apart. Light-duty scaffolding must be able to support 25-pound-per-square-foot loads.
Medium-duty scaffolding normally uses 2-inch-outside-diameter steel-tubing bearers. Posts should be spaced no more than 5- to 8-feet apart. If 2 1/2-inch-outside-diameter steel-tubing bearers are used, posts are be spaced 6- to 8-feet apart. Medium-duty scaffolding must be able to support 50-pound-per-square-foot loads.
Heavy-duty scaffolding should have bearers of 2½-inch-outside-diameter steel tubing with the posts spaced not more than 6-feet to 6-feet 6-inches apart. This scaffolding must be able to support 75-pound-per-square-foot loads.
To find the load per square foot of a pile of materials on a platform, divide the total weight of the pile by the number of square feet of platform it covers.
The bracket, or carpenters scaffold (figure 4-46), is built of a triangular wood frame not less than 2- by 3-inch lumber or metal of equivalent strength. Each bracket is attached to the structure in one of four ways: a bolt (at least 5/8 inch) that extends through to the inside of the building wall; a metal stud attachment device; welded to a steel tank; or hooked over a secured supporting member.
Figure 4-46.Carpenters portable bracket for scaffolding.
The brackets must be spaced no more than 8-feet apart. No more than two persons should be on any 8-foot section at one time. Tools and materials used on the scaffold should not exceed 75 pounds.
The platform is built of at least two 2- by 10-inch nominal size planks. The planks should extend between 6 and 12 inches beyond each support.
When working on scaffolding or tending others on scaffolding, you must observe all safety precautions. Builder petty officers must not only observe the safety precautions themselves, but they must also issue them to their crew and ensure that the crew observes them.
|David L. Heiserman, Editor||
Copyright © SweetHaven
Revised: June 06, 2015