BLOCKING AND CRIBBING
Block timbers are commonly used to provide a foundation for heavy loads or jacks. Cribbing must be used when a heavy weight must be supported at a height greater than blocking can provide. Cribbing is made up by aligning timber in tiers that run in alternate directions (fig. 6-37). Blocking and cribbing is often necessary as a safety measure to keep an object stationary to prevent accidents and injury to personnel working near these heavy objects.
When selecting blocking as a foundation for jacks, ensure it is sound and large enough to support the load safely. It must be free from grease and thoroughly dry.
Figure 6-36.-Permanent metal roller conveyor.
Figure 6-37.-Examples of the use of cribbing.
Additionally, it must be placed firmly on the ground with the load (pressure) distributed evenly.
A firm and level foundation is a paramount requirement where cribbing is used. Also, equally as 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.
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 when in final position to prevent it from falling.
Figure 6-39.-A platform scaffold.
A needle-beam scaffold consists of a plank platform resting on two parallel horizontal beams, called needle beams, which are supported by lines from overhead. (See fig. 6-40.)
Needle-beam scaffolds should be used on-1 y for the support of personnel doing light work. They are suitable for use by riveting gangs working on steel structures because of the frequent changes of location necessary and the adaptability of this type of scaffold to different situations.
Several types of patent and independent scaffolding are available for simple and rapid assembly, as shown in figure 6-41. The scaffold uprights are braced with diagonal members, and the
Figure 6-40.-A needle-barn scaffold.
Figure 6-41.-Assembling prefabricated independent scaffolding.
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. The patented steel scaffolding is usually erected by placing the two uprights on the ground and inserting the diagonal members. The diagonal members have end fittings, which permit easy assembly. 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 into the main structure.
The boatswain's chair shown in figure 6-42 also comes under the heading of scaffolding. It is sometimes used to provide a seat for a person working above the ground.
The seat of the boatswain's chair should be at least 2 feet long, 1 foot wide, and 1 1/4 inches thick (60 cm
Figure 6-42.-A boatswain's chair.
long, 30 cm wide, and 3.1 cm thick). Make sure you always wear a safety belt when using a boatswain's chair. The safety belt should be attached to a lifeline secured to a fixed object overhead. Use a bowline to secure the lifeline to the person in the chair.
NOTE: A BOATSWAIN' S CHAIR SHOULD BE USED ONLY IF OTHER MEANS ARE NOT AVAILABLE.
When you are using scaffolds, SAFETY is your NUMBER ONE PRIORITY! Failure to observe safety precautions can result in serious injury to yourself or coworkers. Some essential safety measures applicable to scaffolds are given here. Use each of them routinely. THINK SAFETY! Be a SAFE WORKER!
Structural members, support ropes, and scaffold equipment must be inspected carefully each workday before using them on the job. The use of makeshift scaffolds is strictly prohibited.
When personnel are working on a scaffold with other personnel engaged directly above, either the scaffold must have an overhead protective covering or the workers on the lower scaffold must wear Navy-approved, protective hard hats. The purpose is to provide protection against falling material. Where the upper working level is no more than 12 feet (3.6 m) above the lower, hard hats worn by workers on the lower level will satisfy this requirement.
An overhead protective covering consists of a roof of lumber, heavy wire screen, or heavy canvas, depending upon the hazard involved. The covering should extend a suffient distance beyond the edge of the scaffold to catch any material that may fall over the edge. A netting of screen should not be less than No. 18 gauge, U.S. Standard Wire, with a mesh not to exceed 1/2 inch. Screens of heavier wire or smaller mesh should bex used where conditions are such that the No. 18 gauge wire or 1/2-inch mesh will not suppl y adequate protection. Personnel should NOT be required to work underneath a scaffold. Scaffolds erected over passageways, thoroughfares, or locations where persons are working should be provided with side screens and a protective covering. A side screen is a screen paneling from the platform to an intermediate railing or from the platform to the top railing. Screening is formed of No. 16 U.S. gauge wire with 1/2-inch mesh. Screen is used for the purpose of preventing materials, loose or piled, from falling off the scaffolds.
A safe means of access should be provided to all scaffolds by means of standard stairs or fixed ladders. Additionally, ensure that a scaffold is properly secured against swaying.
Personnel should not be permitted on scaffolds which are covered with ice or snow. In such instances, clinging ice must be removed from all guardrails, then the planking sanded or otherwise protected against slipping. Workers should not be permitted on scaffolds during a storm or high wind.
No scaffold should be used for the storage of materials, except that required for the immediate needs of the job. Tools should be placed in containers to prevent their being knocked off and the containers should be secured to the scaffold by line. Always make a special effort to ensure that tools, equipment, material, and rubbish do not accumulate on a scaffold to the point where the safe movement of personnel is jeopardized.
NEVER throw or drop objects or tools from scaffolds. Handlines should be used for raising or lowering objects when they cannot be reached easily and safely by hand. Such things as jumping or throwing material upon a scaffold platform are to be avoided at all times.
Scaffolds must never be overloaded! Furthermore, whenever possible, see that the scaffold load is uniformly distributed and not concentrated at the center of the platform.
Wire ropes and fiber lines used in suspension and swinging scaffolds should be of the best quality steel, manila, or sisal. Manila or sisal line used as lifelines should be 1 7/8 inches (51.2 mm) in circumference. Lifelines and safety belts must be used when working on unguarded scaffolds at heights of 10 feet (3 m) and above (as well as on boatswain's chairs, as explained earlier). If working over water, life jackets must be worn.
All scaffolds and scaffold equipment should be maintained in safe condition. Avoid making repairs or alterations to a scaffold or scaffold equipment while in use. Rather than take a chance, NEVER permit personnel to use damaged or weakened scaffolds!