diagonal members, as shown in figure 5-79, 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. 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 that permit rapid locking in position. The first tier is set on steel bases on the ground. The second tier has 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. Steel and aluminum scaffolding is used to speed up construction and maintenance operations. Many types are available for interior work and erection. This kind of scaffolding is used around many construction sites because  it  can  be  disassembled  and  transported whenever  necessary.  The  scaffolds  are  mounted  on steel casters with brakes and adjustable supports, which permit quick movement and positioning of each tower This scaffolding is equipped with special trussed planks,  12  feet  long,  that  are  placed  to  enable  crew members to be within comfortable and safe working distance of all working space. Prefabricated scaffolding of this type comes in three  categories:  light,  medium,  and  heavy  duty.  Light- duty scaffolding has nominal 2-inch-outside-diameter steel-tubing bearers. Posts are spaced no more than 6 to 10 feet apart. The load it can handle is 25 pounds per square  foot.  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 spaced 6 to 8 feet apart. The load it can handle is 50 pounds per square foot. Heavy-duty scaffolding should  have  bearers  of  2  1/2-inch-outside-diameter steel tubing with the posts spaced not more than 6 feet 6 inches apart. The load it can handle is 75 pounds per square  foot. 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. SAFETY The  following  safety  precautions  must  be  observed by all persons working on scaffolds or tending other persons who are working on scaffolds. Construction Figure 5-79.—Prefabricated independent-pole scaffolding. Electrician  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. Scaffolds must be kept clear of accumulations of tools, equipment, materials, and rubbish. No scaffold may be used for the storage of materials in excess of those currently required for the job. Tools not in immediate use on scaffolds must be stored in containers to prevent tools left adrift from being knocked off. Tool containers must be lashed or otherwise secured to the scaffolds. Throwing objects to or dropping them from scaffolds is absolutely prohibited. Hand lines must be used for raising or lowering objects that cannot be passed hand to hand 5-48