Copyright Goodheart-Willcox Co., Inc. Chapter 18 Concrete Flatwork and Formed Shapes 399 The flat ceiling is economical to form and may be used for the finished ceiling without any addi- tional treatment. Cantilevers (beams supported at only one end that carry a load at the other end or along their length) are easily produced, as well as other architectural projections. Slabs generally range from 5″ to 14″ thick and provide spans up to 35′. Flat plates provide a continuous solid ceiling with complete flexibility for locating partitions and mechanical equipment. Supporting columns need not be in a straight line. This adds flexibility. Electrical ducts and conduits may be embedded in the slab. The flat plate system is well suited for heavy loads such as roof parking. Flat Slab Roof and Floor System The flat slab system is designed for heavy roof loads with large open bays below. The difference between the flat plate and the flat slab systems is that the flat slab has a supporting panel in the area of each column for added support. See Figure 18-43. Flat slab thickness is usually 2.5% or 3% of the span and a minimum of 4″ thick. The size of the supporting panel is about 33% of the span and 25% to 50% of the slab thickness. Flat slabs are designed to span up to 40′ with columns an equal distance apart. This system has all the advantages of the flat plate system and is stronger. Precast/Prestressed Concrete Systems Expansion of industrial processes in building construction has resulted in greater use of modular layout, planning, and cost-conscious construction. Precast units for walls, floors, ceilings, and roofs can be mass-produced at the factory or job site. Precast concrete units can be cast as tilt-up panels, standard-shaped concrete panels, or concrete window walls. A variety of prestressed panels and shapes are produced for use in wall, roof, and floor systems. Only trained and competent individuals are involved in the unloading, storing, placing, erecting, and bracing of concrete panels. OSHA safety stan- dards pertaining to lifting of precast and tilt-up panels include the following: • The lifting of panels must be designed and planned by a registered engineer who has knowledge and experience in lifting of slabs. Designated panel lifting procedures are supplied by the manufacturer prior to their delivery to the construction site. • Slab weight must be known to ensure adequate lifting equipment is used. • A crane or other lifting equipment must be rated to lift 2 1/2 times the load to be lifted. • Only a designated competent person is allowed to rig the slab for lifting. • Only essential individuals are allowed in the lifting area during lifting and placing of the slab. • No one is allowed under the load during lifting or placing of the slab. • The height of wall plus 4′ must be cordoned off as clear space around the perimeter in the event of a wall tipping over. • Bracing of walls must retained until permanent supporting members of the structure are in place. Tilt-Up Construction Tilt-up construction is the process of casting concrete walls in a horizontal position on-site or in another location and lifting them from the casting position or truck to their final location in the building. Tilt-up construction is a popular construc- tion method in the United States due to the reason- able cost, low maintenance, durability, and speed of construction. Tilt-up construction is especially suited for buildings greater than 10,000 sq ft with 20′ or higher side walls that incorporate repetition in panel size and appearance. See Figure 18-44 and Figure 18-45. Goodheart-Willcox Publisher Figure 18-43. A flat slab roof is a two-way reinforced structural system that includes either drop panels or column capitals to carry heavier loads.