Reinforced-concrete, folded-plate construction is a versatile concept applicable to a variety of long-span roof construction. Applications using precast, simple V folded plates include segmental construction of domes and (vertically) walls (Fig. 9.60). Inverted folded plates have also been widely used for industrial storage bins.
(Standard Practice for Design and Construction of Concrete Silos and Stacking Tubes for Storing Granular Materials, ACI 313, and Commentary, ACI 313R, American Concrete Institute.) Determination of stresses in folded-plate construction is described in Art. 5.15.5.
Formwork for folded plates is far simpler than that for curved thin shells. Precasting has also been a simpler process to save formwork, permit mass-production construction, and achieve sharp lines for exposed top corners (vees cast upside down) to satisfy aesthetic requirements. For very long spans, posttensioned, draped tendons have been used to reduce the total depth, deflection, and reinforcing-steel requirements. The tendons may be placed in the inclined plates or, more conveniently, in small thickened edge beams. For cast-in-place, folded-plate construction, double forming can usually be avoided if the slopes are less than 35 to 40.
Since larger transverse bending moments develop in folded plates than in cylindrical shells of about the same proportions, a minimum thickness less than 4 in creates practical problems of placing the reinforcing steel. A number of area in the plates will require three layers of reinforcing steel and, near the intersections of plates, top and bottom bars for transverse bending will be required. Ratios of span to total depth are similar to those for cylindrical shells, commonly ranging from 8 to 15. (See also F. S. Merritt, Standard Handbook for Civil Engineers, Sec. 8, Concrete Design and Construction, McGraw-Hill Publishing Company, New York.)