Placing concrete in Arch. The steel lagging of the forms is carried up in sections as the concrete is deposited. The drain is horseshoe shaped, and is 12 feet 3 inches high and 12 feet 3 inches wide. ] The end of the concrete conveying pipe is provided with a flexible joint the simplest form of which can be made by slipping a section of pipe of larger diameter over the end of the transmission line. The concrete is deposited directly on the invert or into the side-wall forms and can be blown into the arch forms for 20 to 25 feet. =190. Materials for Forms.=—The materials used in forms for concrete sewers are: wood, wood with steel lining, and steel alone. The first cost of wood forms is lower than that of steel but their life is relatively short. If the forms are to be used a number of times steel is more economical. With proper care and repairs steel forms will outlast any other material. Because of the increasing price of lumber and improvements in steel forms, wood forms are not frequently used. A common type of specification under which forms are used is: The material of the forms shall be of sufficient thickness and the frames holding the forms shall be of sufficient strength so that the forms shall be unyielding during the process of filling. The face of the form next to the concrete shall be smooth. If wooden forms are used the planking forming the lining shall invariably be fastened to the studding in horizontal lines, the ends of these planks shall be neatly butted against each other, and the inner surface of the form shall be as nearly as possible perfectly smooth, without crevices or offsets between the ends of adjacent planks. Where forms are used a second time, they shall be freshly jointed so as to make a perfectly smooth finish to the concrete. All forms shall be water-tight and shall be wetted before using. Any material in contact with wet concrete should be oiled or greased beforehand in order to prevent adherence to the concrete. =191. Design of Forms.=—The design of forms for reinforced concrete work requires some knowledge of the strength of materials and the theories of beams, columns, and arches. Forms can be constructed without such knowledge but that they will be both economical and adequate is an improbability. The ordinary beam and column formulas are applicable to the design of forms. The maximum bending moment for sheeting and ribs is taken as (_wl_^2)⁄8, where _w_ is the load per unit length, and _l_ is the length between supports. Sanford Thompson recommends that the deflection be calculated as (_wl_^3)⁄(128_EI_), in which _E_ is the modulus of elasticity of the material, and _I_ is the moment of inertia of the cross-section referred to the neutral axis. The horizontal pressure of the concrete against the forms has been expressed empirically by E. B. Smith,[101] as _P_ = _H_^{0.2}_R_^{0.3} + 120_C_ − 0.3_S_