Precast Ribbed Slabs, Folded Plates, and Shells

Curved shells and folded plates have a thickness that is small compared with their other dimensions. Such structures depend on their geometrical configuration and boundary conditions for strength.
Thickness. With closely spaced ribs or folds, a minimum thickness for plane sections of 1 in is acceptable.
Reinforcement. Welded-wire fabric with a maximum spacing of 2 in may be used for slab portions of thin-section members, and for wide, thin elements 3 in thick or less. Reinforcement should be preassembled into cages, using a template, and placed within a tolerance of 0 in or 1⁄8 in from the nearest face. The minimum clear distance between bars should not be less than 11⁄2 times the nominal maximum size of the aggregate. For minimum concrete cover of reinforcement, see Art. 9.97.
Compressive Strength. Concrete for thin-section, precast-concrete members protected from the weather and moisture and not in contact with the ground should have a compressive strength of at least 4000 psi at 28 days. For elements in other locations, a minimum of 5000 psi is recommended.
Analysis. Determination of axial stresses, moments, and shears in thin sections is usually based on the assumption that the material is ideally elastic, homogeneous, and isotropic.
Forms. Commonly used methods for the manufacture of thin-section, precastconcrete members employ metal or plastic molds, which form the bottom of the slab and the sides of the boundary members. Forms are usually removed pneumatically or hydraulically by admitting air or water under pressure through the bottom form.
(Architectural Precast Concrete, Precast / Prestressed Concrete Institute.)

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