360R-06 Design of Slabs-on-Ground
—Slabs post-tensioned for crack control
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| 9.5—Slabs post-tensioned for crack control
9.5.1 Design methods—For lightly loaded slabs with no rack post loads, the crack control design is used. The practical minimum slab thickness is approximately 4 in. (100 mm) to provide necessary concrete cover to the prestressed reinforce- ment. This minimum slab thickness, combined with residual compression from post-tensioning, will provide a considerable load capacity of the slab. 9.5.2 Post-tensioning force required—The post-tensioning force P r (lb/ft [N/m]) required to overcome the subgrade friction can be calculated by the equation P r = W slab μ (9-1) where W slab = self-weight of the foundation slab, lb/ft 2 (Pa) (unit weight in lb/ft 3 [kg/m 3 ] adjusted to the slab thickness); L = slab length in the direction being considered, ft (m); and μ = coefficient of friction between slab and subgrade. The following coefficients of friction μ are recommended for slabs constructed on polyethylene sheeting after Timms (1964): • Slabs on one layer of polyethylene sheeting: 0.50 to 0.75; and • Slabs constructed on a sand base: 0.75 to 1.00. For longer-ribbed slabs, the restraint due to the beams should be taken into consideration. The residual compressive force after all prestress losses and the subgrade friction losses should be determined by the engineer based on the slab geometry, loading, and usage. The following prestress levels have been used satisfactorily: Residential foundations: 50 to 75 psi (0.3 to 0.5 MPa) Industrial floors up to 100 ft (30 m) long: 75 to 100 psi (0.5 to 0.7 MPa) Industrial floors up to 200 ft (60 m) long: 100 to 150 psi (0.7 to 1.0 MPa) Industrial floors up to 300 ft (90 m) long: 150 to 200 psi (1.0 to 1.4 MPa) Industrial floors over 400 ft (120 m) long: 200 to 250 psi (1.4 to 1.7 MPa) The friction losses, elastic shortening, and the long-term losses in the tendons can be calculated according to Zia et al. (1979). The tendon spacing S ten (ft [m]) required to overcome slab subgrade friction and maintain a residual compression at the center of a solid slab, lightly reinforced for crack control, is given by the equation (9-2) L 2 --- S ten P e f p WH P r + ------------------------- = |