360R-06 Design of Slabs-on-Ground
Wire Reinforcement Institute (WRI) design method 6.2.2.1
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| 6.2.2 Wire Reinforcement Institute (WRI) design method
6.2.2.1 Introduction—The WRI design charts, for interior loadings only, are based on a discrete element computer model. The slab is represented by rigid bars, torsion bars for plate twisting, and elastic joints for plate bending. Variables are slab stiffness factors (modulus of elasticity, subgrade modulus, and trial slab thickness), diameter of equivalent loaded area, distance between wheels, flexural strength, and working stress. 6.2.2.2 Wheel loads—Slabs-on-ground subjected to wheel loadings are discussed in Section 6.2.1.1 . The WRI thickness selection method starts with an assumption of slab thickness so that the stiffness of slab relative to the subgrade is determined. The moment in the slab caused by the wheel loads and the slab’s required thickness are then determined. Appendix 2 shows the use of the WRI design charts for wheel loadings. 6.2.2.3 Concentrated loads—WRI charts do not cover concentrated loads directly. It is possible, however, to deter- mine the equivalent wheel loading that represents a concen- trated loading and thereby using the wheel load charts for this purpose. 6.2.2.4 Uniform loads—WRI provides other charts ( Appendix 2 ) for design of slab thickness where the loading is uniformly distributed on either side of an aisle. In addition to the variables listed in Section 6.2.2.1, the width of the aisle and the magnitude of the uniform load are variables in this method. 6.2.2.5 Construction loads—Various construction loads such as equipment, cranes, concrete trucks, and pickup trucks may affect slab thickness design. As with the PCA design method, these are not directly addressed by WRI. Thickness design, however, may be based on an equivalent loading expressed in terms of wheel loads or uniform loads. 6.2.3 COE design method—The COE design charts are intended for wheel and axle loadings applied at an edge or joint only. The variables inherent in the axle configuration are built into the design index category. Concentrated loads, uniform loads, construction loads, and line and strip loads are not covered. The COE method is based on Westergaard’s formula for edge stresses in a concrete slab-on-ground. The edge effect is reduced by a joint transfer coefficient of 0.75 to account for load transfer across the joint. Variables are concrete flexural strength, subgrade modulus, and the design index category. The design index is used to simplify and standardize design for the lighter-weight lift trucks, generally having less than a 25,000 lb (110 kN) axle load. The traffic volumes and daily operations of various sizes of lift truck for each design index are considered representative of normal warehouse activity and are built into the design method. Assumed values are an impact factor of 25%, concrete modulus of elasticity of 4,000,000 psi (28,000 MPa), Poisson’s ratio of 0.20, the contact area of each wheel, and the wheel spacings. The latter two values are fixed internally for each index category. Appendix 3 illustrates the use of the design index category and the COE charts. Additional design charts for pavements with protected and unprotected corners have been developed by the COE for pavements, although they may be applied to slabs-on-ground in general. tải về 2.35 Mb. Chia sẻ với bạn bè của bạn:
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