360R-06 Design of Slabs-on-Ground
—Special slab-on-ground support problem
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- CHAPTER 4—LOADS 4.1—Introduction
| 3.8—Special slab-on-ground support problem
Placement of slabs on topsoil should generally be avoided because of their low shear strength and high compressibility. Project specifications generally require that the building site be stripped of all topsoil. Expansive soils are defined as fine-grained soils, as shown in Tables 3.1 and 3.2 . Soils with a plasticity index of 20 or higher have a potential for significant volume change. A geotechnical engineer should examine the soil data and recommend appropriate options. Potential problems can be minimized by proper slab designs, stabilization of the soil, or by preventing moisture migration through the slab. Failure to manage the problem can, and often will, result in slab failure. Frost action may be critical to silts, clays, and some fine sands. These soils can experience large changes of volume, and consequently heave due to the growth of ice lenses when subjected to freezing cycles and loss of support due to saturation upon thawing. Three conditions must be present for this problem to occur: • Freezing temperature in the soil; • Water table close to the frost level to provide water for the formation of ice lenses; and • A soil that will transmit water from the water table into the frost zone by capillary action. Possible remedies include lowering the water table, providing a barrier, or using a subbase/subgrade soil that is not frost-susceptible. Properly designed insulation can be beneficial. Volume changes due to frost action occur at building perimeters, under freezer areas, and under ice- skating rink floors (NCHRP 1974). CHAPTER 4—LOADS 4.1—Introduction This chapter describes loadings, the variables that control load effects, and provides guidance for factors of safety for concrete slabs-on-ground. Concrete slabs are typically subjected to some combination of the following loads and effects: • Vehicle wheel loads; • Concentrated loads; • Line and strip loads; • Distributed loads; • Construction loads; • Environmental effects; and • Unusual loads. Slabs should be designed for the most critical combination of these loadings, considering such variables that produce the maximum stress. The PCA guide for selecting the most critical or controlling design considerations for various loads (Packard 1976) is presented in Fig. 4.1 . Because a number of factors, such as slab thickness, concrete strength, subgrade stiffness, and loadings, are relevant, cases where several design considerations may control should be investigated thoroughly. Other potential problems, such as loadings that change during the life of the structure and those encountered during construction (Wray 1986), should also be considered. For example, material-handling systems today make improved use of the building’s volume. Stacked pallets that were once considered uniform loads may now be stored in narrow-aisle pallet racks that produce concentrated loads. The environmental |