360R-06 Design of Slabs-on-Ground
R-20 ACI COMMITTEE REPORT
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- 4.9—Factors of safety
| 360R-20 ACI COMMITTEE REPORT
effects are of particular importance for exterior slabs and for slabs constructed before the building is enclosed. Curling caused by these changes produces flexural stresses due to the slab lifting off the subgrade. Generally, the restraint stresses can be ignored in short slabs because a smooth, planar subgrade does not significantly restrain the short slab move- ment due to uniform thermal expansion, contraction, or drying shrinkage. There are several variables that would affect how short a slab this would be, but 30 times the slab thickness is a generally conservative joint spacing for most conditions. Built-in restraints (such as foundation elements, edge walls, and pits) should be avoided. Reinforcement should be provided at such restraints to limit the width of the cracks in the slab. Thermal and moisture effects are discussed further in Chapter 13 , and expansive soils are discussed further in Chapter 9 . 4.9—Factors of safety Slabs-on-ground are distinguished from other structural elements by unique serviceability requirements. Some of these serviceability requirements minimize cracking and curling, increase surface durability, optimize joint locations and type of joints for joint stability (the differential deflection of the adjacent slab panels edges as wheel loads cross the joint) and maximize long-term flatness and levelness. Because the building codes primarily provide guidance to prevent catastrophic failures that would affect the public safety, the factors of safety for serviceability, while inherent in building codes, are not directly addressed as are those for strength. If the slab-on-ground is part of the structural system used to transmit vertical loads or lateral forces from other portions of the structure to the soil (such as a rack-supported roof), then requirements of ACI 318 should be used for the load case. The factor of safety to minimize the likelihood of a serviceability failure is selected by the designer. Some of the items the designer should consider in selecting the factor of safety are the following: • Consequences of serviceability failure, including lost productivity, lost beneficial use, and the costs for repairing areas in an active facility. For example, crack frequency should be minimized and crack widths should be limited for facilities such as pharmaceutical and food processing facilities; • Concrete mixture proportion and its shrinkage charac- teristics (shrinkage should be tested and minimized to reduce linear drying shrinkage and curling); • Humidity-controlled environment that will increase linear drying shrinkage and curling of the slab; • Subgrade smoothness and planeness to minimize restraint as linear drying shrinkage takes place; • Spacing and type of joints; • Geotechnical investigation to determine the shallow and deep properties of the soil; • Number of load repetitions to allow consideration of fatigue cracking; • Impact effects; and • Storage racks installed at an early stage, which will restrain linear drying shrinkage. Some commonly used safety factors are shown in Table 4.2 for the various types of slab loadings. Most range from 1.7 to 2.0, although factors as low as 1.4 are used for some conditions. A moving vehicle subjects the slab-on-ground to the effect of fatigue. Fatigue strength is expressed as the percentage of Fig. 4.2—Tire contact area for various wheel loads. (Note: 1 in. = 25.4 mm; 1 in. 2 = 645.2 mm 2 ; 1 kip = 4.448 kN.) tải về 2.35 Mb. Chia sẻ với bạn bè của bạn:
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