360R-06 Design of Slabs-on-Ground
—Design of slab-support system
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- DESIGN OF SLABS-ON-GROUND 360R-13
| 3.5—Design of slab-support system
3.5.1 General—After the subgrade soils have been classified, the general range of their k values can be approximated from Fig. 3.3 . Adjustments may be made on the basis of local experience and expected seasonal changes as well as expected construction conditions. With this information, a decision can be made whether to use the existing subgrade in its in-place condition, improve it by compaction or stabilization, use a subbase and a base course, or vary the thickness of these layers. Initially, a wide range of subgrade conditions may exist across the site. The soil-support system is rarely uniform. Therefore, some soil work is generally required to produce a more uniform surface to support the slab. The extent of this work, such as the degree of compaction or the addition of a base course, is generally a problem of economics. Selection of crushed rock or soils in the well-graded gravel (GW) and poorly-graded gravel (GP) groups may appear costly as a base material; however, the selection of these materials has distinct advantages. Not only do they improve the modulus of subgrade reaction and produce more uniform support, but they also provide an all-weather working surface to speed construction during inclement weather. 3.5.2 Economics and simplified design—A prerequisite for the proper design of a slab-support system is identification of DESIGN OF SLABS-ON-GROUND 360R-13 the subgrade material and conditions to which it will be exposed. Without this knowledge, neither the modulus of subgrade nor the potential volume change can be estimated. With knowledge of soil classification and some local experi- ence, the engineer can select an appropriate k value and design for the specific soil conditions. The slab thickness calculation is insensitive to small changes in k, and, therefore, the k value need not be known exactly. Significant variations do not significantly change the design thickness. For small projects, it may be advantageous to assume a relatively low k factor and add an appropriate thickness of subbase and base course material to enhance performance of the slab rather than performing an expensive plate load test. The risk of slab failure increases the more the design is based on assumed conditions, but there are occasions where a simplified design approach is justified. These decisions are a matter of engineering judgment and economics. Compounding safety factors may produce an overly conservative design. Inclusion of cumulative safety factors in the modulus of subgrade reaction, applied loads, compressive or flexural strength of the concrete, or number of load repetitions may produce a very conservative and, consequently, expensive construction. The safety factor is normally accounted only in the allowable flexural stress in the concrete slab. 3.5.3 Bearing support—Calculated bearing pressures under loaded slabs-on-ground are typically significantly lower and are not critical to typical designs as compared with the allowable foundation contact pressures for building elements controlled by ACI 318. Providing uniform support conditions, however, is extremely important for serviceable slab performance. tải về 2.35 Mb. Chia sẻ với bạn bè của bạn:
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