360R-06 Design of Slabs-on-Ground
—Thickness design methods
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| 6.2—Thickness design methods
If the slab is loaded uniformly over its entire area and is supported by uniform subgrade, stresses will be due solely to restrained volumetric changes; however, most slabs are subjected to nonuniform loading. In warehouses, for example, the necessity for maintaining clear aisles for access to stored materials results in alternating loaded and unloaded areas. Rack post and lift truck wheel loads present a more complex pattern of loading. As noted in Chapter 1 , the analysis of slabs supporting concentrated loads is based largely on the work of Westergaard (1923, 1925, 1926). Three separate cases, differentiated on the basis of the location of the load with respect to the edge of the slab, might be considered (Winter et al. 1964). These cases are given herein to illustrate the effect of load location, particularly at free corners or edges. Most of the generally used structural design methods discussed do not provide for loading at free edges and corners. The designer should carefully consider such loading. Case 1: Wheel load close to corner of large slab—With a load applied at the corner of a slab, the critical stress in the concrete is tension at the top surface of the slab. An approx- imate solution assumes a point load acting at the corner of the slab (Fig. 6.1). At small distances from the corner, the upward reaction of the soil has little effect, and the slab is considered to act as a cantilever. At a distance x from the corner, the bending moment is Px; it is assumed to be uniformly distributed across the width of the section of slab at right angles to the bisector of the corner angle. For a 90-degree corner, the width of this section is 2x, and the bending moment per unit width of slab is If h is the thickness of the slab, the tensile stress at the top surface is (6-1) This equation will give reasonably close results only in the immediate vicinity of the slab corner, and only if the load is applied over a small contact area. Px 2x ------ P 2 --- = f t M S ----- P 2 ⁄ h 2 6 ⁄ ------------ 3P h 2 ------- = = = Fig. 6.1—Corner load on slab-on-ground. |