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603284-001 <br />5.2.4 Lateral Resistance <br />Lateral loads can be resisted by soil friction and by the passive resistance <br />of the soils. A coefficient of friction of 0.35 can be used between the -15 <br />footings and the floor slab and the supporting soils.The passive <br />resistance of undisturbed natural soils or engineered fill soils can be <br />assumed to be equal to the pressure developed by a fluid with a density of <br />300 pounds per cubic foot (pcf). A one-third increase in the passive value <br />can be used for wind or seismic loads. The friction resistance and the <br />passive resistance of the soils can be combined without reduction in <br />determining the total lateral resistance. <br />5.3 Slabs-on-Grade <br />Concrete slabs may be designed using a modulus of subgrade reaction of 150 <br />pci provided the subgrade is prepared as described in Section 5.1. From a <br />geotechnical standpoint, we recommend slab-on-grade be a minimum 5 inches S" S l,4 <br />thick with No. 3 rebar placed at the center of the slab at 24 inches on center in w/* 322471 <br />each direction. The structural engineer should design the actual thickness and <br />reinforcement based on anticipated loading conditions.Where moisture- <br />sensitive floor coverings or equipment is planned, the slabs should be protected <br />by a minimum 10-mil-thick vapor barrier between the slab and subgrade. A <br />coefficient of friction of 0.35 can be used between the floor slab and the vapor <br />barrier. <br />Minor cracking of concrete after curing due to drying and shrinkage is normal and <br />should be expected; however, concrete is often aggravated by a high <br />water/cement ration, high concrete temperature at the time of placement, small <br />nominal aggregate size, and rapid moisture loss due to hot, dry, and/or windy <br />weather conditions during placement and curing. Cracking due to temperature <br />and moisture fluctuations can also be expected. The use of low-slump concrete <br />or low water/cement ratios can reduce the potential for shrinkage cracking. <br />Additionally, our experience indicates that the use of reinforcement in slabs and <br />foundations can generally reduce the potential for concrete cracking. <br />To reduce the potential for excessive cracking, concrete slabs-on-grade should <br />be provided with construction or weakened plane joints at frequent intervals. <br />Joints should be laid out to form approximately square panels. <br />-23- <br />Leighton