360R-06 Design of Slabs-on-Ground
R-18 ACI COMMITTEE REPORT
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- 4.2—Vehicular loads
| 360R-18 ACI COMMITTEE REPORT
exposure of the slab-on-ground is also a concern. These effects include subgrade volume changes (shrink/swell soils) and temperature changes. Normally, thermal effects may be minimized by constructing the slab after the building is enclosed. Many slabs, however, are placed before building enclosure. Therefore, the construction sequence is important in determining whether transient environmental factors should be considered in the design. Finally, thermal effect due to in-service conditions should be considered. 4.2—Vehicular loads Most vehicular traffic on industrial floors consists of lift trucks and distribution trucks with payload capacities as high as 70,000 lb (310 kN). The payload and much of a truck’s weight are generally carried by the wheels of the loaded axle. The Industrial Truck Association (1985) has compiled representative load and geometry data for lift truck capacities up to 20,000 lb (89 kN) (Table 4.1). Vehicle variables affecting the thickness selection and design of slabs-on-ground include: • Maximum axle load; • Distance between loaded wheels; • Tire contact area; and • Load repetitions during service life. The axle load, wheel spacing, and contact area are functions of the lift truck or vehicle specifications. If vehicle details are unknown or if the lift truck capacity is expected to change in the future, the values in Table 4.1 may be used for design. The number of load repetitions, which may be used to help establish a factor of safety, is a function of the facility’s usage. Knowledge of load repetitions helps the designer to quantify fatigue. Whether these values are predictable or constant during the service life of a slab should also be considered. Often, the slab is designed for an unlimited number of repetitions. The contact area between tire and slab is used in the analysis for lift truck with pneumatic or composition tires (Wray 1986). The contact area of a single tire can be approximated by dividing the tire load by the tire pressure (Packard 1976). This calculation is somewhat conservative because the effect of tension in the tire wall is not included. Assumed pressures are variable; however, pneumatic non-steel-cord tire pressures range from 85 to 100 psi (0.6 to 0.7 MPa), while steel-cord tire pressures range from 90 to 120 psi (0.6 to 0.8 MPa). The Industrial Truck Association found that the standard solid and cushion solid rubber tires have floor contact areas that may be based on internal pressures between 180 and 250 psi (1.2 to 1.7 MPa) (Goodyear Tire and Rubber Co. 1983). Some polyurethane tire pressures exceeding 1000 psi (6.9 MPa) have been measured. Large wheels have tire pressures ranging from 50 to 90 psi (0.3 to 0.6 MPa). Dual tires have an effective contact area greater than the actual contact area of the two individual tires. There are charts available to determine this effective contact area (Packard 1976). A conservative estimate of this effective contact area, however, can be made using the contact area of the two tires and the area between the contact area. If it is not known whether the vehicle will have dual wheels or what the wheel spacings are, then a single equivalent wheel load and contact area can be used conservatively. An important consideration for the serviceability of a slab subject to vehicular loads is the design of construction and sawcut contraction joints. Joints should be stiff enough and have sufficient shear transferability to limit differential move- ment and prevent edge spalling as a vehicle travels across the joint. Refer to Chapter 5 for more information and joint details. tải về 2.35 Mb. Chia sẻ với bạn bè của bạn:
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