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Design For Strength
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An industrial and commercial pavement may be subjected to various types of loading ranging from dynamic wheel loads through post loads to distributed loading from stacked material.

The objective of thickness design is to ensure satisfactory performance of the pavement under all the applied loads, by preventing the occurrence of:

  • Excessive flexural stresses, resulting in cracking of the concrete
  • Excessive bearing stresses on the concrete surface
  • Excessive punching shear stresses due to concentrated loads
  • Differential deflections at joints
  • Excessive deflections due to settlement of the subgrade

The controlling design consideration varies according to the load types/contact areas, as shown in the Figure 1.1.

For most pavements, the governing design consideration will be the flexural tensile stress induced in the concrete by wheel or post loads. If a slab plate of adequate size is not provided under the leg or post of a storage rack subject to heavy loads, excessive bearing stresses or punching shear may occur.

For distributed loads extending over large areas, such as in stacked storage bays, flexural tensile stresses under the loads may not be as critical as stresses due to the negative moments in the aisles between stacks.

Excessive pressures due to heavy distributed loads may cause faulted joints due to differential settlement of the subgrade, or result in unacceptable total settlements in some situations. Is should be noted that Figure 1.1 provides an approximate guide only. Boundaries between different controlling design considerations are not exact and will vary depending on many factors, including subgrade strength and the thickness and strength of the concrete slab.

Subbase Materials and Thickness

Subgrade Rating  Typical CBR (%) Recommended Nominal Subbase Thickness (mm) 
Poor 2 or less 200
Medium  3 to 10 150
Good  10 or more 100

For concrete pavements, it is seldom necessary or economical to build up the supporting capacity of the subgrade with a thick subbase. This is because increasing the subbase thickness results in only minor increases in subgrade support values, and hence only minor reductions in pavement thickness for given loading conditions.

Simplified Thickness Design

For lightly-loaded commercial and industrial pavements, minimum thicknesses based on previous satisfactory performance may be selected from Table 2. Thickness design of a floor slab is dependent upon the following:

  • Type and loading applied
  • Grade of concrete used
  • Support offered by the sub base and/ or subgrade
Typical Application  Rating of Subgrade  Minimum Pavement Thickness (mm) 
Light (loading class) i.e. offices, shops etc Poor 150
Loading up to 3.5kPa forklift capacity <2 tonnes Medium  125
Medium (loading class) i.e. warehouses, industrial premises Poor 175-200
Loading up to 5kPa forklift capacity <3 tonnes Medium to Good 150-175

Loading

In general, a slab will be subject to a combination of the following types of loading:

  • Wheel loads, i.e. forklift trucks, trolley and other vehicles. Trucks with ratings up to 2 tonnes have negligible effect on the slabs
  • Leg loads from warehouse racking systems.
  • Uniformly distributed loads placed directly on the floor slab.

In Table 3 are guidelines produced by the British Cement Association which simplify the definition of loading categories.

Loading  Class  Limits of Loading 
Light  Pallet Racking 4 levels (one on floor) of 0.75 tonne unit loads, 4.5 tonne end frame
   Mezzanine Floor Design load 3.5 kN/m²
   Shelving End frame of 4.0 tonnes
   Forklift Capacity of 2.0 tonnes
 Medium  Pallet Racking 4 levels (one on floor) of 1.0 tonne unit loads, 6 tonne end frame 
   Mezzanine Floor Design load 5.0 kN/m²
   Shelving End frame of 5.4 tonnes
   Forklift Capacity of 3.0 tonnes