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.
Figure 1.1 Controlling design considerations for various load types/contact areas
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
||Typical CBR (%)
||Recommended Nominal Subbase Thickness (mm)
||2 or less
||3 to 10
||10 or more
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
||Rating of Subgrade
||Minimum Pavement Thickness (mm)
|Light (loading class) i.e. offices, shops etc
|Loading up to 3.5kPa forklift capacity <2 tonnes
|Medium (loading class) i.e. warehouses, industrial premises
|Loading up to 5kPa forklift capacity <3 tonnes
||Medium to Good
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.
||Limits of Loading
|| 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
||End frame of 4.0 tonnes
||Capacity of 2.0 tonnes
|| 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
||End frame of 5.4 tonnes
||Capacity of 3.0 tonnes