Design for Strength

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. 

Figure 1.1 Controlling design considerations for various load types/contact areas

 src=/images/cmsimage/Controlling%20design.jpg
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

Recommended nominal subbase thickness  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.
 Subgrade Rating
Typical CBR (%)
Recommended nominal subbase thickness (mm)
 Poor  2 or less  200
 Medium  3 to 10  150
 Good  10 or more  100  

Table 1. Typical thicknesses of unbound subbases.

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 thickness of pavement(mm)
 Light (loading class) i.e. offices, shops etc  Poor  150
 Loading up to 3.5kPa forklift capacity <2 tonnes  Medium to Good  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

Table 2. Guide to slab thickness

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
Table 3. Loading Definition