| 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.
|
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
|
| 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. |
| 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
Loading up to 3.5kPa forklift capacity <2 tonnes |
Poor
Medium to Good |
150
125 |
| Medium (loading
class) i.e. warehouses, industrial premises
Loading up to 5kPa forklift capacity <3 tonnes |
Poor
Medium to Good |
175-200
150-175 |
|
Table 2.
Guide to slab thicknessLoading
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/mē |
| 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/mē |
| Shelving |
End frame of
5.4 tonnes |
| Forklift |
Capacity of 3.0
tonnes |
|
Table 3.
Loading Definition
|
