Question

1- How does the bandage contact area change with the axle load? Show it by drawing.
2- Draw the stresses created by a tandem axle on the superstructure layers by drawing them.

Answer 1

1.

2.

Stress on Roads

Pavement structure reaction to traffic loading is shown by computing the stresses within its layers. If
excessively, the stresses acted upon the pavement can cause cracking or surface rutting. This may lead to
both structural and functional failure, thus causing a safety hazard to the road users such as motorists and car
drivers. Stress (σ) is defined as load, in this case caused by a wheel per unit area measure in Mega Pascal.
The simplified stress equation for a uniform contact stress with a circular loading is:

Road pavements are usually used for low to high volume highway pavements subjected to different
truck axle/wheel arrangement. There are three sets of axle configuration which are single, tandem and tridem.
As shown in Figure, the stress distributions caused by neighboring in a tandem axle are overlapped
resulting in a different stress distribution if compared to that of caused by adjacent wheel. Due to the small
spacing between the car tyres, the critical pavement responses caused by multiple loading are different than
those that are under a single loading

The intensity of the pressure generated by vehicle is the greatest at the surface of the road and it
spreads out into a pyramid-like shape through the layer of the pavement and the bottom soil, which is the
subgrade. Every moving wheel will make the road deflect dynamically which will cause compressive stresses
in the road structure. There is a system used for determining the design loading, where the damaging effect of
axles loading follows an exponential function, which was derived from AASHO Road test. In the road test,
there is Power Law, which has the following equivalency formula:

Load Equivalency Factor=Actual axle load (ton) 4.5/Standard axle load (8, 2 ton)

By referring to the equation, if the axle load is doubled the damage will not just be doubled but it
will increase twenty times the load due to fact that the damage increases exponentially