Question

Which of the below axles are not to be considered in the design of rigid pavements?

• A. Tandem axle
• B. Single axle
• C. Rear axle
• D. Front axle

Hint:

Pavement design is governed by the heaviest axle loads, as these cause the most significant stresses and fatigue damage. Rear axles, whether single or in tandem/tridem configurations, carry the majority of the vehicle's weight and payload, making them the critical factor in determining pavement thickness and strength. Front axles, carrying lighter loads, are typically less critical for design calculations.

Answer 1

The Correct Option is D

Step 1: Understand pavement design and axle loads.
Pavement design (both rigid and flexible) is primarily concerned with resisting the stresses induced by vehicle loads. The damaging effect of a vehicle on a pavement is directly related to the magnitude of the axle loads. Heavier loads cause greater stresses and require thicker, stronger pavements.
Step 2: Differentiate between front and rear axles in terms of load distribution.

• Front axle: The front axle of a vehicle (especially commercial vehicles like trucks and buses) typically carries a smaller portion of the total vehicle weight compared to the rear axles. It is primarily for steering and generally carries lighter loads.
• Rear axle(s): The rear axles (single, tandem, or tridem) are designed to carry the bulk of the payload. Consequently, they impose much higher loads and stresses on the pavement.

Step 3: Consider the impact of different axle configurations on pavement design.

• Single axle: A common configuration, where a single axle assembly carries a significant load.
• Tandem axle: Two axles grouped closely together, sharing the load. They distribute the load over a larger area, reducing the stress per unit area compared to a single axle carrying the same total load, but still contribute significantly to pavement damage due to their combined load.
• Rear axle (general term): This category includes both single rear axles and multi-axle groups (tandem, tridem) which are responsible for transmitting the major portion of the vehicle's weight (and payload) to the pavement.

Pavement design is based on the concept of "Equivalent Single Axle Load (ESAL)," which converts the mixed traffic of various axle loads and configurations into an equivalent number of repetitions of a standard single axle load (e.g., 80 kN or 18 kip). This conversion primarily focuses on the destructive effect of heavy loads, which are predominantly carried by rear axles.
Step 4: Determine which axles are considered for design.
In pavement design, the damaging effect of traffic is predominantly due to the heavy loads carried by the rear axles (single, tandem, or tridem axles). The lighter loads imposed by the front axles of commercial vehicles are generally considered negligible in comparison to the rear axle loads for design purposes, especially for rigid pavements which are designed for bending stresses caused by heavier loads. While front axles do contribute to fatigue, their impact is much less significant than rear axles, and standard design methods often simplify by focusing on the heavier axles.
Step 5: Select the correct option.
Therefore, front axles are generally not considered as critically in the design of rigid pavements as the heavier, load-bearing rear and tandem axles. $$\boxed{\text{Front axle}}$$