Introduction:

Vehicular loading plays a significant role in the design of bridges, pavements, and other transportation infrastructure. Two commonly used vehicular loadings in the United States are HL-93 and HS20-44. This article will explore the HL-93 and HS20-44 loading specifications, including the HL-93 design truck, truck axle spacing, and their impact on structural design.

HL-93 Loading:

The HL-93 loading, or “H-Load 93,” is a set of vehicular loading specifications developed by the American Association of State Highway and Transportation Officials (AASHTO). It represents a standardized design load model for evaluating the structural capacity of bridges and other transportation structures.

The HL-93 loading consists of three components:

1. Design Truck (HL-93 Design Truck): The HL-93 design truck is a standardized truck configuration used to represent a typical heavy vehicle. It comprises a tandem axle truck with specific dimensions, axle spacing, and axle loads. The key parameters of the HL-93 design truck include:
   • Gross Vehicle Weight (GVW): 80,000 pounds (36,287 kilograms).
   • Tandem Axle Spacing: 14 feet (4.267 meters).
   • Axle Loads: Axle loads are distributed to simulate real-world loading conditions.
2. Design Lane Load: In addition to the design truck, the HL-93 loading includes a uniform lane load. This lane load is represented by a uniformly distributed load of 640 pounds per linear foot (9.6 kN/m) over the entire traffic lane width. The lane load ensures that the entire bridge or structure is subjected to a uniformly distributed load.
3. Design Lane Load (Sidewalk Loading): In cases where sidewalks are present on a bridge or structure, an additional uniform lane load is applied. This sidewalk loading consists of 85 pounds per linear foot (1.3 kN/m) over the width of the sidewalk.

HS20-44 Loading:

HS20-44, also known as H-20 loading, is an older but still widely used loading standard in the United States. It represents a design load model based on a specific vehicle configuration and loading conditions. The key parameters of HS20-44 loading include:

• Design Vehicle: The HS20-44 loading is based on a single vehicle configuration, typically a single-axle truck with a design load of 32,000 pounds (14,515 kilograms).
• Axle Spacing: Unlike HL-93, HS20-44 does not specify a tandem axle configuration. Instead, it focuses on single axle loads.
• Design Lane Load: The design lane load for HS20-44 loading consists of a concentrated load of 32,000 pounds (14,515 kilograms) applied to an area of 8 square feet (0.74 square meters). This concentrated load is placed on one wheel at a time and is moved across the structure to evaluate the worst-case loading scenario.

Impact on Structural Design:

Both HL-93 and HS20-44 loadings are used to assess the structural capacity of bridges and other transportation infrastructure. The choice between these loading specifications can have a significant impact on design and construction:

• HL-93 loading is considered more representative of real-world loading conditions, as it includes a tandem axle truck, lane load, and sidewalk loading. It is often used for the design of new bridges and structures, especially those located on major transportation routes.
• HS20-44 loading is still commonly used for evaluating existing structures and assessing their capacity to handle heavy vehicles. It is considered a more conservative loading model and is used in load rating analysis.
• Axle Spacing: The distribution of loads on axles, especially in the HL-93 loading, can affect the structural response. Engineers must consider the axle spacing and configuration when designing and assessing structures.

In conclusion, understanding the differences between HL-93 and HS20-44 vehicular loading specifications is crucial for structural engineers and designers. The choice of loading model depends on factors such as the type of structure, its location, and its intended use. Accurate evaluation of vehicular loading ensures that transportation infrastructure is safe and can effectively carry heavy loads, contributing to the overall safety and efficiency of the transportation network.