Determining the permissible truck load for bridges and roads adheres to specific bridge and road standards and isn’t simply based on the bridge’s design load. This article clarifies the nature of design load versus permissible operating load, helping you understand how to calculate truck load based on bridge and road standards.
The Nature of Bridge Design Load
The bridge design load isn’t the maximum permissible load for vehicles. It’s a theoretical load, determined based on statistical analysis of the effects of various vehicles and convoys on the bridge structure. Bridge design standards, such as Austroads: Bridge Design Code 1992, define live load as the load of traffic or pedestrians, idealized to simulate the actual impact on the bridge.
The process of determining the design load begins with analyzing the effects that actual vehicles have on the bridge, including internal forces, deformation, displacement, and vibration. This data is statistically processed to find the largest values of these effects, called the envelope. Experts then determine a load arrangement and weights so that the resulting effect is equivalent to (actually greater due to safety factors and anticipated future load increases) the calculated envelope. This load arrangement and weighting is the design load, or design live load.
Design Load According to Vietnamese Standard 22TCN 18-79
Standard 22TCN 18-79 specifies the standard live load for road bridges, such as H30-XB80 for national roads and important trunk roads, and H10-X60 or H13-X60 for local roads and industrial traffic roads.
Specifically, the standard H30 convoy consists of two 3-axle vehicles, each with a total weight of 30 tons, arranged alternately with a distance of 10m. Each vehicle has a front axle of 6 tons, a first rear axle of 12 tons (4m or 6m from the front axle), and a second rear axle of 12 tons (1.6m from the first rear axle). However, the fact that a bridge is designed according to the H30 load doesn’t mean that only vehicles with a maximum weight of 30 tons are allowed to pass.
Mistakes in Applying Design Load
Directly applying the design load (such as 30 tons for H30, 13 tons for H13) to regulate the operating load and set load limit signs is a mistake. This creates difficulties for transportation and unnecessarily increases the size and cost of bridges.
Conclusion
The bridge design load is a theoretical value used for calculation, not the permissible operating load. Determining the permissible vehicle load should be based on specific analysis, compliance with bridge and road standards and current regulations, not based on the design load number. Understanding how to “calculate truck load based on bridge and road standards” will help effectively utilize transportation infrastructure and ensure safety for road users.
