Vehicle live load is a crucial factor in highway bridge design. Accurately determining live load ensures that bridges are strong enough to bear stress and operate safely throughout their service life. This article will analyze vehicle live load in detail and its relationship to the design load of bridges.
Misconceptions About Vehicle Live Load and Design Load
Many mistakenly believe that the design load of a bridge is the load of the heaviest vehicle or vehicle convoy specified in design standards. For example, if a bridge is designed with an H30, H13, or H10 load according to standard 22TCN 18-79, many assume the maximum allowable load is 30 tons, 13 tons, or 10 tons, respectively. This understanding is completely incorrect.
Vehicle Live Load: Concept and Principles of Determination
According to the Australian Bridge Design Code (Austroads: Bridge Design Code 1992), live load is defined as the load of traffic flow (single vehicles or vehicle convoys) or pedestrians. The value and arrangement of the load are theoretical, specified in standards to create effects in the structure equivalent to the effects caused by actual vehicles.
The principle for determining vehicle live load is based on statistical analysis of the effects of vehicles traveling on roads on bridge structures. These effects include internal forces, deformations, displacements, vibrations, etc. Based on the set of maximum values of these effects (envelopes), the load arrangement and value are determined such that their effect on the bridge structure is equivalent to, or even greater than, the calculated envelope. This load arrangement and value is the design live load.
Design Live Load and Standards
Design live load can be a series of concentrated or uniformly distributed forces as in standards CHnII 84 (Russia), AASHTO (USA), 22TCN 272-05 (Vietnam). It can also be single vehicles and assumed design vehicle convoys as in standards 22TCN 18-79 (Vietnam), CHnII 200-62 (USSR), AASHTO 1992 (USA), DIN 1072 (Germany), AUSTROADS 1992 (Australia).
For example, according to standard 22TCN 18-79, the H30 live load is a standard convoy of 2 types of 3-axle vehicles, each with a total weight of 30 tons, arranged alternately at 10m intervals.
Vehicle Live Load and Bridge Load Limits
Using the load value in design standards to regulate operating loads and set bridge load limit signs is inaccurate. Vehicle design live load is theoretical, determined to simplify calculations. Bridges are designed to meet normal traffic requirements for all vehicles manufactured for road transport duties.
Conclusion
Vehicle live load is a critical element in highway bridge design. It is essential to correctly understand the concept and principles of determining live load to avoid errors in bridge design, construction, and management. Setting load limit signs should be based on a specific analysis of the bridge’s condition and actual vehicle live load, not on the load value in design standards.
