Axle load shifts during truck braking and their implications for bridge and pavement design

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Abstract
The axle load shifts of four truck types during braking are estimated using theoretical braking models and the implications of these axle load shifts for bridge and pavement design are explored. The truck types examined are a three-axle straight truck, a five-axle tractor semi-trailer, a seven-axle tractor semi-trailer with two air-lift belly axles, and a seven-axle B-train tractor-double trailer, each loaded to legal Ontario limits with weigh-out commodities. Each of the truck types experienced a substantial load transfer to the front steering axles under braking, mainly from the rear tandem axle groups, where the front axle load increase varied from 38 to 48%. The bridge design implications of the load shifts are examined in terms of the Ontario Bridge Formula and the moments induced in simple-span bridges compared with the moments induced by a reference design truck. For the braking model used, the decelerations and associated shifts in axle load do not lead to higher forces for bridge design and evaluation. The pavement damage implications are analyzed in terms of load equivalency functions derived from some load tests conducted at a number of pavement sites across Canada in 1986. These equivalencies increased up to 50% of the static values for the three-axle truck at maximum braking. It is suggested that the major damage implications are at intersections in urban areas and that increased structural sections could be designed to handle the increased equivalencies. Key words: bridge design, pavement design, truck braking, truck loads, pavement damage, bridge damage.

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