Abstract
Extremely large trucks with a weight exceeding the standard require a permit before they are allowed to cross the bridges of a specific route. For the purpose of safety, an escort is often employed to maintain a distance between vehicles and to ensure that the bridge load remain below the allowed maximum. Given that the speed of these large vehicles is quite slow and that the amplitude of vibrations normally declines when the vehicle mass is large, a minor dynamic amplification of the bridge response is expected. However, some of these large trucks have a unique feature characterized by “multiple equally-spaced axles”, something that is uncommon in normal vehicle. The application of axle forces at equal intervals can dynamically excite bridges to a considerable extent, even at low speeds. These “critical” low speeds are estimated a priori from the axle spacing of the truck and the main frequency of vibration of the bridge. This paper demonstrates that when the “critical” speed is unavoidable, a relatively high dynamic allowance must be added to static calculations before granting a permit to a long heavy vehicle.
Highlights
Special vehicles above normal legal weight limits have been generally found to govern bridge loading in shortand medium-span bridges (Enright & O’Brien, 2013)
The road authorities must carefully examine how particular truck configurations affect the safety of the bridges across the route decided for the vehicle
In the case of large forces generated by the truck being close to the loading capacity of certain bridges, the bridge response must be evaluated accurately, i.e., allowing for the dynamic increment resulting from the interaction between the vehicle and the bridges
Summary
Special vehicles above normal legal weight limits have been generally found to govern bridge loading in shortand medium-span bridges (Enright & O’Brien, 2013). Dynamic Amplification Factor is defined as the ratio of the maximum total response to the maximum static response during a vehicle crossing, for the load effect of the bridge being examined (i.e., bending moment at mid-span section) This is the definition employed throughout the paper. Cantero, González, and O’Brien (2011) compare DAF due to articulated 5-axle trucks to large cranes at typical highway speeds, but they overlook the resonance effect. This paper fills this gap by analysing the impact of speed, weight, configuration and transverse location of a permit vehicle on the bridge response. Results are compared to a conventional 5-axle truck configuration, commonly found in the road network
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