Bridges

Bridge 1197 is a 143m long six-span bridge remotely located in the Pilbara region in WA. The bridge superstructure comprises prestressed concrete I-beams and a compositely acting in-situ concrete deck. A load rating of the bridge has indicated that the rating of the bridge beams is marginally inadequate for loading from 60m long, 220 tonne road trains.

A monitoring system was installed at the bridge to gather beam strain measurements and video footage for vehicle passes. The system computes the applicable DLA for each vehicle-pass and also provides information on vehicle axle configuration, speed and mass for categorisation purposes. Traditionally, calculating dynamic load allowance (DLA) requires knowledge of both dynamic and static responses. Therein lies the challenge of calculating the DLA using output-only methods, namely, to estimate the static response based purely on the measured response.

The static response is extracted using a low pass filter to remove dynamic content from the measured response as the vehicle traverses the bridge at speed. The optimal cut off frequency for the low pass filter is determined using a static influence line and virtually calculated vehicle axle loads and spacing. The accuracy of this method was tested using staged passes with road trains of known mass, speed and configuration. Testing showed that the system and method are reliable, but typically only when examining the average results for many events, as anomalies can occur.

The system showed that the applicable DLA is 38% in span 1 and 28% in span 2 of the bridge. As such, the value of 40% specified in AS 5100 cannot be reduced for beams in span 1. The high dynamics in span 1 is due to inconsistencies in the running surface on the approach to the bridge. Maintenance works are therefore required to smooth the running surface.