STRAAM Accelerometer equipment

Using STRAAM to monitor movement in bridges

Austroads Bridge Conference review

In April, some of the Mainmark team attended the annual Austroads Bridge Conference to present a paper on STRAAM, which was introduced by Mainmark into Australia and New Zealand late last year, and how it has been used to assess the structural condition of bridges in the USA.

A bridge in Chicago had been struck by a vehicle, causing a crack on one of the abutments. STRAAM (an acronym for Structural Risk Assessment And Management) was used to determine the extent of damage caused by the vehicle. The analysis revealed that it had only caused a minor loss of capability and that the bridge’s performance remained above the highway code.

The response from the Australian bridge and roads industry to this case study was very positive, with many expressing interest in the ability of STRAAM to measure and compare the current strength of structures against their original ‘as built’ design.

Mainmark can use STRAAM to provide full scale structural integrity assessment and continuous monitoring for bridges in Australia and New Zealand. Advances in measuring extremely low amplitude vibrations combined with methods for extracting the unique dynamic signature have now enabled the rapid measurement of the response of steel and concrete bridge structures.

The STRAAM method uses the normal traffic conditions and wind loads on the bridge, often without any lane closures, to measure the frequency response of the structure. This allows the quick calibration of Finite Element Models that can be used to accurately assess the strength of a bridge. This information also allows bridge owners to efficiently track changes in the capacity of their bridges due to ageing, impact, earthquake or flood activity through changes in the vibration of the bridge and associated natural frequencies, mode shapes and damping ratios.

Because of the sensitivity of the instrumentation and the novelty of the analysis techniques, the information available to bridge owners and assessors allows information-based decisions to be made in a way that optimises the financial implications. In addition, the techniques are non-invasive and non-destructive and they give additional information about the connectivity of the bridge with the surrounding terrain, and whether that connectivity is compromised.

The conference, which was highly focussed and well-attended by key influencers in the bridges sector, also discussed the new Australian Bridge Code. Released in March 2017, the Bridge Code addresses a number considerations, such as changes in the Australian climate, sustainability and the use of a wider range of building materials, that will allow bridge designers to design more efficiently.

The use of weathered steel for constructing bridges emerged as a key area of interest for the bridge design community. By removing the need to manage corrosion, weathered steel maintains its aesthetic over the longer term, however there are a number of design aspects around condensation that must be carefully considered.

By James O’Grady

James O’Grady is a Sales and Business Development Manager at Mainmark. He is a civil engineer with 25 years’ experience in structural engineering, construction materials and ground treatment.