Engineers and public authorities face a mounting challenge to shore-up the civil structures supporting transport infrastructure. Indeed, the construction and engineering industry finds itself at an evolutionary crossroads: on the one hand being tasked with pushing existing infrastructure to its structural limits; and on the other, having to adopt new approaches to accurately monitor the integrity of key structures supporting rail and road networks.
Perhaps the most recent example of the risk posed to public safety by depreciating civil structures is the tragedy that unfolded when the Interstate-35W (I-35W) Bridge in Minneapolis collapsed into the Mississippi River on August 1, 2007, killing 13 people and injuring more than 100. The subsequent National Transportation Safety Board (NTSB) investigation concluded that a metal gusset plate had been too thin to serve as a junction of several girders. Designed and constructed in the 1960s, I-35W had gradually gained weight as concrete structures were added to separate lanes, while further changes had served to create additional strain. At the time of the collapse, maintenance crews had brought tons of equipment and material onto the bridge deck to make repairs.
Engineering at a Crossroads
In developed nations, many civil structures are in urgent need of strengthening, rehabilitation, or replacement due to factors including the corrosion of steel reinforcement and consequent breakdown of the concrete, or the fact that some structures may be sound, but have become functionally obsolete. Moreover, many developed nations are unable to expand their current transport infrastructure, resulting in increasing pressure to optimize and extend the life of the civil structures in place today.
According to the American Society of Civil Engineers (ASCE), one in four bridges in the U.S. is either structurally deficient or functionally obsolete, while more than 40% of the operational bridges in Canada were built more than 30 years ago and have been impacted by the adverse climate and extensive use of de-icing salts. In the U.K., an increasing number of bridges and other structures need to be strengthened to comply with legal minimum requirements set-out by European Community legislation. The U.K.'s Cabinet Office is also leading an initiative to reinforce the resilience of key infrastructure to extreme weather events, following recent flooding that resulted in the collapse of a bridge and the death of a policeman.
Innovation in Structural Health Monitoring
A structure is said to have structural integrity if localized damage does not lead to widespread collapse. Conventional methods of assessment have relied on visual inspections, evaluations using conservative codes of practice, and the use of large machines to 'excite' a structure so that various parameters can be measured and analyzed. While such techniques enable engineers to identify and assess the physical manifestation of an issue, identifying the actual cause of the issue is more complex by an order of magnitude.
The relatively new field of Structural Health Monitoring (SHM) provides information on demand about any significant change or damage that occurs in a structure. Although SHM instruments have been employed for many years, difficulties in managing the huge volumes of data they generate have made efficient monitoring in civil engineering applications a challenge. However, major advances in communications, data transmission, and computer processing have enabled SHM systems that can acquire vast volumes of data in relatively short periods of time and transfer it via high-speed fiber-optic or wireless connections to a central database. Most importantly, they can also provide analysts with the means to interpret these data and diagnose potential problems early, and to a high degree of accuracy.
For the past few decades, closed-loop sensors using a servo torquer mechanism have proven themselves in a variety of applications where extremely precise measurements are required. As a result of technology improvements, these devices have become smaller, more cost effective, and so sensitive that there is no longer a need to excite a structure to gain vital information about its integrity. By placing sensors in the appropriate positions on a bridge, analysts can now acquire the necessary raw data. Moreover, advanced algorithms allow asset owners and managing authorities to acquire both short- and long-term structural integrity assessments.
New Applications for Proven Technologies
Meaningful interpretation of the SHM data acquired is the cornerstone of a new proprietary SHM data collection and analysis system developed by U.S.-based STRAAM (Structural Risk Assessment And Management). STRAAM's structural integrity assessment systems provide the information that analysts use to compare how baseline measurements with predictions of how vibrations will dissipate thorugh the structure.
Customised servo accelerometers developed by Sherborne Sensors used with STRAAM's new data collector devices, enable users to establish whether a structure transfers loads as designed. Placed on a few positions on most bridges for a few hours, these devices record a structure's 3D movement in extreme detail. The accelerometers can measure both static and dynamic linear acceleration, with F.S. ranges as low as ±0.1g, providing high-quality data when making the type of long-constant measurements needed for a system such as STRAAM's to be effective.
Bridge the Divide Between Cause And Effect
SHM's benefits were demonstrated recently at a remote steel bridge in the heart of Brazil's Amazon basin. Supporting freight trains that carry 10% of the world's iron ore each year, the bridge had been rolling back and forth whenever an ore-carrying train was crossing. A horizontal crack had also appeared in one of the supporting concrete girders, with train drivers returning to the mines reporting increasingly violent vibrations as they crossed—despite the freight cars being empty. Brought in to monitor the bridge over a period of time, STRAAM discovered that the crack in the concrete was not causing the vibrations. Rather, it was the frequency of the movement induced by the returning trains coupled with that of the bridge. The solution was to reduce the speed of the trains by 20 km/hr. when they crossed the bridge unladen, eliminating the vibration and removing the need for engineering work on the bridge.
Using conventional methods, a meter would have been placed over the crack to measure how it responded to ambient vibration over time. But such a device would not have told the bridge owners why the crack had come about, and whether it had anything to do with the movement in the structure. STRAAM's SHM system takes raw vibration data and turns it into valuable information, enabling a holistic diagnosis of a structure's health, ensuring that asset owners and management authorities have the knowledge to establish the most appropriate strategy for modifying a structure to repair current weaknesses, minimize further issues, and thus prolong the life of the structure.
In an economy where budgets remain under severe constraint, refurbishing critical transport infrastructure takes on a renewed emphasis. Although implementing change in the civil engineering and construction industry takes time, new approaches to SHM can deliver immediate benefits to asset owners, financiers, and public authorities in reducing the risk of litigation, improving public safety, and increasing the sustainability of critical transport infrastructure.
Jeff Matros is CEO of STRAAM, New York, NY. He can be reached at 845-661-4311.