APPLICATION NEWS
 


        

          
 

          


          Rethinking approaches to sustainable transport infrastructure



          An examination of how making sense of structural health monitoring
          data is crucial to the safety and sustainability of civil structures
          supporting strategic transport links

          
Engineers and
          public authorities face a mounting challenge to shore-up the
          civil structures supporting transport infrastructure. Bridges
          in particular play a crucial role in the transport network, and
          their removal from service can result in severe disruption or
          worse. Perhaps the most recent example of the risk posed to public
          safety by depreciating civil structures is the tragedy that unfolded
          in the US, 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
          investigation by the National Transportation Safety Board (NTSB)
          concluded that a metal 'gusset' plate had been too thin to serve
          as a junction of several girders. Designed and constructed in
          the 1960s to accepted codes of practice at that time, I-35W had
          gradually gained weight as concrete structures were added to
          separate lanes, whilst 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 deck to make repairs.

          
In August 2010,
          URS Corporation, the engineering and construction company hired
          by the state of Minnesota to assess the safety of its bridges,
          agreed to pay $52.4 million to settle claims that it failed to
          spot problems with I-35W. Meanwhile, the State of Minnesota and
          URS still have claims pending against Jacobs Engineering Group,
          which took over the firm that designed the bridge. This is one
          instance of the far-reaching consequences of failing to protect
          the public, faced by asset owners and public authorities in constructing
          and maintaining civil structures supporting critical transport
          links.

          Engineering at a crossroads

          In developed nations, many civil structures are in urgent need
          of strengthening, rehabilitation, or replacement. This is due
          to multiple 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
          - e.g. a bridge that is no longer able to support growing traffic
          volumes, vehicle sizes and weights.

          
According to
          the American Society of Civil Engineers (ASCE), one in four bridges
          in the US is either structurally deficient or functionally obsolete,
          whilst more than 40% of the operational bridges in Canada were
          built over 30 years ago and have been impacted by the adverse
          climate and extensive use of de-icing salts. In the UK, 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 UK'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.

          
Moreover, many
          developed nations are unable to expand their current transport
          infrastructure, resulting in increasing pressure to optimise
          and extend the life of supporting civil structures in place today.
          Thus 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 the accurate
          monitoring of the integrity of key structures supporting rail
          and road networks.



          Innovation
          in structural health monitoring

          A structure is said to have structural integrity if localised
          damage does not lead to widespread collapse. Conventional methods
          of assessment have centered 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 analysed. Such techniques have limitations in that while
          they 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.

          
Structural Health
          Monitoring (SHM) is a system that provides information on demand
          about any significant change or damage occurring 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 solutions that now provide
          the ability to acquire vast volumes of data in relatively short
          periods of time and transfer it via high-speed fibre-optic or
          wireless connections to a central database. Most importantly,
          advanced SHM solutions are now able to provide analysts with
          the means to interpret this data and diagnose potential problems
          early, and to a high degree of accuracy.

          
A field-proven
          technology lies at the heart of SHM innovation. In the past few
          decades, sensors based on MEMS (Micro-Electro-Mechanical Systems)
          technology have become smaller, more cost effective, and so sensitive
          that there is no longer a need to excite a structure in order
          to gain vital information about its integrity. Simply placing
          a sensor on a bridge will provide analysts with all the data
          required via ambient sources such as gusts of wind, foot falls,
          and traffic flows. Moreover, advanced algorithms have been developed
          that allow analysts to evaluate this simple, specific data set,
          to make both short- and long-term structural integrity assessments
          that provide essential for asset owners and managing authorities
          in taking decisions regarding repairs and upgrades, strengthening
          projects, financing, insurance, and dispute resolution.



          New applications
          for proven technologies

          Meaningful interpretation of the SHM data acquired is where the
          rubber hits the road in civil engineering and infrastructure
          management applications, and is the cornerstone of a new proprietary
          SHM data collection and analysis system developed by US-based
          STRAAM (structural risk assessment and management). STRAAM's
          innovative structural integrity assessment solutions provide
          the vital data that analysts use to compare the dissipation of
          vibrations with either the predicted behaviour of the structure
          given its design and materials, or with baseline measurements
          captured earlier.

          
Key to capturing
          these baseline measurements are customised servo accelerometers
          developed by leading UK manufacturer, Sherborne Sensors. Renowned
          for their extreme reliability and long-life within critical applications,
          Sherborne Sensors' custom linear servo accelerometer technology
          powers STRAAM's new data collector devices, which enable users
          to establish whether a structure transfers loads as designed.
          Simply placed on a single position on most bridges for a few
          hours, these devices record a structure's three-dimensional movement
          in extreme detail. The highly accurate accelerometers can measure
          both static and dynamic linear acceleration, with full-scale
          ranges as low as ±0.1g. And when making the type of long
          constant measurements needed for a system such as STRAAM's to
          be effective, these very low-range servo accelerometers provide
          a superior data stream compared to sensors using other design
          technologies.

          
Further successful
          applications within these environments include road deck frequency
          and mode shape determination; seismic structural monitoring;
          vertical, lateral and rotational acceleration measurements of
          decks, cables and bridge towers; and integration with GPS systems
          to improve deflection frequency response. A long-span suspension
          bridge currently under construction in Asia for example, employs
          a sensor network that includes Sherborne Sensors' precision servo
          inclinometers and accelerometers. This sensor network enables
          the identification of structural problems at an early stage,
          prolonging the life of the structure, identifying areas of concern,
          and improving public safety.



          Bridge
          the divide between cause and effect

          SHM's benefits were clearly demonstrated recently at a remote
          steel bridge in the heart of Brazil's Amazon basin. Supporting
          freight trains carrying 10% of the world's iron ore each year,
          the bridge had been rolling back and forth whenever a heavy-laden
          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 their cars being empty. STRAAM was brought
          in to monitor the bridge over a period of time and, using its
          data collector devices and advanced analysis techniques, discovered
          that the crack in the concrete was not the cause. Rather, it
          was the frequency of the movement of the returning trains coupled
          with that of the bridge. The solution was simply to reduce the
          speed of the trains by 20km per hour when they crossed the bridge
          un-laden, and the vibration was eliminated, without the need
          for costly engineering works to 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. Using Sherborne Sensors' field-proven and trusted
          technology, STRAAM has created an SHM solution that takes raw
          vibration data and turns it into valuable information enabling
          analysts to provide a holistic diagnosis of a structure. This
          ensures asset owners and management authorities are fully-equipped
          with the knowledge to establish the most appropriate strategy
          for modifying a structural system to repair current weaknesses,
          minimise further issues and thus prolong the life of the asset.

          
In an economy
          where budgets remain under severe constraint, the refurbishment
          of critical transport infrastructure takes on 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 the sustainability of critical transport infrastructure.

          _____________________________________________________________________

          
November 2010

          

            

              
                
Home - Search - Suppliers - Links - New Products - Catalogues - Magazines

                Problem
                Page
                - Applications - How they work - Tech Tips - Training - Events