Futurtec's First Alert turnkey solution has been chosen as the structural health monitoring solution for the second bridge , which will form part of the planned St Petersburg Ring Road.

The two structures are designed as a graceful pair of bridges which cross the river side by side, each carrying traffic in one direction and providing extra capacity for vehicles in the city. Each bridge is a twin pylon structure with a total length of 730m and a main cable-stayed span of 382m plus two cable-stayed back spans each 174m long. The deck is supported by a total of 112 stay cables in 56 pairs; 28 pairs on each pylon.

Both the deck and the pylons are made of welded steel. During construction, the deck segments were fabricated by bolting together several elements to create a 25m-wide, 2.4m-deep and 12m-long double box girder segment weighing 120t. The two pylons are also built of welded steel segments of varying sizes and geometries, also bolted together to the full height of 124m. The A-shaped pylons flare from 6m wide at the top to 30m at deck level, reducing gradually again to a width of 20m at the base of the structure. The second bridge is being built to the same design as the first.

St Petersburg's Design Institute Giprostroymost was responsible for the design of the bridges, under a contract for the St Petersburg Ring Road Construction Authority. The main construction contract for the structure was awarded to contractor Mostootryad 19 in 2001.

The Futurtec F1A First Alert BSHM solution is reliable and flexible, and the standard package can be expanded on. It is a turnkey solution for monitoring and data analysis of the bridge structure and is composed of a neural network that includes the necessary measurement devices and the neural nodes for their connection. These latter are also transducers for translating the analogue signals into digital format.

The neural nodes are connected to a neural calculation engine which synchronises the data to within one millisecond and calculates the input operating parameters up to the alarm level. The system supplies easy-to-read data informing the operator of the situation and allowing him or her to make rapid decisions on traffic flow or structural intervention. The standard package allows for 100-channel monitoring; this usually creates a degree of redundancy which can be used for future expansion of the system. All data is stored for 20 years and can be reviewed in fully synchronised format, by a simple 'click and drag' method of setting start and finish times for the analysis period, for a selection of channels. Remote control is also possible through safe internet connection and GSM mobile phone network.

The first of the two Neva River bridges was subject to quite severe financial constraints, so the basic package that was chosen measured wind speed and direction, which was of particular importance, given the initial aerodynamic concerns; lateral acceleration of the pylon legs; horizontal displacement at the pylon top; stresses on the inner surface of the pylons; lateral and vertical acceleration of the cantilever tip, and stresses in the steel deck. All these measurements were taken during the construction phase of the project. Hence the designers were able to compare the data generated during construction of the bridge against the predicted data, in order to ensure that the integrity of the bridge was not put at risk during construction. A low-cost solution was adopted which involved moving the sensors as work progressed was adopted. For example, the accelerometers used on the deck were relocated once a further 50m of deck was completed. Wind sensors were also relocated as work progressed. All sensors which were permanently embedded were also used during the operating phase. No strain or vibration sensors were installed on the stay cables on the first bridge, but they will be used on the second crossing.

Given the design parameters of Futurtec's F1A First Alert system, one of the principle considerations of the structural health monitoring solution was to ensure that any system installed would be 'future-proof'. To try and achieve this, Futurtec has based its system on an open Suse Linux platform and all other parts of the operating system are designed around commercially-available programs, which are expected to be available for many more years. In this way it is possible to avoid frequent, expensive software upgrades in future years.

Further cost savings were achieved from the method used to connect the sensors and the nodes with the server: instead of a system where every node is connected separately, F1A First Alert uses connections where the nodes are in series with one another, reducing the amount of cabling that is necessary.

Whilst the same approach will be used on the second bridge, the client took the opportunity to increase the monitoring capacity on the first bridge, through the addition of one long-range laser-displacement sensor and more temperature sensors in the deck.

The new bridge will incorporate a further three laser-displacement sensors for the pylons, allowing total coverage of the two pylons. Another improvement which has been made to the system since the installation on the first bridge is the user interface. A 'control-room friendly' display shows the state of the bridge in real time and highlights any problems with multi-level audio and visual alerts.

Once the data is in the 'tank', it can be stored for years or displayed and analysed, as the operator requires. Security systems for accessing the monitored data are retained at the highest possible level. Everyone who has the security access code and who needs the data can come to the bridge control room to get it. The standard Futurtec package makes it possible to limit control room data to the 'read-only' feature and makes the SLS server information available through a secure, high-speed internet connection. Special authorisation is required to access the data, in order to maintain high security.

It is also possible to monitor the bridge from anywhere in the world through the internet connection. Should the client decide in future to centralise bridge monitoring into a single control room, then the internet connection could be used to ensure real-time data flow from the bridge or bridges. Alternatively, it would be possible to review the data from a university or technical institute for the benefit of future students.

The First Alert bridge structural health monitoring system is reliable, safe, easy to operate and flexible; it can be modified and expanded according to the needs of the individual structure and can be added to years after its installation.

The data is collected, synchronised, stored, analysed and displayed using a unique neural calculating engine which can inform in real time of developing situations or which can be used to analyse the data from a single event. Data can be monitored or analysed either on site or long distance on the internet, with the proper authorisation. The software employs Linux-based programs and other industry-standard operating systems.

Peter Butters is sales and marketing director of Futurtec.

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