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HAZARD MITIGATION AT BRIDGE STRUCTURES USING FIBER OPTIC MONITORING SYSTEM

General description

Fiber-optic geotechnical monitoring system (FOGMS) includes 3 basic components: analyzer (or interrogator), distributed (uninterrupted) fiber-optic sensor and a dedicated software (Fig.1). The analyzer is a powerful diagnostic instrument for distributed measurement of strain and temperature over 65 km per channel, allowing measurement of thousands of locations by means of a single sensing cable. Use of internal optical switch featuring 2 measuring channels allows uninterrupted measurement of up to 135 km in two opposite directions from the analyzer. The number of measurement channels can be further extended by using an external multiple optical switch module and star topology. Once an external switch is connected to the analyzer, up to 21 sensors can be used for distributed sensing.

Distributed sensor is an uninterrupted fiber-optic cable that is often custom-designed for a specific application and environment. Each millimeter of sensor is used as a sensitive element offering a great alternative to numerous point sensors. Taking into account high spatial resolution of the analyzer each 50 centimeters of sensor can be considered as an individual point sensor, therefore 50 centimeter sensor section is equivalent to 100 000 point sensors. Fiber-optic distributed sensors are fully passive and do not require connection to power supply. One sensor is usually connected to the analyzer from one or two ends and is adjacent to the entire length of a monitoring facility. Optical fiber integrated in sensor design is sensitive to various external parameters (temperature, elongation/compression, acoustic pressure, etc.) by changing its optical properties. Thus, fiber-optic geotechnical monitoring system offers a wide range of performances and suitability for different applications.

The system automatically controls an extended facility in every section with an installed sensor providing accurate measurement capabilities in real time. The superior sensing technique and operation flexibility makes FOGMS a unique and unrivalled solution to most demanding applications. For the details of system’s operation principles and advantages of fiber-optic sensors, please refer to Section “Resources”.


A)  

B)

 

C)

Fig. 1. FOSGTM includes a set of components which consist of:

A) Analyzer unit and supporting equipment mounted in a 19” rack cabinet; B) Fiber-optic sensor; C) Dedicated software to be installed in a server room.

 

Benefits of monitoring

Monitoring of complex bridge structures offers many advantages to the owners, such as:


Reduced uncertainty. Monitoring helps to understand real state of the bridge materials and structure, induced loads, etc. This information can be taken into account by bridge owners to take informed decisions based on actual data and reduce insurance costs.
Monitoring discovers hidden structural reserves. The advantage of better material properties, appropriate design solution and synergetic effects usually helps to increase predicted lifetime capacity of a bridge structure. The assessment of load-bearing capacity of a structure without intervention can be obtained by structural monitoring. In practice most of the inspected bridges are in much better conditions than expected. In these cases, monitoring enables to increase durability of bridge operation or to safely extend its load-bearing capacity.
Monitoring enables to discover hidden deficiencies. There are structural deficiencies which cannot be identified by visual inspection or modeling. Timely identification of such deficiencies by the monitoring system can guarantee safety to the structure with reduced repair costs.
Monitoring optimizes maintenance costs. By providing continuous and quantitative da-ta of bridge structure, monitoring system helps to increase quality and reduce costs asso-ciated with construction, operation, maintenance, replacement and repair.
Monitoring increases knowledge. Timely and thorough knowledge about structural condition of a bridge leads to cheaper, safer and more reliable design in the future.
Monitoring helps to make a decision based on facts rather than assumptions. Moni-toring system installed at the construction site and commissioned at the earliest stage of bridge operation provides benefits for unbiased resolutions of controversies arising be-tween customer, design company and contractor.
Monitoring reduces the overall life-cycle-cost and handles unexpected increases dur-ing construction. The use of monitoring system at the construction phase is also advan-tageous for both contractors and subcontractors as it gives them the opportunity to opti-mize project solutions with lower expenses when unexpected factors or breach of the pro-ject conditions are revealed.

Cost estimation for new bridges

The typical initial investment for a bridge monitoring system ranges between 0.5 % and 3% of the total construction cost. This includes hardware, installation and configuration of the system.
Management of the monitoring system and of the resulting data as well as the data analysis typically adds 5% to 20% of the SHM system cost every year. Over the first 10 years in life of a structure, having a bridge monitoring system installed on a medium-size bridge will typically require an in-vestment about 2% to 5 % of the total construction cost.


The financial investments to installation of the monitoring system are paid back to a bridge owner in the following ways:

Free of charge elimination of defects and construction faults within warranty period (typ-ically 2 to 5 years) made by the contractor;
A monitoring system controlling construction process enables to follow design and tech-nological rules of the project whilst improved quality of service is guaranteed;
Observing the bridge condition during the first 10 years of its life provides opportunities to make a qualitive assessment of its distinctive features and performances as well as to reduce maintenance and repair costs.

Cost estimation for bridges under replacement or repair

Payback of the monitoring system installed on the bridge:

The cost for replacing bridge structure is as much as 100% of its prime cost;
The cost for installing monitoring system (equipment, installation, measuring, data analy-sis): 3%;

Conservative estimate of the monitoring system outcomes after performance analysis:

Ratio of bridges in normal/good condition: 20%;
Ratio of bridges being candidates for repair: 20% (assuming that repair costs are: 30%);
Ratio of bridges requiring replacement: 60%.
Total cost including monitoring system installation: 3% + 20% Х 30% + 60% = 69%.
Savings of investment to monitoring system: 31%.

Key parameters of monitoring bridge structures

Typical bridge structural members are exposed to creep and permament processes (such as soil processes occuring in the area of bridge foundations, corrosion and fatigue, etc.), as well as to quick but occasional processes (wind load, temperature gradients, heavy traffic (mass distribution), etc.).
Therefore, critical elements of the bridge structure and their control parameters depend on the bridge design (suspended, beam, arch, pylon, cantilever, etc; span lenghs, etc.), material type (pre-strained concrete or common concrete, all-metall structure, etc.), area specifics (soil conditions, environmental conditions) and operation conditions (transport traffic).
For example, suspended bridges (and their types such as cable-stayed bridges) are con-trolled through strain monitoring of suspensions supporting road or highway to mitigate over-stress events; vibration of road or highway as such bridges are having low stiffness; angular dis-placement of pylons (due to high to a high torsional trend induced by wind loads). As for arch bridges, knowing behaviour of bearings is a pivotal inegrity factor. Both vertical and horizontal loads are transferred to the bearings from a heaivier bridge part (archs and bridge road).


The table below summarizes the main risks associated with each type of the bridge:

 

          Risks

 

Concrete beam bridge

 

Steel beam bridge

 

Concrete cantilever bridge

 

Arch bridge

 

Cable-stayed bridge

 

Suspended bridge

Correspondence between Finite Element Model and real condition

**

**

**

**

*

*

Dynamic strain due to traffic, wind, earthquake, explosion

 

***

 

*

***

***

Creep, relaxation of pre-stress

***

 

***

 

 

 

Change in cable forces

 

 

 

 

**

**

Correspondence between calculated vibration modes and real condition

*

*

*

*

*

*

Non-working bearings or expansion joints

**

**

**

 

**

*

Cracking of concrete or steel

*

*

*

*

*

*

Temperature changes and temperature gradients in load bearing elements

***

***

***

***

***

***

Differential settlements between piers and foundations

*

*

**

**

 

 

Change in water table or pore water pressure around foundations

*

*

*

*

*

*

Stability of slopes around foundations and abutments

*

*

*

*

 

 

Change in the concrete chemical environment: carbonation, alkali-silica reaction, chlorine penetration

***

*

***

**

**

**

Environmental conditions

 

 

 

*

***

***

 

*** based on the work of Daniele Inaudi, Role of Sensing and Measurement for Bridges.

 

Distributed strain monitoring system

A distributed fiber optic sensor (FOS) in a combination with monitoring system is justified to fulfill the aims of integrity monitoring at long-distance bridge structures. Implementation of fiber optic monitoring system guarantees safety and operational integrity, scalable and qualitative data as well as saving MRO costs.


The monitoring system features:


Multiple tasks performed in real time and along the entire length of the controlled structure, such as:
The stressed-deformed state of reinforced-concrete structures (static tension, com-pression, temperature distribution);
Identification and localization of cracks;
Loss of structural stability, tilting (~ up to 1 Hz) leading to misalignment of struc-ture’s design elements.


Diverse sensor installation methods:
Embedded in reinforced-concrete (mounted to reinforcement and then poured with concrete) ;
Glued to reinforced-concrete structure or in the groove (additional protection is achieved by applying steel or polymeric band);
Mounted on reinforced-concrete surface with mechanical fixators (additional pro-tection is achieved by applying steel or polymeric band).


Case study

It was decided to prevent replacement of the multi-span bridge illustrated in Fig.1 by installation of the monitoring system combined with uninterrupted fiber optic sensors (FOS). Optimized con-trol of the bridge behavior is translated into extended life over another 15 years. The total length of the girders is ~ 9 km, whilst length of the monitored supporting girders is ~ 4.5 km. Monitor-ing system performs a long-term integrity monitoring of new crack development and unusual strain development (polling every 2 hours).

Fig. 2. Stringent multi-span bridge GÖTAÄLV, Sweden (54 spans, total length 950 m).

 

 

 

 

   

 

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