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Abstract:Temperature, one of the most important parameters in building fires, is now mostly measured with hightemperature thermocouples, which have the typical drawbacks of electric sensors, such as their sensitivity to electrical and magnetic interference. Fiber optic sensors are an alternative to electric sensors and offer many advantages, although their use in fire engineering is somewhat limited at the present time. This paper presents a set of new fiber optic sensors for measuring high temperatures, based on Regenerated Fiber Bragg Gratings (RFBGs). The sensors were placed near the surface of two concrete specimens and then tested under ISO 834 fire curve conditions for one hour. We consider this an important step forward in the application of hightemperature fiber optic sensors in fire engineering, as the sensors were subjected to direct flames and temperature increments of the order of 200 ºC/min, similar to those in a real fire. The FBG sensors measured maximum gas temperatures of circa 970 ºC, in good agreement with those provided by thermocouples in the same position. The gas temperature measurements of the FOSs were also compared with the adiabatic temperatures measured by plate thermometers and concrete specimens surface temperatures calculated with numerical heat transfer models.

The response of bridges subject to fire is an under researched topic despite the number of bridge failures caused by fire. Since available data shows that steel girder bridges are especially vulnerable to fire, this papers delves into their fire response by analyzing with a 3D numerical model the response of a typical bridge of 12.20 m. span length. A parametric study is performed considering: (1) two possibilities for the axial restraint of the bridge deck, (2) four types of structural steel for the girders (carbon steel and stainless steel grades 1.4301, 1.4401, and 1.4462), (3) three different constitutive models for carbon steel, (4) four live loads, and (5) two alternative fire loads (the hydrocarbon fire defined by Eurocode 1 and a fire corresponding to a real fire event). Results show that restraint to deck expansion coming from an adjacent span or abutment should be considered in the numerical model. In addition, times to collapse are very small when the bridge girders are built with carbon steel (between 8.5 and 18 minutes) but they can almost double if stainless steel is used for the girders. Therefore, stainless steel is a material to consider for steel girder bridges in a high fire risk situation, especially if the bridge is located in a corrosive environment and its aesthetics deserves special attention. The methodology developed in this paper and the results obtained are useful for researchers and practitioners interested in developing and applying a performance-based approach for the design of bridges against fire.

Gas temperature is a key variable in many high temperature applications. Sensors for measuring gas temperatures must be selected according to many different criteria, response time being one of the most important. Response time quantifies the time that the sensor needs to react to a sudden temperature variation. When rapid temperature fluctuations are expected, as in the case of fire tests, significant instantaneous errors can occur if the sensor response time is longer than the duration of the temperature fluctuation. Despite the importance of response time, there is no general agreement on how to quantify this value in high temperature fiber optic sensors. This paper proposes a methodology to estimate the response time of fiber optic temperature sensors based on an analytical model of the heat transfer between the sensor and its surroundings. The method is validated by an experimental study. In addition, the response times of three different high temperature fiber optic sensors developed by the authors are compared with each other and with the response time of some widely used thermocouples. The results show i) that fiber optic sensors have a significantly shorter response time than thermocouples with similar packaging, ii) that the response time is shorter during the heating phase than the cooling phase, and iii) highlight the importance of considering this parameter in the sensor selection process.

Monitoring temperatures during tunnel fires is of major importance for both the firefighters extinguishing the fire, and the engineers in charge of the subsequent repair work. However, current methods of assessing fire damage have limitations when applied to tunnels and only provide estimates of the maximum fire temperatures at specific locations of the tunnel. This is not a desirable situation, as the temperature-time curves associated with the fire event should be available for use in assessing the residual strength of the tunnel structure. This is the key parameter in defining repair work and the length of time the tunnel will need to be closed and thus the socioeconomic cost of the tunnel fire. In addition, real-time recording of the temperature-time curves would provide valuable information to the firefighters engaged in extinguishing the fire. This paper presents a new general methodology for the optimal placement of sensors in a tunnel to obtain the temperature evolution at any point along its lining during a fire. The methodology was applied to the Virgolo Tunnel in Italy, in which 100 potential high-temperature sensor configurations were tested and a set of optimal sensor configurations was proposed. The results of the analysis show that: (a) the proper location of the sensors is crucial; (b) it is possible to define a set of sensor configurations that minimize the cost of the monitoring system and maximize the accuracy of the estimated temperatures; (c) it is important to place at least three high-temperature sensors in each monitored cross section (at the crown and symmetrically on the haunches/side walls). The proposed methodology improves tunnel resilience against fires, as it enables safer infrastructure and a faster and more economic recovery of the tunnel after a fire event.