Structural health monitoring of continuous prestressed concrete bridges using ambient thermal responses Nonthachart Kulprapha ⇑ , Pennung Warnitchai School of Engineering and Technology, Asian Institute of Technology, P.O. Box 4, Klong Luang, Pathumthani 12120, Thailand article info Article history: Received 23 September 2010 Revised 31 January 2012 Accepted 2 February 2012 Available online 22 March 2012 Keywords: Structural health monitoring (SHM) Damage detection Ambient thermal responses Continuous prestressed concrete bridges abstract The feasibility to monitor the structural health of multi-span prestressed concrete bridges by using their ambient thermal loads and responses is investigated. An 8-m-long, 2-span continuous concrete bridge is designed and constructed to represent typical full-scale bridges. The bridge is equipped with various sen- sors to continuously monitor temperatures, strains, deflections, and support reaction forces, and is exposed directly to sunlight, rain, wind, and dewfall in order to attain a realistic ambient thermal loading condition. Five states of distributed flexural damage, ranging from slight to severe, are created in the experimental bridge by applying two overloaded concentrated forces. At every damage state as well as the initial undamaged state, the thermal loads and responses of the bridge are continuously monitored for several days, and these responses are compared with those predicted by a newly developed analytical model. This model takes into account of nonlinear temperature distribution in bridge cross sections, pres- ence of prestressing forces, support flexibility, and initial crookness of bridge span. An excellent agree- ment between model predictions and monitored responses are obtained for the initial undamaged state. The monitored responses in five damage states are again compared with the model predictions of the undamaged state. As expected, the discrepancy between them increases with increasing of damage level. A scheme to account for distributed flexural damage is then developed for the analytical model. By using this scheme, it is possible to tune the model predictions to match with the monitored responses. Through this model tuning process, approximate damage distribution pattern and damage severity along the entire bridge length can be identified. The study clearly demonstrates that an effective structural health monitoring system based on ambient thermal loads and responses can be successfully developed for multi-span prestressed concrete bridges. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction To properly maintain the serviceability and safety condition of a major bridge over its long expected service life, it would be ideal to have a reliable measure to regularly access the structural condition of the bridge, in which the premature aging, structural deteriora- tion, or reduction in load-carrying capacity of the bridge could be accurately diagnosed. The conventional measures such as periodic visual inspections and controlled load tests are, however, known to have limitations and shortcomings. A new technology called struc- tural health monitoring (SHM) has recently gained more attention as a promising new measure in this regard. Several existing SHM systems detect structural deterioration of the bridge by observing changes in its dynamic characteristics, e.g. shifts in natural frequencies and changes in vibration mode shapes. Since the characteristics of fundamental vibration modes with low frequencies are not so sensitive to minor structural changes, the monitoring system has to cover higher frequency modes to detect such changes [1]. This inevitably requires a consid- erable number of high sensitivity transducers and a high sampling rate data acquisition system [2]. A rather complicated data analysis procedure is also required to identify the changes in dynamic char- acteristics [2]. Moreover, the changes in natural frequencies caused by structural deterioration can be easily masked by normal envi- ronmental effects, mainly those from temperature and humidity changes [1–4]. These difficulties and limitations are the motivation for exploring other approaches for SHM. In this paper, the feasibility of SHM based on ambient thermal responses is investigated. The approach is thought to be able to overcome the difficulties and limitations mentioned above. Solar radiation and heat exchange with the surrounding environment cause change in temperature in each part of the bridge. Such change occurs continuously and slowly as a diurnal phenomenon [5,6]. Difference in temperature among various parts causes ambi- ent thermal responses of the bridge, which include thermal- induced deformation, stress, and variation of reactions at supports [7]. Since these responses change gradually, they can be easily 0141-0296/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.engstruct.2012.02.001 ⇑ Corresponding author. Tel.: +66 2 524 6415; fax: +66 2 524 6059. E-mail address: st100300@ait.ac.th (N. Kulprapha). Engineering Structures 40 (2012) 20–38 Contents lists available at SciVerse ScienceDirect Engineering Structures journal homepage: www.elsevier.com/locate/engstruct