## 2 Modal Based Damage Detection: Development & Challenges
### Problematic Effects of Environmental & Operational Conditions  
External conditions affect changes to a bridge's stiffness and mass that in turn vary its modal parameters. Temperature causes expansion and contraction, resulting in observed daily frequency shifts of 5% and seasonal shifts of well over 10% due to temperature fluctuations [75; 76; 77; 78]. Moisture content can also alter the temperature's effect, which [PERSON]'s [75] discovered when a bi-linear distribution of modal frequencies was observed for the Z-24 Bridge centred around freezing point; the cause of which was attributed to the freezing moisture content in the bridge deck and supports, which contributed to the global stiffness of the structure. Generally, an increase in temperature will cause modal frequencies to drop, with flexural modes being more sensitive than torsional modes [79]. Changes in frequency due to temperature can often be mistaken for structural damage [80; 81] and, in some cases, can also mask true damage events, as [PERSON] et al. [82] discovered when investigating a bridge girder subjected to incrementally induced damage events. The expectations of the assessment were that the inflicted damage would reduce the girder's stiffness and thus reduce its natural frequencies. However, the actual observed results did not match expectations; instead, the girder's natural frequency rose for the first two damage scenarios, before falling. It was later discovered that the ambient temperature in the laboratory caused the initial increase in the girder's frequency. Daily operational variability can be a more dominant driver of frequency fluctuation than daily environmental variability [83], with over 5% of daily frequency fluctuation being due to the operational conditions observed [84]. [PERSON] et al. [85] observed that mass variations induced by traffic loading affect shorter span bridges much more than stiffness variations induced by environmental conditions, as shorter span bridges are less susceptible to ambient excitations. In another study, [PERSON] et al. [86] demonstrated that natural frequency variations due to traffic loads were almost negligible for middle to long span bridges. This is due to the mass ratio between the overall bridge structure and the vehicle, and implies that heavier bridges are less affected by variations in mass, which has been reiterated by numerous authors [87; 88; 89; 90]. Other concerns regarding operational conditions relate to their fatigue inducing stress cycles in welded connections. [PERSON] et al. [91] highlighted that this issue is of particular interest in orthotropic steel decks, where their complex geometry coupled with the stochastic nature of traffic loads result in complex stress fields. [PERSON] and [PERSON] [92] observed a highly linear correlation between the traffic volume and number of stress cycles in orthotropic steel deck joints, especially for rib-to-deck welded joints. A similar correlation between stress amplitude and ambient temperature was also observed and attributed to temperature-induced changes in asphalt material properties. Traffic-induced modal variation can be considered to occur in both static and dynamic form. Static variations can be directly related to mass; however, traffic-induced dynamic variation has been shown to be nonlinear and can decrease with an increasing load effect [93]. This further complicates the vibration-based monitoring of in-service bridges, as perceived changes in a bridge's modal parameters may actually be the interaction response of a healthy bridge with a passing vehicle [85]. This relationship is difficult to discern, as it is based on a number of variable factors such as, vehicle weight, velocity, number of vehicles, bridge weight, and temperature. Furthermore, [PERSON] et al. [94] observed that the amplification of frequency response due to a single passing vehicle is considerably more than for two vehicles simultaneously passing in opposite directions, thereby indicating the influence of vehicle direction.