# Non-Destructive Testing Applications for Steel Bridges
## 1 Introduction  
Because of an increased reliance of communities and businesses on surface transportation, and with the presence of ever-increasing environmental impacts caused by aging, maintaining the safety and reliability of civil infrastructures has become a critical issue. Among all civil infrastructures, bridges can be deemed as the most affected type. From over 617,000 accounted bridges in the United States, about 42% of those bridges are at least 50 years old, and 7.5% of them are considered structurally deficient. Although the number of structurally deficient bridges has been declining over the past 50 years, the rate of their improvement is still considered slow [1]. Out of the total number of bridges in the United States, around 33% are steel bridges [2]. A similar condition also exists for bridges in other parts of the world. Throughout the years, steel bridges have been evolved and adapted by their designers, eliminating issues that were consistently reoccurring in the designs. However, there are still many issues that can decrease the structural integrity of a steel bridge, with corrosion being the most prevalent, followed by fatigue and fracture in sensitive connections, some exacerbated by welding issues. Accordingly, bridges must be inspected periodically for the on-time detection of possible structural damage and apply the needed repairs before the damage becomes significant and costly. The National Bridge Inspection Standards (NBIS) mandates that all bridges with a span of longer than20 ft and located on public roads need to be inspected every 24 months by a qualified bridge inspector [1,3].  
Over the years, various techniques have been implemented for the structural health monitoring of the bridge systems. Among various methods, non-destructive testing (NDT) methods incorporate the non-invasive testing of structural components and structural systems without causing any damage, changing their material composition or shape of the inspected components. The Federal Highway Administration (FHWA) also defines NDT as a \"means of analyzing and assessing the condition of various structural components of in-service highway infrastructure assets--pavement, bridges, and tunnels--without impairing their future usefulness\" [4]. These methods bring an innovative and rapid mode of inspection with which to analyze and diagnose structural deficiencies such as cracks, fatigues, delamination, voids, and corrosion. Various non-destructive testing (NDT) methods have been implemented for the inspection of bridge elements. The required type of NDT method differs from one bridge to the next, depending on the structural type, the environment, and the preference of the owner.  
This paper compiles in one place all the information regarding the application of NDT methods for steel bridges. Although both global and local damage detection methods are discussed, the focus of this paper is on the methods applicable to local NDT methods. As such, the methodology, advantages, and disadvantages of each technique are thoroughly investigated. Furthermore, recent innovations in this regard, including the application of drones, sensors, or robots for rapid and efficient assessment of damages on small and large scales, are presented. This is an attempt to compile in one place all the NDT techniques, including state-of-the-art and advanced techniques, for the inspection of steel bridges. It aims to keep researchers up-to-date with existing methods and pave the way for further innovations in this regard. Importantly, this study can help engineers and inspectors select the most proper NDT methods at the site, potentially saving considerable cost and time.