**1. Introduction**

The bridges, which link past to present and age gracefully, are one of the most important engineering structures. The bridges with different characteristics, thanks to their views, effects, and feelings during passing on, holds around and locations bring together the people for ages. In early applications, the bridges were designed as short span and narrow with stone and wood

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

materials, and be able to carry light loads. But, nowadays, these conventional bridges have been replaced to steel and reinforced concrete.

There are various bridge types constructed during the last century according to the carrier system type, span lengths, and material properties such as masonry arch bridges, long span concrete/steel/composite highway bridges, base isolated bridges, footbridges, steel bridges, suspension bridges, cable‐stayed bridges, and wooden/timber bridges. Masonry bridges have been built worldwide for social, economic, and strategic purposes. Originally intended to carry only pedestrian and horse‐drawn vehicles, many of these historical bridges currently serve as critical components of transportation systems and, thus, must withstand significantly larger loads. Among various types of civil engineering structures, long span highway bridges, which are commonly used for passing large rivers, dam reservoirs, and deep valleys, attract the greatest interest for study particularly in terms of structural performance. Footbridges are generally situated to allow pedestrians to cross water or railways in areas where there are no nearby roads to necessitate a road bridge, and also across busy roads to let pedestrians cross safely without slowing down the traffic. Steel offers many advantages to the bridge builder, not only the material itself, but also its broad architectural possibilities such as high strength‐ to‐weight ratio, high‐quality material, speed of construction, versatility, modifications, recycling, durability, and aesthetics. Suspension and cable‐stayed bridges are widely used across long spans (>550 m) and give rise to the usage of domains under the bridge. For this reason, the uses of suspension and cable‐stayed bridges have increased recently. Wood is one of the most used and common materials for bridge constructions from the ancient times when humans first started finding ways on how to cross rivers and hard terrains.

Determination of dynamic response of bridges under static and dynamic loads, such as wind, earthquake, or traffic, is very complex and requires special studies. Finite element method has been widely used in civil engineering application since 1950s. Static, dynamic, linear, and nonlinear behavior can be obtained and illustrated using this method. It is generally expected that finite element models (FEMs) based on technical design data and engineering judgments can yield reliable simulation. However, because of modeling uncertainties, these models often cannot predict dynamic characteristics with the required level of accuracy. This raises the need for verification of finite element models using nondestructive experimental measurement tests.

There are two basically different methods available to experimentally identify the dynamic system parameters of a structure: experimental modal analysis (EMA) and operational modal analysis (OMA). In the EMA, the structure is excited by known input forces and the structural behavior is evaluated. In the OMA, the ambient vibrations such as vehicle load, wind, or wave loads have been used to actuate the structures. Heavy forced excitations may become expensive and sometimes may cause damage to the structure. Ambient excitations and their combination are environmental or natural excitations. Structural identification using this method gains the major importance. In this case, only response data of ambient vibrations are measurable while actual loading conditions are unknown. A system identification procedure will therefore need to base itself on output‐only data.

It is well accepted that the finite element model updating is used to minimize the differences between analytically and experimentally determined dynamic characteristics by changing some uncertain parameters such as material properties, boundary conditions, section and connection details, and some additional structural elements and weights. In the finite element model updating, determination of the uncertain parameters and their ratios/values can be decided according to the nondestructive testing methods such as visual inspection, half‐cell electrical potential method, Schmidt rebound hammer test, carbonation depth measurement test, permeability test, penetration resistance or Windsor probe test, resistivity measurement, electromagnetic methods, radiographic testing, ultrasonic testing, infrared thermography, ground penetrating radar, radioisotope gauges, acoustics emission, computed tomography, strain sensing, and corrosion rate measurement. The detailed information can be found in the related literature.
