Effects of end restraint on eyebar behavior in bridges

Abstract

Many steel truss railroad bridges use tension members consisting of eyebars. After years of service, these eyebars may exhibit significant wear at the pin holes, leading to the development of unequal tension stresses among eyebars within the same set. To restore bridge performance, eyebar tensions are often equalized through flame-shortening, where the tension in any eyebar is determined by measuring its fundamental natural frequency and then converting this frequency to the corresponding tension through a simple analytical model. In practice this model assumes that the ends of the eyebar are pinned; however, for transverse vibration about the minor axis, a significant degree of rotational restraint will often exist, potentially resulting in very inaccurate tension estimates. This paper presents the results of analytical studies, model testing, and field observations that focus on improving the prediction of eyebar tensions from observed natural frequencies. Besides supporting the flame-shortening process, the ability to achieve greater accuracy in tension measurements also provides for valuable applications to bridge rating.