Modal Parameter Variations due to Joist Bottom Chord Extension Installations on Laboratory Footbridges

For steel-framed structural systems, strength and deflections have been the two fundamental limit states for the design. However, the use of high-strength steel, lighter sections, and low inherent damping resulted in more lively structures, and floor vibration serviceability has become a governing factor for structural design. While annoying floor vibrations can be significant in open web steel joist-supported footbridges; a common application in the North American construction industry is the installation of bottom chord extensions in an attempt to strengthen the floor and increase the natural frequency of the floor systems. Even though extending joist bottom chords is very common, there has not been any publication on the topic studying the dynamic effects of this application. The study presented in this paper aims to fill this gap and identify the effect of bottom chord extensions on the modal parameters of joist-supported structures, by studying two laboratory footbridges. The goal is not to solve a potential vibration problem of a footbridge but to identify the modal parameter variations due to bottom chord extension installations. Extensive experimental and analytical studies were conducted on single-span and three-span joist-supported laboratory footbridges with different bottom chord extension configurations. Three-dimensional finite-element computer models were created to simulate and compare the results of stiffness and dynamic tests with the philosophy of testing the footbridges and updating the finite-element models, thus minimizing the discrepancies between the test results and the finite-element models for better prediction. Results from the static and dynamic tests on the single-span and three-span footbridges indicate that installing the bottom chord extensions to the joists increases the natural frequencies of the flexural stiffness and flexural bending. Considering the low cost, and quick and easy application at the construction site, extending bottom chords can be considered as an option to reduce the deflections and increase the governing bending mode natural frequencies of footbridges. While bottom chord extensions do not guarantee that the vibration problem (if any) would be solved, with the higher natural frequencies, the footbridges are relatively less vulnerable to be put in resonance by human induced excitations. As a result, extending bottom chord extensions is an advantage; however, the acceleration response of the footbridges needs to be checked separately, for conformance with the code limits.