Effects of Bottom Chord Extensions on the Static and Dynamic Performance of Steel Joist Supported Floors

Abstract

The use of lightweight and high strength materials in building construction have resulted in larger bays and longer spans in floor systems. Lighter materials in addition to the trend towards a paperless office decreases damping and the amount of live load on the floors. As a result, floors have become more vulnerable to annoying vibrations induced by human occupancy. The number of complaints by occupants has increased in recent years and floor vibrations have become an area of serviceability concern. Activities like walking, dancing, running, jumping, aerobics, etc. generate floor vibrations and humans have different tolerance levels for these vibrations. Because humans are both the source and the sensor, human-induced vibration can not be isolated from the structure; vibration must be controlled by the floor system. Extensive research has been conducted on human perceptibility of floor vibrations and dynamic behavior of floor systems. The procedures in the AISC/CISC Design Guide 11- Floor Vibrations Due to Human Activity are available for designers to determine acceptable acceleration levels and minimize excessive floor vibrations. The best approach is to design floors that do not allow annoying vibrations, as remedies for floors susceptible to excessive floor vibrations are very expensive. Possible modifications of existing problem floors include adding mass, increasing damping (using partitions, damping posts or tuned mass dampers) and increasing structural stiffness of the floor systems. Open web steel joists are very popular in steel building construction. The open web allows the duct work to be run through the web and maintenance can be done easily after the construction is completed. This makes open web steel joists very feasible and economical; however, vibrations of joist supported floor systems, and in particular, the effect of extended bottom chords, are still not very well understood. Extending joist bottom chords in an attempt to improve problem floors has been reported but without success. However, these retrofits were done after fit-out of the buildings and without introducing a preload into the extensions. The purpose of this study is to examine the effect of bottom chord extensions on deflections and vibration characteristics of joist supported floor systems when joist bottom chord extensions are installed (a) before the concrete was placed, (b) after the fit-out with jacking for a single span footbridge, and (c) after the fit-out without jacking for a three span footbridge. Extensive analytical and experimental studies are performed on two laboratory footbridges with different bottom chord extension configurations to understand the effect of extended bottom chords on deflection, natural frequency, damping, mode shape and effective mass. Finite element computer models are created to simulate and compare the results of stiffness and vibration tests.