Effect of Bottom Chord Extensions on the Static Flexural Stiffness of Open-Web Steel Joists

Annoying floor vibrations, a growing issue in today’s world of lighter structural members and low inherent damping because of modern office fit-outs, may also be significant in open web steel joist supported floor systems. In the presence of these vibrations, sometimes bottom chord extensions are installed in an attempt to stiffen the floor and correct the annoying vibrations problem. This study focuses on the effect of bottom chord extensions on the static flexural stiffness of joists under relatively light loads associated with occupant-induced floor vibrations. Extensive analytical and experimental 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 the stiffness tests. Testing was done with (1) the bottom chord extensions installed prior to concrete placement, (2) all or part of the bottom chord extensions removed, and (3) the bottom chord extensions reinstalled with jacking for the single-span footbridge and without jacking for the three-span footbridge. The primary objective was to determine the possible increase in flexural stiffness when bottom chord extensions are installed in an attempt to reduce annoying floor vibrations. The experimental results for the two footbridges with bottom chord extensions installed after the concrete placement are compared with results when the bottom chord extensions are installed prior to concrete placement and to footbridges without bottom chord extensions. Results from the stiffness tests on a single-span footbridge indicate that installing the bottom chord extensions to the joists after concrete placement and with the center of the span jacked up increases the flexural stiffness considerably, but not as much as installing the bottom chord extensions before the concrete placement. Similar results were found when bottom chord extensions were installed after concrete placement but without jacking on a three-span footbridge. The magnitude of the increases in flexural stiffness was sufficient to consider the installation of bottom chord extensions as a remedy to stiffen a floor and reduce floor vibrations because of occupancy movements.