Analysis of floor vibration problems that compromise tenant comfort is complicated; retrofit solutions are expensive and largely untested. Dr. Avci’s much-needed research focuses on extending bottom chords of popular and economical joist-supported floors.

New York, NY - “Effects of Bottom Chord Extensions in the Static and Dynamic Performance of Steel Joist Support Floors,” by Dr. Onur Avci of Weidlinger Associates, was chosen “Best Overall Paper” at the 2008 conference of the Architectural Engineering Institute, held in Denver, Colorado, on September 24-27. Avci’s coauthors were his dissertation advisors at the Virginia Polytechnic Institute (Virginia Tech) in Blacksburg, Virginia: Drs. Mehdi Setareh of the School of Architecture and Design and Thomas M. Murray of the Department of Civil and Environmental Engineering.

Dr. Mohammed Ettouney, who chaired the conference, explained the jury’s choice: “This paper is exemplary for several reasons. Floor vibrations are a growing problem and one that affects immediate use. Vibrations of joist-supported floor systems haven’t been studied and tested extensively, despite their widespread use in steel construction. And, the report itself is concise but comprehensive. It discusses the basic concepts and the process of testing and analysis; it offers guidelines to design engineers; and it suggests avenues of further research.”

Multihazard engineering research typically focuses on the structural effects of earthquakes, hurricanes, and wind, which are only potential problems, whereas floor vibration problems are a serviceability issue that affects owners’ profits directly. A building can become uninhabitable because of tenant discomfort, and even costly retrofits can be unsuccessful. One day, an optimized building design may include analysis to ensure that vibrations are controlled, but more research is needed. Although there are published guidelines on human perception of excessive vibrations and design of structures to mitigate their effects (AISC/CISC Design Guide 11- Floor Vibrations Due to Human Activity, Murray et al., 1997), they are not up to date on joist-supported floors.

Designers have seized the opportunity presented by lightweight concrete and high-strength steel to enlarge bays and lengthen floor spans, but the reduced mass and damping of these floors make them susceptible to excessive vibrations. When tenant discomfort results, the remedies are to add mass, increase damping (with partitions, damping posts, or tuned mass dampers), or increase the structural stiffness of the floor system. That explains why 21st-century “paperless” offices, which provide less live load and/or damping than offices in the past, add to the problem.The analysis is complicated, because human comfort levels vary and are unpredictable. Also difficult to predict is the level of floor acceleration response, even by the latest finite-element models.

Avci’s paper focuses on floors supported by open-web steel joists, which are economical because the ducts that thread through the web are easily accessed for post-construction maintenance. Unfortunately, they are also subject to vibration problems, the cures of which are not well understood. Extending joist bottom chords is a mildly successful but mostly unexplored remedy.

To conduct this research, Avci built two laboratory footbridges with different bottom chord extension configurations and submitted them to extensive analytical and experimental studies to understand the configurations’ effects on deflection, natural frequency, damping, and mode shape. Finite element models were created to simulate and compare the results of stiffness and vibration tests. Re-installing the bottom chord extensions to the footbridge joists reduced deflections; but placing the bottom chord extensions before placing the concrete gave better results. It also increased the natural frequencies for all three governing bending modes, which was intended. Although the extensions helped to stiffen the footbridges, one cannot assume that acceleration levels and peak vibrations decrease as a result.

The 10-year-old Architectural Engineering Institute (AEI) is one of seven institutes sponsored by the American Society of Civil Engineers, bringing together members of the structural, mechanical, electrical, and architectural engineering communities to address problems of interest to all. More than forty industry experts presented technical research papers at the AEI 2008 conference. Avci received his BS in Civil Engineering from Middle East Technical University (METU) in Ankara, Turkey. During and after graduate studies at Virginia Tech, he published reports and papers on a variety of structural engineering subjects, including cold-formed steel decks, roof systems, and composite floors.