Keywords: dynamic loads, vibrations, floor systems, perception criteria, vibration control.

Sports stadiums, discotheques, gymnasiums, aerobic dance studios, shopping malls, and airport terminal corridors are all subjected to significant dynamic loads produced by occupants either while remaining in one location or traversing the structure. Coherent crowd harmonic movements can produce resonant or near- resonant structural vibrations that are uncomfortable and intolerable for some occupants. There have been serviceability problems that required costly remodeling or revision of building regulations.

At the present, U.S. codes and standards are primarily concerned with avoiding structural failure (i.e., a strength requirement), and deal with excessive vibrations (i.e., a serviceability requirement) only to a limited degree. Empirical serviceability requirements usually do not involve the frequency of the loading or the natural frequency of the structure. This paper provides a survey of the historical developments in modeling human dynamic loads, perception criteria used in structural vibrations, and various techniques that are used to mitigate the human- induced vibrations. Brief summaries of these sections are included in this extended abstract, as follows.

Human-Induced Dynamic Loads

Live loads are produced by the use and occupancy of a structure. Human loads comprise the large portion of the live loads in floors of offices and residential buildings. In general, the human live loads are classified into two broad categories: in situ and moving. Tilden (1913) and Fuller (1924) were among the first researchers to experimentally quantify the dynamic load effects of individuals and groups, respectively. Tilden considered both in situ and moving loads. Fuller attempted to experimentally quantify the crowd dynamic effect due to a group of people on a gymnasium balcony. Tuan and Saul (1985) defined various types of in situ movements by measuring the load-time histories for individual subjects on a small piezoelectric force platform. Ebrahimpour and Sack (1989) used a large instrumented force platform to measure in situ loads by individuals and groups of two and four people. In a subsequent study, Ebrahimpour and Sack (1992) constructed a 3.7 m by 4.6 m floor system and measured forces of up to forty people performing in situ harmonic movements. They also recommended simple design values for coherent crowd harmonic movements. Only a very few studies of human moving loads have been reported. Canadian researchers have measured dynamic forces of individuals and small groups of people (Pernica, 1990). In a recent study, Ebrahimpour and Sack (1996) measured the input forces imposed by moving groups of people using a set of instrumented platforms, mathematically modeled the loads, performed simulations, and suggested simple design loads for serviceability criteria.

Human Perception of Structural Vibrations

The most frequently cited reference for human perception of vibration is by Reiher and Meister (1931). The modified Reiher-Meister scale was proposed by Lenzen (1966) for vibrations due to walking impact. For floors with less than 5% critical damping, he suggested the original scale be applied if the displacement is increased by a factor of ten. Wiss and Parmelee (1974) suggested that a constant product of frequency and displacement existed for a given combination of human response and damping. Allen and Rainer (1976) developed vibration criteria in terms of acceleration and damping intended for quiet human occupancies such as residential buildings and offices. Murray (1979) suggested a new human perception scale for required damping as a function of the product of initial displacement and frequency, which are the same parameters used in the Wiss-Parmelee scale. The International Standards Organization (ISO, 1989) recommends vibration limits in terms of acceleration root-mean-squared (rms) and frequency. A baseline curve is used by ISO and different multipliers are used for different occupancies. The paper categorizes the recent work in vibration serviceability criteria for floors into two broad categories. These are: criteria for steel beam and concrete slab construction, and wood/lightweight construction.

Structural Vibration Control

Floor vibration control may be achieved in several ways: passive, semi-active, and active. A tuned mass damper (TMD) is one way of passively controlling the floor vibration. Lenzen (1966) used a TMD to provide artificial damping in a floor to dampen the vibration in less than five cycles. The spring and dashpot unit had a frequency of about one cycle per second below the natural frequency of the floor and had a viscous damping equivalent of 7.5 Using a semi-active control scheme, Sack and Patten (1993) significantly reduced vibration of a 4 m narrow floor. The low-power device consisted of a closed hydraulic control system that used no pump but had an adjustable valve within the shock tube. Changing the opening size of the valve (at an extremely fast rate) provided the variable levels of damping. In another study, Hanagan (1998) developed an active electro-magnetic actuator that uses a piezoelectric velocity sensor and a feedback loop to generate control forces, thus adding damping to the supporting structure. Significant results were obtained on the office floor of a light manufacturing facility and a chemistry laboratory. Recently Ebrahimpour and Martell (2002, 2003) retrofitted laboratory-constructed floors with laminates of carbon fiber reinforced polymer (CFRP) and constrained layers of viscoelastic (VE) material. The CFRP and VE retrofit uses high damping with some increase in stiffness to improve the vibration performance of the floor. The damping ratio of the floor increased from 2.4% to 11.7% (i.e., by 388%) with the addition of CFRP and VE layers.

purchase the full-text of this paper (price £20)

go to the previous paper
go to the next paper
return to the table of contents
return to the book description
purchase this book (price £123 +P&P)