Floor Vibrations

Construction technology advancements in the last couple of decades have led to the use of lightweight and high-strength materials in structural systems. Although longer spans and lighter materials result in floor systems with less mass, stiffness, and damping , the trend toward a paperless office decreases damping and the amount of live load on the floors even more. Consequently, structures have become more vulnerable to annoying vibrations, and vibration serviceability has become an area of serviceability concern. For vibration serviceability calculations, the damping value of the structural systems is a critical parameter. Damping in structures has proved to be dependent on the amplitude of the applied force on the structure. This condition is referred to as nonlinear damping, or amplitude-dependent damping. Although damping is constant at low and high amplitudes, for in-between amplitudes, the damping value increases with the levels of excitation amplitude. For wind and earthquake excitations, the amplitude-dependent characteristics of damping have been studied extensively in the literature. For floor vibration serviceability applications, even though the nonlinear behavior of damping has been accepted to exist and mentioned in some publications, it is not closely looked at or discussed in detail. The floor vibration serviceability calculations are very sensitive to damping values, but vibration serviceability researchers and practicing engineers are often uncomfortable with assigning a specific number as a damping ratio for a specific mode because of the inconsistency of damping values obtained from different methods. This paper presents a closer look at the amplitude-dependent damping in vibration serviceability and focuses on a laboratory footbridge with experimental and analytical studies. The laboratory footbridge was studied extensively with static and dynamic tests. Three-dimensional finite-element (FE) models were developed, updated, and fine-tuned for two bottom chord extension configurations for both static and dynamic tests. The amplitude-dependent damping behavior of the laboratory footbridge is shown for different amplitudes of sinusoidal excitations. The amplitude-dependent damping ratio values obtained from effective mass calculations proved to be correct with the FE model acceleration predictions. The FE model predictions successfully matched the test results with the nonlinear characteristic introduced for modal damping. One of the most difficult tasks in vibration serv-iceability research is matching the measured acceleration responses with the FE models, and the success of this paper in matching the acceleration responses for various levels of excitations (with corresponding amplitude-dependent damping values) with the FE model is unique. Successful verification and clarification of the amplitude-dependent phenomenon and FE model matching of measured acceleration responses reinforce the confidence in the FE models in vibration serviceability research by showing that the FE models are reliable not only for natural frequency predictions but also for acceleration response predictions.

Slender monumental stairs are major architectural features in many high-end building structures. Architectural requirements for these are usually very aggressive, with long spans and slender stringers being the norm. The resulting low natural frequencies are often within reach of the lower harmonics (which have high amplitudes) of the vertical force caused by a person descending the stair. Also, some configurations with very low frequency lateral vibration modes might allow high lateral accelerations. Because the potential for annoying vibrations is high, it is critical that stair designers have access to evaluation methods. Current evaluation methods rely on finite-element analysis-based response prediction methods that are outside the reach of many structural engineering firms. This paper presents a simplified vertical acceleration prediction method, based on the fundamental natural mode of the stair, which is suitable for manual calculations. Response predictions are compared to experimental measurements to calibrate the prediction method for design use. Lateral vibrations are discussed, and an evaluation method is proposed. Finally, a numerical example is provided. DOI: 10.1061/(ASCE)ST.1943-541X .0001256. © 2015 American Society of Civil Engineers.

Floor vibration is widely recognized as an important limit state in the design of steel-framed floors, and various evaluation methods have been developed over the years. These methods range from simplified methods that are suitable for routine usage during the design phase to complex computerized or probabilistic models. The former are in common usage in the structural design community. It is critically important to structural engineers that the accuracies of the commonly used methods are known; however, a study of these methods based on a large database of recorded observations is not available in the literature. Therefore, the authors investigated the evaluation accuracy of four well-known simplified methods by comparing predicted and observed acceptability (whether or not occupants complained) of 50 floor bays in real buildings framed with W-shaped members subjected to walking excitations. The percentage of correct predictions is used to judge the accuracy of each method. It is observed that the AISC Design Guide 11 accurately predicted acceptability of floors; two methods from the SCI P354 (Simplified Method and Vibration Dose Value Method) fairly accurately predicted acceptability of floors. The HIVOSS method provided unconservative predictions. In addition, modified version of the P354 method was investigated in an attempt to improve the accuracy of these methods.

With the use of lighter construction materials, more slender architectural designs, and open floor plans resulting in low damping, vibration serviceability has become a dominant design criterion for structural engineers worldwide. In principle, assessment of floor vibration serviceability requires a proper consideration of three key issues: excitation source, system, and receiver. Walking is usually the dominant human excitation for building floors. This paper provides a comprehensive review of a considerable number of references dealing with experimental measurement and mathematical modeling of dynamic forces induced by a single pedestrian. The historical development of walking force modeling—from single harmonic loads to extremely complex stochastic processes— is discussed. As a conclusion to this effort, it is suggested that less reliance should be made by the industry on the deterministic force models, since they have been shown to be overly conservative. Alternatively, due to the random nature of human walking, probabil-istic force models seem to be more realistic, while more research is needed to achieve enough confidence to implement in design practice.

Walking-induced loads on office floors can generate unwanted vibrations. The current multi-person loading models are limited since they do not take into account nondeterministic factors such as pacing rates, walking paths, obstacles in walking paths, busyness of floors, stride lengths, and interactions among the occupants. This study proposes a novel video-vibration monitoring system to investigate the complex human walking patterns on floors. The system is capable of capturing occupant movements on the floor with cameras, and extracting walking trajectories using image processing techniques. To demonstrate its capabilities, the system was installed on a real office floor and resulting trajectories were statistically analyzed to identify the actual walking patterns, paths, pacing rates, and busyness of the floor with respect to time. The correlation between the vibration levels measured by the wireless sensors and the trajectories extracted from the video recordings were also investigated. The results showed that the proposed video-vibration monitoring system has strong potential to be used in training data-driven crowd models, which can be used in future studies to generate realistic multi-person loading scenarios.

Disturbing walking-induced vibrations have been observed more frequently in recent times on long span lightweight floor systems as evidenced by the development of a number of new design guidelines for floor vibration assessment. This paper discusses a simple probability-based vibration analysis of a real office composite floor, taking into account the variability in walking excitation and dynamic characteristics of the floor. Some aspects of randomness in gait parameters are determined via a statistical analysis of measured gait data obtained from a biomedical research program; and the likely change in serviceability load and the uncertainty in the estimation of floor damping and frequency are considered. Consequently, the probability distribution of the floor response is determined with good agreement between the predicted and measured floor responses. However, response levels can be translated inconsistently in terms of human comfort by various acceptance criteria.

Abstract Floor vibration is widely recognized as an important limit state in the design of steel-framed floors, and various evaluation methods have been developed over the years. These methods range from simplified methods that are suitable for routine usage during the design phase to complex computerized or probabilistic models. The former are in common usage in the structural design community. It is critically important to structural engineers that the accuracies of the commonly used methods are known; however, a study of these methods based on a large database of recorded observations is not available in the literature. Therefore, the authors investigated the evaluation accuracy of four well-known simplified methods by comparing predicted and observed acceptability (whether or not occupants complained) of 50 floor bays in real buildings framed with W-shaped members subjected to walking excitations. The percentage of correct predictions is used to judge the accuracy of each method. It is observed that the AISC Design Guide 11 accurately predicted acceptability of floors; two methods from the SCI P354 (Simplified Method and Vibration Dose Value Method) fairly accurately predicted acceptability of floors. The HIVOSS method provided unconservative predictions. In addition, modified version of the P354 method was investigated in an attempt to improve the accuracy of these methods.