FOOTFALL INDUCED VIBRATION IN LONGSPAN COMPOSITE STEEL BEAMS USED IN TWO PROJECTS AT THE UNIVERSITY OF AUCKLAND
Author: V.N.Patel and R.J.Built | Size: 1.3 MB | Format:PDF | Quality:Unspecified | Publisher: Steel Innovations Conference 2013 Christchurch, New Zealand 21-22 February 2013 | Year: 2013 | pages: 13
Floor vibration due to human activity has become increasingly recognised by structural engineers, architects, and building owners as an inherent issue in long-span steel framed floor systems. In the past, attention was primarily focused on strength and deflection serviceability limits. However, as designers seek to push the limits on structural spans, grid spacings and adopt light-weight, low damping structural steelwork floor systems, more detailed consideration is required of the design tools and processes available to analyse and predict the vibration performance of floor systems.
Selection from published criteria of an “acceptable” vibration limit is sometimes possible depending upon the intended use of the space and the availability of manufacturers’ data for any vibration sensitive equipment. Building Owners and User Groups often have little understanding or quantitative “feel” for what performance the proposed “acceptable vibration limit” actually represents.
The theoretical predication of vibration performance against actual measured performance can sometimes vary significantly. This can lead to dispute post-construction as to whether the floor has an “acceptable” level of vibration. Post construction remediation of a space that is deemed to be “too lively” is often difficult, therefore, it is important that the vibration design criteria proposed are discussed and agreed and the limits of theoretical predications of vibration performance are clearly understood by all parties at the outset.
Beca Carter Hollings and Ferner Ltd (Beca) are currently in the process of designing two projects at the University of Auckland, utilising long-span partial-composite cellular steel beams. Both buildings will utilise existing structural frame layouts and foundations. As the new structure is to be built on the existing foundations, there is a necessity to keep it as lightweight as possible. The question of vibration sensitivity has been raised as a potential issue as both buildings contain research laboratories. An in-depth investigation has been conducted into the factors affecting vibration performance in order to give the Client and User Groups confidence that footfall induced vibration will not be an issue with the proposed floor structure.
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Over the last years, the trend in footbridge design has been towards greater spans
and increased flexibility and lightness. As a consequence, stiffness and mass have
decreased which has lead to smaller natural frequencies and more sensitivity to
dynamic loads. Many footbridges have natural frequencies that coincide with the
dominant frequencies of the pedestrian-induced load and therefore they have a potential
to suffer excessive vibrations under dynamic loads induced by pedestrians.
The main focus of this thesis was on the vertical and horizontal forces that
pedestrians impart to a footbridge and how these loads can be modelled to be used
in the dynamic design of footbridges. The work was divided into four subtasks. A
literature study of dynamic loads induced by pedestrians was performed. Design
criteria and load models proposed by four widely used standards were introduced
and a comparison was made. Dynamic analysis of the London Millennium Bridge
was performed using both an MDOF-model and an SDOF-model. Finally, available
solutions to vibration problems and improvements of design procedures were studied.
The standards studied in this thesis all propose similar serviceability criteria
for vertical vibrations. However, only two of them propose criteria for horizontal
vibrations. Some of these standards introduce load models for pedestrian loads
applicable for simplified structures. Load modelling for more complex structures,
on the other hand, are most often left to the designer.
Dynamic analysis of the London Millennium Bridge according to British and
International standards indicated good serviceability. An attempt to model the
horizontal load imposed by a group or a crowd of pedestrians resulted in accelerations that exceeded serviceability criteria.
The most effective way to solve vibration problems is to increase damping by installing a damping system. Several formulas have been set forth in order to calculate
the amount of damping required to solve vibration problems. However, more
data from existing lively footbridges is needed to verify these formulas.
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Author: Delft University of Technology The Netherlands Faculty of Civil Engineering and Geosciences Department of Design & Construction Section of Structural Engineering Arup Brisbane, Australia Infrastructure, Civil Structures | Size: 18.6 MB | Format:PDF | Quality:Unspecified | Year: 2009 | pages: 140
The last few decades the community demands for more interesting bridges. Improvements in material properties, design methods, building techniques and the involvement of
architects led to longer and slender footbridges. These bridges tend to be more sensitive to dynamic forces induced by pedestrians, resulting in vibrations of the bridge deck. These vibrations can in some cases attain high proportions, especially when the walking pace of the pedestrians approaches the natural frequency of the bridge. Such a case could result in a situation where the pedestrian feel uncomfortable or even unsafe. This topic has thus become an important issue for the Serviceability Limit State of footbridges.
Some Codes of practice nowadays refer to this topic but it is still a developing field.
Dynamic analyses during the design phase have become inevitable. This report compares and evaluates three load models described in the codes (or proposal for the codes):
Proposal Annex C (to EN 1991-2:2003), the British National Annex (to EN 1991-2:2003) and the Australian Standard (AS 5100.2-2004). All three codes have different approaches to this topic. Proposal Annex C considers walking pedestrians (single and groups) and crowds and represent all of them by non moving harmonic loads. The British National Annex also considers joggers which have total different walking pattern. The fundamental difference with the load models described in Proposal Annex C is that the loads are represented by moving harmonic loads. So does the Australian Code, but this one only considers the model of a single pedestrian.
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Research (academic and industrial) and healthcare facilities contain a number of equipment and procedures that are sensitive to footfall-induced vibrations. Vibration thresholds for these equipment and procedures impose demands on the structural design beyond the typical requirements for strength and serviceability. Vibration engineers typically conduct site measurements and numerical modeling of the floor to assist with the design. The development of practical and accurate numerical models for estimating floor responses is an important component of this process.
Variations in key modeling assumptions can produce impractical and potentially incorrect design recommendations. It is therefore important to be aware of the impacts of modeling assumptions on response predictions. These impacts can be effectively assessed through comparisons of experimental and full scale measurements with numerical prediction models.
In this study, measured floor vibration data from an existing laboratory are used for the development and evaluation of numerical prediction models for footfall vibrations. An existing vibration-sensitive laboratory floor was measured in three configurations: un-furnished pre-retrofit, unfurnished post-retrofit and furnished postretrofit.
Inaccuracies that can arise during floor vibration modeling are highlighted, in addition to the effects of changes to the structural system and the introduction of laboratory furnishings on footfall response levels.
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Floor vibration serviceability has become an area of interest over recent years. Lightweight electronic offices, especially those with slender architectural forms, and high-technology buildings with low vibration environments can often have excessive floor vibration due to human walking. This can result in a floor which is not fit-forpurpose.
There exist a number of design guides which offer simplified formulae toanalyse these floors. However, the simplistic methods are often shown in be inaccurate and inappropriate. The alternative, finite element analysis (FEA), which is often considered more accurate, can be expensive to implement and time consuming. In addition, modelling techniques required for dynamic analyses are different to that required for static analysis. This is often overlooked, resulting in poor inaccurate models. An accurate, simplistic, method is required which is suitable for everyday design.
This paper examines the current simplified methods of predicting floor response to a single human walking. The inaccuracies are highlighted, as well as where the methodology is inappropriate. The paper also introduces new techniques and ideas to improve design guides in the future.
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Several ski companies have recently introduced vibration damping
features in their product lines. These features are designed to improve the
performance of the ski and lessen felt shock. Three brands of vibration damping
systems were tested to determine the effectiveness of the damping features.
Similar skis without damping systems were also tested. The systems were tested
in the lab and on the hill. In the lab the systems were subjected to a variety of
random inputs and their responses recorded. On the hill the data was collected
from the ski and binding as the ski was being used. A portable data collection
system carried by the skier was employed to gather information. It was found that
two of the systems did reduce vibrations and transmitted shock over ranges of the
frequency spectrum. An error in the data collection process prevented a
determination of the effectiveness of the third damping system.
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DYNAMIC RESPONSE OF BEAMS WITH PASSIVE TUNED MASS DAMPERS
Author: Mustafa Kemal Ozkan | Size: 2.5 MB | Format:PDF | Quality:Unspecified | Publisher: Purdue University West Lafayette, Indiana | Year: 2010 | pages: 275
Passive tuned mass damper (TMD) is a stand-alone vibrating system attached to a primary structure and designed to reduce vibration of the structure at selected frequency. This study focuses on the application of single or multipleTMDs on Euler-Bernoulli beams and examines their effectiveness based on free
and forced vibration characteristics of the beams, i.e., the primary structures. There is a gap in the existing literature in terms of free and forced vibration analysis of beams carrying any number of concentrated elements. There are methods developed for the free vibration analysis but they are not practical due to the complex mathematical expressions. Numerical assembly method (Wu and Chou, 1999) is used to determine free vibration characteristics of beams in order to get over the drawbacks of other approaches in the literature and forced vibration response is obtained based on modal analysis approach and orthogonality condition.
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PREDICTION OF FLOOR VIBRATION RESPONSE USING THE FINITE ELEMENT METHOD
Author: Michael J. Sladki | Size: 737 KB | Format:PDF | Quality:Unspecified | Publisher: December 1999 Blacksburg, Virginia 24061 | Year: 1999 | pages: 162
Several different aspects of floor vibrations were studied during this research. The focus of the research was on developing a computer modeling technique that will predict the fundamental frequency of vibration and the peak acceleration due to walking excitation as given in AISC Design Guide 11, Floor Vibrations Due to Human Activity (Murray, et al., 1997). For this research several test floors were constructed and tested, and this data was supplemented with test data from actual floors. A verification of the modeling techniques is presented first. Using classical results, an example from the Design Guide and the results of some previous research, the modeling techniques are shown to accurately predict the necessary results. Next the techniques were used on a series of floors and the results were compared to measured data and the predictions of the current design standard. Finally, conclusions are drawn concerning the success of the finite element modeling techniques, and recommendations for future research are discussed. In general, the finite element modeling techniques can reliably predict the fundamental frequency of a floor, but are unable to accurately predict the acceleration response of the floor to a given dynamic load.
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Energy Harvesting from Random Vibraitions of Piezoelectric Cantilevers and Stacks
Author: In Partial Fulfillment of the Requirements for the Degree Master of Science in Mechanical Engineering Georgia Institute of Technology August, 2013 | Size: 3.9 MB | Format:PDF | Quality:Unspecified | Year: 2013 | pages: 100
Electromechanical modeling efforts in the research field of vibration-based energy harvesting have been mostly focused on deterministic forms of vibrational input as in the typical
case of harmonic excitation at resonance. However, ambient vibrational energy often has broader frequency content than a single harmonic, and in many cases it is entirely stochastic. As
compared to the literature of harvesting deterministic forms of vibrational energy, few authors presented modeling approaches for energy harvesting from broadband random vibrations. These
efforts have combined the input statistical information with the single-degree-of-freedom (SDOF) dynamics of the energy harvester to express the electromechanical response
characteristics. In most cases, the vibrational input is assumed to have broadband frequency content, such as white noise. White noise has a flat power spectral density (PSD) that might in fact excite higher vibration modes of an electroelastic energy harvester. In particular, cantilevered piezoelectric energy harvesters constitute such continuous electroelastic systems
with more than one vibration mode.
The main component of this thesis presents analytical and numerical electroelastic modeling, simulations, and experimental validations of piezoelectric energy harvesting from
broadband random excitation. The modeling approach employed herein is based on distributed-
parameter electroelastic formulation to ensure that the effects of higher vibration modes are
included. The goal is to predict the expected value of the power output and the mean-square
shunted vibration response in terms of the given PSD or time history of the random vibrational
input. The analytical method is based on the PSD of random base excitation and distributed-
parameter frequency response functions of the coupled voltage output and shunted vibration
response. The first one of the two numerical solution methods employs the Fourier series representation of the base acceleration history in a Runge-Kutta-based ordinary differential
equation solver while the second method uses an Euler-Maruyama scheme to directly solve the
resulting electroelastic stochastic differential equations. The analytical and numerical simulations
are compared with several experiments for a brass-reinforced PZT-5H cantilever bimorph under
different random excitation levels. In addition to base-excited cantilevered configurations,
energy harvesting using prismatic piezoelectric stack configurations is investigated.
Electromechanical modeling and numerical simulations are given and validated through experiments for a multi-layer PZT-5H stack. After validating the electromechanical models for specific experimentally configurations and samples, various piezoelectric materials are compared theoretically for energy harvesting from random vibrations. Finally, energy harvesting from
narrowband random vibrations using both configurations are investigated theoretically and experimentally.
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