WIND-INDUCED VIBRATIONS IN HIGH-RISE …

1 Master s DissertationStructuralMechanicsFREDRIK STEFFENWIND- induced VIBRATIONS IN HIGH-RISE BUILDINGSDEPARTMENT OF CONSTRUCTION SCIENCESSTRUCTURAL MECHANICSISRN LUTVDG/TVSM--16/5211--SE (1-66) | ISSN 0281-6679 MASTER S DISSERTATIONS upervisor: PETER PERSSON, PhD, Div. of Structural Mechanics, LTH. Examiner: Professor PER-ERIK AUSTRELL, Div. of Structural Mechanics, 2016 Division of Structural Mechanics,Faculty of Engineering LTH, Lund University, by Media-Tryck LU, Lund, Sweden, June 2016 (Pl).For information, address:Division of Structural Mechanics,Faculty of Engineering LTH, Lund University, Box 118, SE-221 00 Lund, : STEFFENWIND- induced VIBRATIONS IN HIGH-RISE BUILDINGSA bstractBuildings are getting taller due to increased urbanisation and densification of advanced construction methods and the desire to construct impressive build-ings is also supporting the trend.

2 Due to their inherent slenderness resulting in loweigenfrequencies, these buildings are susceptible to WIND-INDUCED VIBRATIONS whichcan be highly disturbing for occupants. Already barely perceivable acceleration levelswithin the low-frequency range relevant to whole-body VIBRATIONS can cause nauseaand discomfort, while high acceleration levels can cause alarm and fear amongst theoccupants. The thesis summarises acceptable acceleration levels in HIGH-RISE build-ings (here referred to as buildings over 200 m in height) stated in different buildingcodes and previous work on the subject. Equations for estimating acceleration in tallbuildings in an early design stage are formulated. Accelerations of a HIGH-RISE buildingsubjected to wind -loads are evaluated using a full numerical model and one reducedwith Ritz-vectors and the results are compared. The thesis focusses on wind -load dy-namics in early stages of the design process, with an intent to give an indication ofthe dynamic properties of a building .

3 Finally some actions to reduce VIBRATIONS master thesis was carried out at the Division of Structural Mechanics, Facultyof Engineering LTH at Lund University, and concludes five years of civil and foremost, I would like to thank my supervisor Peter Persson, who has notonly helped me through the work of this thesis, but also career wise. Your time andeffort have been truly would like to thank Per-Erik Austrell for suggesting the idea of using Ritz-vectorsand helping me throughout this thesis. Furthermore I would like to thank Kent Perssonfor always having his doors open for but not least I would like to thank the other students writing their theses, both atthe Division of Structural Mechanics and the Division of Structural Engineering. Youknow who you are, and you have made this final semester at Lund University a , June SteffenContentsAbstractiAcknowledgements iii1.

4 Of the thesis ..42 VIBRATIONS in HIGH-RISE studies and experiments .. experiments and surveys .. simulators and shaker table experiments .. and codes .. tunnel testing ..113 Acceleration response .. response ..214 Numerical modelling and vectors .. static wind -load .. time series .. analysis and results ..335 vibration -reduction methods .. methods .. methods ..476 Conclusion and discussion49A Appendix A .. Appendix B .. Appendix C ..591. BackgroundWith advancement in material and construction sciences, buildings have been gettingtaller and taller during the last century. high -strength steel, lighter cladding and mod-ern construction techniques have resulted in tall buildings, this also giving more slen-der buildings with lower natural frequencies. Buildings exposed to stochastic loadingsuch as earthquake or wind load have a tendency to vibrate in the first natural fre-quency, the fundamental frequency.

5 For HIGH-RISE buildings the fundamental fre-quency can be lower than 1 Hz. In designing these tall buildings serviceability criteriaare often harder to fulfil than the survivability ones. Given the development, manyinternational building codes have been revisited to estimate acceleration levels in tallbuildings. The Eurocode on the other hand, has not been updated to accommodatethe need of estimating acceleration levels in taller buildings in the preliminary designstage. It is only valid up to 200 meters and does not provide a way of estimating re-sponse in the across- wind direction due to vortex shedding. Furthermore, the Eurocodedoes not provide any guidelines on allowed acceleration levels at certain frequencies,nor their to their tall and slender shape, thus a low first natural frequency, tall buildings havea tendency to vibrate in their first mode.

6 This combined with numerical models havinga large number of dofs, the use of Ritz-vectors may be suitable for reducing the systemin dynamic analysis. A building with many dofs has been modelled and subjectedto the wind -load. The timeseries of the wind -load will be created using an invertedFFT procedure (IFFT) and applied to the finite element method (FEM) model. Thetime to complete the analysis will be measured and compared to the time to completesimulations in a system reduced by the last chapter of the thesis, some measures of damping tall buildings are 1. Scope of the thesisThe thesis covers wind -loads generated by synoptic winds. Extreme wind -events suchas hurricanes, tropic cyclones and downbursts are not covered. The wind load wasapplied in two direction, the along- and across- wind direction respectively. The FEM-model has a symmetric cross-section with centre of mass as well as moment of inertiacoinciding on each floor.

7 The system was reduced to its first mode of vibration , highermodes being VIBRATIONS in HIGH-RISE buildingsSince tall buildings with low natural frequencies vibrates due to wind excitation, it isimportant to have an understanding how it affects the users. This chapter aims to givea fundamental understanding on human response to low frequency accelerations andwhat might be considered acceptable accelerations in tall Performed studies and experimentsThroughout the years several studies has been conducted on the human perception ofvibrations in the frequency span below 1 Hz. The studies usually fall within one ofthree types of studies: [1] Field experiments and surveys of occupants in tall building . Motion simulator and shaker table experiments. Field experiments in artificially excited results of several studies have been summarised by Kwok et al.

8 [1] and someof those which fall within the first two categories mentioned above is discussed , if any, field experiments in an artificially excited building with a natural frequencybelow 1 Hz, which is usually associated with very tall buildings, have been done andwill therefore not be Field experiments and surveysField experiments and surveys can be divided into two categories. The first being sur-veys conducted in buildings during or after a passing storm. The results of the surveyare later on compared to data measured from wind -tunnel testing of that particularbuilding. More useful result have been derived from buildings that are known to havecomplaints from tenants regarding uncomfortable acceleration levels. This has allowedtesters to properly install equipment to be able to register the actual accelerations andvibrations, and be able to compare these to what the occupants 2.

9 VIBRATIONS IN HIGH-RISE BUILDINGSF igure : Reported symptoms of motions sickness in the study [7].Hansen et al. [2] installed one out of two 167 m tall office buildings with accelerom-eters and recorded a RMS acceleration of 2 mg. Surveys showed that occupants haddifferent cues for perceiving the motion, such as feeling, hearing, seeing etc. Withowners and developers it was established that 2% of the occupants in the top third ofthe building could object to VIBRATIONS once a year without affecting the rental limit of RMS acceleration of 5 mg every six years was et. al [3] [4] and Denoon [5] conducted experiments in three airport towersequipped with accelerometers and anemometers. With help of the test it was concludedthat the average threshold of perception varied with the natural frequency of the build-ing. It is noteworthy that even though two of the buildings experienced accelerationsthat were acceptable according to the ISO 6897:1984 [6] which applied back then, oneof the buildings received more complaints.

10 It is believed that the tower was exposedto winds during a longer period of time, suggesting that exposure duration affects theperception of vibration . This is an important consideration, since the characteristics ofwindstorms differ greatly across the Lamb et al. [7] conducted surveys of 47 office workers in 22 buildings witha control group of 53 workers. The study was performed in Wellington, New Zeeland,a notoriously windy city. With a risk of earthquakes in the country, designers areforce to allow for more structural flexibility which makes buildings more prone todynamic response. 1909 surveys were collected during an eight month period andinvestigated. It was noted that there is a significant increase amongst the workersexperiencing nausea and dizziness when building motion was possibly or definitelyperceivable as shown in Figure asked to rate their own work performance there was a clear indication that PERFORMED STUDIES AND EXPERIMENTS7ers in a higher Combined Motion Sickness Scale (CMSS) score group experienced adrop in their overall work performance, shown in Figure : Work performance in the study [7]The study showed that 60 70% of the participants detected building motion by vestibu-lar or proprioceptive cues, by feeling the sensation of motion.