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Journal ArticleDOI

Wind-induced vibrations and building modes

01 Aug 1966-Bulletin of the Seismological Society of America (GeoScienceWorld)-Vol. 56, Iss: 4, pp 793-813

AbstractThis paper outlines the methods that have been used to determine the frequencies and modes of vibration of multistory buildings from their wind-induced vibrations. Three buildings of ten, thirty-eight and forty-seven stories were investigated. A simple theoretical model was used to calculate the frequencies of vibration of the buildings; the model was based on the assumption that there was no joint rotation in the building frames. A comparison of the theoretical and measured values of the frequencies showed that this simple model was a realistic representation of only the smaller building. It is concluded that a model that includes joint rotation would be more realistic for the taller buildings. Auto-correlation and power spectrum analysis of the vibration records were used to obtain an estimate of the damping characteristics of the buildings. The values obtained were 1 to 3 per cent of the critical amount of damping.

Topics: Vibration (53%), Normal mode (51%)

Summary (1 min read)

DESCI~IPTIOS OF TIIE I~UILDISGS

  • Thc three buildings investigated have been c.onstructc~c1 ~vithiil the last five years-One of the buildings, the Sir Alcsaildcr Cam~~bcll Buildiilg is the headquarters of the Cunndi:ul Post Office Dcp:u.tilleilt 211 Ottawa; the other two buildiilgs, the Cnnacliail Imperial I3~1lli of Com~ncrcc Builcliitg ant1 CIll IIouhe, arc 1oc.atctl 011 Dor-c11estc.r Boulevard i~t 3101ltre~~l.
  • \Trllcii the vibrntioli had becil built up to a suitable lcvel, thc motion was allon~cd to dccny and thc dampingv:\lue was again obini~icd by the logarithmic decrement mcthod.
  • This assuillptioil was made, togcthcr i~ith the further assuinptioll that a11 thc Inasscs of cach floor for n give11 building were eclual.

I%ESULTS FOR I\~ODES AS11 FIEEQUENCIES OF VIBIEATIOS

  • The wind records havc bceil stcacly on each occasion when readings have bccn taken froill the aiiemonleter, a fact that could be related to its isolatcd position.
  • The claillpirig valucs obtained by these mcthocls arc shoxvn in Table 4 -Tllc ~ncastued and theoretical valucs of the pcriotls of tlic fulidaillclital inodcs of lateral vibration of the buildiiigs are sho~vii in the first two lilies of T~~blc 5.
  • The ~ne:~surecl results in Tablc 1 show that the ratios of tlic natural frccluerlcies of vibration to the fundanlentnl freciuency :ire of the o~,cler 1, 3, 5, 7, etc. 31oclc shapes of buildings :Ire also inlportant in carthqualic engineering design l~ccause they cletermine t'lic manner in which eart'llqualic loatls are distributed thl~ougli the kicight of the st,ructure.
  • One of the best ways of achicviag this knomledgc is to continue the comparison of the predictions of theory with the nleasurecl dy~ian~ic ~haractcristics of buildings.

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Bulletin of the Seismological Society of America, 56, 4, pp. 793-813, 1966-07-01
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Wind-induced vibrations and building modes
Ward, H. S.; Crawford, R.
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'794
BULLETIS
OF
TIIE
SEISSIOLOGICAIJ
SOCIETY
OF
ASIEIIICA
importance
irl
the ~arth(l~:lli~ :111d wintl resihtant clcsig~l of
structures,
it
is impera-
tive that every opportunity should be talie~l to try to unclcrstand tho dynamic hc-
llaviour of buildi~~gs. HOUSILC~ aid 13racly
(19G3)
have studied exiitillg information,
and have co~ic.ludcd that
sonic
of the existing empiricxl formulae used
ill
builtling
codes to determine the periods of vibration of buildings can 1,c in error by as inuch
as
100
per cent.
Another c.onclusion was that soillc inoclcrn steel frame builcliilgs behave esseriti:~lly
as
if
they had rigid floor girc1el.s. Calculi~tiolls have bee11 made to t,est this assump-
tion for the buildii~gs dcscribccl
ill
this p:~prr, ancl it is sho~vil that fair agreement
was obtui~lcd for the ten-story building, but that there ~vas an overcstimatiorl of the
fundamental frequency of the order of
50
per cent for the multi-story structures.
)p
Some possible causes for the discrepancy arc discussed, and the measured values are
comparcd with sonlc of the dcrivccl formulae obt:~ined by Hous~ler and Brady.
There is
perhaps less l~no~vn about the damping properties of buildings th;~n of
t~ny other clyil:~mic c11ar:~ctcristic. Ail attempt has been made to determine damping
values froin the rccorcls of the wild-iilducctl vibrations of the building by two meth-
ods: power spectrum :ulalysis and auto-correlatioll a~lalysis.
111
one instance the rc-
sults have been compared with those obtained by inail-induced vibrations. The
incthods need to be applied to illore builclillgs before general co~lclusioris c:~n be
drawl, but the initial fi~idiiigs iiltlic~atc damp~ng values of the ordcr of
1
to
3
per cent
of criticsal.
DESCI~IPTIOS
OF
TIIE
I~UILDISGS
Thc three buildings investigated have been c.onstructc~c1 ~vithiil the last five years-
One of the buildings, the Sir Alcsaildcr Cam~~bcll Buildiilg is the headquarters of
the Cunndi:ul Post Office Dcp:u.tilleilt
211
Ottawa; the other two buildiilgs, the Cn-
nacliail Imperial I3~1lli of Com~ncrcc Builcliitg ant1 CIll IIouhe, arc 1oc.atctl
011
Dor-
c11estc.r Boulevard i~t 3101ltre~~l.
The
Posl
O.@iic.e
Ru~lrlrnq
is
266
f
by
74
ft, c~lcvcn bays by thrcc b:~ys ill pl;ul di-
mcnsio~l, ant1
147
ft
Ci
in. high. Tllcrc :we trrl floors :~bovc grouncl, including
a
pent
-
house ancl oile b:lselncilt; arlcl
t
yl~ical sto1.y height is
12
ft
1
ill. The h:~nic ;lnd floor
slabs :~i.c rciriforc~ccl conc.i3dc, and the cxtcrnal I\-:dls arc 11011-load-bc:~ri~ig 4-h~. or
8-in. bric.1; ~valls.
Tllc buildii~g rcsls
011
groups of piles
22
in.
ill
diitmdcr, e:~cl1 of
125
toils capac.it y,
that pass through
20
f
of
(21:~~
:11lc1
'LO
ft of gravel and s:lild to solid liinesto~lc rocsli.
Tllc colum~ls :we rcc-tar~gular
:LI~
oricutcd so that the stiffest usis is parallel to the
loilg clinlcnsiorl of tllc builcli~~g; the colu~nils arc s~pprosim:ltcly twicac as stiff about
this asis as they arc about the one at right angles to it. The c~olum~l stift'iic~ss is
ap-
proximately cdolistalit Roil1 the fou~~tlation lo the
fif
h
floor, where the values :we
approximately 11:llvctl a~itl Illen ~tlnniil colistailt lo the roof.
?'he
Canatlian
Insperin1
Banlc
oJ'
Co117111el*ce
Builclinq
ii
140
ft by
100
ft,
scvcil b:lys
1,y four bays iri plaii,
slid
rises
GO3
ft
above the street lcvcl. There arc
44
stories
:~bovc ground lcvcl wit11 a typical story height of
12
ft
3
ill., cxcocpt for the first floor
aiid the five ~nec~hailic.:~l floors
:L~OUI~
the fifteeilth floor level arid the top.
A
sub-
structwc thi*cc floors in tlcptll covers thc entire site of
'243
ft
by
185
ft.
1Yic building is fo~~~idctl
011
bed r0c.1;
4S
ft brlonr street level, with footi~lgs desig~~rd

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Abstract: The ambient and forced vibration techniques for testing full-scale structures are critically compared. Both methods, based on small level excitation, may be used to determine many mode shapes and frequencies of vibration and the corresponding damping values, with adequate accuracy for most purposes. The two techniques give mutually consistent results. The mode amplitudes determined by ambient and forced vibration tests show systematic departure for high modes and near the top levels of buildings tested. This phenomenon is attributed to the participation of all mode shapes and is a consequence of excitation by a concentrated force near the top of a building and at a frequency differing by only a few per cent from a natural frequency of vibrations. A new way of showing the effect of unwanted modes on the response near resonance of the mode being sought is developed. It is particularly useful for the analysis of steady, forced vibration tests of structures using eccentric mass vibration generators.

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Abstract: Results of two detailed ambient vibration surveys of a 7-story reinforced concrete building in Van Nuys, California, are presented. Both surveys were conducted after the building was severely damaged by the 17 January 1994, Northridge earthquake (ML=5.3, epicenter 1.5 km west from the building site) and its early aftershocks. The first survey was conducted on 4 and 5 February 1994, and the second one on 19 and 20 April 1994, about one month after the 20 March aftershock (ML=5.3, epicenter 1.2 km north–west from the building site). The apparent frequencies and two- and three-dimensional mode shapes for longitudinal, transverse and vertical vibrations were calculated. The attempts to detect the highly localized damage by simple spectral analyses of the ambient noise data were not successful. It is suggested that very high spatial resolution of recording points is required to identify localized column and beam damage, due to the complex building behavior, with many interacting structural components. The loss of the axial capacity of the damaged columns could be seen in the vertical response of the columns, but similar moderate or weak damage typically would not be noticed in ambient vibration surveys. Previous analysis of the recorded response of this building to 12 earthquakes suggests that, during large response of the foundation and piles, the soil is pushed sideways and gaps form between the foundation and the soil. These gaps appear to be closing during “dynamic compaction” when the building site is shaken by many small aftershocks. The apparent frequencies of the soil–foundation–structure system appear to be influenced significantly by variations in the effective soil–foundation stiffness. These variations can be monitored by a sequence of specialized ambient vibration tests.

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TL;DR: The time domain classification and quantification of Broad-band urban seismic noise is capable to resolve the influence of wind on seismic noise and a known site effect variation in the metropolitan area of Bucharest, Romania.
Abstract: SUMMARY Broad-band urban seismic noise (USN) must be considered as a temporally and spatially non-stationary random process. Due to the high variability of USN a single measure like the standard deviation of a seismic noise time-series or the power spectral density at a given frequency is not enough to characterize a sample (time-series) of USN comprehensively. Therefore, we use long-term spectrograms and propose an automated statistical classification in the time domain to quantify and characterize USN. Long-term spectrograms of up to 28 d duration are calculated from a broad-band seismic data set recorded in the metropolitan area of Bucharest, Romania, to identify the frequency-dependent behaviour of the timevariable processes contributing to USN. Based on the spectral analysis eight frequency ranges between 8 mHz and 45 Hz are selected for our proposed time domain classification. The classification scheme identifies deviations from the Gaussian distribution of 4-hr-long timeseries of USN. Our classification is capable to identify Gaussian distributed seismic noise timeseries as well as time-series dominated by transient or periodic signals using six noise classes. Four additional noise classes are introduced to identify corrupt time-series. The performance of the method is tested with a synthetic data set. We also apply the statistical classification for the data set from Bucharest in three time windows (0–4, 8–12 and 13–17 EET) at 11 d in the eight frequency ranges. Only 40 per cent of the analysed time-series are observed to be Gaussian distributed. Most common deviations from the Gaussian distribution (∼47 per cent) are due to the influence of large-amplitude transient signals. In all frequency ranges between 0.04 and 45 Hz significant variations of the statistical properties of USN are observed with daytime, indicating the broad-band human influence on USN. We observe the human activity as a dominant influence on the USN above and below the frequency band of ocean-generated microseism between 0.04 and 0.6 Hz. Our time domain classification and quantification is furthermore capable to resolve the influence of wind on seismic noise and a known site effect variation in the metropolitan area of Bucharest. The information about noise amplitudes and statistical properties derived automatically from broad-band seismic data can be used to select time windows containing adequate data for seismic noise utilization like H/V-studies or ambient noise tomography.

88 citations


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Abstract: SUMMARY An understanding of the precise nature of the non -linear response of structures is essential for future improvement of earthquake resistant design procedures. This paper presents a summary of observations of dynamic behaviour which were made on two typical modern buildings during a period of about ten years. These structures underwent numerous tests and experienced three strong earthquake ground motions . The data presented should prove useful for calibration of parameters in theoretical non-linear models. For buildings having an apparent soft -spring-type non -linearity, a partial or complete recovery of the structural stiffness appears to occur following the large strains created by strong ground shaking . The rate and extent of this recovery appear to depend strongly on the strain levels throughout the excitation.

78 citations


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TL;DR: The main results include the evidence that urban seismometers can be used as a easy-to-use, robust monitoring tool for road traffic and subway activity inside the city, and the interest to understand the propagation of seismic waves generated by those rather particular sources.
Abstract: Urban seismology has become an active research field in the recent years, both with seismological objectives, as obtaining better microzonation maps in highly populated areas, and with engineering objectives, as the monitoring of traffic or the surveying of historical buildings. We analyze here the seismic records obtained by a broad-band seismic station installed in the ICTJA-CSIC institute, located near the center of Barcelona city. Although this station was installed to introduce visitors to earth science during science fairs and other dissemination events, the analysis of the data has allowed to infer results of interest for the scientific community. The main results include the evidence that urban seismometers can be used as a easy-to-use, robust monitoring tool for road traffic and subway activity inside the city. Seismic signals generated by different cultural activities, including rock concerts, fireworks or football games, can be detected and discriminated from its seismic properties. Beside the interest to understand the propagation of seismic waves generated by those rather particular sources, those earth shaking records provide a powerful tool to gain visibility in the mass media and hence have the opportunity to present earth sciences to a wider audience.

53 citations


References
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Book
01 Jan 1959

121 citations


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Abstract: Random wind excitation has been used to find the first three modes of vibration of a nineteenstory building about the two major translational axes and the torsional axis of the building. Vibration records were obtained with Willmore electromagnetic seismometers feeding into a multichannel magnetic tape recorder. A harmonic analysis was then performed with the aid of an analogue computer to determine the first few vibration modes. Concurrently with the experimental program the building modes were computed two ways using different simplifying assumptions regarding the lateral stiffness of the structure.

61 citations


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Abstract: The measured periods of vibrations of a large number of buildings are used to compare the merits of existing formulas and of equations derived using a rational approach. It is concluded that no single, simple, empirical equation will give reasonably accurate estimates for the periods of buildings having shear wall characteristics. The calculated periods of steel frame buildings are found to be proportional to the square root of the number of stories rather than directly proportional as is usually assumed. However, observed periods of such buildings indicate that the steel frame alone would contribute only about 25% of the effective stiffness. The dependence of the period on the flexibility of the floor girders is also demonstrated using a digital computer. Calculations show that these buildings behave essentially as if they had rigid floor girders, although the girders of the steel frames themselves are very flexible, so far as the period of vibration is concerned. Additional studies of the natural periods of actual structures are recommended.

58 citations


01 Jan 1962
Abstract: The general considerations behind the design of a rotating weight sinusoidal vibration generator for dynamic studies of full-scale structures have been given in a previous report which pointed out the advantages of a system of multiple synchronized machines that would permit a distribution of exciting forces throughout a structure so as to most efficiently excite various modes of vibration. Under the sponsorship of the California State Division of Architecture a set of four synchronized vibration generators has been completed and tested, and the purpose of the present report is to summarize the design information on this new system, and to give detailed operating instructions for its use.

24 citations


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Abstract: It is shown that the inertia force obtained by a man moving his body back and forth in synchronism with the natural period of vibration of a large structure is sufficient to build up a measurable amplitude of motion. By recording such structural vibrations versus time, the natural period and damping of several of the lower modes of vibration can be determined. The amplitudes of motion set up in this way are for many structures significantly larger than can be obtained from wind excitation, which has been used in the past for the measurement of the period of the fundamental mode.

7 citations