Wind-induced vibrations and building modes
Abstract: This 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.
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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