In this paper, the dynamic characteristics of 30m high pole lamp are studied through the response analysis of environmental excitation. Then, under the actual conditions of wind-induced vibration, the vibration frequency, vibration mode and wind speed at that time were measured, and it was confirmed that the vibration was caused by the structural resonance caused by the wind wake shedding excitation formed by fluid structure coupling. This paper discusses the theoretical basis, measurement method, data acquisition and analysis technology of environmental excitation response measurement, and gives the theoretical basis and relevant design formulas for avoiding wind-induced vibration. Key words: highly flexible structure, wind load, environmental excitation, modal analysis, random response 1. Introduction Environmental excitation often causes vibration of highly flexible structures, which attracts the attention of many engineers. Research work in this field has been published in recent years [1-5]. Based on the random vibration theory of linear structures, this paper discusses some problems in field measurement of highly flexible structures, such as experimental methods, data acquisition, modal analysis, etc. In this paper, the theoretical basis of structural vibration mode analysis using response spectrum analysis of ground environmental excitation is demonstrated in detail, and the method to determine the sampling length when measuring low frequency modal with small damping is given to ensure the accuracy of damping analysis. In this paper, the reason of wind vibration of 30 meter high pole lamp in Beijing is analyzed by field measurement, and the relevant formula for the design of high pole lamp is given. 2、 Analysis of structural vibration response caused by random ground motion horizontal vibration is the main vibration of highly flexible structures. Only horizontal vibration is analyzed here. This paper was commissioned by Beijing Municipal Architectural Design Institute for research, and was measured on site in cooperation with the Institute. Date of receipt: July 1997

Figure 1 High flexible structure excited by ground motion If the structure shown in Figure 1 is excited by horizontal ground motion u (t), then the motion differential equation of the structure is

[ M] {¨x}+[ C] {﹒ δ}+ [ K] { δ}= {0} ( 1)

Here{ δ} Is the relative displacement vector of the structural mass point relative to the moving ground, and {x} is the absolute displacement vector. So there are

x i = δ i +u, ¨x i =¨ δ Substitute i+¨ u (2) into equation (1) to get

[ M] {¨ δ}+ [ C] {﹒ δ}+ [ K] { δ}=- [M] {1} ¨ u (3) - [M] {1} ¨ u at the right end of the above formula can be considered as the equivalent force vector acting on the structure due to ground motion. According to the random vibration spectrum analysis theory, the relative displacement vector{ δ} The power spectral density matrix of is

[ S δδ] = [H *] [Sf f] [H] T (4) Where [H] is the frequency response function matrix of the structure, [H *] is its conjugate matrix.

[ H] =[ Υ] [ Y r] [ Υ] T [ Υ] = [ { φ 1}

{ φ 2}…{ φ n}] [ Y r] =diag[ Y 1

Y 2 …Y n] Y i =[ k r -mr ω 2 +j ω Cr] - 1 k r, mr, cr are modal parameters and have

Ψ r = kr / mr, ζ r = cr 2 mrk r { φ r} Is the mode shape of the r-th order{ φ r}=[ φ 1 r φ 2 r … φ Nr] T When using environmental excitation response analysis to obtain structural dynamic characteristics, the main content is to obtain modal shape{ φ r} , modal frequency Ψ R and modal damping ratio ζ r 。 [Sf f] is the power spectral density (PSD) matrix of the excitation force, which should be [Sf f]=[M] [I] [M] TS (5) where S can be determined as follows:（ τ) = E[ ¨u ( t) ( ¨u( t - τ) )] = ω 4E[ u( t) u( t - τ)] S¨u¨u = 12π∫∞-∞R¨u¨u ( τ) e-j ωτ d τ= ω 4 In Suu (6), R and E [¨ u (t) ¨ u (t- τ)] Represents autocorrelation functions and mathematical expectations. According to formula (4), (5) and (6), [S δδ] = [ H] ＊[ M] [ I] [ M] T[ H] T ω 4 Suu (7) However, the absolute displacement {X}, rather than the relative displacement, can be directly measured in measurement{ δ}。 Therefore, we are directly concerned with the absolute displacement PSD matrix [S x x]. It is not difficult to prove.