METHOD FOR IDENTIFYING MODAL FREQUENCY OF BEAM BRIDGE BY CONSIDERING INFLUENCE OF ENVIRONMENTAL TEMPERATURE

Abstract

The disclosure provides a method for identifying a modal frequency of a beam bridge by considering influence of environmental temperature. The method includes the following steps: installing a sensor on a newly-built beam bridge without damage, measuring a dynamic response of the nearn bridge cinder ambient excitation, recording temperature data, processing by a modal parameter identification method to obtain a modal frequency value at the temperature, and titarling from a modal frequency corresponding to the temperature, carrying out iterative calculation to obtain the modal frequency at any temperature. The modal frequency value at any temperature is obtained by arranging a small number of sensors and canying out a small number of tests, so that the influence of the temperature on the modal frequency is quantified, furthermore, the part of environmental influence is eliminated in future damage evaluation of the beam bridge, which allows for a more accurate isamage evaluation rttsult,

Description

TECHNICAL FIELD

The present disclosure relates to the field of vibration-based health monitoring of beam bridges, and in particular to a method for identifying the modal frequency of a beam bridge by considering the influence of environmental temperature.

BACKGROUND ART

With the rapid development of the social economy, an increasing nwnber of large-span beam bridges have been put into use. In the meanwhile, more and more attention has been paid to structure safety. Modal analysis is of great significance to safety diagnosis of large-span structures. Modal parameters include frequency, damping ratio, and mode shape, which can be used to monitor and detect the health status of structures, and are widely used in damage identification and health assessment of beam structures. As one of the main modal parameters, structural modal frequency, featured by convenient measurement and high precision, is widely used as a common performance index of structures, and as a basic physical measurement method used in various structural evaluation. However, in the actual process, the change of environmental temperature may also cause the change of structural modal frequency, and even cover up the change caused by damage, which makes it difficult to implement a vibration-based damage identification method. Therefore, quantifying the influence of environmental temperature on structural frequency to further eliminate the influence of environmental temperature on modal frequency is an essential step in the process of damage identification.

SUMMARY

An objective of the present disclosure is to provide a method for identifying a modal frequency of a beam bridge by considering the influence of environmental temperature without the need for real-time monitoring of the beam bridge, so as to eliminate the influence of the environmental temperature in damage evaluation and obtain the modal frequency of the beam bridge at any temperature, thereby improving the accuracy of damage identification.

In order to achieve the foregoing objective, the present disclosure employs the following technical solution:

The present disclosure provides a method for identifying a modal frequency of a beam bridge by considering the influence of environmental temperature, including the following steps:

Step 1: determining a length L and a section parameter of the beam bridge; and in a length direction of the beam bridge, arranging 1 acceleration sensor and 1 thermocouple in each span;

Step 2: obtaining an acceleration response of the beam bridge subjected to a moving load at different temperatures using the acceleration sensor and the thermocouple with a sampling frequency of f_s, selecting any of the temperatures as a reference temperature T₀, and obtaining the reference temperature T₀ and a corresponding acceleration response;

Step 3: conducting Fourier transform on the acquired acceleration response to obtain first n orders of frequencies of the beam bridge which are denoted as {f₁*|n=1,2,3} where f_n* denotes an nth order measured frequency; and

selecting the reference temperature T, and corresponding first n orders of frequencies {f_n(T₀)|n=1,2,3}, where f_n(T₀) denotes an nth order frequency corresponding to the reference temperature T₀;

Step 4: obtaining a slope k₀ between the nth order frequency f_n(T₀) and the reference temperature T₀ according to formula (1):

$\begin{array}{cc}{k}_0=\frac{1}{2}{\beta }_E·f_n\left(T_0\right)& \left(1\right)\end{array}$

where in formula (1), β_E denotes a thermal coefficient of an elasticity modulus of a beam bridge's material;

Step 5: starting from a given point (T₀, f_n(T₀)), constructing the slope k₀ in a frequency-temperature relation at an interval of T₀˜T₀+ΔT to obtain an nth order frequency L(T₀ +AT) corresponding to a temperature T₀+ΔT, where ΔT denotes a temperature increment;

f _n(T₀+ΔT)=f(T₀+k₀ΔT   (2)

Step 6: defining a variable i, and initializing to 1;

Step 7: obtaining an nth order frequency f_n(T_i) corresponding to an ith temperature T_i between the reference temperature T₀ and a target temperature T_m according to formula (2):

f _n(T_i)=f_n(T_i-1)+k_i-1ΔT_i-1   (3)

where in formula (2), ΔT_i-1 denotes a temperature interval between an (i-1)th temperature T_i-1 and the ith temperature T_i, and when i=1, T_i-1=T₀; f_n(T_i-1) denotes an nth order frequency corresponding to the (i-1)th temperature T_i-1, and when i=1, f_n(T_i-1)=(T₀) ; and k_i-1 denotes an instantaneous slope between the nth order frequency f_n(T_i-1) and the (i-1)th temperature T_i-1 and when i=1, k_i-1=k₀;

Step 8: obtaining an ith slope k, according to formula (3):

$\begin{array}{cc}{k}_i=\frac{1}{2}{\beta }_E·f_n\left(T_i\right)& \left(4\right)\end{array}$

Step 9: after assigning i+1 to i, returning to Step 7, and performing sequential execution until i=m; and.

Step 10: obtaining an nth order frequency f_n(T_m) corresponding to the target temperature T_m according to formula (3) and formula (4):

$\begin{array}{cc}{f}_n\left(T_m\right)=f_n\left(T_0\right)+\frac{1}{2}{\beta }_E\sum _{i=0}^{m-1}{f}_n\left(T_i\right)·\Delta T_i& \left(5\right)\end{array}$

Compared with the prior art, the present disclosure has the following beneficial effects.

1. Given a frequency corresponding to a single temperature, the frequency at any temperature can be obtained by arranging a small number of sensors and carrying out a small quantity of tests and iterative solutions, so as to quantify the influence of temperature on modal frequency without the need for real-time monitoring of the frequency change under the temperature change, thereby obtaining the influence of temperature on the frequency more directly and effectively.

2. With the complexity of the modal frequency identification process taken into consideration, all that is required is to process a group of data of a newly-built beam bridge without damage at a single temperature to obtain a modal frequency value at the temperature, and then a modal frequency of the beam bridge at any temperature is available, which simplifies the process of modal frequency identification.

3. After the beam bridge is put into operation, all that is required is to process a group of data at any temperature, obtain a modal frequency value at the temperature, and compare it with a modal frequency at the corresponding temperature of a newly-built beam bridge without damage. In this way, the part of environmental influence is eliminated in damage evaluation of the beam bridge, which allows for a more accurate damage evaluation result.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a simply-supported beam bridge with a constant section of a finite element model according to the present disclosure;

FIG. 2 is a diagram of the acceleration response of a simply-supported beam at 25° C. according to the present disclosure;

FIG. 3 is a spectrogram of the acceleration response of a simply-supported beam at 25° C. according to the present disclosure;

FIG. 4 is a comparison diagram of first three orders of frequencies of a simply-supported beam obtained through theoretical iterations and tests according to the present disclosure;

FIG. 5 is a diagram of a two-span continuous beam bridge with a constant section of a finite element model according to the present disclosure;

FIG. 6 is a diagram of the acceleration response of a two-span continuous beam bridge at 25° C. according to the present disclosure;

FIG. 7 is a spectrogram of the acceleration response of a two-span continuous beam bridge at 25° C. according to the present disclosure; and

FIG. 8 is a comparison diagram of first three orders of frequencies of a two-span continuous beam obtained through theoretical iterations and tests according to the present disclosure.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In this embodiment, a method for identifying the modal frequency of a beam bridge by considering the influence of environmental temperature is provided. A T-shaped simply-supported beam with a constant section is taken as Embodiment 1, and a concentrated force with a magnitude of 80 kg passes through the beam bridge at a uniform speed of 0.1 m/s ; a model is established by finite element software; the elasticity modulus under the influence of different temperatures is set to obtain a model at different temperatures; and the beam bridge is divided into 20 planar Euler beam elements at an equal spacing, The finite element model of the simply-supported beam is shown in FIG. 1. The method includes the following steps:

Step 1: determine the length L and the section parameter of the beam bridge. In this embodiment, the beam has a length of 4.08 m , and moment of inertia, of the section of 1.355×10⁻³m⁴, the elasticity modulus E of the beam changes with temperature, and the mass per linear meter of 331.5 kg/m , In the length direction of the beam bridge, 1 acceleration sensor and 1 thermocouple are arranged in each span, as shown in FIG. 1.

Step 2: as shown in FIG. 2 obtain the acceleration response of the beam bridge subjected to a moving load at different temperatures using the acceleration sensor and the thermocouple with the sampling frequency of f_s=100Hz, select any of the temperatures as a reference temperature T₀=19° C. and obtain the reference temperature T₀ and the corresponding acceleration response;

Step 3: conduct Fourier transform on the acquired acceleration response, and obtain, by Peak-Picking (PP), first n orders of frequencies of the beam bridge at different temperatures which are denoted as {f_n*|n=1,2,3}, where f_n denotes the nth order measured frequency, frequency spectrum of the acceleration response of a simply-supported beam is shown in FIG. 3, and the first three orders of frequencies of the beam bridge measured at certain temperatures are shown in Table 1.

Select the reference temperature T₀=19° C. and the corresponding first n orders of frequencies {f_n(T₀)|n=1,2,3}, where f_n(T₀) denotes the nth order frequency corresponding to the reference temperature T₀. In this embodiment, reference temperature 19° C. and the corresponding first three orders of frequencies f_n(19) are selected, as shown in Table 1.

TABLE 1
First three orders of frequencies of a beam
bridge measured at certain temperatures
	T	−8	7	19	28	47
	f1*	36.279	34.848	33.660	32.740	30.709
	f2*	145.117	139.394	134.641	130.963	122.836
	f3*	326.523	313.646	302.950	294.674	276.389

Step 4: obtain a slope k₀ between the nth order frequency f_n(T₀) and the reference temperature T₀ according to formula (1):

$\begin{array}{cc}{k}_0=\frac{1}{2}{\beta }_E·f_n\left(T_0\right)& \left(1\right)\end{array}$

In formula (1), β_E d denotes the thermal coefficient of the elasticity modulus of the beam bridge's material. In this embodiment. 4.5×10⁻³° C⁻¹ is taken as β_E, and according to formula. (1), k₀ corresponding to three orders of frequencies of the beam bridge at 1.9° C. is −0,0757, 0,3029 and −0.6816.

Step 5: starting from a given point (T₀, f_n(T₀)), construct the slope k₀ in a frequency-temperature relation at an interval of T₀˜T₀+ΔT to obtain the nth order frequency f_n(T₀+ΔT) corresponding to the temperature T₀+ΔT where ΔT denotes the temperature increment;

f _n(T₀+ΔT)=f_n(T₀)+k₀ΔT   (2)

In this embodiment, ±1° C. is taken as ΔT, based on first three orders of frequencies f_n(19) corresponding to the reference temperature 19° C., the relation of the first three orders of frequencies and the temperature at the interval of 19° C.-18° C. and the relation of the first three orders of frequencies and the temperature at the interval of 19° C.-18° C. are constructed with the corresponding slope k₀ ; so that first three orders of frequencies f_n(20) and f_n(18) corresponding to temperatures of 20° C. and 18° C. are obtained.

Step 6: define a variable i, and initialize i to 1;

Step 7: obtain the nth order frequency f_n(T_i) corresponding to the ith temperature T_i between the reference temperature T₀ and the target temperature T_m according to formula (2):

f _n(T_i)=f_n(T_i-1)+k_i-1ΔT_i-1   (3)

In formula (2), ΔT_i-1 denotes the temperature interval between an (i-1)th temperature T_i-1 and the ith temperature T_i, and when i=1, T_i-1=T₀; f_n(T_i-1) denotes the nth order frequency corresponding to the (i-1)th temperature T_i-1, and when i=1, f_n(T_i-1)=f_n(T₀); and k_i-1 denotes the instantaneous slope between the nth order frequency f_n(T_i-1) and the (i-1)th temperature T_i-1, and when i=1, k_i-1=k₀. In this embodiment, ΔT_i-1 is ±1° C.

Step 8: obtain an ith slope k_i according to formula (3):

$\begin{array}{cc}{k}_i=\frac{1}{2}{\beta }_E·f_n\left(T_i\right)& \left(4\right)\end{array}$

Step 9: after assigning i+1 to i, return to Step 7, and perform sequential execution until i=m.

Step 10: obtain the nth order frequency f_n(T_m) corresponding to the target temperature T_m according to formula (3) and formula (4):

$\begin{array}{cc}{f}_n\left(T_m\right)=f_n\left(T_0\right)+\frac{1}{2}{\beta }_E\sum _{i=0}^{m-1}{f}_n\left(T_i\right)·\Delta T_i& \left(5\right)\end{array}$

In this embodiment, when the target temperature T_m is 47° C., ΔT_i is 1° C.; and when the target temperature T_m is −8° C. ΔT_i is −1° C. According to formula (3) and formula (4), first three orders of frequencies of the beam bridge at the target temperature and certain intermediate temperatures are calculated through theoretical iterations, as shown in Table 2.

TABLE 2
First three orders of frequencies of a beam bridge at certain
temperatures calculated through theoretical iterations
	T	−8	7	19	28	47
	f1	35.766	34.580	33.660	32.985	31.603
	f2	143.064	138.322	134.641	131.939	126.411
	f3	321.903	311.232	302.950	296.871	284.433

Step 11: compare the theoretical iteration result f with the test result f, , as shown in FIG. 4 . In the meanwhile, calculate the relative error e, according to formula (6), as shown in Table 3:

$\begin{array}{cc}{\epsilon }_n=\frac{❘f_n-f_n^{*}❘}{f_n^{*}}& \left(6\right)\end{array}$

TABLE 3
Relative error between the theoretical iteration result
and the test frequency result at certain temperatures
	T	−8	7	19	28	47
	εn	1.41%	0.77%	/	0.75%	2.91%

A two-span continuous beam with a constant rectangular section is taken as Embodiment 2. A concentrated force with a magnitude of 80kg passes through the beam bridge at a uniform speed of 0.1 m/s; a model is established by finite element software; the elasticity modulus under the influence of different temperatures is set to obtain a model at different temperatures; and the beam bridge is divided into 40 planar Euler beam elements at an equal spacing. The finite element of the two-span continuous beam with a constant section is shown in FIG. 5. As shown in FIG. 5, the length of the beam bridge is 4in , the beam bridge has a length of 2m each span, a width of the section of 0.3m a height of 0.1m , and a sectional moment of inertia of 2.5×10⁻⁵m⁴ the elasticity modulus E of the beam bridge changes with the temperature, and the mass per linear meter of 78 kg . In the length direction of the beam bridge, 1 acceleration sensor and I thermocouple are arranged in each span, as shown in FIG. 5. Refer to Embodiment 1 for the remaining operating steps. The acceleration response is measured at each span, and the sampling frequency is 100Hz, as shown in FIG. 6. The spectrogram of the acceleration response is shown in FIG. 7, and a comparison diagram of frequencies of the two-span continuous beam obtained through theoretical iterations and tests is shown in FIG. 8. First three orders of frequencies of the beam bridge measured are shown in Table 4 . In the process of theoretical iterations, ΔT_i is 1° C., ΔT_j is −1° C., and β_E is 4.5×10⁻³° C.⁻¹. The first three orders of frequencies calculated through theoretical iterations are shown in Table 5; and the relative error of frequencies obtained through theoretical iterations and tests is shown in Table 6.

TABLE 4
First three orders of frequencies of the beam
bridge measured at certain temperatures
	T	−8	7	19	28	47
	f1*	45.320	43.533	42.049	40.900	38.362
	f2*	70.799	68.007	65.688	63.893	59.929
	f3*	181.283	174.133	168.195	163.600	153.448

TABLE 5
First three orders of frequencies of the beam bridge at certain
temperatures calculated through theoretical iterations
	T	−8	7	19	28	47
	f1	44.679	43.198	42.049	41.205	39.478
	f2	69.797	67.484	65.688	64.370	61.673
	f3	178.718	172.793	168.195	164.820	157.915

TABLE 6
Relative error between the theoretical iteration result and first
three orders of the tested frequencies at certain temperatures
	T	−8	7	19	28	47
	εn	1.41%	0.77%	/	0.75%	2.91%

Embodiment 1and Embodiment 2 fully illustrate that the method of the present disclosure can identify the modal frequency at any temperature without the need for real-time monitoring of the beam bridge at any environmental temperature. At the same time, as lona as the modal frequency value under the influence of an individual temperature is obtained, the modal frequency value under the influence of arbitrary temperature can he obtained, which greatly simplifies the modal frequency processing process, In this way. ^-the influence of environmental temperature on modal frequency is accurately quantified, and the problem that a. vibration-based health monitoring system is affected by environmental temperature in damage identification is solved.

Claims

1. A method for identifying a modal frequency of a beam bridge by considering the influence of environmental temperature, comprising the following steps: Step 1: determining a length L and a section parameter of the beam bridge; and in a length direction of the beam bridge, arranging 1 acceleration sensor and 1 thermocouple in each span;Step 2: obtaining an acceleration response of the beam bridge subjected to a moving load at different temperatures using the acceleration sensor and the thermocouple with a sampling frequency of fs, selecting any of the temperatures as a reference temperature T0, and obtaining the reference temperature T0 and a corresponding acceleration response;Step 3: conducting Fourier transform on the acquired acceleration response to obtain first n orders of frequencies of the beam bridge which are denoted as {fn*|n=1, 2,3}, wherein fn* denotes an nth order measured frequency; andselecting the reference temperature T0 and corresponding first n orders of frequencies {fn(T0)|n=1,2,3}, wherein fn(T0) denotes an nth order frequency corresponding to the reference temperature T0;Step 4: obtaining a slope k0 between the nth order frequency fn(T0) and the reference temperature T0 according to formula (1):