Download - CIVE 801 - Course Project Report (Teal)
CIVE 801 DYNAMICS OF STRUCTURES I COURSE PROJECT
Cyle Teal CIVE 801, Winter 14-15 March 16, 2015
Table of Contents INTRODUCTION ..................................................................................................................... 2
1. FRAME DESIGN ............................................................................................................. 2
Girder Sizing ......................................................................................................................... 4
Longitudinal Girder weight .................................................................................................. 4
Deck Weight ......................................................................................................................... 4
Overall Weight ....................................................................................................................... 5
2. NATURAL FREQUENCY AND PERIOD ....................................................................... 5
Stiffness .................................................................................................................................. 5
Mass ....................................................................................................................................... 6
Natural Frequency ................................................................................................................ 6
Natural Period ....................................................................................................................... 6
3. UNDAMPED FREE VIBRATION .................................................................................. 6
4. VISCOUSLY DAMPED FREE VIBRATION ................................................................... 7
Part (i) ................................................................................................................................... 8
Part (ii) .................................................................................................................................. 9
5. STRUCTURE SUBJECTED TO SINUSOIDAL HORIZONTAL FORCE ...................... 10
6. LIMITS FOR FREQUENCY OF VIBRATING MACHINE ............................................. 11
7. PERIODIC RECTANGULAR WAVE ............................................................................ 12
8. STRUCTURAL RESPONSE TO ARBITRARY EXCITATION ...................................... 15
CONCLUSION ........................................................................................................................ 17
APPENDIX............................................................................................................................... 18
1. System Characteristics ................................................................................................. 18
2. Determine natural frequency and period of structure ............................................... 18
3. Subject structure to initial conditions ......................................................................... 18
4. Assume damping ratios of 2%, 5%, 15%, 70%, 100% and 150% .................................. 19
5. Subject structure to a sinusoidal horizontal force ...................................................... 21
6. Limits for the freqency of vibrating machine for TR = 20% condition ...................... 22
7. Fourier representation of periodic rectangular wave ................................................. 23
8. Response of structure to arbitrary excitation ............................................................. 25
PAGE 1
INTRODUCTION CIVE 801 covered dynamics of single-degree-of-freedom (SDF) systems. The course project, detailed herein, required a realistic frame to be designed and idealized as a SDF system. This report presents the results of the analyses and computations completed to fulfill the requirements for the project. The MATLAB code used to complete the project is provided in the appendix.
1. FRAME DESIGN The structure chosen for the project, a bridge supported by two abutments and two bents, is an adaptation of the one in Example 1.3 of the course text1 and shown in Figure 1.
Figure 1. 375-ft-long concrete bridge on four supports.
1 Chopra, Anil K. Dynamics of Structures, Theory and Applications to Earthquake Engineering, 4th ed., 2012. Pearson Education, Inc.
PAGE 2
The bridge can be represented as a frame (see Figure 2) with each bent providing a longitudinal stiffness ๐๐bent.
Figure 2. Frame representation of the bridge.
In the problem statement for Example 1.3, it was given that each bent consisted of three 25-ft-tall columns of circular section with ๐ผ๐ผ๐ฆ๐ฆโฒ = ๐ผ๐ผ๐ง๐งโฒ = 13 ft4. These values were assumed for the project.
The problem also gave the following specifications:
โข Box-girder bridge โข Cross-sectional area of the bridge deck = 123 ft2 โข Unit weight of concrete = 150 lb/ft3
Of these three, only the last was assumed. A multi-girder design was selected instead of a box-girder. The multi-girder cross section shown in Figure 3 was assumed for the project. Four longitudinal plate girders of equal depth are arranged at uniform spacing across the width of the bridge. The deck slab spans transversely across the longitudinal girders and cantilevers outside the outer girders. The girders were assumed to be spaced 12 ft on center and the deck was assumed to have a constant thickness of 1 ft across the 43-ft-width. Before the weight per unit length of the cross section could be calculated, the girders had to be realistically sized for a bridge of this span.
Figure 3. Cross section of a multi-girder bridge2
2 Taken from steelconstruction.info
PAGE 3
GIRDER SIZING
The longitudinal girders were proportioned using the recommended girder proportions found on steelconstruction.info3. The following table presents the values used to calculate the cross-sectional area of each longitudinal girder.
Table 1. Girder Element Sizes.
Element Value (in)
Girder Depth 50
Top flange width 14
Top flange thickness 1
Bottom flange width 25
Bottom flange thickness 2.25
Web 0.75
The sizes in Table 1 resulted in the following girder cross-sectional area
๐ด๐ดst = 105.3 in2
LONGITUDINAL GIRDER WEIGHT
Assuming a density of steel, ๐๐st = 0.28 lbin3
, the longitudinal girder contribution to the overall weight per unit length was calculated
๐ค๐คg = 4 ๐๐st ๐ด๐ดst = 4 ๏ฟฝ0.28lb
in3๏ฟฝ (105.3 in2) ๏ฟฝ
1 kip1000 lb
๏ฟฝ ๏ฟฝ12 in1 ft
๏ฟฝ = 1.42 kipft
DECK WEIGHT
Assuming a density of concrete, ๐๐c = 150 lbft3
, the deck contribution to the overall weight
per unit length, ๐ค๐คc, was calculated
๐ค๐คc = ๐๐c ๐ด๐ดc = ๏ฟฝ150lbft3๏ฟฝ (43 ft)(1 ft) ๏ฟฝ
1 kip1000 lb
๏ฟฝ = 6.45 kipft
3 http://www.steelconstruction.info/Bridges_-_initial_design, based upon a typical highway bridge of two or more spans carrying a single carriageway over a dual 2 or dual 3 lane highway.
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OVERALL WEIGHT
The overall weight per unit length, ๐ค๐ค, was calculated by taking the sum of the deck contribution and the girder contribution multiplied by a factor to account for the undetermined weight of the cross girders and bracing
๐ค๐ค = 2๐ค๐คst + ๐ค๐คc = 2(1.42) + 6.45 = 9.29 kipft
Multiplying by the total length of the bridge gives the total weight that is to be lumped at the deck level
๐๐ = ๐ค๐ค๐ค๐ค = ๏ฟฝ9.29 kipft๏ฟฝ (375 ft) = 3484 kips
2. NATURAL FREQUENCY AND PERIOD
STIFFNESS
The stiffness of each column in a bent is calculated
๐๐col =12๐ธ๐ธ๐ผ๐ผโ3
In Section 1, the column height, โ, and moment of inertia, ๐ผ๐ผ, were specified as 25 ft and 13 ft4, respectively. The assumed elastic modulus of concrete was ๐ธ๐ธ = 3000 ksi. Substituting these values, the column stiffness is
๐๐col =12 (3000 ksi) (13ft2)
(25 ft)3 ๏ฟฝ144 in2
1 ft2๏ฟฝ = 4313
kipft
Each bent consists of three columns, therefore the bent stiffness, assuming the bridge displaces rigidly as shown in Figure 2, is
๐๐bent = 3 ๐๐col = 3 ๏ฟฝ4313 kipft๏ฟฝ = 12940
kipft
The structure has two bents, therefore the system stiffness is
๐๐ = 2๐๐bent = 2 ๏ฟฝ12940 kipft๏ฟฝ = 25800
kipft
PAGE 5
MASS
The mass lumped at the deck level is simply the weight divided by the acceleration due to gravity. Written mathematically,
๐๐ =๐๐๐๐
The weight was specified at the end of Section 1 as 3484 kips. Assuming a value ๐๐ =32.2 ft
sec2, the system mass was computed as
๐๐ =3484 kips
32.2 ftsec2
= 108.2 kip sec2
ft
NATURAL FREQUENCY
The following equation shows the natural frequency, ๐๐๐๐, being a function of the two system characteristics just presented:
๐๐๐๐ = ๏ฟฝ๐๐๐๐
Substituting the values for the system mass and stiffness, the natural frequency is
๐๐๐๐ = ๏ฟฝ25800 kip/ft
108.2 kip sec2/ft= 15.4
radsec
NATURAL PERIOD
The natural period, ๐๐๐๐, can easily be calculated by
๐๐๐๐ =2๐๐๐๐๐๐
Given the calculated natural frequency, the natural period is
๐๐๐๐ =2๐๐
15.4 sec= 0.41 sec
3. UNDAMPED FREE VIBRATION The undamped free vibration response of the structure to two distinct cases is shown in Figure 4.
PAGE 6
Figure 4. Free vibration response.
The initial conditions for the two distinct cases were
i. ๐ข๐ข(0) = 0.5 ft, ๏ฟฝฬ๏ฟฝ๐ข(0) = 10 ftsec
ii. ๐ข๐ข(0) = 0.2 ft, ๏ฟฝฬ๏ฟฝ๐ข(0) = โ10 ftsec
The mathematical solution of the structureโs response in both cases is
i. ๐ข๐ข(๐ก๐ก) = 0.5 cos 15.4๐ก๐ก + 1015.4
sin 15.4๐ก๐ก ft
ii. ๐ข๐ข(๐ก๐ก) = 0.2 cos 15.4๐ก๐ก โ 1015.4
sin 15.4๐ก๐ก ft
4. VISCOUSLY DAMPED FREE VIBRATION In this section, the structure was assumed to have a damping ratio of 0.02, 0.05, 0.15, 0.7, 1.0 and 1.5. For each damping ratio, the natural frequency of damped vibration can be calculated
๐๐๐ท๐ท = ๐๐๐๐๏ฟฝ1 โ ๐๐2
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PART (I)
For the selected initial conditions in case (ii), the mathematical solution for the three distinct cases of damped motion (underdamped, critically damped, and overdamped) is given
1) Underdamped, ๐๐ < 1
๐ข๐ข(๐ก๐ก) = ๐๐โ๐๐๐๐๐๐๐ก๐ก ๏ฟฝ๐ข๐ข(0) cos๐๐๐ท๐ท๐ก๐ก +๏ฟฝฬ๏ฟฝ๐ข(0) + ๐๐๐๐๐๐๐ข๐ข(0)
๐๐๐ท๐ทsin๐๐๐ท๐ท๐ก๐ก๏ฟฝ
2) Critically damped, ๐๐ = 1 ๐ข๐ข(๐ก๐ก) = ๐๐โ๐๐๐๐๐๐๐ก๐ก [๐ข๐ข(0) + (๏ฟฝฬ๏ฟฝ๐ข(0) + ๐๐๐๐๐ข๐ข(0))๐ก๐ก]
3) Overdamped, ๐๐ > 1
๐ข๐ข(๐ก๐ก) = ๐ถ๐ถ1๐๐๏ฟฝโ๐๐+๏ฟฝ๐๐2โ1๏ฟฝ๐๐๐๐๐ก๐ก + ๐ถ๐ถ2๐๐
๏ฟฝโ๐๐โ๏ฟฝ๐๐2โ1๏ฟฝ๐๐๐๐๐ก๐ก where
๐ถ๐ถ1 =๐ข๐ข(0)๐๐๐๐๏ฟฝ๐๐ + ๏ฟฝ๐๐2 โ 1๏ฟฝ + ๏ฟฝฬ๏ฟฝ๐ข(0)
2๐๐๐๐๏ฟฝ๐๐2 โ 1
๐ถ๐ถ2 =โ๐ข๐ข(0)๐๐๐๐๏ฟฝ๐๐ โ ๏ฟฝ๐๐2 โ 1๏ฟฝ โ ๏ฟฝฬ๏ฟฝ๐ข(0)
2๐๐๐๐๏ฟฝ๐๐2 โ 1
A solution was obtained for each damping ratio with MATLAB. The results are shown in Figure 5. The result for each underdamped case was plotted blue in color to help make a comparison of the underdamped cases. As expected, a response of an underdamped system shows a decrease in response amplitude with time. A second observation is the rate at which the amplitudes decreases follows the damping ratio (i.e., the underdamped solution with the largest damping ratio dies out the fastest). Also, it can be observed from the response of the critically damped and overdamped system that no oscillatory motion is present for these cases, another expected result.
PAGE 8
Figure 5. Damped responses to initial conditions.
PART (II)
For each underdamped case, the ratio of two consecutive amplitudes was evaluated using
๐ข๐ข๐๐๐ข๐ข๐๐+1
= exp๏ฟฝ2๐๐๐๐
๏ฟฝ1 โ ๐๐2๏ฟฝ
and the logarithmic decrement, ๐ฟ๐ฟ, using
๐ฟ๐ฟ =2๐๐๐๐
๏ฟฝ1 โ ๐๐2
The following table presents the ratio and logarithmic decrement for each underdamped case.
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Table 2. Ratio of two consecutive amplitudes
Damping ratio, ๐ป๐ป ๐๐๐๐๐๐๐๐+๐๐
๐น๐น
0.02 1.13 0.13
0.05 1.37 0.31
0.15 2.59 0.95
0.7 472.84 6.16
5. STRUCTURE SUBJECTED TO SINUSOIDAL HORIZONTAL FORCE The frequency of the exciting force was varied as 0.5, 1, 2 and 3 times the natural frequency of the system. For the purposes of this project, this quantity is termed the frequency ratio ๐๐. The amplification factor for each frequency ratio was determined using
๐ ๐ ๐๐ =1
๏ฟฝ(1 โ ๐๐2)2 + (2๐๐๐๐)2
where
๐๐ =๐๐๐๐๐๐
and
๐๐ = 0.1
The following table presents the amplification factor for each frequency ratio.
Table 3. Amplification factors.
๐๐ ๐น๐น๐ ๐
0.5 1.32
1.0 5.0
2.0 0.33
3.0 0.12
PAGE 10
6. LIMITS FOR FREQUENCY OF VIBRATING MACHINE The limit for the frequency of a vibrating machine installed on the deck of the bridge was determined so that the amplitude of the transmitted force to the deck is 20% of the exciting amplitude. Stated differently, for a given damping ratio, the frequency at which the transmissibility, ๐๐๐ ๐ , is equal to 0.2 was determined. Transmissibility vs. frequency ratio is shown in Figure 6. The red squares show the intersections of each curve with the line ๐๐๐ ๐ = 0.2. The frequency at each of these locations is presented in Table 4.
Figure 6. Transmissibility Plot.
Table 4. Frequencies for TR = 0.2.
๐ป๐ป ๐๐ [๐๐๐๐]
0.02 6.03
0.05 6.09
0.15 6.69
0.70 17.30
PAGE 11
In summary, for a lightly damped, ๐๐ โค 0.15, vibrating machine, the excited frequency should be
๐๐ > 6.69 Hz
For a significantly damped, ๐๐ โค 0.7, machine, the exciting frequency should be
๐๐ > 17.30 Hz
7. PERIODIC RECTANGULAR WAVE A periodic rectangular wave (plotted black in color in the following four figures) with unit amplitude and period ๐๐ was solved in class. The period was selected as 2 sec for the project. From observation of the results presented in Figure 7 - Figure 10, seven Fourier terms are needed to sufficiently reproduce its shape. No noticeable difference is seen after increasing the number of terms from seven to eight, therefore seven was the number chosen.
Figure 7. Fourier series with n = 1 and n = 2 terms.
PAGE 12
Figure 8. Fourier series with n = 3 and n = 4 terms.
PAGE 13
Figure 9. Fourier series with n = 5 and n = 6 terms.
PAGE 14
Figure 10. Fourier series with n = 7 and n = 8 terms.
8. STRUCTURAL RESPONSE TO ARBITRARY EXCITATION The arbitrary excitation selected for the project was
๐น๐น(๐ก๐ก) = ๐น๐น0(1 โ ๐ก๐ก2) + ๐น๐น1 sin๐๐1๐ก๐ก + ๐น๐น2 sin๐๐2๐ก๐ก
where
๐น๐น0 = 15 kip, ๐น๐น1 = 4 kip, ๐น๐น2 = 2 kip, ๐๐1 = 18 radsec
and ๐๐2 = 59 radsec
This function is shown in Figure 11.
PAGE 15
Figure 11. Arbitrary excitation.
The response of the bridge to this excitation was determined by numerically integrating Duhamelโs integral with MATLAB. A damping ratio of 0.1 was assumed for the bridge. The excitation was applied for a duration of 0.8 sec and the response is shown in Figure 12; the portion of the response during the excitation is plotted in blue and the portion after in orange.
PAGE 16
Figure 12. Response to arbitrary excitation.
CONCLUSION The CIVE 801 course project was an alternative way to demonstrate knowledge of dynamics of SDF systems. A significant effort was made to be thorough and clear in the presentation of the work completed to fulfill the requirements of the project. From a studentโs perspective, the author considers the project more beneficial, and much more enjoyable, than studying for and taking a final exam. All parts of the project, except for the frame design, were completed in MATLAB. The appendix contains the MATLAB code for the project.
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APPENDIX This appendix was published with MATLABยฎ R2014b
1. SYSTEM CHARACTERISTICS
Stiffness, [kip/ft]
k = 25800;
Mass, [kip-sec2/ft]
m = 108.2;
2. DETERMINE NATURAL FREQUENCY AND PERIOD OF STRUCTURE
Natural frequency, [rad/sec]
w_n = sqrt(k/m)
w_n =
15.4417
Natural period, [sec]
tau_n = 2*pi/w_n
tau_n =
0.4069
3. SUBJECT STRUCTURE TO INITIAL CONDITIONS
Mathematical solution for free vibration without damping
syms u0 v0 t
u_nodamp = u0*cos(w_n*t)+(v0/w_n)*sin(w_n*t);
t = linspace(0,3*tau_n);
PAGE 18
Case (i):
u0 = 0.5; % [ft]
v0 = 10; % [ft/s]
u_i = double(subs(u_nodamp));
Case (ii):
u0 = 0.2; % [ft]
v0 = -10; % [ft/s]
u_ii = double(subs(u_nodamp));
Plot the response from case (i) and case (ii)
figure(1)
plot(t,u_i,t,u_ii)
xlabel('t (s)')
ylabel('u (ft)')
legend('Case (i)','Case (ii)')
4. ASSUME DAMPING RATIOS OF 2%, 5%, 15%, 70%, 100% AND 150%
4(i)
Mathematical solution for underdamped, , cases
syms zeta t
w_D = w_n*sqrt(1-zeta^2);
u_underDamp = exp(-zeta*w_n*t)*(u0*cos(w_D*t)+...
(v0+zeta*w_n*u0)*sin(w_D*t)/w_D);
Mathematical solution for critically damped, , case
u_critDamp = exp(-w_n*t)*(u0+(v0+w_n*u0)*t);
Mathematical solution for overdamped, , case
C1 = (u0*w_n*(zeta+sqrt(zeta^2-1))+v0)/(2*w_n*sqrt(zeta^2-1));
C2 = (-u0*w_n*(zeta-sqrt(zeta^2-1))-v0)/(2*w_n*sqrt(zeta^2-1));
u_overDamp = C1*exp((-zeta+sqrt(zeta^2-1))*w_n*t)+...
C2*exp((-zeta-sqrt(zeta^2-1))*w_n*t);
PAGE 19
Plot results
zeta_vec = [0 .02 .05 .15 .7 1 1.5];
t_vec = linspace(0,6*tau_n,200)';
u_results = zeros(length(t),length(zeta));
for j = 1:length(zeta_vec)
zeta = zeta_vec(j);
for k = 1:length(t_vec)
t = t_vec(k);
if zeta < 1
u_results(k,j) = double(subs(u_underDamp));
elseif zeta == 1
u_results(k,j) = double(subs(u_critDamp));
else
u_results(k,j) = double(subs(u_overDamp));
end
end
end
figure(2)
plot(t_vec,u_results(:,1),'k',...
t_vec,u_results(:,2),'b--',...
t_vec,u_results(:,3),'b:',...
t_vec,u_results(:,4),'b-.',...
t_vec,u_results(:,5),'b-',...
t_vec,u_results(:,6),'r',...
t_vec,u_results(:,7),'g')
legend('\zeta = 0',...
'\zeta = 0.02',...
'\zeta = 0.05',...
'\zeta = 0.15',...
'\zeta = 0.7',...
'\zeta = 1',...
'\zeta = 1.5',...
'Location','NorthEastOutside')
axis([0 6*tau_n -1 1])
set(gca,'XTick',0.5:0.5:2,'YTick',-1:0.5:1)
text(.6,.8,'ICs: u(0) = 0.2 ft, v(0) = -10 ft/s')
xlabel('Time (s)')
ylabel('Displacement (ft)')
4(ii)
Expression for ratio two consecutive amplitudes
r = @(zeta) exp(2*pi*zeta/sqrt(1-zeta^2));
ln_r = @(zeta) 2*pi*zeta/sqrt(1-zeta^2);
zeta_vec = [.02 .05 .15 .7];
PAGE 20
Evaluate ๐ข๐ข๐๐๐ข๐ข๐๐+1
at underdamped cases
for i = 1:length(zeta_vec)
ratios(i) = r(zeta_vec(i));
decrements(i) = ln_r(zeta_vec(i));
end
Print table
fprintf('%15s%15s%25s\n','damping ratio','u_i/u_(i+1)','logarithmic decrement')
fprintf('%15s%15s%25s\n','-------------','-----------','---------------------')
for i = 1:length(ratios)
fprintf('%15.2f%15.2f%25.2f\n',zeta_vec(i),ratios(i),decrements(i))
end
fprintf('\n')
damping ratio u_i/u_(i+1) logarithmic decrement
------------- ----------- ---------------------
0.02 1.13 0.13
0.05 1.37 0.31
0.15 2.59 0.95
0.70 472.84 6.16
5. SUBJECT STRUCTURE TO A SINUSOIDAL HORIZONTAL FORCE
Assume damping ratio
zeta = 0.1;
Expression for amplification factor
R_d = @(r) 1/sqrt((1-r^2)^2+(2*zeta*r)^2);
Vector containing values of ๐๐๐๐๐๐
at which ๐ ๐ ๐๐ will be evaluated
r_values = [0.5 1 2 3];
Evaluate
PAGE 21
for i = 1:length(r_values)
Rd_values(i) = R_d(r_values(i));
end
Table
fprintf('%24s%24s\n','Frequency ratio (w/w_n)','Amplification Factor')
fprintf('%24s%24s\n','-----------------------','--------------------')
for i = 1:length(r_values)
fprintf('%24.1f%24.2f\n',r_values(i),Rd_values(i))
end
fprintf('\n')
Frequency ratio (w/w_n) Amplification Factor
----------------------- --------------------
0.5 1.32
1.0 5.00
2.0 0.33
3.0 0.12
6. LIMITS FOR THE FREQENCY OF VIBRATING MACHINE FOR TR = 20% CONDITION
Expression for transmissibility, TR
syms r positive
syms zeta
TR = sqrt((1+(2*zeta*r)^2)/((1-r^2)^2+(2*zeta*r)^2));
Output vector for ratios and forcing frequencies that satisfy TR condition
r = zeros(1,length(zeta_vec));
w = zeros(1,length(zeta_vec));
Results
for i = 1:length(zeta_vec)
[S.r,~]=solve(TR==0.2,zeta==zeta_vec(i),'Real',true,...
'IgnoreAnalyticConstraints', true);
r(i) = double(S.r);
w(i) = r(i)*w_n/(2*pi);
end
TR = @(r,zeta) sqrt((1+(2*zeta*r)^2)/((1-r^2)^2+(2*zeta*r)^2));
r_plot = linspace(0,8,200);
PAGE 22
TR_plot = zeros(length(zeta_vec),length(r_plot));
for i = 1:length(zeta_vec)
for j = 1:length(r_plot)
TR_plot(i,j) = TR(r_plot(j),zeta_vec(i));
end
end
Table
fprintf('%15s%31s\n','damping ratio','frequency for TR = 0.2 [Hz]')
fprintf('%15s%31s\n','-------------','---------------------------')
for i = 1:length(w)
fprintf('%15.2f%31.2f\n',zeta_vec(i),w(i))
end
fprintf('\n')
damping ratio frequency for TR = 0.2 [Hz]
------------- ---------------------------
0.02 6.03
0.05 6.09
0.15 6.69
0.70 17.30
Plot
figure(3)
plot(r_plot,TR_plot(1,:),...
r_plot,TR_plot(2,:),...
r_plot,TR_plot(3,:),...
r_plot,TR_plot(4,:),...
[r_plot(1) r_plot(length(r_plot))],[0.2 0.2],'k:',...
[r_plot(1) r_plot(length(r_plot))],[1 1],'k-',...
[1 1],[0 15],'k-',...
r(1),TR(r(1),zeta_vec(1)),'rs',...
r(4),TR(r(4),zeta_vec(4)),'rs')
ylabel('TR')
xlabel('\omega/\omega_n')
axis([0 8 0 15])
set(gca,'YTick',[0.2,1:15])
legend('\zeta = 0.02',...
'\zeta = 0.05',...
'\zeta = 0.15',...
'\zeta = 0.7')
7. FOURIER REPRESENTATION OF PERIODIC RECTANGULAR WAVE
Rectangular wave parameters
PAGE 23
Fo = 2; % [kip]
tau = 2; % [s]
Data for rectangular wave plot
t_top = -2*tau:tau/2:3*tau/2;
t_bot = -3*tau/2:tau/2:2*tau;
x_top = (Fo/2)*ones(1,8);
x_bot = -1*x_top;
Fourier Series
f = @(t,n) (2*Fo/(pi*n))*sin(2*pi*n*t/tau);
Number of terms for approximation
num = 8;
Approximations
f_approx = 0;
t_values = linspace(-2*tau,2*tau,200);
f_values = zeros(num,length(t_values));
for i = 1:num
syms t
n = 2*i-1;
f_approx = f_approx + f(t,n);
for j = 1:length(t_values)
t = t_values(j);
f_values(i,j) = double(subs(f_approx));
end
end
Plot
for i = 1:4
figure
% Rectangular wave
plot(t_top(1:2),x_top(1:2),'k-',t_bot(1:2),x_bot(1:2),'k-')
hold on
plot(t_top(3:4),x_top(3:4),'k-',t_bot(3:4),x_bot(3:4),'k-')
plot(t_top(5:6),x_top(5:6),'k-',t_bot(5:6),x_bot(5:6),'k-')
plot(t_top(7:8),x_top(7:8),'k-',t_bot(7:8),x_bot(7:8),'k-')
for j = 2:8
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plot([t_top(j) t_top(j)],[1 -1],'k--')
end
axis([-2*tau 2*tau -2 2])
xlabel('Time (s)')
ylabel('Force (kip)')
set(gca,'YTick',-2:2)
% Fourier approximations
plot(t_values,f_values(2*i-1,:))
plot(t_values,f_values(2*i,:))
legend(['n = ' num2str(2*i-1)],...
['n = ' num2str(2*i)])
end
8. RESPONSE OF STRUCTURE TO ARBITRARY EXCITATION
Excitation function
F0 = 15; % [kip]
F1 = 4; % [kip]
F2 = 2; % [kip]
w1 = 18; % [rad/s]
w2 = 59; % [rad/s]
F = @(x) F0.*(1-x.^2)+F1.*sin(w1.*x)+F2.*sin(w2.*x);
Plot function
t = linspace(0,0.8);
F_values = zeros(1,length(t));
for i = 1:length(t)
F_values(i) = F(t(i));
end
figure
plot(t,F_values)
xlabel('Time (s)')
ylabel('Force (kip)')
Unit impulse-response function
zeta = 0.1;
w_D = w_n*sqrt(1-zeta^2);
h = @(x,t) -(1/w_D).*exp(-zeta*w_n.*(t-x)).*sin(w_D.*(t-x));
Function for Duhamel's integral
fun = @(x,t) F(x).*h(x,t);
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Response during excitation
t1 = linspace(0,0.8);
u1 = zeros(1,length(t1));
for i=1:length(t1)
u1(i) = integral(@(tau)fun(tau,t1(i)),0,t1(i));
end
Response after excitation
idx = length(u1);
u0 = u1(idx);
v0 = (u1(idx)-u1(idx-2))/(t1(idx)-t1(idx-2));
t2 = linspace(0,4);
u_underDamp = @(t) exp(-zeta*w_n.*t).*(u0*cos(w_D.*t)+...
(v0+zeta*w_n*u0).*sin(w_D.*t)/w_D);
u2 = u_underDamp(t2);
figure
plot(t1,u1,t2+0.8,u2)
xlabel('Time (s)')
ylabel('Displacment (ft)')
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