Plot bending moment diagram based on provided formula

Hi, I have 2 questions regarding this. I already have equations (I upload it as a picture below please check it out) but apparently I am making mistakes on applying them cause the result are quite different than my expectation.( I guess i am making mistakes based on L and L_span) Could you please tell me what part of my code is wrong ? I am about to upload the equation . And my second question is aside from making mistakes in formula I can not plot it with non-integer steps . Means id i put x=1:0.5:14 then M(1.5) make an error so I gotta stick with only 14 valid integer in my preferred domain.thanks!

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x=input('Enter your preferred span : ') q=25; p=400;

    if (x>=0 && x<=4)
    L=4;
    mu=(q*L*x/2)-(q*x*x/2);
    mleft=-69.14;
    mright=-238.27;
    M=mu+(mleft*(4-x)/4)+(mright*x/4)

    elseif (x>4 && x<7)
    L=6;
    mu=p*x/4;
    mleft=-238;
    mright=-218;
    M=mu+(mleft*(L-x)/L)+(mright*x/L)

    elseif (x>=7 && x<10)
    L=6;
    mu=p*(L-x)/4;
    mleft=-238.27;
    mright=-218.52;
    M=mu+(mleft*(L-x)/L)+(mright*x/L)

    elseif (x>=10 && x<=14)
    L=4;
    mu=(q*L*x/2)-(q*x*x/2);
    mleft=-218.52;
    mright=0;
    M=mu+(mleft*(L-x)/L)+(mright*x/L)

    else 
    disp('Warning! your span is over than the length of the beam')
    end

    %%Plot Bending Diagram

       q=25;
    p=400;
for x=1:14

    if (x>=0 && x<=4)
    L=4;
    mu=(q*L*x/2)-(q*x*x/2);
    mleft=-69.14;
    mright=-238.27;
    M(x)=mu+(mleft*(4-x)/4)+(mright*x/4);

    elseif (x>4 && x<7)
    L=6;
    mu=p*x/4;
    mleft=-238;
    mright=-218;
    M(x)=mu+(mleft*(L-x)/L)+(mright*x/L);

    elseif (x>=7 && x<10)
    L=6;
    mu=p*(L-x)/4;
    mleft=-238.27;
    mright=-218.52;
    M(x)=mu+(mleft*(L-x)/L)+(mright*x/L);

    elseif (x>=10 && x<=14)
    L=4;
    mu=(q*L*x/2)-(q*x*x/2);
    mleft=-218.52;
    mright=0;
    M(x)=mu+(mleft*(L-x)/L)+(mright*x/L);

    end
end

x=1:14; plot(x,M,'rs-','MarkerSize',3,'MarkerFaceColor','m') grid on xlabel('x (m)') ylabel('M (kN.m)') title('Bending moment diagram')

Answer 1

[KSSV]

This :

 put x=1:0.5:14 then M(1.5) 

Should be replaced with:

  x=1:0.5:14 ;
  for i = 1:length(x)
      M(i) = % some caclulation using x(i)
  end

Note, MATLAB indices should be always positive integers or logicals.

You can modify your code as below:

  x=input('Enter your preferred span : ') 
q=25; p=400;
if (x>=0 && x<=4)
    L=4;
    mu=(q*L*x/2)-(q*x*x/2);
    mleft=-69.14;
    mright=-238.27;
    M=mu+(mleft*(4-x)/4)+(mright*x/4)
    
elseif (x>4 && x<7)
    L=6;
    mu=p*x/4;
    mleft=-238;
    mright=-218;
    M=mu+(mleft*(L-x)/L)+(mright*x/L)
    
elseif (x>=7 && x<10)
    L=6;
    mu=p*(L-x)/4;
    mleft=-238.27;
    mright=-218.52;
    M=mu+(mleft*(L-x)/L)+(mright*x/L)
    
elseif (x>=10 && x<=14)
    L=4;
    mu=(q*L*x/2)-(q*x*x/2);
    mleft=-218.52;
    mright=0;
    M=mu+(mleft*(L-x)/L)+(mright*x/L)
    
else
    disp('Warning! your span is over than the length of the beam')
end

%%Plot Bending Diagram

q=25;
p=400;
X = 1:0.1:14 ; 

for i=1:length(X)
        x = X(i) ; 
    
    if (x>=0 && x<=4)
        L=4;
        mu=(q*L*x/2)-(q*x*x/2);
        mleft=-69.14;
        mright=-238.27;
        M(i)=mu+(mleft*(4-x)/4)+(mright*x/4);
        
    elseif (x>4 && x<7)
        L=6;
        mu=p*x/4;
        mleft=-238;
        mright=-218;
        M(i)=mu+(mleft*(L-x)/L)+(mright*x/L);
        
    elseif (x>=7 && x<10)
        L=6;
        mu=p*(L-x)/4;
        mleft=-238.27;
        mright=-218.52;
        M(i)=mu+(mleft*(L-x)/L)+(mright*x/L);
        
    elseif (x>=10 && x<=14)
        L=4;
        mu=(q*L*x/2)-(q*x*x/2);
        mleft=-218.52;
        mright=0;
        M(i)=mu+(mleft*(L-x)/L)+(mright*x/L);
        
    end
end

plot(X,M,'rs-','MarkerSize',3,'MarkerFaceColor','m') 
grid on 
xlabel('x (m)') 
ylabel('M (kN.m)') 
title('Bending moment diagram')

[Fatemeh Salar]

Thank you so much.