the global stiffness of two 4x4 matrices of truss

Question

Mohammad 2023-9-21

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https://ww2.mathworks.cn/matlabcentral/answers/2024162-the-global-stiffness-of-two-4x4-matrices-of-truss

评论： Mohammad 2023-9-22

How do I make a code to assemble these two 4x4 matrices to get the 6x6 global stiffness matric as shown?

clear all;
clc;
K1=[1 1 -1 -1;
    1 1 -1 -1;
    -1 -1 1 1;
    -1 -1 1 1];
K2=[1 -1 -1 1;
    -1 1 1 -1;
    -1 1 1 -1;
    1 -1 -1 1];
%here are the two matrices

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Answer 1

Fifteen12 2023-9-21

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https://ww2.mathworks.cn/matlabcentral/answers/2024162-the-global-stiffness-of-two-4x4-matrices-of-truss#answer_1315452

编辑：Fifteen12 2023-9-21

在 MATLAB Online 中打开

To add the matrices together, just arrange them by indices.

% Dummy values for k1 and k2
k1 = ones(4,4);
k2 = ones(4,4) * 2;
% Calculate stiffness matrix
K = zeros(6,6);
K(1:4, 1:4) = k1;
K(3:6, 3:6) = K(3:6,3:6) + k2
K = 6×6
     1     1     1     1     0     0
     1     1     1     1     0     0
     1     1     3     3     2     2
     1     1     3     3     2     2
     0     0     2     2     2     2
     0     0     2     2     2     2

You can make this more robust by using variables instead of hard coding the indices, but this minimal example shows the concept. Good luck with your finite elements!