How do i create Apply Boundary Conditions

How do I create a Boundary Conditions with GSM.

And here is my previous setup for Element 1,2,and 3 and a GSM with those 3 element.

Answer 1

[Mario Malic]

It seems that you know force boundary conditions for your nodes.

Following the equation:

[K] {u} = {F}

You do not know your displacements - the x vector, but you know the external forces at the nodes, the F vector.

What you are asked is to assign the external forces as

% Edit 4 nodes
Ext_F = [0; 0; 0 ;F] % Keep in mind, this is a row vector

The hint given to you about, that you don't need the complete global stiffness: you can check on the paper first, how can you obtain i.e. displacement u1.

[Shing Kwan Tang]

ok. But it seems like it said I need to extract the smaller matrix from the full GSM matrix. Cause I have a 4x4 matrix for GSM. For u1,u2,and u3 is the first 3 row and 3 column from the full GSM. in this situation, is there anny way to just get the 3x3 matrix from a 4x4 matrix? or I need to type like

mini_GSM = [ k11 k12 k13 ; k21 k22 k23 ; k31 k32 k33] 

to create a small matrix? thank you

[Mario Malic]

Oh, now I see. Mat = GSM(1:end-1, 1:end-1)

[Shing Kwan Tang]

ok, i get it , thx so much

[Mario Malic]

Please accept the answer if it solved your problem.