Why would it assemble? i:i+1 is a vector of length 2. You use it for both indices of K, so K(i:i+1,i:i+1) is going to talk about a 2 x 2 array. You take that 2 x 2 array extracted from K and you add all of ke to it. If ke is not a compatible size to the 2 x 2 array you extracted, then the addition is going to fail.
The compatible sizes of array that can be added to a 2 x 2 array are:
- 1 x 2 x anything
- 2 x 1 x anything
- 2 x 2 x anything
If you want to add all of ke to something extracted from K, then you need the extracted size to be compatible with the size of ke. For example,
NE=2;
NN=NE+1;
NDOF=NN*2
ke=[1 -1 3;-1 1 5];
[r, c, ~] = size(ke);
K=zeros(NDOF);
for i=1:NE
K(i:i+r-1,i:i+c-1)=K(i:i+r-1,i:i+c-1)+ke;
end
K
