you really only need 1 additional shipping rule to add the additional shipping.
lets say the base shipping is $20 for <= 15kg and $1 for each additional kg
Create the first rule using the trigger:
Subtotal for any column
Subtotal for column 'TotalWeight' is '>' a value of '0'
and use a flat rate calculation of 20.
For the second rule, use the trigger:
Subtotal for any column
Subtotal for column 'TotalWeight' is '>' a value of '15'
for the calculation, use:
Multiple Of Column Subtotal
Subtotal of column 'TotalWeight' 'times' '1' 'minus' '15'