Contributions to and payments from the general reserve fund shall be as follows: On account of each member there shall be paid annually into the fund by the city an amount equal to a certain percentage of the earnable compensation of the member to be known as “the normal contribution” and an additional amount equal to a percentage of his earnable compensation to be known as “the accrued liability contribution”. The rates percent of the contributions shall be fixed on the basis of the liabilities of the retirement system as shown by actuarial valuations.