The City and County shall contribute jointly with the members of the Retirement System to meet the liabilities accruing under the system because of service rendered to the City and County by persons after becoming members of the system. Members of the system shall contribute not to exceed 10 percent of their salaries or wages, provided that members may, at their option, elect to contribute at rates in addition to those fixed as normal by the retirement board. The City and County shall contribute an amount equal to normal contributions of members as provided for in the preceding sentence, but the City and County shall not contribute any amount because of additional contributions by members.