Sec. 18. PROPORTIONAL RETIREMENT PROGRAM.    (a)  In this section:
(1)  "Combined service credit" means the total amount of service credit a member has for participation in the fund plus service credit the member has in any participating retirement system.
(2)  "Participating retirement system" means the retirement system established under Article 6243g-4, Revised Statutes, or Chapter 358, Acts of the 48th Legislature, Regular Session, 1943 (Article 6243g, Vernon's Texas Civil Statutes), or a successor statute to either of those laws.
(3)  "Program" means a proportional retirement benefits program established under this section that permits members to establish service credit for a proportional retirement benefit using combined service credit.
(4)  "Service credit" means service or participation that is credited under the fund or a participating retirement system to establish service or participation requirements for a proportional retirement benefit.
(b) The board may maintain a proportional retirement program under this section.
(c) Under the program, a member who is eligible to participate may use combined service credit to determine eligibility for a benefit under this article.  The member must have at least 20 years of combined service credit to receive a proportional retirement benefit.  The member is subject to the same requirements and receives the same benefits, including enhancements, as a member who establishes retirement eligibility for the same amount of service credit without using combined service credit.
(d) A member may not:
(1)  use service credit in a participating retirement system to meet the eligibility requirements for participating in a DROP under Section 5 of this article;
(2)  receive service credit in the fund for the same service for which the member receives service credit in a participating retirement system;  or
(3)  receive a benefit under the program in an amount that is greater than the amount the member would have received for the same benefit without the program unless the greater amount results from a modification under Subsection (j) of this section.
(e) A person is eligible to participate in the program if the person is employed by the city, is covered by a participating retirement system, and is or has been a member of the fund.
(f) A member who is retired or participating in a DROP under Section 5 of this article may not participate in the program.  A member may not receive a disability retirement benefit and a service retirement benefit under the program.
(g) In determining proportional retirement benefits under the program for a member who has participated in the fund for less than 20 years, the member is entitled to a monthly benefit in an amount equal to 1.7 percent of the member's average monthly salary multiplied by the number of the member's years of participation in the fund.
(h) A member who receives a disability benefit under a participating retirement system may receive a proportional disability benefit under the program as provided by Subsection (i) of this section.
(i) In determining disability retirement benefits under the program, the member is entitled to a monthly benefit in an amount equal to 1.7 percent of the member's average monthly salary multiplied by the number of the member's years of participation in the fund.
(j) The board may modify the program only to make the program's provisions compatible with the provisions of a participating retirement system.  The board may not modify the program for the purpose of providing a new benefit to a member.
(k) If the board determines that the provisions governing a participating retirement system are not compatible with the provisions governing the fund under this article, the board may terminate the program.  The board shall provide written notice to the executive director of the participating retirement system before the 30th day preceding the date of the program's termination.  The board may reestablish the program at its discretion, subject to the requirements of this section.
(l) The board may adopt rules to implement and administer this section.