Wash. Rev. Code § 41.48.010
In order to extend to employees of the state and its political subdivisions and to the dependents and survivors of such employees, the basic protection accorded to others by the old age and survivors insurance system embodied in the social security act, it is hereby declared to be the policy of the legislature, subject to the limitations of this chapter, that such steps be taken as to provide such protection to employees of the state and its political subdivisions on as broad a basis as is permitted under the social security act. Persons now members of or protected by any state or local pension or retirement plan or system may be covered under the federal social security act only as provided by the federal social security act amendments of 1954. (Public Law No. 761.) It is hereby declared to be the policy of the legislature in enacting the succeeding sections of this title that the protection afforded the employees in positions covered by a retirement system on the date an agreement under this title is made applicable to service performed in such positions, or receiving periodic benefits under such retirement system at such time, will not be impaired as the result of making the agreements so applicable whether the agreement provides for supplementation, integration or coordination.
RCW 41.48.010