agreed, it can be called a subset but in fact it is a specialty and most civil engineers cannot claim to be structural engineers (by law).
from 2010 California Rules
PROFESSIONAL ENGINEERS ACT
(Business and Professions Code §§ 6700 – 6799)
6736. Title of structural engineer
No person shall use the title, “structural engineer,” or any combination of these words or abbreviations thereof, unless he or she is a licensed civil engineer in this state and unless he or she has been found qualified as a structural engineer according to the rules and regulations established for structural engineers by the board.
6763. Structural, soil, soils, geotechnical authority
Application for authority to use the title “structural engineer,” “soil engineer,” “soils engineer,” or “geotechnical engineer” shall be made to the board on forms prescribed by it and shall be accompanied by the application fee fixed by this chapter.
An applicant for authority to use the title “structural engineer,” “soil engineer,” “soils engineer,” or “geotechnical engineer” who has passed the examination prescribed by the board, or an applicant for authority to use the title “soil engineer,” “soils engineer,” or “geotechnical engineer” whose application is submitted prior to July 1, 1986, and who has otherwise demonstrated that he or she is qualified, shall have a certificate of authority issued to him or her.
For purposes of this chapter, an authority to use the title “structural engineer,” “soil engineer,” “soils engineer,” or “geotechnical engineer” is an identification of competence and
2010 Professional Engineers Act 25
specialization in a subspecialty of civil engineering and necessitates education or experience in addition to that required for registration as a civil engineer.
6763.1. Structural Engineer examination requirements
An applicant to use the title “structural engineer” shall have successfully passed a written examination that incorporates a national examination for structural engineering by a nationally recognized entity approved by the board, if available, and a supplemental California specific examination. The California specific examination shall test the applicant’s knowledge of state laws, rules, and regulations, and of seismicity and structural engineering unique to practice in this state. The board shall use the national examination on or before December 31, 2004.