SCHEMA:
CREATE TABLE table_name_6 (floors INTEGER, name VARCHAR, rank VARCHAR, year VARCHAR)
USER:
What is the average amount of floors of the building that is ranked larger than 30, a year prior to 1977 and an address of 100 Van Ness Avenue?

QUERY:
SELECT AVG(floors) FROM table_name_6 WHERE rank > 30 AND year < 1977 AND name = "100 van ness avenue"