CREATE TABLE table_name_75 (year_opened NUMBER, material VARCHAR2, span_feet VARCHAR2)

What is the sum of Year opened when steel was the material with a Span feet of 1476?
SELECT SUM(year_opened) FROM table_name_75 WHERE material = "steel" AND span_feet = 1476
SELECT SUM("year_opened") FROM "table_name_75" WHERE "material" = "steel" AND "span_feet" = 1476