Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_name_75 ( year_opened INTEGER, material VARCHAR, span_feet VARCHAR )

### Question ###
What is the sum of Year opened when steel was the material with a Span feet of 1476?

### Accurate SQL ###

SELECT SUM(year_opened) FROM table_name_75 WHERE material = "steel" AND span_feet = 1476