CREATE TABLE table_7204 ("Rank" FLOAT, "Name" TEXT, "Span metres" FLOAT, "Span feet" FLOAT, "Material" TEXT, "Year opened" FLOAT, "Location" TEXT, "Country" TEXT)

What is the sum of Year opened when steel was the material with a Span feet of 1476?
SELECT SUM("Year opened") FROM table_7204 WHERE "Material" = 'steel' AND "Span feet" = '1476'
SELECT SUM("Year opened") FROM "table_7204" WHERE "Material" = 'steel' AND "Span feet" = '1476'