What is the Main span of the Qincaobei Bridge
CREATE TABLE "bridges_under_construction" (
  "name" text,
  "main_span_metres" text,
  "main_span_feet" text,
  "year_to_open" real,
  "country" text
);
SELECT "main_span_feet" FROM "bridges_under_construction" WHERE "name"='qincaobei bridge';