The Senate on Friday joined the House of Representatives in approving legislation providing pension fund relief to companies. Aimed principally at old industrial companies such as automakers, airlines, and steel producers, the new pension contribution formula could save firms as much as $80 billion over the next two years. The legislation, a compromise version created by House and Senate negotiators last Thursday and passed by the House the following day, is expected to be signed into law by President Bush. Companies that have been underfunding their pensions will be able to use higher-yield corporate bond rates -- instead of the 30-year Treasury bond interest rate that now serves as the benchmark -- to calculate their pension funds' projected assets. In total, some 31,000 single-employer pension plans will be able to use the new formula.