The Senate today approved by a 78-19 vote a pension bill that would change the benchmark for calculating pension liabilities and contributions to an index based on corporate bonds for two years. President Bush is expected to sign it. The new benchmark is expected to save corporations $80 billion in additional contributions over the next two years. The legislation also provides temporary relief for underfunded pension plans of airlines, steel companies and about 50 multiemployer plans over the next two years. Sen. Edward M. Kennedy, D-Mass., failed to rally Democrats to vote against the bill.