General Motors Co. plans to freeze its U.S. pension plan for longtime white-collar workers and give all salaried employees annual bonuses but not pay raises in an effort to hold down expenses, officials announced Wednesday.

The Detroit-based automaker said roughly 19,000 salaried workers hired before 2001 will move from a traditional pension with guaranteed payments to a 401(k)-type plan with contributions based on salary and bonuses. Employees hired after 2001, which represent about 30 percent of the company’s salaried workforce, already are in that defined contribution plan.