WASHINGTON—Planes have been grounded at all three Washington, D.C.-area airports after a strong chemical smell forced workers to evacuate the region’s air traffic control center. Flights were abruptly grounded at Reagan National, Dulles, and Baltimore-Washington International Airports late Friday, and inbound flights were diverted, causing major delays at one of the nation’s largest air travel hubs. It is unclear what caused the smell, or how long the airports will remain locked down. Flights were also grounded at Richmond International Airport.