After graduation, Baker worked with the Air Ministry on structural problems of airships. At 28, in 1929, he contracted tuberculosis. Upon recovering, he became a Technical Officer with the Structural Steel Research Committee, which was investigating why measured stresses in structural steel bore little resemblance to theoretical ones. During his time in this post, he developed the plastic theory of design, a revolutionary method of design of steel structures which gives a lower bound on the collapse load, and is hence always safe. Until then, all design of steel structures was based on elastic theory of design. In 1932 he was awarded the Telford Gold Medal of the Institution of Civil Engineers for the work.