Contracts totaling $31,100,000 (equivalent to $428 million in 2020[1]) were finalized with McShain and the other contractors on 11 September 1941, and ground was broken for the Pentagon the same day. Among the design requirements, Somervell required the structural design to accommodate floor loads of up to 150 psi (1,000 kPa), which was done in case the building became a records storage facility at some time after the end of the current war. A minimal amount of steel was used as it was in short supply during World War II. Instead, the Pentagon was built as a reinforced concrete structure, using 680,000 tons of sand dredged from the Potomac River, and a lagoon was created beneath the Pentagon's river entrance.[32] To minimize steel usage, concrete ramps were built rather than installing elevators.[33] Indiana limestone was used for the building's façade.[35]
Architectural and structural design work for the Pentagon proceeded simultaneously with construction, with initial drawings provided in early October 1941, and most of the design work completed by 1 June 1942. At times the construction work got ahead of the design, with different materials used than those specified in the plans. Pressure to speed up design and construction intensified after the attack on Pearl Harbor on 7 December 1941, with Somervell demanding that 1 million sq ft (9.3 ha) of space at the Pentagon be available for occupation by 1 April 1943. David J. Witmer replaced Bergstrom as chief architect on 11 April after Bergstrom resigned. Unrelated to the Pentagon project, he was charged with improper conduct while having served as president of the American Institute of Architects. Construction was completed 15 January 1943.[38]