 The building was constructed above a Con Edison substation that had been on the site since 1967 . The substation had a caisson foundation designed to carry the weight of a future building of 25 stories containing 600 @,@ 000 sq ft ( 56 @,@ 000 m2 ) . The final design for 7 World Trade Center was for a much larger building than originally planned when the substation was built . The structural design of 7 World Trade Center therefore included a system of gravity column transfer trusses and girders , located between floors 5 and 7 , to transfer loads to the smaller foundation . Existing caissons installed in 1967 were used , along with new ones , to accommodate the building . The 5th floor functioned as a structural diaphragm , providing lateral stability and distribution of loads between the new and old caissons . Above the 7th floor , the building 's structure was a typical tube @-@ frame design , with columns in the core and on the perimeter , and lateral loads resisted by perimeter moment frames . 
