"It's a pretty awesome feeling, " Juan Estevez said from a temporary platform on the roof of the tower where he and other workers watched the milestone.
Members of Cambridge University Officers' Training Corps (CUOTC) have agreed to build a platform representing the roof of the 70ft (21m)-high Senate House, and a vehicle will be hoisted to the top.
Loud applause and cries of joy erupted from construction workers assembled on a temporary work platform on the roof of the building as the huge, silver spire was gently lowered and secured into place.