Construction of the Trump International Hotel & Tower Chicago will offer a three-year spectacle of modern construction as the building climbs, level by level, above its neighbors and into the ranks of the world's tallest buildings. When done, it will stand as tall as the John Hancock Center even before its controversial spire is added.
Now, passersby watch as drills the size of telephone booths spin into muddy ground where the newspaper offices once stood, spewing damp earth into piles that steam briefly in the chilly air.
Unlike Chicago's other three giants--the Sears Tower, the Aon Center and the Hancock building--the Trump tower will be supported not by a framework of steel but by a spine and outriggers of concrete.
Without high-quality concrete, the structure would never support the building's 360,000-ton bulk--the weight of four aircraft carriers.
Without new chemical processes that make the wet cement more fluid or new pumping techniques to move it, it could never be pumped 92 stories and 1,125 feet into the air.
Without concrete, the building could never climb so high and still stay so thin.
The footprint of Trump's building will be 348 feet by 135 feet--not much bigger than the squat Sun-Times building.
"On a steel building, it would have had to be much wider," said William Baker, a structural engineer at Skidmore, Owings & Merrill, the architectural firm that designed both buildings.
"We wouldn't have been able to put a steel building on this site."
When the building is finished, a skin of stainless steel and glass will reflect the sun. The tower's shape will mirror the buildings around it, its faceted setbacks nodding to the 1920 Wrigley Building and monolithic 1973 IBM Building next door.
The building's designers are using the tower's concrete-swollen weight to defy heavy breezes. By making it too heavy to tip easily, the designers have pitted one of the oldest enemies of skyscrapers, gravity, against its other foe, the wind.
In addition, the setbacks and rounded edges will prevent vortices from organizing into mini-tornadoes, reducing the wind's power.
To further secure it, the building will be cantilevered into a section of the Earth's crust, a limestone formation 420 million years old and 110 feet underground, so the building will touch sky and bedrock at the same time.
Though it will outreach its neighbors, the Trump tower must start far beneath them, on pillars extending like stilts into the ground. The holes for those 4-foot-wide pillars are being drilled now. Under the building, every 30 feet around its perimeter, steel-reinforced cement will be poured.
On top of that, an 8,400-ton concrete pad the size of a river barge will be built. From that pad the building's spine will rise, climbing as Wabash Avenue is rebuilt between the IBM Building and the future Trump tower.
The spine will consist of five gigantic concrete walls, each shaped like an I-beam standing on its 45-foot-wide end. At about eight stories, the exterior columns will begin to follow, with a concrete slab between the columns at each story for a floor.
It will climb like this: Spine, columns, slab. Spine, columns, slab. Eight to 10 floors below the highest slab, the curtain wall will rise on the outside of the structure. As the building climbs, it will narrow, the spine dwindling to two parallel I-beams from five.