CHICAGO - In the annals of tall skyscrapers, there is no doubt that the soon-to-open Burj Dubai will be the world's tallest. But how tall is known to only a few.

It's still a secret, William Baker of Skidmore, Owings and Merrill LLP, the tower's structural engineer, said in an interview ahead of the Burj Dubai's January 4 opening.

The client (Emaar Properties PJSC) will only let us say it's more than 800 meters (2,625 feet) tall. It's part of the mystique of the project, he said.

Baker said the architects wondered if someone would try to figure out the slender tower's height by measuring its shadow.

Chicago-based Skidmore developed the buttressed core structural system that resembles a gigantic Y to support the super-tall tower, which is nearly twice as tall as Skidmore's 442-meter (1,389 feet) Willis Tower in Chicago.

The 35-year-old Willis Tower, formerly the Sears Tower and once the world's tallest building, will be bumped down to fifth spot, according to the Council on Tall Buildings and Urban Habitat, which tracks such things. The current record-holder, the Taipei 101, will be dropped to second.

Seven of the world's 10 tallest towers are in Asia, all built in the past 13 years.

Jan Klerks of the Council on Tall Buildings said the group has asked to know the tower's exact height.

We have expressed this a number of times, but if there are reasons not to disclose it, then I guess we have to do without an official number. We also do not know why they chose not to disclose the number. The only thing we are sure of is that it is the tallest building in the world, and that it is at least 800 meters, Klerks said in an email.

The Burj Dubai really cannot be rationalized through urban scarcity and land prices, Klerks said. As such it is very much an iconic building, aiming at those who want an iconic address.

ABOUT 164 FLOORS

Apparently, the emirate's recent financial problems have not hurt sales of the approximately 1,100 one- to three-bedroom residences in the tower, which are pretty much 100 percent sold out, Baker said. Those include corporate apartments near the summit topping out at around the 164th floor.

A luxury Armani hotel will occupy the bottom floors.

The cost of the tower has been put at around $1.5 billion in published reports.

From the 124th floor observation deck of the tower viewers can see 50 miles on a clear day.

Terraces are located at setbacks spiraling up the tapered tower, which is based on the geometries of the desert flower and the patterning systems embodied in Islamic architecture, according to its promotional literature.

The air is noticeably cooler and fresher on the terraces compared to the stifling heat and humidity at ground level during Dubai's summer, Baker said.

He said it takes about two minutes to get to the summit on some of the fastest elevators in the world, which travel at up to 25 miles per hour (40 kilometers per hour).

The tower's exterior is glass and steel that would cover 17 soccer fields, and will require six to eight weeks to clean.

Concrete was used extensively in the core, enough to build a sidewalk 1,283 miles long, and the steel reinforcing bar used would stretch a quarter of the way around the Earth. The cooling system produces enough condensation to fill 20 Olympic swimming pools a year, which will be used to water the grounds.

(Editing by Eric Beech)

Dubai