Design work started again last June. By the end of October, the team had secured a permit for the foundations.

Plans call for a mostly residential, mixed-use tower, including a hotel, office space and condominiums. The tower would also contain the world's highest observation level. Smith calls the architecture an evolution of the Y-in-plan concept used on the burj and first introduced in the early 1920s by architect Mies Van Der Rohe. The evolution mostly relates to the structure and the perimeter wall, he adds.

To understand the evolution, it is necessary to compare the burj and the Kingdom Tower. The burj consists of a 585-m-tall, structural-concrete superstructure, with setbacks, topped by a 200-m-plus structural steel spire that culminates in a steel-pipe pinnacle. Through level 156, the burj is a high-performance concrete tower with a Y-shaped footprint. A hexagonal, shear-wall core is at the nexus of the Y’s wings. Every seven floors, an outer bay "peels away," creating a setback and 27 different floor plate sizes. Outrigger shear walls for stability are in three-floor mechanical levels, every 30 stories or so. Slabs are generally two-way, reinforced-concrete flat plates. Core slabs have beams. The tower is founded on a high-performance, reinforced-concrete raft on piles.

The Kingdom Tower's design also calls for a three-legged building. But gone are the setbacks, the outriggers and the transfers, which wreaked havoc with the construction pace. Instead, the legs are sloped and connected to each other by three tilted planes, which form the tower's outer walls. Each wall, or face, is a single plane, tilted at a slightly different angle, so that each wall reaches the tower's top at different heights. The only asymmetry is the three tilted planes, which form the gradual taper.

In plan, the three-winged building looks like three hammerheads because, near the tips of the wings, there are "serious walls" that cut across, said Sinn.  

Inside, throughout each floor plate, a series of concrete walls would contain 12,000-psi concrete and be interconnected by link beams.

Avoiding setbacks and transfers translates to minimizing formwork resetting time. On the burj, the contractor lost three weeks at each setback redoing formwork for walls and slabs, said Smith. On the Kingdom Tower, a day, at most, will be lost to resetting forms, he added.

Thanks to the burj's shape, the terraces at the setbacks are very windy at the higher elevations. For the Kingdom Tower, the terraces are between the legs in so-called quiet zones, where there is not a lot of wind pressure. "That should be much more accommodating," said Smith. It also helps to increase the value of the rental units.

Another lesson from the burj relates to its 3.2-m floor-to-floor height. "It was a coordination nightmare to get the utilities, structure and headroom within the 3.2 meters," said Smith. That required too many beam penetrations, which were difficult within the robust structure, he added.

The Kingdom Tower has 4-m floor-to-floor heights. That eases coordination and means fewer beam penetrations.