The Spire
1.5 miles. 660 floors. The center column to which all five Pillars buttress.
The spine of the formation — 7,920 feet from foundation to the Crown, where the Telescope and the Apex civic program live in the top 60 floors above all other occupied space on Earth.
The Spire cannot stand alone. Tied to the five Pillars via skybridges at every 100 floors, its maximum unsupported vertical length is roughly 1,200 ft. That tying is what makes 1.5 miles structurally possible at all.
Slender at the top, broad at the base
Outer diameter approximately 200 ft at the lower floors, tapering as the load above decreases. Base wall thickness in ultra-high-performance concrete: roughly 30–50 ft. Floor plate ~30,000 sf — modest by Pillar standards, but Spire floors are premium occupancy by elevation alone.
660 floors, six tiers
The Spire's floors mirror the skybridge tiers: Foundation (1–100), Common (101–200), Mid (201–300), Upper (301–400), High (401–500), Crown (501–600), Apex (601–660). Use intensity climbs with elevation; population density falls. The bottom serves the most people; the top serves the most prestige.
The top 60 floors
Floors 601–660 stand alone above everything else in the formation — above the highest Pillar by 0.25 miles. The Apex hosts the Telescope, civic observation, the highest residential addresses ever built, and the Crown bell. See The Apex.
Compression at the base
Self-weight produces ~8,250 psi at the Spire base. With live load, wind transfer, and safety factors, design pressure is 14,000–16,000 psi. Ultra-high-strength reinforced concrete handles up to ~20,000 psi today (Burj Khalifa lower core). Carbon-fiber UHPC pushes 30,000+ psi in lab. Inside the envelope. Tight, but inside.
UHPC is pumped to 7,920 ft via multi-stage relay pumping — Burj reached 1,970 ft in single-stage, so this is engineering, not research.