Security was as important a concern as stature for this 541m-tall New York City landmark. The key to both is a massive, incredibly robust core, made of ultra-high-strength concrete clocking in at 14,000psi. “It’s much stronger than any rock you could find,” says Ahmad Rahimian, director of building structures at WSP. That allowed the walls to be thinner, maximizing lettable area and minimizing the weight of materials required. More than 50% of the carbon-intensive cement content was replaced with industrial by-products, and over 95% of the structural steel was recycled.