Dragline spider silk has a tensile strength around 1.6 GPa at a density of 1.3 g/cm³. That gives a specific strength of roughly 1230 kN·m/kg — about 24× better than structural steel (51 kN·m/kg) and 5× better than Kevlar. It also has a toughness (energy absorbed before failure) of ~350 MJ/m³, higher than any known engineering fiber. So: should we frame skyscrapers with the stuff?

The self-supporting height ceiling. For a uniform column in pure tension, h_max = σ / (ρg). Plugging in silk: 1.6×10⁹ / (1300 × 9.81) ≈ 125 km. Steel: ~5.2 km. Silk could hold up its own weight from low Earth orbit. The Burj Khalifa (830 m) is laughably easy.

The catch is compression. Silk is a polymer fiber — it buckles like wet rope under axial load. Euler buckling stress for a fiber of diameter d and length L goes as π²E(d/L)² / 16. With silk's modulus of ~10 GPa, a 10 μm fiber 1 mm long buckles at ~600 kPa. Useless as a column. So a silk skyscraper has to be a pure-tension structure: a central concrete or basalt-fiber compression mast surrounded by silk cables in a hyperboloid suspension cage, like Shukhov's tower scaled up. Every floor hangs from silk; nothing rests on it.

How much silk does a 500 m tower need? Estimate the working tension load at ~200,000 tonnes (lateral wind plus floor weights). With a 3× safety factor and 1.6 GPa allowable stress, cross-section required: (2×10⁶ N × 3) / 1.6×10⁹ Pa ≈ 3.75 m². Distributed across, say, 400 main cables, each cable is ~10 cm diameter. Total silk mass for the cable cage: 3.75 m² × 500 m × 1300 kg/m³ ≈ 2,400 tonnes.

Can we actually make 2,400 tonnes of silk? A live spider produces ~1 mg/day, so this would require 2.4×10¹² spider-days — call it 6.5 billion spiders working a year (and not eating each other). Hard pass.

Recombinant silk from engineered E. coli or yeast (Spiber, Bolt Threads) now hits ~10 g/L fermenter yields. At that density, 2,400 tonnes needs 240 million liters of culture — roughly 12 large brewery-scale bioreactors running for a year. Capital and energy cost: comparable to a chip fab, but plausible.

Other gotchas. Silk creeps under sustained load (5–15% over years), loses ~30% strength when saturated with humidity, and degrades under UV. So every cable lives inside a sealed, climate-controlled sheath — essentially the building is wearing a raincoat that is the cladding. Fire is the killer: silk denatures at ~230 °C. The building needs aggressive intumescent coatings, or the whole skeleton softens in a hot fire.