Concordia
A raised plate district with balustrades and planters, an inhabited truss bridge and rows of blocks on the next plate.

The megaframe city

Above the streets, a lattice of piers and ring beams holds a second city on plates, bridges and terraces. The lattice is the megaframe. It was built first, and the city grew inside it.

See on the map

What you see

The frame is pale sintered regolith and steel. Primary piers stand on the ground, 45 m square at the base, and rise toward the axis in three stages, with ring beams at 625 m, 1,000 m and 1,500 m. Between them, bays 654 m around and 480 m along the rotor hold plates, atria and inhabited bridges.

On a plate at 625 m you weigh about three quarters of your ground weight. Housing, offices, universities and hotels fill the plates up to about 800 m. Higher up, the frame turns to service halls, farms and process vaults, covered in Upper city farms. Balconies reach further as gravity drops: 40 m at 1 g, 100 m at 0.4 g.

Pale structural piers and ring beams rise through the mid levels of the city, with partial decks, bridges and bay infill.
Piers and ring beams at mid height, with partial decks, bridges and bay infill. Open this view

The oldest parts of the frame are monumental and repetitive, with large joints and a restrained Art Deco look. Newer infill is organic: retrofits, bridges and converted spaces added over the years since the first residents arrived. Towers wear crowns and spires, and the tallest ones on the plates carry beacons. Nothing on the ground reaches through the sky. Inhabited bridges join the plates, with gate towers at their ends, and on the flat roofs sit penthouses, cooling towers, dishes and glasshouse pavilions. From the street you see only the foot of a pier rising into the sky. The plates sit above the artificial sky, so the second city is hidden from the streets below. From a plate, the upper city on the far side of the rotor hangs overhead.

The end walls

Each rotor ends in a disc, and the disc is a city face. The end wall is an inhabited cliff 2,436 m tall, from the ground to the 64 m bore at the axis. Its 24 cells are 654 m wide at the rim and narrow to 131 m at 2,000 m of height. Ledges, viaducts and terraces step up the face, with trains on the track strips.

A viaduct terrace on the end wall cliff, with a train on its track strip, planters and terraced blocks above the city.
A viaduct terrace on the end wall with a train, planters and terraced blocks above the city. Open this view

Most of the face is near the ground: 64 percent of the disc area lies in the outer 1,000 m of height, so the cliff is broad at the foot and narrows like a tower toward the axis. Low-gravity halls, vaults and gardens fill the upper ring ledges, where the ribs thin toward the bore.

Gravity follows the same rule as everywhere: it falls in a straight line with height. The viaduct terrace at 241 m is at about 0.9 g. Two walls are closed: the forward wall of Noordung (Rotor 2) and the aft wall of Bernal (Rotor 3) face each other across the counter-rotating gap, and they are signed COUNTER-ROTATING / CROSS VIA AXIAL SPINE.

How it works in brief

The frame is a load path. Every building rests on its bay structure, the bay on the primary frame, and the frame on the rotor's outer tension structure. All permanent loads go outward, because outward is down. Structure explains the pressure vessel, hoop stress and the frame in detail.

A tall open atrium of long-span steel framing with sparse process facilities and hazy air above a city below.
The frame thins as weight falls. At about 0.32 g the bays are long-span and sparse, and process facilities replace housing. Open this view

Because the frame was built while the rotor stood still but was designed for full spin, the city could be filled in over time. Try the overlay: Open in the simulation: Structural overlay

Key numbers

1,500 mwhere the frame ends
800 mtop of the housing plates
24cells around each end wall
2,436 mheight of the end wall cliff
3ring beams, at 625, 1,000 and 1,500 m
Go deeper: Piers, beams and gravity

Pier size shrinks as the load above it shrinks. These are the design values: 45 m square up to 625 m, 28 m square up to 1,000 m, and 15 m square up to 1,500 m. The load at the base of a primary pier is about 2 GN. The whole frame is about 6 million tonnes per rotor, roughly 150 kg per square metre of floor, about fifty times lighter than the same volume built solid. Axial members run every 480 m.

Level Height Gravity
First ring beam 625 m 0.75 g
Second ring beam 1,000 m 0.60 g
Third ring beam 1,500 m 0.40 g

Gravity at height h above the ground is g = 1 − h / 2,500 m.

Visit

Open in the simulation: Urban Megaframe Base Open in the simulation: Mid-Gravity Megaframe Open in the simulation: Megaframe Plate District Open in the simulation: End Wall: Foot of the Cliff Open in the simulation: End Wall: Viaduct Terrace Open in the simulation: End Wall: Upper Wall from the Axis Side

Nearby

Arbor Quarter

Arbor Quarter is a neighbourhood on the floor of Tsiolkovsky (Rotor 1), where the ground weighs what it does on Earth.

Ascenders and the sky

The ascender is a lift 2.4 km tall. It carries 150 people from the street through the artificial sky to the axis in about ten minutes, and on the way your weight drops to almost...

Upper city farms

Food is grown high in each rotor, where gravity is 0.6 g and the air is the same air the city breathes.

  • Arbor Quarter Arbor Quarter is a neighbourhood on the floor of Tsiolkovsky (Rotor 1), where the ground weighs what it does on Earth.
  • Ascenders and the sky The ascender is a lift 2.4 km tall. It carries 150 people from the street through the artificial sky to the axis in about ten minutes, and on the way your weight drops to almost...
  • Structure and pressure A rotor is a pressure vessel 5 km across and 2.5 km long. It holds in a full atmosphere while spinning under the weight of a city.