Railway engineers work with a simple equation. Put a heavy mass on soft springs and the system has a natural frequency of f₀ = (1/2π)·√(k/m). Vibration well above roughly √2 times that frequency is filtered out before it reaches whatever sits on top. Concert halls next to rail lines rely on the same physics. In Zürich-West it holds up a neighbourhood. At the switches of the Depot Hard tram hall, a 430 t concrete block rests on 110 steel springs, and towers of 65 and 68 metres rise above it.

I am a building too, up on the Hönggerberg ridge, and this one moves me. Most cities hide their tram depot. Zürich made this one the ground floor of a public housing scheme.

What the building is

Depot Hard was completed in 2025 for the City of Zürich. Morger Partner Architekten designed it and Ulaga Weiss engineered the structure; both offices are in Basel. In June 2026 the city reported that the flats are occupied. The depot below goes into service from summer 2027 and has space for 25 Flexity trams. espazium counts 197 flats for about 550 residents, with 46,660 m² of floor area (SIA 416) in a 22-storey southwest tower and a 23-storey northeast tower. Voters approved the CHF 203.525 million credit in February 2020, with 70.9% in favour.

The 2015 competition brief asked for a continuous 30 m perimeter block. Matteo Moscatelli describes the architects’ answer in espazium (13 October): “Morger Partner verschmilzt dafür Block- und Hochhausbauweise zu einem zusammenhängenden Ensemble”. In English: block and tower fused into one ensemble. Two days later, structural engineer Dominik Weiss of Ulaga Weiss explained in the same magazine how that fusion stands up. This piece follows his account upward from the bottom.

Climbing up the load path

The ground. The towers stand on 111 bored piles, each 1.18 m in diameter, which reach the Molasse at 30 to 42 m. Measured settlement is up to 10 mm. The piles are not designed for tension, so nothing anchors the towers against tipping. Weiss explains how they resist sideways loads instead: “Die Hochhäuser stabilisieren sich horizontal mit ihrer Schwerelast”. Their own weight keeps them stable. During construction, each tower had to stand on its own.

The springs. Weiss describes three variants of mass-spring system (Masse-Feder-Systeme). The heaviest is the 430 t mass on 110 springs. Another uses 380 m of 18 cm steel troughs. In Star Wars, Cloud City floats on repulsorlifts. Depot Hard floats its rails on steel springs, and here the physics is real.

The slab. At about 8 m height, the depot slab is 137 m long and up to 30 m wide. It spans up to 14.5 m with no columns and is 60 to 80 cm thick. 127 tendons prestress it in one direction at a mean 3.5 MPa. In the final state the slab ties the two towers together.

The turn. Weiss puts the constraint this way: “Die Tramfahrgassen orientieren den Sockel streng in West-Ost-Richtung”. The tram lanes fix the base strictly west to east, so the towers’ bracing walls cannot run straight down. On floors 3 to 5, walls 40 to 80 cm thick form a transfer and outrigger structure that carries the forces around the lanes. Where those walls cross, they carry up to 36 MN. Max Ricci, foreman for contractor Marti Zürich, on the roughly one-third extra reinforcement: “This extra reinforcement is extended up to the fifth floor.” Marti’s crews placed 34,000 m³ of concrete and 5,300 t of reinforcement.

The body. According to Weiss, 66% of the concrete is recycled (RC-C, at least 25% recycled content). A further 25% uses strongly CO₂-reduced CEM III/B cement, mostly in the massive members, and 13% is C45/55 high-strength concrete. Most of it was mixed at a batching plant on site, which cut truck traffic.

←TODAY: In 2026, about 550 Zürchers live above a hall whose rails sit on springs. →3012: Cities will stack housing on every working layer they own, and separate each layer from the one below. Fulcrum: Stacking only works when every layer can be traced back to the one beneath it.

The trade-off is simple. Every turn in a load path costs wall thickness, reinforcement and coordination. Here it took up three whole storeys of the section. PAZ has followed this lineage before. At Kalkbreite, another Zürich block built over a working VBZ depot, which we read from the rails up, the transfer level was also the real design problem. Depot Hard takes that idea up to 68 m.

Building-sense: A building like this would feel its trams as a low hum at the springs and never in its bedrooms. It would know which three floors work hardest.

Atelier: When infrastructure sits underneath, the load path is no longer something the engineer calculates at the end. It becomes design material from the first sketch, and the architect and engineer have to draw it together. Monday move: before any massing over a depot, station or car park is frozen, put the load-path routing on the agenda of the first architect–engineer coordination meeting.

Hack: Trace the load path from roof to pile and flag every place it turns. Draw it in Rhino as one polyline and feed it into a GhPython component as path. The script marks each vertex where the path bends by more than 5°:

import rhinoscriptsyntax as rs
v = rs.PolylineVertices(path)
turns = [v[i] for i in range(1, len(v)-1)
         if rs.VectorAngle(rs.VectorSubtract(v[i], v[i-1]),
                           rs.VectorSubtract(v[i+1], v[i])) > 5]

Each point in turns is a place where your structure will need its own version of floors 3 to 5. We teach this kind of section reading in our Rhino and Grasshopper courses at PAZ, a Rhino centre in Switzerland.

Open your current section, trace the load path, and count its turns before your engineer has to.