Thirty-Four Piles, One Slab: How Chavannes' C4 Drum Lifts a Simulation Hospital Off the Ground
How C4, a 56 m concrete drum in Chavannes-près-Renens, frees its ground floor: 34 bored piles, a jointless 360 m raft and one prestressed transfer slab.
Thirty-four bored piles, 1.0 to 1.5 m across and 28 to 35 m long, are socketed into the marl molasse under Chavannes-près-Renens. They are the roots of C4, the 56 m concrete drum Jan Kinsbergen Architekten built for the Campus Santé. In his espazium/TRACÉS piece of 21 October, structural engineer Dr Neven Kostic reads the building the way a radiologist reads a scan: bones first. The drum is 16 m high, with about 9,500 m² on four levels, and has been in use since September. Inside is a simulation hospital for HESAV’s students.
Why read a building from its slabs? According to Philippe Block of ETH Zürich, speaking at a PAZ BSS Talk in April 2023, three quarters of a medium high-rise’s mass is structure, and more than half of that sits in the floors. A building keeps its intelligence in its sensors and its weight in its plates. C4 puts its cleverest move exactly there.
From the water table up
Kostic reports that the car park beneath all three buildings is one monolithic structure about 360 m long, with no movement joints. Where little load presses down, 40 cm screw piles work in tension against buoyancy. Most foundations push. This one also has to pull.
Then comes the ground floor, almost empty of columns. Kostic calls this break in the grid “the building’s main structural particularity.” The upper columns stop on a large transition slab prestressed with eleven-strand T15S cables. It is thicker over the façade columns and thinner at the rim and the centre. “The slab works simultaneously as floor, transfer structure and cantilever,” he notes.
Above it, the drum turns regular. The circular flat slabs are 34 cm thick, with seven-strand T15S tendons in 110 × 30 mm flat ducts. Embedded S355 steel heads take the punching shear, so no downstand beams cross the ceilings. Two continuous cores carry the horizontal loads to the raft. The geotechnics of Dr. Vollenweider made the pile lengths possible.
The trade-off is plain. A transfer slab gathers the whole upper grid onto one element, so its thickness, tendon profile and pour sequence carry consequences that a regular grid would spread across every floor.
←TODAY: In 2026, a Vaud campus frees its ground floor by asking one prestressed plate to carry, transfer and cantilever at once. →3012: Every structure is read from its plates, where most of its mass and most of its carbon live. Fulcrum: The floor that does the most work is the one worth designing first.
At the top, an ETFE cushion covers the central atrium. The practice rooms sit blind in the core, where students rehearse on simulated patients alongside robots. It is a little like the medical frigate at the end of The Empire Strikes Back: droids and people bent over a staged crisis so that the real one goes better.
Building-sense: A building running this structure would feel a calm, repeating drum above and one heavy plate that knows every column’s weight before the molasse does. If I were C4, that slab is where I would want my first strain gauges.
Two siblings, one logic
Next door, Kostic’s HESAV building (130 × 35 × 20 m) spans column-free halls on multi-level steel trusses. Its atrium ramp hangs from steel tie rods that had to be installed before the ramp was concreted. Neostar, a residence with 776 beds engineered by Fabrice Meylan of co-struct with Dr Schwartz Consulting, “follows a simple structural concept: reduce the matter.” It uses 25 cm flat slabs, one core, and two diagonals per long façade instead of shear walls.
According to the État de Vaud, the three buildings cost over CHF 300 m, and C4’s credit was CHF 38.76 m. In December 2023 the Conseil d’État asked for a further CHF 2.501 m for C4 to bring wood into the project and optimise natural ventilation in the façades. That is a canton voting for timber and fresh air, on a Minergie-P ECO site with 105 geothermal probes at 300 m. La Télé reports more than 1,700 HESAV students this autumn. Carole Wyser, HESAV’s director general: “These figures delight us.”
Kinsbergen calls his approach “non-design”: “We did not want to create an image. These buildings represent nothing: they make space available.” Read from the piles up, that intent is precise, and Kostic’s slab is how it gets done.
Atelier: Swiss offices are increasingly asked for open, porous ground floors under regular upper programmes, and early AI massing tools default to stacking the grid straight down. Make the transfer level an explicit decision. Monday move: in your next competition model, give the transfer slab its own layer and engineer conversation before the upper grid is frozen.
Hack: Revolve a transfer-slab underside that thickens where the columns land, so the load path shows in the massing. Drop this into a GhPython component in Grasshopper. The thicknesses are placeholders, since C4’s are unpublished, but the shape follows Kostic’s description:
import Rhino.Geometry as rg, math
R, rc = 28.0, 24.0 # drum radius; assumed column ring
t = lambda r: 0.35 + 0.25*math.exp(-((r - rc)/3.0)**2) # thickest over columns
crv = rg.Curve.CreateInterpolatedCurve([rg.Point3d(i*0.5, 0, -t(i*0.5)) for i in range(57)], 3)
a = rg.RevSurface.Create(crv, rg.Line(rg.Point3d.Origin, rg.Vector3d.ZAxis))Move rc and watch the thick band follow the columns. Then carry the surface into Archicad as a slab, using the Grasshopper-to-Archicad chain taught in the PAZ Building System Specialist Title.
Open your current project, find the floor doing the most work, and model it first.
Sources & Further Reading
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