Concrete, Steel and Precast

When do you use cast-in-place concrete, precast and structural steel in the same building?

The walls are filled-cell CMU — block with the cells grouted solid around vertical reinforcement — tied into cast-in-place concrete columns and beams on a cast-in-place slab on grade. Heavy structural steel beams and steel joists carry the long spans. Every other slab in the building is precast concrete: the second floor, the third floor, the roof deck, the folded planes and the arch, flown in by crane over several mobilizations, welded in place, grouted and waterproofed. The folded planes visible from the street are the structure itself; there is no separate frame behind them.

Why filled-cell CMU

Plain block is stacked and mortared, and its strength stops at the joints. Filled-cell CMU is a different animal: vertical reinforcement runs down through the cells, the cells are pumped solid with grout, and the whole wall is tied into cast-in-place concrete columns and a continuous poured beam at the top. What results behaves far closer to a poured element than to stacked units — which is the point in a High-Velocity Hurricane Zone, where the wall has to take suction and impact without opening along a joint.

Where steel takes over

Concrete is strong in compression. Stretched across a long opening without support, it cracks. So wherever this building needs a span that concrete alone cannot carry — across wide window-wall openings, across the loaded pool deck — heavy structural steel takes over. Steel beams carry the load in bending across those openings. Steel joists, lighter than solid beams and open enough to run mechanical services through their depth, handle the longer deck spans and carry precast above them. The transition between masonry, poured concrete and steel span is a drawing-stage decision; none of the three substitutes for another where the other is used.

Precast, flown and welded

Other than the slab on grade, every slab in this building is precast concrete — the second floor, the third floor, the roof deck, and the folded and sloped planes that give the house its profile. They were made in a plant, cured under controlled conditions, and delivered numbered for the position each one occupies.

They arrived by crane, over the roof of the adjacent building, onto an infill lot with no lay-down yard, across several mobilizations rather than one dramatic day. Each piece was set onto the masonry, the poured columns and the steel already in place, then welded at its embedded connections, grouted along its joints, and waterproofed. Precast is not a cheaper substitute for pouring on this job; on a lot this tight it is the only way these elements could have been built at all.

The folded planes are the structure

The roof geometry that reads from the street as an architectural surface is also doing structural work. The folded planes are not a skin over a hidden frame; they are the load path.

That is why the tolerances are what they are. A surface that resolves forces through its own geometry has to arrive already correct — the shape is decided in the casting yard, not on site, and a piece that comes off the truck wrong cannot be adjusted into position seventy feet up. The shoring in these photographs is not formwork waiting for a pour; it is falsework holding finished precast exactly where the drawings put it until the welds and the grout make the connection permanent.

Built by Academia Development Corp, licensed Florida general contractor CGC1528882.