Skin and Bones

Steven Holl’s Glassell School of Art in Houston moves seamlessly between precast panels on its striking geometric facade and an exposed structure of in-situ concrete within, writes Tony Whitehead

It is not everybody’s concrete that gets to feature on the front cover of Vogue – so naturally the team at US-based McCarthy Building Companies were tickled to see their 8,000psi self-compacting mix supporting the substantial form of rapper DJ Khaled as he posed on concrete steps at the newly built Glassell School of Art.

But it was not such a surprising location for a fashion shoot. Part of the Museum of Fine Arts, Houston, the Glassell is a hip, contemporary building, surrounded by sculptures by the likes of Anish Kapoor. Designed by US star architect Steven Holl, the school‘s unusual plan and dramatic facades make it something of an artwork in itself.

The staircase favoured by Vogue, for example, forms part of a grand, triple-height forum lit by a spectacular skylight, and the school is L-shaped in plan, the longer side inclined to allow visitors to walk up to a roof-level event space. The 8,640m2 three-storey school also includes 36 art studios, gallery spaces, auditorium and a double basement of parking.

But the most striking elements of the building are the facades. Constructed from giant, trapezium- shaped panels of white precast concrete, they are interspersed with similarly enormous sheets of translucent glass. The rhythmic puzzle of glass and concrete dominates the aesthetic – so it comes as a surprise to hear the building was not originally conceived in concrete at all.

“The L-shape and the slope [plan and section] – they were there from the beginning,” says Olaf Schmid, Holl’s project architect for the Glassell. “But the exterior was first envisaged as clad in perforated metal. Then Steven decided he didn’t like that any more. For a while it was going to be plain grey concrete, and we went back and forth before finally choosing white.”

This key decision was strongly influenced by the neighbouring sculpture garden, by acclaimed artist and landscaper Isamu Noguchi, which dates from the 1980s and features white concrete for its walls and planters. The notched character of the panels also references neighbouring art, specifically the sculpture by Eduardo Chillida that adorns the space created within the Glassell’s L.

“Once we went for the concrete it became key to the expression of the building’s skin and structure,” says Schmid. “The large-scale precast panels pick up the angle formed by the sloping roof, and having these big elements fitting together – it’s a very tectonic idea.”

Precast concrete was chosen because of the dimensional precision and finish control offered by factory production. “At first we wanted to construct the whole building in precast,” Schmid says, “but this proved impracticable. The building has some large spans, cantilevers and odd angles. It was just too difficult to figure out the connections, so the beams and cores became cast-in-place. The challenge then was to get the in-situ concrete to match the white of the precast.”

They considered an acid-etch for the precast, but in the end chose a sandblasted finish. The in-situ concrete was also sandblasted, “so both the colour and the roughened surfaces help to tie the two kinds of concrete together”.

Colour-matching concrete is challenging, however, and McCarthy conducted several test pours before settling on the mix that best mimicked the precast panels. Unfortunately this was not a mix that the local ready-mixed concrete supplier could produce – so, unconventionally, some 90 bags of white cement were added by hand to each truckload. Cemex, the ready-mixed concrete producer, provided a quality-control specialist on site to ensure consistency for all the white concrete pours.

Once the basement car park was complete, the first rows of precast units and glass panels could be placed (see box, overleaf). An in-situ perimeter beam was then cast on top of the panels, which proved structurally complex. The floors of the Glassell are built from hollowcore extruded- concrete plank-shaped units which rest on the beams. “In the upper floors though, the facade kinks in and out,” says Schmid, “so the perimeter beam has to widen to accommodate that. Where that happens, the bearing point of the planks gets pushed back from the facade, introducing a torsional force – meaning that the beam grew in size. In some cases it had to be tied back to the spine centre of building with another cast-in- place element.”

The central support for the beams and floor slabs comes not from in-situ columns but from precast “wall columns”, as Saman Ahmadi, principal at associate architect Kendall/Heaton, explains: “These are actually made in a similar way to the facade panels, but like much of the interior concrete, they are natural grey, not white.”

There are several other differences: “The concrete of the facade panels is only visible on the exterior of the building,” says Ahmadi. “The team did consider having the concrete exposed on the interior of some of the facade panels, but it would have been more expensive to sandwich insulation inside the panels and, in any case, simple interior insulation covered by plasterboard suited the school’s requirements for attaching art to the walls.”

So while the facade panels are only sandblasted on the exterior face and edges, the interior wall columns are sandblasted on both sides and the concrete left exposed. “These panels are also made from a stronger concrete because of the extra structural work they do supporting the floors,” he adds.

The hollowcore units that make up the floors span up to 9.1m and weigh up to 9.1 tonnes each. Routinely seen making up the floors in multi- storey car parks, they are known as a rapid and inexpensive way of creating floors – though Schmid says that because of the torsional issues created by the irregular perimeter beam, “it might, with hindsight, have been easier, structurally, to go for a cast-in-place slab. But we liked the expression of the exposed planks on the ceilings and we have been able to use the grooves between them for lighting tracks.”

Other electrical conduits, and piping for a water heating and cooling system, have been laid on top of the planks before being covered with a 9cm layer of concrete. There is no false ceiling or raised floor, and Schmid says that exposing the concrete floors helps to control the building’s environment. “It means the air is in contact with
the thermal mass of the concrete, which helps even out temperature swings.

We are big proponents of this and use it whenever we can. The climate in Houston is challenging – it can get very hot. But running cold water through pipes buried in the ectopic slab cools the building very efficiently. It is particularly effective where you have sunlight falling directly onto the floor and thermally activating it. Then the water cooling comes into its own and transfers out large amounts of heat very efficiently.”

Because the local climate is also very humid, it is important that the slabs do not become too cool, however. “Condensation is a risk, so the slabs are not allowed to cool to the dew point,” says Schmid, adding that the radiant slab cooling is boosted with more traditional air-conditioning when required.

Far from all of the servicing at the Glassell School of Art is hidden, however. In most of the teaching studios, an industrial look prevails with considerable amounts of ductwork visible on the ceilings and walls. In some studios, the use of specialist equipment and materials requires additional exhaust ventilation, and this is provided via a plenum through a series of holes cast into some of the facade panels. Each hole is fitted with a bird screen and is sloped to drain rain away from the plenum.

In contrast, there are virtually no services visible in the more public areas, particularly in the forum, which is characterised by a large feature staircase, triple-height atrium and a huge triangular skylight. “Holl’s office wanted this uncluttered so the services have been routed up from the basement levels to feed each of the two sides of the building,” says Ahmadi. “This leaves the feature concrete in the forum area clear.”

The forum includes some of the largest beams in the building, which span from the corners of the L to make the triangular openings on level one and two for the skylight. The longest of these is a post-tensioned beam of some 21.3m with a section 1,370mm high and 660mm wide. This beam was placed monolithically with the entire skylight structure due to the way the 100 post-tensioning cables interacted with other skylight beams.

Like much of the building, the design of the staircase below the skylight evolved over time. Originally conceived as precast, it is now in-situ with the exception of some precast vertical panels that rise from it. “It was always going to be difficult to make large steps like this into a seamless staircase,” says Winston Hesch, McCarthy’s project superintendent.

He explains that a grey 8,000psi self-compacting mix was chosen to help create a smooth airhole-free finish, but as always with in-situ concrete, the final result came down to attention to detail. “We had skilled carpenters making the formwork, we triple-checked every measurement, we poured in small flat sections, and we got a result we are really proud of.”

To create the treads, the forms were stripped a little early while the concrete was relatively soft, and trowel-finished before being finally being given a grit polish. The result is pleasingly monolithic – and smooth enough for even a fashionista’s posterior.

Solving the panel puzzle

The 178 precast reinforced concrete panels that make up the facades of the Glassell School of Art are 300mm thick, up to 5.2m high and weigh up to 19 tonnes. All are subtly different, however, meaning that the manufacturer, Gate Precast, had to create a bespoke timber mould for each before transporting the finished units horizontally by road from its facility some 200 miles north of Houston.

Once on site, they were treated with reverence by the contractor, McCarthy Building Companies: “We would pick them up carefully with two cranes at first to minimise the forces acting on them, and then unsling the second crane to finally position each one,” says Winston Hesch, project superintendent at McCarthy.

The exact placement of each panel was a key concern. “The panels are cast with sleeves or holes in the bottom and top,” explains Hesch. “These fit onto rebar dowels. So for each panel we would have, for example, some 40 rebar dowels sticking up from the slab and our concern was how to keep these within tolerance so the panel would slide on neatly.”

The low-tech solution to this problem proved highly reliable: “Gate would send us the formwork from the top and bottom of the panel, and we would use it as a template for the dowels.” It worked so well that, with practice, McCarthy could place a panel in just 30 minutes.

When the panel was perfectly aligned it would be braced prior to final fixing. “Small holes leading from the sleeves allowed us to inject grout to fix the rebar. We could then place rebar dowels in the top sleeves and erect formwork for the beam over the top of the panel.”

Design architect Steven Holl Architects
Associate architect Kendall/Heaton Associates
Structural engineers Guy Nordenson and Associates, Cardno
Contractor McCarthy Building Companies
Precast concrete manufacturer Gate Precast

Photos Richard Barnes