Urban poetry: new offices for Caltrans in LA explore notions of economical energy use and urban monumentality.

Los Angeles sprawls across a desert and its survival depends on water and transport. The city's Department of Water and Power commissioned a landmark headquarters building from A. C. Martin 40 years ago; now the California Department of Transportation (Caltrans) has moved into an even bolder structure, symbolically located across from City Hall, and a few blocks east of Disney Hall (AR March 2004). Speed, economy of construction, flexible work spaces and energy conservation were the priorities; the architects met all these goals while creating a building and piazza that enrich downtown and its meagre public realm. It's also a breakthrough for Morphosis, the firm Thom Mayne co-founded in 1975, which has finally (like Gehry) won a place at the top table in its home city. Mayne is working with David Childs of SOM, developing plans for vacant lots on Grand Avenue, which links Moneo's Cathedral (AR March 2005) to Disney Hall, Isozaki's MOCA and the public library. With luck and creativity, it could become the major axis of a revitalised downtown.

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Caltrans (officially the DoT's District 7 HQ) is the latest of the firm's urban monuments, and it develops ideas of structure and skin previously explored in the Hypo Adria-Alpe complex in Klagenfurt, Austria, and the Toronto University student housing block. Soon to come is the Science Centre School in the inner city, a federal courthouse in Oregon (AR April 2005) and the innovative federal office building in San Francisco. Caltrans comprises a 13-storey slab of offices that arches over a four-storey wing, defining the east and south sides of the piazza. A 9m cantilever carries the upper storeys over First Street to the north, supporting a light bar that serves as a marquee to this piece of urban theatre. The east and west sides of the slab are wrapped in a thermoplastic membrane, and this is concealed by a scrim wall of three different gauges of perforated metal which shades the sun and is set forward to enclose a thermal air curtain. The porous surface, which can be washed down from gantries, is punctuated with hydraulic flaps that pop open when the sun is not shining on that side. The steel frame and the scrim it supports are folded out at the base and extended around the open edges of the piazza as a canopy. To reduce energy consumption at peak hours, photovoltaic panels are set at an angle across the board-marked shotcrete of the south facade, with spaces between each row to open up views for the occupants. Boldly sculpted steel escape stairs jut from the facade and a low wall encloses a play area for the daycare centre.

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Tough, sharp-edged, and frugal, Caltrans is infused with urban poetry. The scrim shimmers in sunlight and glows from within at night. When the flaps open, they generate an irregular pattern as though these were manually operated shutters on a medieval warehouse, or rocks artfully positioned in a vertical Zen garden. Concrete benches and bent steel posts are scattered across the piazza, doing double duty as seating and security barriers. At the junction of the two wings, bleachers are sunk into the paving, providing additional seating and direct access to the four levels of subterranean parking. Art and signage are integrated with the architecture. A neon mural by Keith Sonnier evoking the lighted streams of traffic on a freeway is wrapped around the glass facades of the lobby and the low wing, with a supergraphic of Caltrans etched on the glazing fronting the piazza and a giant steel '100' to indicate the street address (on First and Main) standing up from the angled end wall.

In the entrance lobby, backlit fibreglass panels wrap round a suspended reception desk. An atrium reaches up four storeys and you glimpse shadows of people moving over the glass floor of a conference room high overhead. Express lifts stop at every other floor, speeding vertical circulation, and encouraging occupants to use the stairs and socialise with their neighbours. The building, constructed in just over two years on a budget of about $170 million, houses 2300 staff in the 105 000sqm interior which is notable for its openness and transparency. The grey hulk of the exterior is dematerialised by light and becomes, from within, a veil that seems to hover over the city.