At BCAM, inclined aluminum sunshades shield the galleries from direct sunlight which would otherwise penetrate the glazed ceilings (shown here prior to installation of sunshades). Photo © Museum Associates/LACMA

The three-story BCAM, opened earlier this year, is made up of two nearly identical travertine-clad, linked, boxlike volumes. A north-facing skylight system illuminates the building's top-floor galleries. Piano intends visitors to travel to these daylit spaces first, via an exterior zigzagging collection of escalators and stairs that he refers to as "the spider."

The building's roof is an adaptation of the sawtooth roofs found on many industrial buildings. But instead of the typical combination of a sloped opaque surface and a vertical glazed surface, each 8,500-square-foot ceiling of the top-floor, column-free galleries is entirely glazed, serving as skylight and weather enclosure. Above the glass, 17-foot-tall aluminum sunshades, sloped 45 degrees, admit light from the north but block direct light.

The skylight ceiling, cambered to permit water to drain, is made of low-iron, insulated glazing units. In addition to a polyvinyl butyl (PVB) interlayer that filters nearly all of potentially art-damaging ultraviolet light, the units have a translucent frit pattern that reduces visible light transmission to 38 percent. "The ceiling is working hard from a thermal and daylighting point of view," says Arup's Davies, the lighting design team leader for the BCAM project.
The all-glass ceiling posed challenges for seismic designers because it did not allow for a roof slab that would serve as a structural diaphragm in earthquake-prone Los Angeles, explains Simon Rees, a structural engineer in Arup's local office and the project's multidisciplinary team manager. So, in order to maintain the stability of the building during a temblor, engineers devised a system of east-to-west spanning trusses and tension-and-compression bracing. In conjunction with an unbonded brace frame, the components help "limit the forces the roof will experience during a seismic event," says Rees.

Section Through Third Floor Gallery To determine the primary direction of light within the galleries, the consultants conducted an illumination vector analysis (both images below). The analysis takes into account light passing through the skylight from the north and light reflected between the inclined sunshades. Images courtesy ARUP

 

Although the BCAM daylighting system is primarily passive, it includes motorized exterior blinds that exclude almost 90 percent of visible light. The blinds can be lowered to shut out daylight during nonoperating hours, or to meet the curatorial and conservation demands of a particular exhibition. The blinds are also programmed to close during summer early morning and late afternoon hours-those few periods when direct sunlight will pass through the inclined roof panels.

A potential drawback of a north-facing skylight system is that it can create diffuse light conditions that lack uniformity. "One disadvantage of the sawtooth system is that the light is directional," says Davies. "The south-facing wall does not ‘see' the sky at all," he says.

To determine the primary direction of illumination within the gallery space Davies and his team conducted an illumination vector analysis. This analysis takes into account light passing through the skylight from the north and light reflected between inclined sunshades. As a result of the study, designers refined the sunshade, adding a "kicker" at its bottom edge. This 3-foot-tall vertical element bounces light back to the south-facing wall, creating more uniform daylighting conditions.

The top-floor galleries' electric lights, which include both wall washers and spotlights for highlighting individual pieces, are integrated into the skylight mullions. Photo censors control the dimmable fixtures, adjusting electric lighting levels as daylight varies. "To maximize energy savings and provide adequate lighting, it is important to have a coordinated approach, not only for museums, but for any building type," says Davies.