CASE STUDIES

The "sails" on north facade help reduce sky glare and undesirable direct-beam solar radiation during the summer months in the morning and evening, when the rising and setting sun actually strikes the north facade. Photo: Bill Timmerman

Project: Phoenix Central Library, Phoenix, AZ

Owner: City of Phoenix

Architect: bruderDWLarchitects

Mechanical and Electric Engineer: Ove Arup & Partners California

Daylighting Consultant: Tait Solar Company

Structural Fabric Consultant: FTL/Happold

CASE STUDY FEATURES:
Automated Shading
Orientation
Daylighting
Glare

 

Typical Glazing Properties:
Green-tinted insulating low-E glass
U-factor=0.31
SHGC=0.37
VT=0.65

Entry Glazing Properties:
Clear insulating low-E glass
U-factor=0.31
SHGC=0.53
VT=0.76

Phoenix Central Library

With a total area of 280,000 sq ft spread out over five floors and basic rectangular form, the Phoenix Public Library is outwardly an air-conditioned big box. However, underpinning this simple form is a design wed to the hot-dry desert at 33° N latitude. Facades and roof are designed to control solar heat gains, a significant factor in the climate; building faces were conceived mindful of sun angles and sun path, these factors and orientation informing development.

Context and climate have the capacity to shape design. In fact, the design team initially studied an Adobe concept—a library with thick walls and small windows —a nod to the lessons from traditional vernacular architecture of the region. An architecture of thermal mass and minimal openings has advantages in the desert. However, such a strategy did not yield good contact with the outdoors, and the team wanted to capitalize on vistas of Phoenix's mountains enjoyed at the library site. The library design had to limit solar gains and glare while admitting light and views.

The design resolution ultimately recognized that it is possible to let in light on the facades where it can be tempered and controlled, but it is not feasible to harness light on all orientations. For instance, at Phoenix's latitude (and others for that matter) low sun angles, characteristic in the rising and setting sun, are the most difficult to control. Neither overhangs nor vertical fins fully block the intense sun on building elevations facing east or west. This fact shaped the design.

On the east and west facades, copper-clad saddlebags—mechanical rooms, emergency stairs, toilets, service elevators, and other servant spaces—buffer the primary, served spaces of the main library block. The auxiliary areas bear the brunt of the intense summer sun, offering protection from the sun's light and heat. Their reflective copper cladding provides the first line of sun defense, with their depth—about 25 feet at the widest—also providing protection. The saddlebags are naturally ventilated, unconditioned spaces (except in the rest rooms), with the mass of 12-inch concrete panel walls acting as a thermal damper.

The elevations explore the issues of shade, reflectivity, and solar gains. The east and west cladding consists of perforated copper sheets, which reduce the solar gains normally associated with those orientations. In contrast, the west entry point—the incision that curves into the building—is clad in highly reflective stainless steel, a surface that accentuates heat and light. On the north, "saddlebags" jut out to protect the glazing and the fabric "sails," shade the glazing. Photo: Bill Timmerman

The saddlebags, on multiple levels, are designed to protect the main library spaces from the early morning and late afternoon sun. In contrast, the north and south facades are designed to admit light and views, but are again conceived with orientation in mind. The fully glazed north face is fitted with external, fixed vertical shading—Teflon-coated acrylic fabric sails. The saddlebags also jut out to protect the glazing. As the rising and setting summer sun strikes the north facade, both elements shield glazing from direct sun penetration in the critical time frame of March through September.

In contrast, the south facade is fitted with adjustable, horizontal louvers. Externally mounted, the aluminum louvers are computer controlled to eliminate direct sun penetration, while maximizing views and daylight. High sun angles on the south facade make horizontal elements effective means to control and temper sunlight, and even drive light deeper into space if properly positioned to act as light shelves. These customized louvers are integrated with the conventional window system, offering a cost-effective, striking solution to sun control. These customized louvers are integrated with the conventional window system, offering a cost-effective, striking solution to sun control.

South facade with horizontal louvers, designed to optimize daylighting while minimizing the glare. The louvers are motorized and automated to position them to block direct beam solar radiation and allow for natural lighting. In fact, they are not operated most of the time and provide sun control in a stationary, horizontal position. Photo: Bill Timmerman

Toplighting for the fifth-floor reading room is also important, and is accomplished with narrow, linear skylights running along the saddlebag walls and circular skylights set above each column. These small, simple skylights are not fitted with louvers or other controls, but their depth limits direct sun penetration. Only the high sun at solar noon fully enters the reading room through the linear skylights. Direct sun fully penetrates the deep circular oculi only at solar noon on the summer solstice. The atrium skylights, however, are fitted with reflective louvers that track the sun and direct appropriate amounts downwards, with a lower set that diffuses the light and eliminates direct glare within the spaces below.

The Phoenix Central Library illustrates how the sun path and facade orientation can shape design decisions. Daylight is important in library spaces, yet it must be tempered within the desert context. As the north and south facade glazing is shading appropriately for each orientation, a relatively high-VT glazing is possible—to offer light and views. Toplighting is also carefully considered and controlled.

The project was a joint venture between william p. bruder, architect, ltd. and DWL Architects & Planners

Source:
Carmody, J. S. Selkowitz, E. Lee, D. Arasteh, T. Willmert. Window Systems for High-performance Buildings. Norton, 2004.

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