The new San Diego Central Library project features a broad range of unique structural solutions, from its cast-in-place architectural concrete frame beams and columns to its exposed concrete waffle slabs. Its chief crowning glory, however, is its distinctive silver dome.
The iconic building features 504,000 square feet of space across 10 storeys and includes reader seating for 1,200 people, 407 computer stations, and 22 WiFi-enabled study rooms, meeting rooms and gallery/exhibition spaces. It also houses a 350-seat community auditorium building and underground car parking for 250 cars.
The dome that sets the building off, providing shade and acclimatising the eighth floor main reading room, is the largest steel post-supported segmented dome in the world. The complex dome structure was engineered by Endrestudio Architecture & Engineering, with a design by San Diego architect Rob Quigley.
The dazzling structure measures 140 feet (43 metres) in diameter and rises 221 feet (67 metres) above ground level. It is constructed from more than 3,000 individual pieces of steel, weighing a total of 285 tons and clad in 1,500 perforated aluminium panels.
EndreStudio’s conceptual design of the dome explored six different circumferential and segmented options. The chosen solution employs two tiers of post-tensioned, three-dimensional, moon-shaped truss elements. Eight unique truss “ribs” rise from base to apex in varying heights from 72 to 113 feet (22 to 34 metres) and eight unique “sail” structures are located between the ribs. Unfurled, the largest sail is 123 feet by 53 feet (38 by 16 metres) wide and made up of 175 members of tubular steel as well as 60 cable segments.
Architecturally, it is desirable for the lower edges of the sails to be as thin as possible. This is achieved through the use of the three-dimensional truss spanning diagonally from rib to rib, and appears visually as a six-inch (15 centimetre) edge thickness. To further reduce the mass of the sails, steel cables are introduced to minimize large members on the interior surfaces close to the glass of the reading room.
The sails were built on-site, two at a time, on large temporary platforms on the ground and lifted into place one sail at a time.
Adjacent to the dome, the vertical stair tower forms a strong structural core which anchors the two wings of the building with a system of triangulated arms. A special events assembly hall tops this rectilinear cluster with a stunning 30-foot high glass wall that angles out from the building’s exterior.
Behind this glass plane, and following its angle, is a single central column with multiple connections. This column holds the cantilever ring to horizontal steel triangular vierendeel trusses as well as two composite precast double tee beams flanked by similar trusses. This structure defines two square roofs separated by a central strip of glass which illuminates the special events hall below.
Given the complexity of the dome — it has very few identical members — 14 months of detailing were required to prepare 3,300 drawings. The structure took 27,000 man hours to fabricate and more than 19,000 hours to field assemble and erect.
The project won the 2013 National Award for Excellence in Structural Engineering.