The Westside Media Center is one of the new flagship office buildings in the trendy West Los Angeles/Santa Monica, California, area. In addition to the rehabilitation of an existing warehouse, this project consists of two new four-story buildings that provide 300,000-square-foot office space and over 340,000-square-foot parking in two subterranean levels. At the ground level, an additional 100,000-square-foot landscaped area creates a campus-like feel that helps attract employees of the entertainment and software industries.
Concrete was selected for the framing system to create large uninterrupted interior spaces, commonly preferred by prospective tenants. The floor system was required to span 45 feet with no vertical supports between the core and the edges of the building. The shallow concrete floor system allowed greater than usual open office space, since the required floor to floor heights are 15 feet.
The floor framing consists of 7-inch thick cast-in-place one way slabs spanning between precast beams. Using modular formwork supported at the edges of the precast beams allowed the elimination of conventional full-height shores and reshores, thereby saving both time and money. The 16-by-30-inch precast, prestressed beams are supported by 20-by-30-inch precast, prestressed girders. Aesthetic demands of the project required all the beams and girders to have a dapped end, supported by rectangular corbels flush with the bottom of the framing members. Where only one beam frames into a column, pockets were provided in the column, which eliminated the need for corbels. Precast concrete columns 24-by-24-inches are utilized for the entire 85-foot height.
Two types of lateral force resisting systems are utilized in these structures. In the long direction, precast hybrid moment-resistant frames are provided. Cast-in-place concrete shear walls are used in the short direction. Precast columns at each end of the shear walls constitute boundary elements.
The precast hybrid moment resisting frames utilize post-tensioned cables anchored at both ends of the frame, running concentrically with the axis of the beams. In addition to the restoring force it provides, the post-tensioning steel, which remains elastic during a seismic event, generates a clamping force between the beams and columns, and enables the shear forces to be transferred by friction at the beam-column interface. Mild reinforcing steel, the straining of which provides the necessary energy dissipation during a seismic event, is placed at the top and bottom of the beam through the joint and is grouted in place.
Typical frame columns are 36-by-36-inches with a concrete strength of 6,000 psi. Frame beams are 24-by-41-inches with 24 strands (0.6 inches diameter) at typical floors, and are 24-by-48-inches with 19 strands at the roof level. The beams have three Number 9 mild reinforcing bars at the top and bottom, placed inside the 3-inch diameter duct filled with high-performance non-shrink grout.
These buildings, which are the first application of the International Conference of Building Officials (ICBO)-approved precast hybrid cement resisting frame system in Los Angeles, were each constructed in approximately 13 months. Concrete framing satisfied the needs of the owner, and provided cost-effective buildings that were occupied in a relatively short amount of time.
Kilroy Realty Corporation, Los Angeles, Calfornia
HKS, Los Angeles, California
Englekirk Partners Consulting Structural Engineers, Inc., Los Angeles, California
Charles Pankow Builders, Ltd., Altadena, California