Situated on the exclusive shoreline of downtown Long Beach, California, is the Ocean Villas Condominium project. Two 18-story towers house a total of 556 residential units with over 723,000-square- feet of living area, and 264,000 square feet of parking is accommodated in three subterranean levels. At ground level, the entry driveway, two swimming pools, and a seaside promenade walkway are part of an 85,000-square-feet landscaped plaza area.
One of the main goals of the project was to reduce overall construction time so that occupancy could commence as soon as possible. It was determined early in the development stage that a reinforced concrete structure utilizing a tunnel formwork system would meet the aggressive schedule, and would save approximately 30 percent in direct construction costs. At 230 feet in height, the buildings are the tallest formed by this type of system in a region of high seismic risk.
Tunnel forms, which can usually be reused 500 to 1,000 times, are an effective way to construct buildings that have repetitive elements or layouts. A typical construction cycle, which covered approximately 5,000- to 6,000-square feet of surface area, was as follows:
- Cast foundation and starter walls used to position tunnel forms.
- Install tunnel forms between starter walls with cranes and align with screw jacks.
- Install electrical conduits, blockouts for doors and slab openings, reinforcement for the walls and slabs.
- Form starter walls for the next level.
- Pour concrete for the slabs and walls, and suspend portable heaters inside the forms to help cure the concrete so that it reaches a compressive strength of 2,200 psi in 16 hours.
- Remove one L-shaped form segment, shore the slab, and remove the other L-shaped segment
With 21,000 square foot per floor, it took four days on average to complete one level.
Floor framing consists of 5½-inch thick cast-in-place one-way slabs spanning between walls, which are centered 12 feet or 16 feet apart. At the plaza levels, a cast-in-place two-way slab system is utilized to support heavy landscape and fire truck loading. Both 10½-inch and 14½-inch thick slabs with 10-inch. thick drop panels span 24- to 33-feet and are supported on 12-by-30-inch concrete columns.
The foundation system for each tower is a 5-foot thick concrete mat, which is 110 feet wide by 230 feet long. Conventional spread footings are used under the columns supporting the three plaza levels and parking levels outside the footprint of the towers. To support the unbalanced soil pressure and to minimize lateral forces resisted by the tower walls, buttresses were added every 25 feet on center along the north basement wall.
Special reinforced concrete shear walls are used as the lateral force resisting system. Since the 1997 Uniform Building Code limits the height of bearing wall systems utilizing shear walls to 160 feet, the structural engineer demonstrated through experimental data and analysis that the proposed system possesses the ductility required to exceed code performance objectives for the 230-foot tall towers. A displacement-based design procedure confirmed the available curvature ductility of nine for the shear walls. The City of Long Beach Building Department subsequently granted an exception based on this evidence.
The shear walls are 12-inch thick in the subterranean levels, 8 inches ground to level 7, 6 inches levels 7 to 13, and 8-inches thick level 13 to the roof. The walls, typically 24 feet long, are coupled with 24-inch wide by 72-inch beams at the roof level. The 8- and 12-inch shear walls are reinforced with two layers of No. 5 bars at 12 inches on center and 10 No. 8 jamb bars. The 6-inch walls are reinforced with one layer of No. 5 bars at 12 inches on center and six No. 8 jamb bars.
In addition to faster construction time and lower costs, concrete framing provided other important advantages, including resistance to fire with no need for additional fireproofing. Since approximately 60 percent of the interior walls are concrete, a 30- to 40 percent reduction in sound transmission is realized compared to stud and drywall construction, resulting in quieter units. Also, due to the inherent thermal mass characteristics of concrete, utility costs are also reduced. In short, concrete framing satisfied the needs of the owner and the residents of the buildings, providing a cost-effective solution for both.
Genesis Real Estate Group, Inc., Dallas, Texas
EDI Architecture, Inc., Houston, Texas
Englekirk Partners Consulting Structural Engineers, Inc., Orange County, California
Summit Builders Construction Co., Irvine, California
Concrete Subcontractor (Tunnel Forms):
Highrise Concrete Systems, Inc., Irving, Texas