hotels_borgataThe Borgata Hotel Casino & Spa opened with a splash July 3, 2003. It is the first new casino to hit the shores of Atlantic City in 13 years. The 4.1 million-square-foot complex comprises an impressive hotel tower, a sizeable low-rise casino and spa, and a 5,300 vehicle parking facility. The buildings utilize three types of structural systems, each selected to meet the needs of the specific structure. The tower is a post-tensioned, cast-in-place concrete structural system, the low-rise uses a concrete-steel composite construction, and the parking facility is precast concrete.

The Borgata features a 1.53 million-square-foot, 46-story, 2,400-room hotel tower. An investigation of numerous structural systems for the tower determined that post-tensioned cast-in-place concrete construction would best suit the project’s needs. This system provided the lowest floor-to-floor heights, the thinnest floor slab construction possible, and the most efficient use of materials. It also offered the flexibility to adapt the structure to meet the architect's design requirements.

A cylindrical zone at each end of the tower lends a unique architectural appearance. Despite irregular column locations and numerous floor openings, the circular floor areas in these zones are economically designed and constructed with cast-in-place post-tensioned concrete. Spaced 35 feet on center, the perimeter columns in the circular areas required an 8.5-inch-thick slab. Designed as 8.5-inch-deep in-slab beams, the curved slab edges were constructed with a curved band of tendons along the floor radius to support a "mullion-free" glass curtain wall façade. Despite being the thinnest floor system investigated for this project, the post-tensioned floor was the best suited for meeting the stringent slab edge deflection requirements established by the curtain wall manufacturer.

More than 79,000 cubic yards of concrete and 579 tons of post-tensioning tendons were used in the hotel tower and low-rise casino superstructures. Post-tensioned cast-in-place concrete construction provided the hotel design team with flexibility to accommodate irregular column locations and slab openings, curved slab edges, low floor to floor heights, and stringent slab edge deflection requirements with efficiency and economy.

The $1.1 billion, 4.2 million-square-foot resort includes a 43-story tower on a 30-acre site with 3,100 trees. It features 2,002 guestrooms and suites, 125,000 square feet of gaming space, parking for 7,100 cars, and a 50,000-square-foot European-style spa. It has 11 retail boutiques, 11 restaurants, a 1,000-seat theater, a 15,000-square-foot indoor pool/garden complex and 70,000 square feet of event space.

The unique shape of the hotel and strong winds off the Atlantic Ocean meant that the building’s structural system had to be fine-tuned using wind tunnel analysis. A wind tunnel study was conducted to investigate the structure’s response to hurricane-force winds. Shear walls were strategically located to best resist the wind loads, and 9,000-psi concrete was used in the walls.

The hotel tower utilized several concrete mix designs. The columns and shear walls were constructed using concrete varying in strength from 9,000 psi on the lower floors to 5,000 psi on the upper floors. Higher strength concrete on the lower levels greatly reduced the amount of reinforcing steel required in columns and walls to eliminate rebar congestion and expedite construction. The tower’s height, geometry, location on a hurricane coastline, and distinctive full-height glass curtain wall system necessitated the use of a sufficiently stiff lateral load resisting system to prevent excessive building drift during hurricane-force winds. Minimum modulus of elasticity values were specified and tested for each of the specified concrete strengths to ensure the proposed mix designs would yield the necessary concrete stiffness and strength. An investigation of the concrete long-term creep and shrinkage was carried out to address the differential shortening between adjacent columns and shear walls. With a 5 percent material cost premium over conventional Grade 60 reinforcing steel used for #10 bars and smaller, Grade 75 reinforcing steel was specified for #11 and #14 bars. The extra cost helped reduce the amount of reinforcing required by 20 percent.

Column sizes and locations were configured to optimize the design of the floor slab, where the largest percentage of the hotel tower concrete is contained. Framed with 5,000-psi, 7.5-inch-thick post-tensioned slabs, the 510-foot-long floor was constructed without expansion joints and assisted by the central location of the elevator core. Temporary pour strips divided each floor slab into three independent segments to allow creep and shrinkage shortening to occur in each segment for thirty days before joining the floor plate together as an uninterrupted 510-foot-long slab. The pour strips also allowed simultaneous construction of each floor in three segments to facilitate post-tensioning operations. Slab tendons were banded along the column lines in the short direction of the floor and were distributed uniformly in the long direction. With a span of 30 feet 9 inches in the long direction, the slab achieves a span-to-depth ratio near 50. None of the structural systems investigated during the project schematic design phase came close to matching this value.

Attachment of the façade to the floor slabs required careful attention to slab edge deflections and the means by which these deflections could be controlled. Additional tendons were added at perimeter slab edges to provide the required stiffness and to reduce deadload and long-term-creep deflections that would otherwise have occurred. In order to maximize efficiency of the slab design, mild reinforcing steel was placed in the top and bottom layers of the slab in the longer span direction. The uniformly distributed tendons were given priority in placement over perpendicular mild reinforcing steel, permitting installation with the maximum possible drape.

A massive, curved, staged post-tensioned transfer girder was required on the tower’s third floor to support a 43-story tower column, which architectural design dictated could not extend below level three. The large concentrated load on the transfer girder required pre-stressing force in the girder of 3,000 kips. The tendons were stressed in several stages as tower construction progressed. By gradually stressing the tendons as loads increased on the girder, designers were able to keep stressed within allowable limits. Perpendicular concrete beams were arranged to brace the curved transfer girder against torsional rotation.

Columns were sized to be large enough so that the required floor slab thickness was driven by flexural considerations and not slab punching shear stresses around the columns. Slab openings were kept away from columns wherever possible; however, when slab openings adjacent to columns could not be avoided, prefabricated shear reinforcing was installed around the columns. This slab shear reinforcing eliminated the need for drop panels and allowed continued use of the most efficient slab thickness.

Architectural design in levels one and two below the hotel tower required a shift in location of two shear walls. The level three floor slab was designed as rigid diaphragm to transfer the horizontal shear wall forces through the slab between the offset walls. A 16-inch post-tensioned slab was used to provide the floor slab diaphragm with sufficient strength and stiffness for lateral load transfer between the offset shear walls.

Credits

Owner:
Borgata

Architect:
Cope Linder Architects, Philadelphia, Pennsylvania

Structural Engineer:
Cagley Harman & Associates, King of Prussia, Pennsylvania

Construction Manager/General Contractor:
Yates Construction, North Mobile, Alabama/Tishman Construction, New York, New York