Las Vegas always has been a “concrete town.” Nearly all the hotel towers on the Strip are constructed with structurally reinforced concrete and the MGM Mirage's City-Center project, located on the Strip between the Bellagio Casino to the north and the Monte Carlo Casino to the south, follows that tradition. But it's the size and scope of this $7.8 billion construction that is most challenging. Over 1 million cubic yards of concrete will be placed by the time it's completed in 2009. Shelton Grantham, vice president of field operations, Perini Building Co., Phoenix, says they are placing 2000 cubic yards of concrete every day to keep up with this schedule.
A total of 17 buildings, including six towers, are being built at the same time. All will be ready at the time of the grand opening in 2009. Congestion both below- and aboveground provides many challenges for the contractors.
Credit: Perini Building Co.
When the MGM Grand Inc. acquired Mirage Casinos in 2000 to become MGM Mirage, the company not only took ownership of several world-famous hotels, it also acquired considerable real estate in the transaction. This was repeated when MGM Mirage acquired Mandalay Resort Group in 2005. All told, MGM Mirage now owns close to 800 acres of land on the Las Vegas Strip, 150 of which are either totally undeveloped or under-developed, meaning the structures on the land are no longer justified by the astronomical prices now being demanded for strip acreage.
Gordon Absher, vice president of public affairs for MGM Mirage, says the 76-acre site where CityCenter is being built was the last significantly sized property left for development in the center portion of the Strip. It originally was part of the Dunes Casino golf course, with a helicopter ride business occupying the front portion next to Las Vegas Boulevard. The Boardwalk Hotel and Casino was the only significant building on the site that required demolition and removal before construction began.
At first, MGM Mirage wondered how the property could best benefit its shareholders and considered selling it. “With land this valuable you, don't put up a parking ramp,” says Absher. “Even building another casino similar to other Strip properties wouldn't justify the land value.” They decided to build a mixed-use high-density development, putting more into less space—building up instead of out. So the concept for City-Center was born. The 18 million square feet of livable space would include retail and dining districts, provide the feeling of energy and excitement that other large city downtown areas have and avoid the concept of a mall development. In contrast, the Bellagio Casino, which the MGM Mirage owns, has 6 million square feet of space on roughly the same acreage.
Until now, casino developments on the Strip have tended to be themed structures—buildings in costumes. But the MGM Mirage team decided it was time for a change so they invited several world-class architects to design buildings in their own signature styles.
Designing the project
Absher says that each design architect works within the space their building occupies. In addition they also were asked to work in the context of the whole development. So the architects met together with the MGM Mirage development team to consider the impact of their designs on each others' work. Absher says the discussions also revealed how different architectural approaches addressed common problems such as managing desert heat in the buildings.
Credit: Joe Nasvik
Table forms are being used for all tower decks. Hardware mounted on columns and shear walls support the tables, providing easy access for workers below. Bolt holes for the supports are cast into placements for easy attachment.
CityCenter is also a Leadership in Energy and Environmental Design (LEED) project, currently the largest LEED development in the U.S. The idea to apply for LEED certification was encouraged by the design architects. They suggested that building high-performance buildings represents cutting-edge technology so the MGM Mirage team agreed. This meant, among other things, that the buildings demolished to prepare the site for construction would be recycled and all interior spaces would be healthier to live in by excluding materials and furnishings that generated toxic fumes. Absher says the decision is winning favor with their customers who are buying condos and leasing retail space. An added bonus is that the state of Nevada recently passed legislation offering tax credits to companies who use green building practices. When the development is complete, the project application will be for a Silver LEED Certification.
The story about building CityCenter is as much about the organization required to build it as it is about the actual construction. It's considered usual practice to build developments in phases or stages but Perini's contract requires that all construction proceed at the same time and be completed for the grand opening. Grantham says this includes a total of 17 structures plus the infrastructure. Six of these structures are towers with the rest being low-rise buildings. The towers include Harmon, Mandarin, Pelli, Vdara, and the twin towers of Veer.
The project is divided into three blocks: A, B, and C. Construction of the infrastructure is additional and includes heating/cooling, electrical, an overhead tram connecting several MGM Mirage properties, communications, water, and sewer service to all the blocks. Also included is the construction of a new Harmon Street that will run adjacent to the project and cross over the I-15 expressway to the west. A managing architect and engineer are assigned to each block. Tishman, N.Y., is the executive construction manager representing the owner and Gensler, N.Y., is the executive architect for the entire project. Here is what is included in each block:
Credit: Joe Nasvik
With so much under construction, there isn't much room to maneuver. Notice Veer Towers on the right, one is beginning to tilt to the right and other (just behind) is beginning to tilt to the left. Pelli Tower is in the background.
- Pelli Tower—a 4000-room hotel
- The Convention Center
- The Show Room—a large live theater
- West Podium—an underground parking area with swimming pools and leisure areas on a deck above the parking ramp approximately 20 feet above ground
- East Podium—casino, restaurants, and a porte-cochere, or architectural entryway
- Central Plant—power, heating, and cooling for the entire complex
- Sinatra Parking Garage
- Vdara Tower and low-rise—a condo-hotel
- Fire station
- Bellagio employee parking garage—replaces demolished parking
- Harmon Circle—elevated entrance to CityCenter
- Mandarin Tower and Podium—hotel, service condominiums, and dining
- Veer Tower—condominiums
- Harmon Tower and low-rise—hotel, service condominiums, and dining
- Dining, Entertainment, and Retail District—encompassing 500,000 gross square feet
- Garage #6 and service tunnels—approximately 2000 feet of underground tunnels
Each construction in each block has its own architect and engineer. Perini is the primary general contractor for the entire development and is self-performing the structural concrete for the Pelli, Veer, and Vdara Towers. Bomel, Anaheim Hills, Calif., is overseeing the structural concrete for the Mandarin Tower, and Ceco, Las Vegas, is building the Harmon Tower.
Credit: Joe Nasvik
The towers are structural concrete construction while several of the low-rise buildings are structural steel. If you look closely you can count 10 tower cranes and three track-mounted cranes.
The first challenge for Perini was to provide office space for a project this size. Grantham says two office buildings on the edge of the property were needed: a 100,000-square-foot building to house MGM Mirage construction-related staff and an 88,000-square-foot structure for the Perini construction staff. In addition, Perini has 20,000 square feet of office space inside the project area, with another 15,000 square feet planned. The goal was to keep the entire construction team of approximately 480 salaried staff close together.The construction is already a small city. Clark County Fire Department will be located onsite, the county has an office for permits and inspections, emergency medical help is available, and there is a ready-mix plant. But with everything under construction at the same time and congestion a major problem, employee parking is offsite and building materials are stored in staging areas on properties near the jobsite. Keeping track of materials and getting them to the site when needed is a large concern. Grantham says that one staging area inventories a large number of structural steel pieces. “We use a bar code system to track each piece so that we will know where they go,” he says.
CityCenter is Perini's first experience with a LEED project so several staff members had to become LEED certified in order to understand the expectations. They had to locate new sources for materials and find out what was used to make them. For instance, Grantham says they had to search out new sources for plywood.
Workers place wood wedges between the table trusses and the form beams to provide camber. Floors are level after the forms are removed and no underlayment material is needed.
Credit: Joe Nasvik
Concrete is the most used material on the site. Here's how some of the productivity and engineering issues are being handled.
Foundations. In the past, casino foundations consisted of either mat slabs or spread footings. That changed several years ago to include caissons, in part because buildings are taller now. The depth of soil and silt in the Las Vegas valley makes it impossible for caissons to reach bed rock so engineers rely on the “skin friction” between concrete and soil plus attachment to caliche lenses (a local calcium aggregate-based rock) where they occur, to provide additional support. At CityCenter, the foundation system includes 3226 caissons to date, 60 to 100 feet deep and 4 feet in diameter connected in clusters by pile caps and grade beams, supporting mat slabs that the towers are constructed on.
Grantham says that underground congestion has been a major issue involving caissons, underground tunnels, electrical lines, and utilities. In addition, there are sewer lines running through the construction area serving Bellagio.
Credit: Joe Nasvik
Core Forms for the core walls of the towers are self-climbing. Placing booms are attached to a deck over the top of the forms, eliminating the need to move the boom for each new floor placement.
The tower floors are all “flat plate” construction to reduce the vertical height needed between floors. Aluma Systems', Concord, Ontario, Canada, column-supported table forms are being used for all of the tower construction. For core wall construction, self-climbing EFCO steel forms, Des Moines, Iowa, are being used. EFCO forms also are being used for shear walls and both cylindrical and rectangular columns. The Veer Tower poses special forming challenges due to the tilt of the towers and Doka USA, Little Ferry, N.J., is supplying the forming system for both horizontal and vertical work. The Mandarin Oriental Tower walls and special areas is using Peri Formwork Systems, Elkridge, Md., and Waco Scaffolding Equipment, Cleveland, forming systems.
Concrete requirements. Rinker Material, Las Vegas, is the ready-mix producer for the job and has a batch plant located onsite that provides most of the concrete needs for the project. They supply the rest of it from other locations in the area. More than 60 mixes were developed, with all but a couple of them including some portland cement replacement with fly ash to meet the requirements of 5000, 6000, and 7500 psi compressive strength decks, and 7000, 8000, and 10,000 psi compressive strengths for vertical applications, all depending on location.
Credit: Joe Nasvik
This hardware piece is cased into the outside edges of decks. The track on the exposed side makes it very easy to make connections to curtain wall panels-a way to make tolerance issues go away.
Slumps can be as high as 9 inches with the inclusion of superplasticizers for placements with congested reinforcing steel. There is controversy in the concrete industry about where concrete samples should be taken for testing: at the ready-mix truck or at the point of placement. On this project, Perini requires them to be taken at the point of placement to ensure specified performance.
In addition, some architectural concrete is being placed at the Veer tower, requiring special care with regard to forming and placement. Closer to the grand opening, there will be significant amounts of decorative concrete.
Perini is using both crane buckets and concrete pumps with deck-mounted placing booms to place concrete at the towers. Both are in use for the Pelli Tower floors, which are long and narrow. Workers place concrete in the center deck areas with a placing boom and at the ends by crane bucket. For low-rise construction, truck-mounted boom pumps also are used.
The Veer and Pelli towers involve additional construction challenges. Veer consists of two towers, each rising 5 degrees off vertical, one toward the south and one toward the north direction. Each floor level is 6 inches out of plumb with the one proceeding, adding to the challenge of both layout and construction. There is more rebar congestion as a result and some reinforcement includes #18 bars (2¼ inches in diameter).
The Pelli Tower is the largest and tallest building onsite. It consists of two intersecting arcs, one 900 feet long and the other 993 feet long. The curving of the building and the intersecting arcs provide layout challenges. Floor areas started at 109,000 square feet and are now 70,000 square feet. Perini is completing a floor every seven days.
Credit: Joe Nasvik
This "delay strip" is cased in two locations for each deck of the Pelli Tower providing a place for shrinkage to occur. Post-tensioned reinforcement is tensioned here also. The strip is concreted after 21 days.
At the two intersection points of the arcs on each deck there are 6-foot “delay strips” running the width of the floor. Concrete is placed in these areas 21 days afterward to provide time for shrinkage so that the floors don't overly stress the columns. The floors also are post-tension reinforced so Perini includes five additional “stressing strips” that are 6 feet long, crossing the width of each floor. The tendons are stressed when concrete strengths reach 3000 psi—generally by the third day and the stressing strips then are filled with concrete four days later.
Building all at once
Though the site is small for the size of the development, it also feels enormous. There are 13 tower cranes and numerous track-mounted cranes. Several concrete pumps and placing booms serve the tower constructions and truck-mounted boom pumps serve the many low-rise areas. There are perhaps more “mules” (four-wheel drive transports for construction personnel) onsite than anyone can count.
At the present time, 80 subcontractors are working on the project and Grantham says that the number will greatly increase as construction proceeds. For now the workforce is 4800 construction workers but Grantham says the number eventually will rise to 7000.
With all the intense activity, the project is running smoothly. Absher says the whole team is working well together and there are ongoing discussions about keeping the team together for the next MGM development.