The Magic of Making a Super-Tall Building

Very few contractors have the opportunity to build a super-tall building (defined by the Council on Tall Buildings and Urban Habitat as being taller than 984 feet, or 300 meters, tall). However, most of us are fascinated by the process and many would love to get into a construction “skip hoist” attached to the side of the building, ride to the top and see how the magic happens.

The building’s name is its address: 432 Park Avenue, an upscale condominium projected located in the heart of Manhattan, close to Central Park, Rockefeller Center and Broadway. When completed it will be the tallest residential structural concrete building in the western hemisphere at just under 1,400 feet in height. It is also remarkable because of its small, square footprint of just 93 feet. Vertically the building is rectangular in shape. The exterior faces consist of white concrete columns and spandrel horizontal beams (requiring no additional decorative finishes) with 10- by 10-foot tri-pane windows allowing wide range views of Manhattan.

The development is on the site of the old Drake Hotel. Developers Macklowe Properties and CIM Group conceived the idea for the $2.7 billion project. The hotel was demolished in 2010 and construction for the building began in the fall of 2012. The building is expected to “top out” at the end of the year.

The structural concrete work is being completed by Roger and Sons (R&S), a family business started in 1975, with third generation family members currently working for the company. They began as concrete contractors performing curb and sidewalk work. Constructing house and commercial building foundations followed and then they moved on to mid-rise concrete construction. Completing two 500-foot tall buildings in White Plains, New York, provided them with the credentials to bid, and then contract, the concrete work for the “Building 4” project at the World Trade Center. This project provided them with the necessary references to bid the Park Avenue job — a super-tall, record-setting, structural concrete construction.

Sustainability throughout

432 Park Ave. will receive a LEED certification upon completion. An underlying message throughout the projects is sustainability and an important measure will be how much energy the building will use over time, including its ultimate destruction and recycling. It could “live” for hundreds of years with low maintenance and low energy costs, which is a benefit for the use of concrete.

There are several buildings in the world taller than 432 Park Ave., but it’s thought to be the tallest all-residential structure in the world. The new 1 World Trade Center building stands at 1,335 feet tall from the ground to the roof, making 432 Park Ave. the second tallest building in the U.S., behind the Willis Tower in Chicago. The design is sleek; the construction and engineering are state-of-the-art; and its occupants will enjoy living in an energy-efficient environment.

Engineering the structure

The WSP Group is the engineering firm of record for the project. Silvian Marcus, a Principal of the company, says the building consists of two vertical structural elements; the core and the external columns (two structural tubes; one inside the other). The core is a robust, two-foot-thick structural concrete tube housing the elevators, emergency stairs and utilities. It provides safe access for occupants and provides primary strength for the building. The 44-inch wide exterior columns, spaced 15 foot and 6 inches apart, are connected by horizontal spandrel perimeter beams (also 44 inches wide) to form the exterior tube. The depth of these elements decreases as the building gets taller. The core and the columns are joined together by 10-inch-thick, highly reinforced concrete floors acting as a diaphragm, adding stiffness to the building.

Engineers must consider wind shear (winds pushing against the building) forces for every tall building design. The affect of wind shear is  controlled by calculating the strength requirements of the structure necessary for the wind forces and by designing building shapes that minimize wind pressure. This building also includes two open floors every twenty stories that house mechanical equipment, allowing the wind to pass through the building to reduce wind shear. In addition, to counter building movement, a “tuned mass damper” is included on the top floor to reduce building acceleration. Marcus explains that people are sensitive to acceleration so it’s important to minimize it.

Grade 60 steel rebar reinforcement is normally used for building construction, but for this project the decision was made to use #20 (2-1/2-inch-diameter) grade 97 rebar for vertical reinforcement, made by SAS Stressteel, to reduce congestion. Tom Deysher, Director of Sales and Marketing, says the bars (referred to as “thread bars”) are delivered to the site pre-cut and have deformations shaped in a threaded pattern to allow bars to be joined by threaded couplers for transferring load.

"The material costs more but saves on labor, making it a cost productive choice," says Pete Rodrigues, R&S’s manager for the project (and part of the family ownership). Using stronger steel reinforcement also means that less steel is needed for the job, which contributes to the sustainability of the building.

Caissons are normally a part of every tall building construction, but bedrock is close to the surface for most of the Manhattan area, so there are no caisson foundations for this building. Instead it has three levels of building below the street level and conventional spread footings and rock anchors to resist building movement.

Planning for construction

Building a project of this magnitude requires much preparation, careful planning and value engineering. Rodrigues says it was decided from the start to include the latest in forming technology. Sales engineers from DOKA and PERI Formwork Systems were asked to help to solve the challenges.

The specifications required a four-day cycle (one floor constructed every four days) for the first 50 floors and a three-day cycle after that. A cycle starts with casting the core structure two floor levels above floor construction. The external columns and perimeter beams follow by one day and are cast using white portland concrete. Casting the floor is the last operation before moving all the formwork up to the next level.

Establishing and tracking horizontal and vertical geometry for a building like this can be very difficult because it constantly vibrates due to wind and construction activity. “We decided to use a fully integrated system from Leica: GR-10 GPS receivers, AS-10 GPS/GLONASS triple frequency antennas, NIVEL tilt meters and a TS-15 1-second robotic total station with software from Leica to locate points even during building movement,” says Rodrigues.

Working with concrete

The performance requirements for the concrete were developed by the WSP Group. Marcus says their requirements included:

• High compressive strength — up to 14,000 psi
• High modulus of elasticity (MOE) — 7.7 msi (millions of pounds/square inch)
• Self-Consolidating Concrete (SCC) — 30-inch spread requirement
• White portland cement concrete with good color consistency
• Low heat of hydration — columns and other building elements were considered Mass Concrete, not to exceed 160 degrees F
• A pumpable mix for the entire height of the building
• Long transport times to the jobsite — two hours and more
• Low shrinkage
• Concrete floors must be hard enough to walk on within five hours of placement in all weather
• Mixtures must have the same performance criteria from the hottest to the coldest days of the year
• Must be a sustainable mix — 70% portland cement replacement with pozzolan materials.

Developing concrete mixes to meet all the requirements was a challenge involving the joint efforts of the engineer, concrete contractor, ready-mix producer and the BASF Corporation. Marcus says they were responsible for determining and verifying the performance requirements, but they depend on other expert resources to develop the mixes.

Ferrara Bros Building Materials Corp. has a long-standing business relationship with R&S and provided helpful information during the bid phase. When R&S got the job, so did Ferrara. Ferrara Vice President and General Counsel, Joseph J. Ferrara, says they like being involved on projects that challenge the limits of concrete and this was one of their most demanding. Ferrara Bros, through its subsidiary, Aggregate and Concrete Testing, is one of only three New York City concrete producers that are licensed by the NYC Department of Buildings to issue its own mix designs. BASF supplies the admixtures so Ferrara asked them to help with the concrete mix designs. Andreas Tselebidis, BASF’s director of Sustainable Concrete and Technical Solutions started working on mix development eight months prior to the start of construction. His work also included an Eco Efficiency Analysis, which measures the concrete’s impact on the environment, water emissions and transportation impacts.

In the past, designing concrete to meet MOE requirements required casting samples and running tests a year or more in advance of a project. But Tselebidis says their predictive modeling software saves most of this time. The mixes developed provided high early strengths for each of the high-compressive strength mixes, consistent setting times in all ambient temperatures, and concrete temperatures that didn’t exceed 160 degrees F.

The setting characteristics for the white portland cement concrete also presented challenges because they were different than regular portland mixes. White portland imported from Denmark was chosen for the project because of its setting characteristics.

Ferrara Brothers supplied space at one of their batch plants for mock-up panels. Several panels were cast to make the final adjustments to the mix design. R&S crews cast the samples, sometimes bringing their pump to the site to assure pump-ability. Ferrara Brothers, BASF and R&S worked closely on the development of the final concrete mixes.

The mixes included local coarse and fine aggregates, slag, small amounts of silica fume, high-reactivity metakaolin (a calcined clay pozzolan that adds strength, lightens the color, and increases the denseness of concrete), white portland and regular portland cement, and admixtures to manage the low water-cementitious material ratios, provide slump control, and control of setting times. Not all ingredients were used in each mix and Ferrara says none of the mixes were static; performance remained the same, but constant changes were made to manage ambient conditions, transportation time and placement issues. Most of this involved admixture dosage changes, allowed by the latest revision of NYC’s Building Code.

Alex Ferrara, a Sales and Product Support Manager for Ferrara, says this is the first time his company brought all these performance requirements together in one mix.

Placing concrete

Concrete strength changes as the building goes up — 14,000 psi for the first 40 floors; 12,000 psi between the 40^th to the 51^st floors; and 10,000 psi for the 51^st and above floors. For winter floor placements the space below the floor forms is enclosed and heated to ensure good setting times. But Rodrigues says they don’t usually place concrete when the air temperatures drop below 25 degrees F because it’s too hard to keep heat in the concrete.

R&S owns two model 14000 Putzmeister concrete pumps (one stand-by pump is required for most projects) and it delivers all the concrete to the placing boom on the core-form deck through a “slick line” mounted to the side of the building — eventually delivering concrete to the top of the building. The regular concrete mix pumps at 40 to 50 cubic yards per hour and the white portland mix pumps at 20 to 30 cubic yards per hour. As the building approaches its top elevation, many cubic yards of concrete are needed to fill the pump lines. So when placements are complete, the slick-lines are emptied into ready-mix trucks and the concrete is used for suitable applications on other jobsites.