Constructing 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 300 meters or 984 feet tall). But we are all fascinated by the process and many of you 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. Concrete as a material dominates the super-tall market now and we in the industry can feel proud of this amazing building technology and what can be achieved.

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 it’s completed it will be the tallest residential structural concrete building in the western hemisphere at just under 1,400 feet in height, also remarkable because of its small, square footprint — 93 feet. Vertically the building is rectangular in shape. The exterior faces consist of white concrete columns and spandrel horizontal beams with 10 x 10 foot windows allowing wide range views of Manhattan.

The development is on the site of the old Drake Hotel. Developers Macklowe Properties and CIM Group, New York City, 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 done by Roger & Sons (R&S), New York City, a family business, started in 1975, with third generation family members working for the company now. They began as concrete contractors doing curb and sidewalk work, constructing house and commercial building foundations followed, and then mid-rise concrete construction. Completing two 500-foot tall buildings in White Plains, NY 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.

Engineering the structure

The WSP Group, New York City, 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 large 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 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 goes up. The core and the columns are joined together by ten-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. They control it by calculating the strength requirements of the structure needed to handle wind forces and by designing building shapes that minimize wind pressure. This building also includes two open floors every 20 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 the decision was made to use #20 (2-1/2” diameter) grade 97 rebar for vertical reinforcement made by SAS Stressteel, Fairfield, N.J. 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 allowing bars to be joined by threaded couplers for transferring load. Pete Rodrigues, R&S’s manager for the project (and part of the family ownership) says the material costs more but save on labor, making it a cost productive choice.

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. It has three levels of building below the street level and then 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 they decided from the start to include the latest in forming technology, telling sales engineers from DOKA, Little Ferry, N.J., and PERI Formwork Systems, Valley Cottage, N.Y., what they wanted to do, asking their help to solve the problems.

The specifications required a 4-day cycle (one floor constructed every 4 days) for the first 50 floors and a 3-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. So Rodrigues says they 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.

Forming systems

R&S is using DOKA’s new “SCP Super Climber” forming system to build the core structure. It consists of inside and outside gang forms surrounding the core, along with three completely decked out levels of working and material storage platforms all moving as one unit. I-beams are placed on top of the forms supporting a deck or gantry level. The deck carries the concrete placing boom and a 3 ton, 50-foot telescopic crane boom for lifting reinforcing steel. Also, five floor levels of temporary stair towers for worker access and egress are located in two of the elevator shafts, attached to the deck’s under-side. Rodrigues says the forms are stripped from the concrete each day after placement. “At the touch of a button the 16-foot long hydraulic cylinders lift the entire assembly to the next floor level to start the next cycle,” he adds.

Mike Schermerhorn, a Senior Account Manager for DOKA, says their self-climbing “X-Climb 60” rollback system is also being used on this project. The five floor level climbing system surrounding the perimeter of the building, provides workers with multiple levels of up to 9-foot wide safe work decks for setting and stripping forms, removing anchors, patching exterior surfaces, and even assist with the installation of the 10 x 10 foot windows surrounding each floor level. It also supports the columns and perimeter beam forms. The custom made hinge-forms made with stainless steel face sheets allow stripping of the columns and perimeter spandrel beams as a unit which is then rolled backward on the X-Climb 60 platform in order to be hydraulically climbed to the next floor along with all five levels of working decks in a single operation.

As you might expect, using these forming systems requires training. Schermerhorn says workers who operate these hydraulic climbing systems to move the formwork upward must be certified by DOKA.

For forming floors R&S chose PERI Formwork Systems’ “SKYDECK” drophead system. The entire system can be set up quickly and efficiently from the floor below, creating a safe working environment. It consists of three key form elements; props (vertical supports), beams, and panels — all lightweight and easy for workers to place. Because the floor-to-floor height for this project is 15 foot 6 inches with a finish ceiling height of 12 foot 6 inches — apartments feature high ceilings — workers use rolling ladders similar to those used in “big box” stores to assemble the forms. The drophead hardware allows workers to quickly lower form panels the day after concrete placement (depending on concrete strength and depth) without moving the props—releasing them from the concrete while it’s easy to do so. The beams are also released, leaving only the props or shores to support the slab while it gains the necessary strength.

Concrete, the most important part

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°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 percent 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 depended on other expert resources to develop the mixes.

Ferrara Bros Building Materials Corp, Flushing, New York, 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. Their 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 LLC, is one of only three NYC concrete producers licensed by the NYC Department of Buildings to issue its own mix designs. BASF supplies their admixtures so Ferrara asked them to help with the concrete mix designs. Andreas Tselebidis, BASF’s director of Sustainable Concrete and Technical Solutions said he 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° 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 its setting characteristics.

Ferrara Bros 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 40thto the 51st floors, and 10,000 psi for the 51st 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 go below 25° 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. Rodrigues says their regular concrete mix pumps at 40 to 50 cubic yards per hour but the white portland mix is more difficult, pumping at 20 to 30 cubic yards per hour.

The weather variable

For most construction, weather is the uncontrolled variable. This is especially true for super-tall building construction. If the weather is warm and balmy on the ground, 1,000 feet up conditions can be very different. For instance, rain at ground level might be sleet, snow or ice up on the construction level. Wind speed and drops in temperature occur too, changing setting times for floor placements or causing surface crusting problems. But weather isn’t as much of an issue for concrete, adjustments can be made, as it is for worker safety and comfort — freezing hands and feet in the winter or heat exhaustion during the summer.

Concrete vs. structural steel

Today almost all super-tall buildings are constructed with structural concrete, especially residential buildings. Concrete costs less to build with now, primarily due to improvements in concrete pumping and forming technology. Concrete construction provides a very safe working environment too, workers stand on concrete floors or plywood floor deck, no one has to walk out on a steel beam.

Both owners and building occupants appreciate the advantages of concrete construction. If this was a structural steel building, floor thicknesses would increase from the 10-inch thick “flat-plate” method used to 20 inches or more for steel beam construction, and more noise would be transmitted between floors. This translates to fewer floors for the same height building. Structural steel construction would also require a minimum five-day cycle.

Occupants like the fact that concrete buildings sway less due to wind shear than structural steel ones; building movement makes people feel nervous. Energy costs are typically lower too due to thermal mass and the fact that concrete is a poor conductor.

Our world is more concerned about sustainability now and concrete structures have advantages in this area. Over 70-percent of the portland cement in the mixes for this project were replaced with supplementary cementitious materials (SCMs), that included metakaolin in some mixes and by-products from other industries. This minimized the carbon footprint of emissions that can be attributed to the concrete mixes. Portland cement production releases significant amounts of carbon dioxide into the atmosphere, hence the benefit of the SCMs.

432 Park Avenue

It will receive a LEED certification upon completion. But the more important measure will be sustainability — 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. That’s what makes concrete such a great material.

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 is only 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 a safe, energy efficient environment.

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