Two powerful organizations are having great success increasing the use of concrete in high-rise construction in New York City. The Cement League membership includes union concrete contractors who specialize in structural reinforced concrete construction, general contractors, concrete repair contractors, and manufacturers of concrete products. The League acts as the collective bargaining agent for eight different building trade unions, and it employs a professional engineer to assist members with technical issues.
The Concrete Alliance membership includes both trade unions and the Cement League, and its mission is to promote the construction of structural reinforced concrete buildings. The success of their efforts revolves around a building method developed in New York more than 50 years ago, now referred to as the “two-day cycle”—constructing a structural reinforced concrete high-rise building floor every two days.
Promoting concrete residential high-rise buildings isn't difficult because they have been around a long time and tenants prefer them. Construction of these buildings is at an all-time high and is likely remain that way for at least the next few years. Commercial high-rise construction, however, is a different story. Commercial high rises have traditionally used structural steel for the building's frame. But the Concrete Alliance is working to change that, and it was recently instrumental in the owner's decision to go with a concrete frame and structural reinforced concrete floors for a current commercial building project called the 505 Fifth Avenue building (also called Fifth @ 42nd LLC). As a concrete frame building it will have 30-foot clear spans and 15-foot cantilevers.
Why structural reinforced concrete?
Anthony Leichter, a consultant for 505 Fifth Avenue, says the company considered alternates for both steel and structural reinforced concrete construction and decided on concrete. Northside Structures, Brooklyn, is the construction company doing the work. Leichter's company specializes in 30-story, 250,000- to 300,000-square-foot commercial buildings; 505 Fifth Avenue is its third using structural reinforced concrete. Leichter adds that his company prefers concrete construction for the following reasons:
- Structural reinforced concrete buildings in this size range use “flat-plate” construction methods; ceilings are beam-free and smooth, resulting in lower overall height while achieving the same ceiling heights as steel.
- There is significant noise reduction for building tenants.
- In the city that experienced 9/11, there is great concern for structural robustness in a fire, which concrete offers. Structural steel must be protected with fireproofing materials, but during construction and later tenant installations, some is always knocked off.
- It's important to have cast-in-place stairwells for occupant safety— not drywall, block, or other materials. These concrete cores may also serve as shear walls for the structure.
- Speed of construction is almost always important, and the two-day cycle method of concrete construction is much faster than structural steel construction.
- Structural reinforced concrete offers greater design flexibility. Knockout sections within slabs are often designed into areas where the building's occupants might want to add future stairwells to another floor. Also, it's possible to institute changes during construction without altering the construction schedule.
- Structural reinforced concrete buildings are generally tighter than steel buildings since they don't use multiple components.
The two-day cycle
The construction methods needed to perform a two-day cycle were developed in New York City in 1950; this method has been in continuous use ever since. Alfred Gerosa, chairman of the executive committee of the Cement League and president of the Concrete Alliance, says that a basic requirement for the success of this method is the availability of skilled labor that understands the system. The city has a large union labor force that understands the two-day cycle and helps to make it a productive option.
In New York City, cranes are prohibited from hoisting materials or equipment over city streets. As a result there isn't enough space to move pre-assembled floor form tables or other forming systems. Floor forms are therefore hand-built using 4×4 shoring timbers, plywood, and pre-assembled form panels for shear walls, elevator shafts, and stairwells. Joseph Mitrione, president of Pinnacle Industries, N.Y., says that his company provides enough forming material for two floors—one for the floor just placed and one for the next floor up. Column forms and forming lumber are coded for specific locations to streamline the process.
Column forms and rebar cages are preassembled by carpenters and members of the Metallic Lathers and Reinforcing Ironworkers unions onsite, either on the ground level or a few floors below the placement area. On most New York City concrete jobs, workers move concrete into place by crane; few jobsites use pumps and placing booms. Two crane buckets are used; one is lifted while the other is being filled. Mitrione says that using this system they can place between 60 and 90 cubic yards of concrete per hour.
To make a two-day cycle work requires the organized efforts of crews assembling forms and steel reinforcement in staging areas, as well as crews on the placement deck level. Frank Fama, a superintendent with DiFama Concrete, Brooklyn, says that 45 carpenters, 50 laborers, 5 layout engineers, 38 metallic lathers (ironworkers), and 13 finishers are performing as an efficient team to install the 12,000-square-foot decks on his current project.
Here's how a typical two-day cycle works for a floor that is 10,000 to 12,000 square feet per level:
- 7:00 a.m.: Workers begin placing concrete for the deck, typically using 6000-psi compressive strength concrete; superplasticizers and accelerating admixtures are often used to facilitate placement and finishing.
- 10:30 a.m.: Layout engineers begin locating columns and wall in the portion of the floor that was placed first; the concrete is fresh enough that they leave faint foot impressions. Workers continue to place concrete in other parts of the deck.
- 11:30 a.m.: The climbing tower crane starts lifting bundles of column forms, 4×4 shoring lumber, and plywood. Teams of carpenters and laborers begin positioning column forms and setting up shoring for the floor above.
- 12:00 p.m.: Concrete placement for the floor slab is completed.
- 2:00 p.m.: Finishers complete their work on the floors and begin placing concrete into the step forms for the two preceding floor levels by moving concrete to location through a tremmie from the top deck.
- 3:30 p.m.: By the end of the day, formwork erection is complete for the next floor deck and columns.
- 7:00 a.m.: Workers place and secure rebar cages in columns and shear walls. Metallic lathers begin placing top and bottom layers of rebar for the deck. Electricians begin rough electrical work (most conduit is now plastic), and carpenters place blockouts for plumbing and HVAC.
- 12:00 p.m.: Workers begin concrete placement for the columns typically using 7000-psi compressive strength concrete.
- 3:30 p.m.: All floor formwork and reinforcement is in position, ready for concrete placement the next morning.
Structural reinforced concrete cores for structural steel buildings
Concrete use is also increasing in building with structural steel frames. Two problems that engineers face with structural steel buildings are reducing sway from wind forces and providing a safe exit for people in the event of a fire or blast. Structural reinforced concrete buildings naturally exhibit very little sway as the result of wind forces. With structural steel construction, the addition of reinforced concrete cores and shear walls greatly dampens the amount of sway. The stairwell problem was highlighted at the World Trade Center where drywall-enclosed stairwells were inadequate to allow people to escape after the fire and blast. Building codes now require robust stair enclosures that guarantee safe passage in the event of a disaster.
Another benefit of concrete cores in steel buildings is that the construction can be very fast—preceding steel erection by several floor levels. Sorbara Construction, Lynbrook, N.Y., recently completed the reinforced concrete core for the new 7 World Trade Center. Steel fabrication started before the concrete work so the first four floors were formed with conventional wood lumber. But above the fourth floor Sorbara switched to a self-rising gang-form system and was able to easily keep up with the four-day cycle at which the steel was being erected. The core walls in this building are 2 feet thick—encasing elevators, stairs, and power systems and providing protection from fire and blasts.
In August 2005, construction of the core on One Bryant Park, the new headquarters building for the Bank of America, will begin. Ulma Form Works, Hawthorne, N.J., devised a self-rising forming system that adjusts to a changing geometry as the core rises. Construction won't be disrupted during this process. Century Maxim, New York City, is the concrete contractor for the project.
Marketing high-rise buildings
After 9/11, the city of New York made a pledge to its citizens that only the safest structures possible would be built. The Concrete Alliance believes that this pledge should lead to all high-rise buildings being structural reinforced concrete. For the past two years it has sent letters and promotional materials to over 1000 developers, engineers, architects, and public officials, addressing the benefits of all-concrete buildings. Safety is the most important message, but the environmental benefits of concrete construction and its soundproofing qualities are also stressed. This promotion is directed at commercial space because most residential towers and hotels are already reinforced concrete structures.
The final message is speed. New York City is one of the world's most expensive places to build, and time is money. Since Joseph DePaola worked out the two-day cycle construction method more than 50 years ago, contractors and unions have learned to turn time into a cost benefit for concrete construction.
The two-day cycle
In 1950, Joseph DePaola, DIC Concrete, N.Y., discovered an on-hold building project on 60th Street in Manhattan. The building had been designed for structural steel, but steel was unavailable. He asked the owners if they would consider building it with structural reinforced concrete, assuring them that he could start Immediately and beat the timetable originally planned for steel. They gave him the opportunity. Farkas & Baron Consulting Engineers, N.Y., re-engineered the building for concrete, and DIC Concrete embarked on a schedule of constructing one floor every two days for the 15-story building. It worked and DePaola went on to use this method on many other projects. His competitors soon followed, and New York City became the undisputed leader in short construction cycles. Today, there is even the occasional one-day cycle.