PT concrete use on the rise.

With several technological advances since the 1970s, post-tension concrete's use in structural framing can be the material of choice for commercial buildings in New York City, said a five-member panel of experts at a recent Concrete Alliance and Lather's Local 46 symposium.

"The changes accomplished over the last 30 years by the various segments of the PT industry are truly impressive when we look at them all collectively," said Michael Russillo, P.E., president of Barker Post-Tensioning LLC.

"The design and detailing advances combined with improved installation practices have made the costing more economical, while the improvements made to the materials and fabrication of PT have created reliable long life of this structural method. The Post-tensioning Institute has been instrumental in these advances all along the way." Although post-tension concrete has been used in the framing of buildings nationwide for several decades, New York City developers and those in the construction and design industry have been slow to jump on the bandwagon.

"There are a lot of buildings nationwide being built with post tension concrete. It's not just for bridges," said Jim Rogers, Director of Certification Programs for The Post Tension Institute.

Structural steel has been the industry standard in New York City, but concrete industry professionals are trying to change that standard.

The Local 46 Metallic Lathers and Reinforcing Ironworkers Union has trained and certified more than 200 field technicians on post-tension concrete at its training center in Queens. One hundred more will be certified by the end of the year, said Robert A. Ledwith; president of Local 46.

"We are looking to show the construction industry that for every 30 floors of structural steel, you can get 33 floors with post-tension concrete," said Ledwith.

Post-Tensioning is a method of reinforcing concrete, masonry, and other structural elements. It is one of the two methods of prestressing. The reinforcing is simply installed on the job site after the contractor forms up the slabs or constructs the walls, versus the second method where the stressing is performed at a precast manufacturing facility.

The first full application of post-tensioning in a building in the five boroughs since the early '70s is Kew Gardens, a twin residential tower with 15-stories in Queens by Desimone Consulting Engineers, PLLC, said Borys Hayda, P.E. managing partner.

"This project is also helping to address the lingering prejudicial perceptions," said Mike Mote. P.E. of The Portland Cement Association "Also, designers realize that the industry is serious about supporting post-tensioning and as a result they feel more comfortable in designing with pest-tensioning."

Several advantages to the use of PT concrete in structural framing:

* Ability to span Class "A" office spans up to 45 feet clear while saving over two feet in floor to floor height as compared to a composite steel floor system: example is 140 West 42nd Street with 40 spans where the post-tensioned floor slab thickness is 12" only and the composite steel floor system (steel beam plus concrete topping) was over 36."

* The ability to have long clear spans (i.e. without interior columns) allows the architect more freedom to organize the interior layout without having interior columns in the way.

* Due to lower floor to floor heights, architects can potentially include additional floors and still comply with zoning height.

Advancements in PT include:

* Low-relaxation steel tendons, which do not loose as much strength over the life of the project as compared to earlier tendons. Remember that the material gets "tired" after years of stress and tends to relax thus loosing strength over time.

* Un-bonded tendons which reduce labor costs as compared to bonded tendons, which require a separate labor crew.

* Fully encapsulated tendons and anchorages which prevent corrosion from setting in, especially important for structures exposed to the environment such as parking garages.