At 50, Post-Tensioning Leader Reinforces Industry Position

Having devoted half a century to the design, manufacture and installation of post-tensioning systems and components, VStructural, LLC (VSL) celebrates

Having devoted half a century to the design, manufacture and installation of post-tensioning systems and components, VStructural, LLC (VSL) celebrates both a 50th anniversary in 2006 and its standing among technical leaders in construction. Known for high-profile post-tensioning projects throughout the U.S., VSL traces its roots to a major Swiss contracting firm, Losinger, circa 1953. Dramatic raw material shortages resulting from the war led Losinger to experiment with prestressing techniques and, ultimately, to devise a new construction method promising material savings.

The new technology, named VSL (Vorspann System Losinger) Wire System, was first applied commercially in 1956 on the Pont des Cygnes bridge in Switzerland. To spur further development, VSL initiated several licensee organizations, including VSL Corp. in the U.S. in 1966.

In the 1960s, with the development of higher strength steel, better attachment hardware, improved construction techniques and simplified design methods, the use of post-tensioning to reinforce structures became more widespread, notes VSL President Scott Greenhaus. By the early 1980s, the mystery of post-tensioning subsided with further refinements to the tensioning process, the development of more corrosion-resistant anchorages, and widespread dissemination of design software. These factors, he contends, helped post-tensioning become a preferred method for reinforcing concrete.

VSL Vice President and Technical Manager John Crigler, P.E., observes that while post-tensioning has been a proven solution in the marketplace for five decades, older systems focused primarily on obtaining the desired prestress force; durability was a lesser priority. Reflecting an evolution within the industry, however, current systems deliver the desired prestress force plus improved durability, he emphasizes.

Advancements in corrosion protection Û a prerequisite for durability Û have played a significant role in improved post-tensioning technology, especially in areas experiencing significant exposure and damage from freeze-thaw cycles, de-icing salts, seawater, salt spray, and other deterioration mechanisms. After it became apparent that a combination of corrosion inhibitor and seamless extruded plastic sheathing used for unbonded post-tensioning systems did not sufficiently address anchorage protection, VSL introduced its first generation of encapsulated components, CP+, in the late 1980s. Improvement in durability was thereby achieved, VSL representatives note, yet the technology did not provide complete encapsulation at construction joints.

Accordingly, company engineers moved toward fully integrated plastic duct systems for bonded tendons with the early 1990s introduction of the P-T Plus duct system. It combines a robust plastic duct with couplers, anchorage connections, grout caps and vents to completely encapsulate bonded tendons. The recently launched Enhanced Protection Plus (EP+) offers end-to-end tendon protection with a coupler that extends encapsulation across construction joints for improved constructibility, durability and quality, VSL officials affirm.

Additional technological advances since post-tensioning was first introduced in the U.S., Crigler reports, include seven-wire strand, low-relaxation strand, improved analysis techniques and design software, banded tendons, extruded sheathing, encapsulated anchors, plastic duct systems, and nonbleed grouts. As a result, he adds, the post-tensioning solution offers a high level of structural flexibility to meet changing user requirements.

Further, new technologies are likely to promote the continued acceptance and expansion of post-tensioning applications, VSL asserts. An area of significant growth likely will be tall buildings (> 20 stories), where framing historically has been largely structural steel, as post-tensioned concrete offers performance benefits with respect to fire resistance, sound transmission and floor stiffness. Post-tensioning floors and frames of tall concrete buildings in combination with the use of high-strength concrete minimizes their weight for optimum constructibility. Û


1956 Pont des Cygnes Bridge Û first commercial application of VSL Wire System
1957 First VSL Rock Anchor application to secure the foundation of a chair-lift mast
1965 Heimiswil Bridge Û first commercial application of VSL Strand System
1967 Founding of VSL Corp., USA
1967/68 World Trade Center Û largest tieback anchor contract executed in the U.S. at the time
1968 Start of VSL Heavy Lifting
1972 First VSL Extruder in the U.S.
CN Tower Û post-tensioning contract for world’s largest freestanding structure
1969/74 Rancho Seco Power Plant Û first contract for post-tensioning of a containment vessel
1974 Brent C & Statfjord A Û first offshore platform post-tensioning contracts
1976/77 Tarbela Dam, Pakistan Û securing of spillway slab with corrosion protected VSL rock anchors
1978 Terminal Island Plant Û first post-tensioned concrete digesters
1979/82 Seven Mile Bridge Û first segmental bridge erection
Columbia River Bridge Û VSL form travelers
1983 Sunshine Skyway Û first strand stay cable bridge
1986 CP+ introduced Û first encapsulated monostrand system
1991 PT+ introduced Û first plastic duct system
1993 Beznau Nuclear Power Plant Û first steam generator replacement with VSL Heavy Lifting
1994 Confederation Bridge Û VSL post-tensioning
1996 VSLAB introduced
1998 Structural Group Acquires VSL U.S. Operations
2000 SSI 2000TM Stay Cable introduced