Thornton Tomasetti (formerly The Thornton-Tomasetti Group, Thornton Tomasetti Engineers, LZA Technology) is an American 1,200+ person structural engineeringconsulting firm headquartered in New York City. Founded in 1956 as Lev Zetlin & Associates, the 61-year-old company has office locations throughout the world.
Thornton Tomasetti works to advance sustainable design and construction practices. About one-fifth of its staff are LEED accredited professionals (LEED AP). Thornton Tomasetti was the first structural engineering company to sign the AIA 2030 Commitment.
The company began using the name Thornton Tomasetti in 1975 when Charlie Thornton and Richard Tomasetti purchased Lev Zetlin & Associates (LZA) from Gable Industries, to which Lev Zetlin had sold LZA in 1971. Thornton Tomasetti immediately began to branch out and enter the highrise market with several innovative designs. 
The New York City office essentially began in 1956 as Lev Zetlin & Associates (LZA), founded by Lev Zetlin upon the completion of his PhD at Cornell University. Zetlin pioneered the use of the double layer bicycle wheel roof system used in the Utica Civic Auditorium (also known as the Utica Memorial Auditorium), as well as the hyperbolic (hypar) roofs used in American Airlines 747 super-hangars at airports in Los Angeles and San Francisco. Lev Zetlin also embraced the creative use of materials, most notably demonstrated in a bridge design made entirely of paper products for an International Paper ad.
In 1993, Thornton Tomasetti acquired Cohen-Barreto-Marchertas (CBM) and created a Chicago office. Founded in 1965 by Eli Cohen, CBM pioneered the use of the now-common composite steel structural system, which combines a reinforced concrete core wall (to resist lateral loads) with a light steel floor framing. The development of 50- to 60-story buildings became more feasible and efficient because the lighter framing reduced construction time and expense, and allowed for large, column-free spans, giving architects greater freedom in exterior expression. In the early 1960s, Cohen's adaptation of the concrete cooling tower design to high-rise offices required a change in the way the unions worked. Until that time. ironworker contracts forbid members of other trades to work at higher elevations at job sites. But the proposed system was reviewed with them and they agreed to let concrete workers work above them.
On September 8, 2015, Thornton Tomasetti announced that it had merged with structural engineering firm Weidlinger Associates. "The combined firm will have 1,200 employees operating in 34 cities internationally", and will operate under the Thornton Tomasetti name.
In addition to founding Thornton Tomasetti, Charlie Thornton founded The ACE Mentor Program, which is a not-for-profit organization, formed to enlighten and motivate students toward architecture, construction, engineering, and related careers by providing mentoring opportunities for future designers and constructors. In 2008 Thornton was awarded the Henry C. Turner Prize for Innovation in Construction Technology from the National Building Museum for his work with this program.
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