Question: John James 'Sean' Kelly (born 24 May 1956) is an Irish former professional road bicycle racer. He was one of the most successful road cyclists of the 1980s, and one of the finest classics riders of all time. From turning professional in 1977 until his retirement in 1994, he won nine monument classics, and 193 professional races in total. He won Paris-

Kelly returned in April to the 1988 Vuelta a Espana which started on the rugged mountainous island of Tenerife where his team struggled in the second stage, losing the influential rider Thomas Wegmuller to dysentery and losing further time in the time-trial around Las Palmas. However, on the Spanish mainland, Kelly concentrated on winning sprint time bonuses, battling with sprinter Jorge Dominguez, the BH teammate of leader, Laudelino Cubino.  After regaining a minute in four days, the race reached the mountains where Kelly relied on help from Robert Millar of team Fagor-MBK to stay within two minutes of Cubino after the mountain trial to Alto Oviedo. He then finished fourth behind stage-winner Fabio Parra and Anselmo Fuerte on stage 13 to the ski-station at Cerler, cutting a minute and a half into Cubino's lead. From this stage, Fuerte had moved into second overall and later took the jersey from Cubino on the 16th stage to Albacete when the leader got caught on the wrong side of a split caused by cross-winds.  Kelly maintained the gap between himself and Fuerte and started the time trial on the second last day 21 seconds behind. Confident that he could overhaul the leader, he "put it in a big gear and gave it everything". He took the leader's amarillo jersey, beating Fuerte by almost two minutes. The following day Kelly won his only grand tour, over West German Raimund Dietzen and also won the points competition. After his Vuelta win Kelly returned to Carrick-on-Suir where a parade was held in his honour.

Using a quote from the above article, answer the following question: why did he only race once?
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Answer: 

Problem: Fazlur Rahman Khan (Bengali: phjlur rhmaan khaan, Fozlur Rohman Khan) (3 April 1929 - 27 March 1982) was a Bangladeshi-American structural engineer and architect, who initiated important structural systems for skyscrapers. Considered the "father of tubular designs" for high-rises, Khan was also a pioneer in computer-aided design (CAD). He was the designer of the Sears Tower, since renamed Willis Tower, the tallest building in the world from 1973 until 1998, and the 100-story John Hancock Center. Khan, more than any other individual, ushered in a renaissance in skyscraper construction during the second half of the 20th century.

Khan's central innovation in skyscraper design and construction was the idea of the "tube" structural system for tall buildings, including the framed tube, trussed tube, and bundled tube variants. His "tube concept", using all the exterior wall perimeter structure of a building to simulate a thin-walled tube, revolutionized tall building design. Most buildings over 40 stories constructed since the 1960s now use a tube design derived from Khan's structural engineering principles.  Lateral loads (horizontal forces) such as wind forces, seismic forces, etc., begin to dominate the structural system and take on increasing importance in the overall building system as the building height increases. Wind forces become very substantial, and forces caused by earthquakes, etc. are important as well. The tubular designs resist such forces for tall buildings. Tube structures are stiff and have significant advantages over other framing systems. They not only make the buildings structurally stronger and more efficient, but also significantly reduce the structural material requirements. The reduction of material makes the buildings economically more efficient and reduces environmental impact. The tubular designs enable buildings to reach even greater heights. Tubular systems allow greater interior space and further enable buildings to take on various shapes, offering added freedom to architects. These new designs opened an economic door for contractors, engineers, architects, and investors, providing vast amounts of real estate space on minimal plots of land. Khan was among a group of engineers who encouraged a rebirth in skyscraper construction after a hiatus of over thirty years.  The tubular systems have yet to reach their limit when it comes to height. Another important feature of the tubular systems is that buildings can be constructed using steel or reinforced concrete, or a composite of the two, to reach greater heights. Khan pioneered the use of lightweight concrete for high-rise buildings, at a time when reinforced concrete was used for mostly low-rise construction of only a few stories in height. Most of Khan's designs were conceived considering pre-fabrication and repetition of components so projects could be quickly built with minimal errors.  The population explosion, starting with the baby boom of the 1950s, created widespread concern about the amount of available living space, which Khan solved by building upward. More than any other 20th-century engineer, Fazlur Rahman Khan made it possible for people to live and work in "cities in the sky." Mark Sarkisian (Director of Structural and Seismic Engineering at Skidmore, Owings & Merrill) said, "Khan was a visionary who transformed skyscrapers into sky cities while staying firmly grounded in the fundamentals of engineering."

Is there any popular buildings that use this structural design?

Answer with quotes:
Khan was a visionary who transformed skyscrapers into sky cities