Dr. N. Subramanian, Consulting Engineer, Gaithersburg, MD 20878, USA
IntroductionWorld's first tall, steel framed building is the 10-storey, 42 m tall The Home Insurance Building, which was built in Chicago (Fig.1), Illinois, USA during 1884. It was designed by engineer William Le Baron Jenney. In 1890, two additional floors were added, increasing its height to 54.9 m. It was the tallest building in the world from 1884 to 1889 and was demolished 47 years later in 1931 to make way for another skyscraper, the Field Building (now known as the LaSalle Bank Building). In addition to being the first of a new generation of steel-framed skyscrapers built in the world, the building set the standard for various other building innovations, including rapid, safe elevators, wind bracing and modern plumbing (www.history.com). It has to be noted that this building relied on its masonry cladding for stability-the iron columns were merely embedded in the walls, and their load carrying capacity was considered to be secondary to the capacity of the masonry, particularly for wind loads.
Figure 1: World’s first steel framed skyscraper- 42 m tall The Home Insurance Building, Chicago
Figure 2: Earlier connections used in moment resistant frames (Hamburger et al., 2009)
Earlier research by Professor Egor Popov at the University of California at Berkeley in the 1960s and 1970s showed that in order to obtain superior inelastic behavior of steel moment resisting frames in strong earthquakes, it is necessary to adopt proper proportioning and detailing of these frames. These design criteria were introduced in the 1988 Uniform Building Code, and the frames designed as per these criteria were designated as Special Moment-Resisting Space Frames, and were assigned the highest R factor. It is important to realize that after 1980s, engineers adopted designs which minimized expensive site welding and economized their designs. This resulted in using fewer bays of moment-resisting frames, which had heavier beams and columns. In some extreme cases, tall structures were provided with only a single bay of moment-resisting framing on each side of the building, resulting in less redundant structures with more concentrated lateral force resistance (Hamburger et al., 2009)
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