Historic structures significant structures of the past Figure 1: Illustration of cross-sectional and perspective views of the Kahn reinforcement bar, along with a diagram of the theoretical “truss action”. ® t the time of this writing, a vacant A recent survey of the area identified buildings former bakery is standing for a few that are potentially historic. A low-rise structure more days at the corner of Fifth Street without any prominent architectural features, North and Seventh Avenue North, a series of mismatched additions, and a leak- Ajust outside of the official boundary of the local ing roof Copyrightmight typically and national historic Warehouse District of be dismissed. However, Minneapolis, Minnesota. The building is typi- a 1909 building permit cal of others in the area: a one- to three-story card for the bakery noted The Kahn System of utilitarian structure with little architectural orna- that the “fireproof Kahn Reinforced Concrete mentation and several additions. The hodgepodge concrete tile system” was of construction styles is reflected by the variety used in the building’s early of structural systems found within, including construction. Furthermore, the bakery is purport- Why it Almost Mattered load-bearing masonry walls, cast iron columns, edly the site of the invention of “sliced bread” and framing of heavy timber, structural steel and and other innovations in the baking industry. By Ryan Salmon, EIT and reinforced concrete; the building is a dictionary of Thus, when a proposal for a new apartment Meghan Elliott, P.E., Associate AIA historic construction techniques. It is a familiar magazinebuilding suggested demolition of the bakery, the scene in many urban industrialS areas: T declining R Minneapolis U Heritage C Preservation T CommissionU R E industries leave behind a decaying infrastructure was charged with making the decision as to and abandoned buildings. For years this bakery whether to allow it: could the use of the Kahn has stood with boarded windows, roof leaks, system be significant enough to merit saving it? squatters, and active plundering of any metals Furthermore, how important is “sliced bread” of value. However, a recent urban revitalization anyway? An engineering condition assessment was has brought redevelopment interest to many of ordered to determine the structural integrity of Ryan Salmon, EIT the buildings nearby. the building, and the importance of sliced bread ([email protected]), is The Warehouse District is notable for its was left to the opinion of the Commission. a project associate and Meghan representation of the growth of industry in The “Kahn System” was invented by Julius Kahn, Elliott, P.E., Associate AIA Minneapolis, which was supported by the early who filed a patent for it on December 11, 1902. ([email protected]), is the arrival of the railroads. The initial construction in Julius was the brother of the more well-known founder and owner at Preservation the area was from the mid-19th century to support Albert Kahn, a nationally prominent architect Design Works, LLC, a consulting the local milling industry – first lumber, and then based in Detroit who is best known for the facto- and project management firm flour. By 1902, the connection of major railroad ries he designed for automotive companies such as in Minneapolis, Minnesota that lines brought industries such as wholesaling and Packard and Ford in the early 20th century. Albert specializes in the preservation and manufacturing; Minneapolis eventually became incorporated his brother’s patented reinforce- redevelopment of historic buildings. the center of distribution for America’s farm ment into many of the buildings he designed. machinery. Growth in the District reached its Meanwhile, Julius formed the Trussed Concrete peak around 1930, and then declined along with Steel Company, also based in Detroit, to manufac- the rest of the American economy. Concurrently, ture, market, and provide structural engineering several changes in interstate shipping legislation services for his proprietary system. It serves as the allowed for independence from the railroads that structure for buildings throughout the country, had been so critical to the growth of the economy such as the Engineering Building (1904) at the in Minneapolis. The District remained relatively University of Michigan and the Blenheim Hotel unchanged between 1930 and 1990, which left (1906) in Atlantic City, New Jersey. many of the original historic structures intact, At that time, reinforced concrete was inherently albeit untended. experimental as engineers and builders worked STRUCTURE magazine 9 the longitudinal reinforcement. While the lack of testing standards and documentation from that time leaves some questions today about the validity of the tests, the intention of the testing was clear: to demonstrate the strength of the product, to create confidence in the product for contractors and building code officials, and to promote a proprietary reinforcing method. Kahn’s reinforcing system drew both attention and criticism from contemporary practitio- ners. In particular, C.A.P. Turner, a nationally prominent engineer based in Minneapolis, vehemently objected to Kahn’s system in his 1909 book, Concrete-Steel® Construction: In theory the Kahn bar is supposed to act with the concrete after the manner of a Warren Truss, and proof of this theory as advanced by the advocates of this type of Figure 2: Illustration of Kahn’s reinforcement as installed in floors, beams and columns. reinforcement, reminds one of the story of Copyright the friendly discussion between two law- yers, in which the question came up as to to understand how to design and construct which resulted in a smooth underside suitable who was recognized as the most prominent safe and efficient structures from the material. for finishing with plaster. The bakery used the attorney in the place. Numerous patented systems were developed, hollow clay tile joist construction method for such as Ernest Ransome’s twisted steel rein- the floor in the 1909 addition. “I am of course,” said the first. “How can forcement and Claude (C.A.P.) Turner’s A publication by the Trussed Concrete Steel you prove it?” asked his friend. “Why, I “mushroom system” of flat-slab floor con- Company claimed that the system offered do not need to prove it, I am willing to struction using smooth round rods. Kahn’s 20-30% greater capacity compared to “beams admit it,” replied the first. reinforcement system was unique: it consisted reinforced with horizontal rods and loose Thus the advocate of this type of rein- of visually distinctive rolled diamond-shaped stirrups with the same area of reinforcement.” forcement apparently advances similarly bars with flat-plate flanges (or “wings”) that These claims were based on tests performed magazineconvincing proof that the bar acts in the were sliced and bent up at regular intervals in 1907 at the University of Wisconsin on S T R U C T U mannerR claimed.E at approximately 45 degrees from the lon- simply supported beams. According to the test gitudinal axis of the reinforcement. Kahn report provided in the Trussed Concrete Steel Turner also pointed out that because of how rationalized that concrete members con- Company publication, the beams with “loose the bar was manufactured, the “wings” at structed with his reinforcement, particularly stirrups” failed due to slippage of the rods and the ends of the bars were shorter. He noted beams, would act in bending as a Pratt or concrete shear near the supports. In contrast, that shear stresses were greatest at the ends Warren truss, with the diagonal wings and the Kahn-reinforced beams were claimed to of a simply supported beam, where the Kahn longitudinal bars serving as tension members fail in flexure near the center (Figure 4 ). A System provided the least amount of shear and the concrete serving as vertical (or diago- review today of the test data shows that the reinforcement. Turner further critiqued it nal) compression members (Figure 1, page test beams with “loose stirrups” were con- for not providing continuous longitudinal 9 ). The diagonal wings were also theorized structed with smooth longitudinal bars and reinforcement for the beams at the columns, as shear reinforcement. The system was incor- U-shaped shear reinforcement spaced at inter- which created a potentially catastrophic porated into reinforced concrete beams, joists vals of at least three-quarters of the depth of and brittle failure mechanism; in contrast, and – in early examples – columns (Figure 2 ). Joists were typically installed with a hollow clay tile block system that consisted of rows of blocks laid with a three- to four-inch space between. Kahn-style reinforcement was placed into the voids between the blocks, which were subsequently filled with concrete (Figure 3). The hollow clay tile blocks were left in place, Figure 3: Cross-section of hollow clay tile joist Figure 4: Load test of a Kahn System beam to failure. floor system. STRUCTURE magazine10 April 2013 STRUCTURE magazine System. Reports published Company designed the structure for the by the Joint Committee on Minnesota State Fair Grandstand, as well Standard Specifications for as the Lowry Building in St. Paul. A descrip- Concrete and Reinforced tion of the Kahn System in Cement Age, a Concrete in 1908, 1912, 1916 trade journal of the time, mentioned that and 1924 provided specifica- the nine-story, 450,000-square-foot Farwell, tions and guidelines for the Ozmum, Kirk & Co. warehouse in St. Paul design and construction of was built in 1906 with a reinforced concrete nonproprietary reinforced structure using the Kahn System throughout. concrete structures. As con- The warehouse was designed to carry 500 crete reinforcement became psf, and was load-tested after construction standardized into the form to 1,500 psf by piling pig iron eight feet most commonly used today, high in a floor bay. It currently serves as the Trussed Concrete Steel the East Building of the Ramsey County Company broadened its Government Center.