Julius Kahn’s "Trussed Concrete Steel" Reinforcement
Recently I was asked to gather background information on an early reinforced concrete system used in a portion of a historic building located in the North Loop area of Minneapolis. The building is a former bakery that has been vacant for many years. Unfortunately, a lack of maintenance and water infiltration has undermined the structural integrity of much of the bakery. The first building in the complex was constructed around 1895, and additions were built in 1909, the late 1920s, and 1968. The 1909 section was designed by local architects Kees and Colburn, and is considered to be the most prominent. It comprises roughly 10-15% of the total building. On the building permit for the 1909 section, it was noted that a “fireproof Kahn concrete tile system” was used in its construction. Questions have arisen about the historic significance of the building’s use of the system, so I was tasked with gathering more information about it.
I have come across the Kahn System previously during research into other types of historic concrete reinforcement. It was invented by Julius Kahn, and patented on August 8, 1903 (Patent No. 736,602). Julius was the brother of Albert Kahn, a nationally prominent architect based in Detroit best known for the factories he designed for Packard and Ford in the early twentieth century. Albert incorporated his brother’s patented reinforcement into many of the buildings he designed. After inventing the system, Julius Kahn formed the Trussed Concrete Steel Company, also based in Detroit, to manufacture, market, and provide structural engineering services for the system.
Kahn’s reinforcement system was unique compared to other systems of the day. It consisted of rolled bars with “wings,” which were sliced and bent up at regular intervals (see the banner image above, excerpted from a publication of the Trussed Concrete Steel Company). The system was incorporated into reinforced concrete beams, joists and, in early examples, columns. Kahn theorized that his system acted in bending as a Pratt truss, with the diagonal wings and main longitudinal bars serving as tension members, and the concrete serving as vertical compression members (see the figure below, also excerpted from a publication of the Trussed Concrete Steel Company). The diagonal wings also served as shear reinforcement at the ends of the members. A publication by the Trussed Concrete Steel Company claimed that the system offered 20-30% greater capacity compared to “beams reinforced with horizontal rods and loose stirrups with the same area of reinforcement,” based on tests performed in 1907 at the University of Wisconsin, Madison.
I have found a few other local buildings that incorporated the Kahn System. In Who’s Who in Engineering 1922-23, by John W. Leonard, an entry for Leon H. Sault mentioned it. Apparently Sault worked for the Northwest branch office of the Trussed Concrete Steel Company from 1909-1911, during which time he designed the structure for the Minnesota State Fair Grandstand, as well as the Lowry Building in St. Paul. A description of the Kahn System in Volume 3 of Cement Age mentioned that the nine-story, 450,000 square foot Farwell, Ozmum, Kirk & Co. warehouse in St. Paul was built with a reinforced concrete structure using the Kahn System throughout. The warehouse was designed to carry 500 pounds per square foot (a very heavy load). I was amazed to find out they load tested entire floor panels of the warehouse up to 1,500 pounds per square foot by piling pig iron eight feet high! Internet sleuthing revealed that the building still stands, and currently serves as the East Building of the Ramsey County Government Center. I was not able to determine if Kahn’s company was involved in designing the 1909 addition for the bakery.
Kahn’s system was one of numerous other reinforced concrete systems that were being developed at the time. Claude Allen Porter (C.A.P.) Turner, a nationally-prominent engineer based in Minneapolis, vehemently objected to Kahn’s system in his 1909 book Concrete-Steel Construction. Turner did not hold back his feelings describing Kahn’s system:
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 reinforcement, reminds one of the story of the friendly discussion between two lawyers, in which the question came up as to who was recognized as the most prominent attorney in the place.
“I am of course,” said the first. “How can you prove it?” asked his friend. “Why, I do not need to prove it, I am willing to admit it,” replied the first.
Thus the advocate of this type of reinforcement apparently advances similarly convincing proof that the bar acts in the manner claimed.
Along with sharing his skepticism of Kahn’s theory, Turner pointed out that because of the nature of the way the bar was manufactured, the “wings” at the ends were shorter. He rightly pointed out that shear stresses were greatest at the ends of a beam, where the Kahn System provided the least amount of shear reinforcement. Turner also lamented the fact that the system did not provide continuous longitudinal reinforcement. Longitudinal reinforcement adds a significant amount of safety to reinforced concrete structures because in the event of a failure of a beam, the reinforcement is embedded in adjacent beams, which can prevent immediate collapse.
Establishing the significance of its use in the building harkens back to my previous post on the significance of engineering. Of the dozens of patented reinforcement systems in use throughout the country at the time of construction of the 1909 bakery addition, the Kahn System was certainly one of the most novel. Julius Kahn’s theories on the mechanics of the “trussed steel bars” are intriguing, however as C.A.P. Turner pointed out, the reasoning behind them was not completely sound. However, significance and engineering competence are separate issues. Historic significance can be established by applying the National Register Criteria for Evaluation. The presence of the Kahn system provides an example of the use, impacts, and trends of one of the numerous patented reinforcement systems used nationally and in Minneapolis. Although the building lies one block outside of the boundary drawn for the Minneapolis Warehouse Historic District, the building’s proximity to the district and its dates of construction justify comparison. C.A.P. Turner’s patented “Mushroom System” of flat slab concrete floor construction is frequently noted by historians in the context of reinforced concrete engineering and construction trends of the early twentieth century used in the Warehouse District. The use of the Kahn system in the bakery building provides an example of another patented reinforcement system, one of perhaps dozens used at the time. Contrasting the Kahn system with others used in the Warehouse District provides an excellent illustration of the widely varying theories and methods of reinforcement used early in the history of reinforced concrete structures.
The Kahn system was brought up and highlighted as one of the significant remaining features of the 1909 building during a public hearing on the bakery building. However, while it may retain its historic integrity, the structural integrity was shown to be significantly compromised. The demolition of the building was ultimately approved. While this physical example of the Kahn system will be lost, we have gained more insight into its history, use, shortcomings, and significance. I hope that the this insight, along with the attention the Kahn system has gathered, will increase the likelihood of recognizing and preserving other historic structural systems in the Twin Cities.
Donald Friedman, Historical Building Construction (New York: W.W. Norton & Co., 2010), 148-155.
John W. Leonard, Who’s Who in Engineering: 1922-23, (New York: John W. Leonard Corporation, 1922), 1102.
Water Mueller, “Reinforced Concrete Construction,” Cement Age 3 (October, 1906):313-332.
Trussed Concrete Steel Company, Kahn System Standards (Detroit: Author, 1913): 5-12.
C.A.P. Turner, Concrete-Steel Construction (Minneapolis: Farnham Printing & Stationary Company, 1909), 115-120.