Twin Cities Patents for Reinforced Concrete Technology

Recently I have been researching the history of reinforced concrete in the Twin Cities, to help Meghan Elliott prepare for a presentation that she will be giving at the annual University of Minnesota Concrete Conference on December 6, 2012. One of the avenues of my research has been an investigation of patents issued prior to 1970 related to concrete filed in Minneapolis, St. Paul, and their surrounding suburbs. This investigation has turned up some interesting results.

The earliest Twin Cities reinforced concrete-related patent that I could find was Patent 662,266 filed November 20, 1899 by Charles F. Haglin of Minneapolis. Haglin’s patent was for what is now known as a “slip form,” which is a type of mold used in the construction of circular concrete grain elevator storage tanks. He utilized this type of formwork to construct the Peavey-Haglin Experimental Concrete Grain Elevator in St. Louis Park, which was the first circular concrete grain elevator constructed in the US, and possibly the world. Haglin was a prominent contractor locally at the turn of the 20th century, and is associated with several other early examples of reinforced concrete construction in Minneapolis. He also patented two variants of wire-reinforced concrete flooring systems in the following year. In 1900, Charles Haglin and Olaf Hoff patented a type of reinforced concrete piles and columns using a novel type of steel reinforcement. Hoff was also a locally prominent engineer and contractor who designed the steel roof trusses for the Minneapolis City Hall and Courthouse.

Because of the size and prominence of the flour milling industry in Minneapolis at this time, it is unsurprising that there was a local focus on inventions and improvements related to milling technology. I came across numerous patents related to reinforced concrete or other “fireproof” grain elevator designs and improvements dated from the late 1800s to early 1900s. Most of the local “fireproof” grain elevators patents that I found were designed to be constructed primarily from brick or clay tile reinforced by iron or steel, such as the system used in the Barnett and Record Company’s Red Tile Elevator, adjacent to the Pillsbury “A” Mill in Southeast Minneapolis. Although other local examples of brick or tile elevators exist (such as the Northwestern Consolidated Elevator A, also known as the Ceresota Mill), ultimately reinforced concrete became the material of choice in “fireproof” grain elevator construction. I came across dozens of patents related to improvements in slip-form construction used in circular concrete elevator construction issued through the 1950s.

Interesting trends were revealed when I compiled my findings into a spreadsheet. I make no claim to locating every concrete-related patent from the Twin Cities, but I believe my search was exhaustive enough to draw some conclusions from my findings. Although they weren’t typically reinforced at the time, concrete masonry units (or “concrete blocks”) had an important part in advancing the acceptance of concrete as a building material. From 1904-1909, there was an explosion of patents filed related to concrete blocks and concrete block making machines. I located fifteen patents related to concrete blocks filed in that five year time span. Local newspapers and advertisements echoed this trend: “Concrete – the Building Material of the Age – Miracle Blocks in the Lead,”  and “Cement Construction.” 

Other interesting trends that I found in the patent record were the differences between Minneapolis and St. Paul. I analyzed the number of patents per capita produced in each city for each decade from 1900-1970 based on the 176 Twin Cities based patents I located, and US Census data. I don’t intend to spur any inter-city rivalry, but based on the historic patent record, it is clear that Minneapolis has spawned a much greater rate of concrete-related patents per capita compared to St. Paul. The average rate of patents per capita weighed by the total number of patents per decade reveals that Minneapolis produced about 200% more patents per capita compared to St. Paul from 1900-1970. Minneapolis gained the lead in the decades prior to 1940, which accounts for 143 of the 176 pre-1970 Twin Cities patents that I found. From 1900-1910, Minneapolis spawned a whopping 684% more patents per capita compared to St. Paul, but as decades passed, the difference between the rate of patents per capita for the two cities decreased, along with the total number of concrete-related patents. Sometime around 1940 the tide turned, from 1940-1970 St. Paul produced more patents per capita compared to Minneapolis. I hope that further research into the context of construction trends at the time will help me to understand why there was such a discrepancy between the two neighboring cities.

Any historical discussion of reinforced concrete in the Twin Cities wouldn’t be complete without mentioning C.A.P. Turner. I’ve mentioned Turner in previous posts related to the history of reinforced concrete. He was a nationally prominent engineer based in Minneapolis who is often credited with the development and implementation of flat slab reinforced concrete floor systems, along with many other notable achievements. The first local patent that I could find related to Turner was patent 795,463, filed December 7, 1904, which was for a reinforced concrete floor system for use in conjunction with steel framework. In total, I found 23 concrete-related patents awarded to Turner. One of the most notable patents he received was 1,003,384, filed June 7, 1907 and issued September 12, 1911, which was for his reinforced concrete “mushroom system” of flat-slab floor construction. Turner’s innovative flat-slab system was used in hundreds of buildings across the US. Most of his patents were variants of his “mushroom system” used for flooring, with some exceptions such as patent 1,351,588, a novel dam design that uses a “mushroom system” style structure for support. Turner’s “mushroom” floor system became the subject of a heated patent dispute with the C.M. Leonard Company of Chicago (who built the Deere & Webber Co. building in Minneapolis), regarding Orlando Norcross’ 1902 patent on a similar flooring system. The prolonged legal battle and defeats in court led to the unfortunate downturn of Turner’s career.

Another innovative system incorporated into local concrete construction was developed by William S. Hewett. Hewett is probably best known for role as a contracting engineer responsible for the design and construction of numerous streetcar bridges for the Twin Cities Rapid Transit Company. In 1921 and 1927, Hewett filed patents on early methods for prestressing the steel reinforcement in concrete pipes and water storage tanks to prevent cracking of the concrete. In 1931 Hewett was hired as a consulting engineer for the construction of the Washburn Park Water Tower in the Tangletown neighborhood of South Minneapolis. He incorporated his prestressing system into the water tower, which continues to perform its original function in the summer months.

Although there are many patents related to concrete issued to inventors from the Twin Cities after 1970, my search stopped at that point to limit my scope. From what I could tell from browsing through search results, a large amount of concrete-related patents continue to be produced in Minneapolis and St. Paul.


Google Patent Search

D. A. Gasparini, “Contributions of C.A.P. Turner to Development of Reinforced Concrete Floor Slabs 1905-1909,” J. Structural Engineering (October 2002): 1243-1252.

Thomas W. Balcom, “A Tale of Two Towers: Washburn Park and its Water Supply,” Minnesota History (Spring, 1984): 19-28.

EngineeringPVN Staff