Under the unlikely headline “Mushroom Infringement Suits,” the trade journal Cement Age published an article by C.A.P. Turner in May, 1911, announcing that he had brought suit against the Deere and Webber Company for building a bootleg version of his mushroom slab system in their new building (still standing) on Washington Avenue in downtown Minneapolis. “It is the intention of Mr. Turner,” he wrote, “to rigidly prosecute all infringers.  This news item will undoubtedly be of interest to general contractors and builders.”¹. This kicked off more than ten years of concrete patent battles between Turner and a Chicago-based group of legal rivals over the development of flat slabs in concrete.

Turner had good reason to be upset. Deere and Webber had been a licensee of his system in their gigantic factory and warehouse in Omaha, finished in 1908. That building had been built by a Chicago contractor, the C.M. Leonard Company, and had employed Arthur Talbot, of the University of Illinois’ Experiment Station, as a consultant. Deere and Webber hired Leonard to build the Minneapolis structure and gave Talbot and his protege, Arthur Lord, permission to load test its floor slabs. But they did not license Turner’s system, even though the structure bore an obvious resemblance to the warehouses and industrial buildings he had been building nearby. Instead, Deere and Webber employed a system of reinforcement supplied by the “Concrete Steel Products Company” of New York and Chicago.

Concrete Steel Products had joined forces with a handful of other engineers and builders who had developed flat slab systems and faced an increasingly combative Turner. The “Flat Slab Patents Company” (seriously) had an office on Michigan Avenue–but no factories. Instead, it provided administrative and legal support for six other systems: Condron and Sinks’ AKME (developed from Chicago’s Studebaker Building), Francis Barton’s “Spiderweb,” and Concrete Steel Products, among them.

Flat Slab Patents had cleverly dug through the archives and found that Boston contractor O.W. Norcross had received a patent in 1902 for “Flooring for Buildings” that proposed using sheets of metal mesh to reinforce flat slabs along orthogonal and diagonal lines–similar in plan to Turner’s system, but without the mushroom-shaped reinforcing cages, and supported by steel columns instead of concrete. Norcross’ claim is intriguing, given Turner’s patents:

“This invention relates to a flooring for buildings which has been designed with a view of securing the advantages, first, of entirely dispensing with all girders or floor-beams, which have heretofore been regarded as absolutely essential for supporting the floors of buildings; second, to provide a form of flooring which will utilize to best advantage the immense crushing strength of concrete, and, third, to provide a strong inexpensive solid inflexible flooring which can be laid in place by unskilled labor.”²

Norcorss went on to explain that the metal mesh was to be arranged so as to “…permit concrete alone to resist compressions and to supply a maximum amount of metal at points where the flooring is to be subjected to greatest tensions and shearing strains.” But, as you can see from his patent drawing, he (or, more likely, his draftsman) failed to show how the reinforcing would be placed to do this. In a flat slab, the greatest positive moment is at the center of the span, while the greatest negative moment is over the column supports; to work properly, metal reinforcement should therefore be placed at the bottom of the slab in the middle, and at the top of the slab over the supports.

Nonetheless, Leonard negotiated with Norcross for rights to his patent, and his company’s secretary, John L. Drum, filed suit against Turner in 1913, claiming that Turner’s entire system had been foreseen by Norcross. Turner sued Leonard for violating his patent by using flat slabs and a suspiciously mushroom-like column in the Minneapolis Deere Building. For good measure, Turner also sued Deere–perhaps out of spite that he’d been left off the construction team–and a handful of other companies that had leased space in flat-slab structures in Minneapolis. While Turner won an initial ruling, public opinion gradually turned against him. He argued that the courts were incapable of understanding the fine points of slab behavior, which may have been true, but which won him no allies in the legal profession–taking out a gloating advertisement in the Western Architect in 1914, he railed against Drum and Leonard’s claims that the Norcross system represented a complete solution:

“the astounding assertion was made by witnesses for the plaintiff that this concrete, unreinforced over the support would resists the same bending moment as though steel were present to give it rigidity, a proposition in defiance of the law of conservation of energy, the theorem of least work and the principle of rigidities.”³

Flat Slab Patents and Condron & Sinks eventually got into the melee as well, generating a slew of claims, counterclaims, and rulings that ultimately found Turner’s patent infringed on Norcross, despite the earlier patent’s technical error. “There is today,’ a federal appeals court held, dismissing one of Turner’s suits,

“neither invention nor novelty in merely placing metal reinforcement in concrete at places at which strains come…one striving to find a new principle, or to invent a new means of concrete reinforcement under the old principle, enters a well-known and widely practiced art, and must do something more than care for tensile strains at places where they are known to come”⁴

Turner eventually was forced to pay royalties to Leonard and the Flat Slab Patents Company, after the cases were finally resolved in 1922, a humiliation and financial penalty that largely ended his career as a builder of warehouses and factories. His work in reinforced concrete, however, did lead to a healthy second career as a bridge engineer. By that time, the Norcross patent had expired (though Norcross himself filed an updated version, with the reinforcement placement corrected, in 1917). Still, Turner remained bitter about the decade-long fight to claim the invention of a technique that would become standard practice in multi-story concrete construction with the patent disputes cleared. In a combination memoir/textbook published in 1919, Turner included a full appendix, written in the third person, that reiterated his case for primacy in the invention of the flat slab and argued for a full Congressional hearing to overhaul the patent system. Titled–none too subtly–“State Of the Art of Reinforced Concrete from the Patent Standpoint and the Menace to Progress by Unscientific Decisions,” the screed concluded that

“competent scientific ability is not required of the judge who passes upon the validity of the patent. Thus the final step which renders the system beneficial on the one hand or a menace to progress on the other, is taken haphazard without the guidance of competent scientific ability.”

The expiration of the Norcross patent in 1919 roughly coincides with the more widespread use of flat slab construction in high-rises other than factories — while steel remained in use for tall residential buildings, the shallower overall depth of flat slabs soon made them standard in apartment hotels and other domestic skyscrapers. While avoiding potential lawsuits surely aided the spread of the technique, a parallel effort to understand the mechanics of flat slabs — still led by Talbot but joined by other researchers at universities and laboratories — was also critical in refining the dimensions and reinforcement of those slabs.

1. “Mushroom Infringement Suits.”  Cement Age, Vol. XII, no. 5.  May, 1911.  285. ↩︎
2. O.W. Norcross, U.S. Patent No. 698,542.  “Flooring for Buildings.”  Filed Nov. 22, 1901, Granted Apr. 29, 1902. ↩︎
3. Advertisement, C.A.P. Turner, The Western Architect, Vol. 20, no. 12.  December, 1914.  XXII. ↩︎
4. Turner v. Lauter Piano Co., 248 F. 930 (3d Cir. 1918)
   U.S. Court of Appeals, January 17, 1918 ↩︎