Part of our conversations about the recent “Modern Concrete Skyscraper” show at the Skyscraper Museum involved the development of flat-slab high-rise structures, which became the standard for residential construction in Chicago (and elsewhere) beginning in the 1920s. The evolution and development of the flat slab as a technique has a long and fairly tortured history, much of which involves Chicago engineers and builders, but one aspect has been particularly interesting to me.

Beams and Girders

Eleven years ago, I wrote a post about punching shear that explained a fundamental problem in concrete construction. While steel gets its strength from members’ depth, concrete structures get their strength more from bulk (as well as a judicious placement of tensile steel reinforcement). That means that concrete structures, and concrete structural members, are relatively heavy. To span a typical 25-30′ residential span, a concrete slab has to be around 10″ deep. That’s a lot of rocks. When those slabs are perched atop columns, they want to “punch” through–that is, fail in shear between the column and the slab.

Not a problem if you’re using concrete to replicate a skeletal frame. Those heavy columns, girders, and slabs above are from Cincinnati’s Ingalls’ Building, the 15-story skyscraper that, when it opened in 1905, was hailed as the “first reinforced concrete skyscraper.” Note the connection between the girder and the column–it’s haunched, which provides extra cross sectional area–and extra room for steel reinforcement–to help resist the shear forces between the slab + girder, and the supporting column.

Mushrooms and Flat Slabs

That girder, though, reduced headroom, and at the edges of buildings it threatened to restrict the amount of daylight entering the interior (this was an era when electric lighting was common but still expensive). It was a common desire among architects, engineers, and owners to let the slab itself do the work of resisting shear loads, which led Minneapolis engineer C.A.P. Turner to develop what he called the “mushroom system” of flat slab construction:

Turner’s idea was to have the column heads flare out, providing extra circumference that would increase the area of interface between the slab and the column–instead of having the slab expand into a girder, which provided extra depth to increase that area. You can see the advantages–more headroom, more height at the building edge for windows, and unobstructed runs for machinery shafts or (in this case) sprinkler pipes.

As Dario Gasparini has noted, Turner’s system was successfully employed in the Johnson-Bovey building in Minneapolis in 1905-6, over concerns by that city’s building department over its experimental construction that were ultimately assuaged by a successful load test of the completed structure.[i]  The flaring capitals were often combined with drop panels in the slab that further spread the shear loads out over a greater cross-sectional area and reinforcing.

Early flat slab construction

Turner quickly found other clients keen to exploit the system’s advantages, constructing flat-slab structures in Milwaukee (Hoffman Building, 1906), Toledo (Bostwick-Braun, 1907), and Philadelphia (Grellet Collins, 1907).[ii]  In Chicago, Turner was hired to consult on a factory building for the Curtis-Leger company, designed by Francis Barton, that employed the “mushroom system” to provide six stories of flat-slab construction in 1906.[iii] 

More widely recognized was Holabird and Roche’s 12-story Born Building, a mercantile structure completed in 1908 that employed Turner’s mushroom system to become the “first tall building erected in Chicago of reinforced concrete” [emphasis added].[iv]  The building’s concrete structure was poured in just three months and, with 8” thick slabs supported on a 16’ x 20’ column grid, Turner’s mushroom system gave “the advantage of 60,000 cubic feet of air more than a similar building of steel and hollow tile construction” over a comparable steel structure of 16” depth.  Pleas Concrete Construction, the subcontractors, estimated that the results saved $35,000, or 15% of the total construction cost of $225,000.[v]

A Chicago Mashup

Turner’s system had one drawback, however. To maximize the slabs’ efficiency, he included reinforcing bars in four directions–along traditional orthogonal lines similar to traditional beams and girders, and diagonal lines across structural bays. This meant that all four sets of reinforcing bars crossed directly over the column heads, leading to crowded conditions that required careful attention to ensure concrete filled all the narrow interstices between the steel.

More importantly, the four sets of reinforcing bars complicated load calculations for the monolithic structure. Engineers had developed imperfect but useful methods for approximating flat slabs by dividing them into theoretical strips and treating each strip as if it were isolated from the others. This ignored the slabs’ two-way action, which made for extremely conservative (but safe) calculations. Adding Turner’s diagonals, however, meant that this method was nonsensical–to take advantage of the added strength of the additional bars, the system had to be calculated as four intersecting “girders”, which added too many variables to be solved algebraically.

Two Chicago engineers–Theodore Condron and F.F. Sinks–addressed this problem with a hybrid system that split the difference between the deep girders of Ransome’s construction and Turner’s flat slabs. In the 1909-1910 Studebaker Building (extant, William Ernest Walker, architect), they proposed “A Unique Type of Reinforced Concrete Construction” that spread Turner’s shallow “drop panels” from column to column–stealthy “girders” that provided little strength on their own, but provided space for a fully orthogonal system of reinforcement. Between these, a shallower central slab eschewed Turner’s diagonal reinforcement. Condron and Sinks’ terminology for these was telling–they called the “girders” “inclosing slabs,” and the central elements “inclosed slabs.” The result was, in some ways, a coffered ceiling, eliminating depth where it wasn’t needed. In others, it replicated the plan geometry of a traditional frame, but with dimensions that approached Turner’s more spatially efficient slabs.

Sinks patented the system, critiquing Turner’s system directly by noting:

“…the manner of placing proposed reinforcements in the floors of such constructions…has been such as to make a careful analysis of the stresses due to the dead weight and to the applied loads practically impossible and a computation of the stresses difficult and inaccurate.”

And his patent documents make clear that the simplified reinforcing system traded a bit of sectional efficiency for this ease of calculation:

Alert eyes will notice that Sinks’ patent was granted in close proximity to Turner’s, despite being filed two years later. That is its own tangled story, one that involved other inventors, engineers, the U.S. Supreme Court, and, eventually, the demise of Turner’s business. That, however, is for another post. In the meantime, the Studebaker Building, now owned by the Chicago Public Schools, still stands–unrecognized–as a key moment in the development of concrete high-rises in Chicago and elsewhere.

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[i] D.A. Gasparini, “Contributions of C. A. P. Turner to Development of Reinforced Concrete Flat Slabs 1905–1909.” Journal of Structural Engineering, Vol. 128, no. 10.  Oct 2002.  1241-1365

[ii] Ibid.

[iii] “The load on the floors is not carried to girders and beams, thence to columns, which is usual in buildings constructed of concrete or of ordinary mill construction, but is carried direct from the point of contact to the column.  The bars cross in six directions, resting on circular hoops which form the head of the column. Ordinarily the greatest stress on the floor is around the top of the column, but this is overcome by the fact that all bars terminate on the head of the column. This is called the mushroom system on account of its peculiar construction and form of the column head, which is the main feature.”  “Mr. Barton Adopts the Mushroom System Concrete Construction.” The Construction News, vol. 22, no. 18.  Nov. 3, 1906. 368. 

[iv] Monthly Bulletin, Universal Portland Cement Co. No. 59, February, 1909.  8.  That year’s January issue carefully noted that the Born would be “with a single exception the highest reinforced concrete building in the country,” that exception being the Ingalls.

[v] Ibid.