Analysis: What to Consider in Cantilever Design

Brief 

Julie Mark Cohen offers a review of enclosed cantilever construction, how it’s used today, how well it performs and key issues that are often overlooked. Some of those issues include bracing compression flanges of beams, directly exposing cladding to outside temperatures and overloading framing.

Insight

From the early 1960s through the 1970s, several U.S. buildings were also designed in the Brutalist style. The predominant feature of this style continued to be the use of concrete, at least in façades.

Another distinguishing feature was large rectangular boxes that protruded (cantilevered) from the main portion of the buildings, typically along one or two bays and over one to two stories in height. Concrete structural members framed the protruding volumes with minimal glazing in the façades.

In these boxes, the structural floor depths of the cantilevers and back spans followed long-standing rules of thumb. The ratios of cantilever lengths to back span lengths were no more than 1:3 and sometimes closer to 1:4 (the maximum ratio the author learned as an architecture student).

These rectangular protuberances read as rigid sub-assemblages. An example of a U.S. Brutalist-style building is the Folsom Library at Rensselaer Polytechnic Institute in Troy, NY, which was designed by Quinlivan, Pierik & Krause and constructed from 1972 to 1976 (Figure 2).

Here, some of the cantilevered volumes appear to be supported from framing just above, but their actual cantilevered lengths are relatively short.

Enclosed Cantilevered Construction Today

By 2000, buildings reminiscent of the Brutalist architectural style started to appear in newly-constructed residential, hotel, and office buildings of about five to twelve or so stories.

The protruding rectangular volumes are larger than in the older buildings; they include many more stories and extend along most or all of the bays in the façades.

For these buildings, the primary framing is either structural steel or reinforced concrete. The cladding typically comprises a significant amount of glazing or is blank (windowless) stucco or other lightweight walls supported on light gauge steel studs.