Curtain wall is the prevailing type of enclosure on modern buildings because of its economy, its independence from structure that allows flexibility in design, its thinness that allows maximum usable floor space, and its speed of erection—in short, its expedience. But ever since the appearance of the steel and concrete frame and the curtain wall in the late 19th century, architects have continued to consistently, insistently, render structure, or apparent structure, on the non-load-bearing surfaces of some of the most influential works of modern architecture. Many high-profile contemporary projects have enclosures that appear to be structural shells of some sort, but are really curtain wall on a concealed frame. There are some significant modern buildings that employ structural enclosure systems, most built prior to the end of the 20th century, but they have been outside the mainstream and with limited influence on the conventions of modern construction. Still, their motives and success in integrating structure and skin are worth studying as examples of doing more with the material we incorporate in buildings, as a basis for developing more integrated and sustainable ways of building in the future.
The growing realization of the need for more sustainable ways of building makes the possibility of the curtain wall doing more, as a genuine structural enclosure, something we should explore from functional and formal perspectives.
This study presents modeling of different patterns of full-bay braced panels of a prototype structural enclosure system on a high-rise steel frame tower, showing a significant contribution to serviceability-level wind stiffness. This builds on previous studies of stick-framed steel mullion systems that tested infill spandrel panels and then bracing rods for in-plane stiffness. A variety of bracing and non-bracing panel configurations were modeled to test efficiency and demonstrate effectiveness in various arrangements that imply adaptability to different design constraints and intentions.
The results provide preliminary metrics for integrating cladding and primary structure to economize on steel in the primary frame and on aluminum in the cladding, with roughly 20% improvement in life cycle cost and embodied carbon in the primary structure and the enclosure compared to a conventional base case. At the same time, the structural details and mix of structural and non-structural panel types creates an animated facade, expressing function in form and speaking to our evident desire to see structure revealed.
In sum, building skin represents a significant amount of material that should, and can, be utilized as part of the structure.
“A skin always remains a skin...Whatever its virtues may be, it can never assume the natural primacy of structure. Structure is obvious, basic, and irreducible. It is the perennial source
A series of schemes was developed, each one analyzed for strength and stiffness with structural skin and a braced-core steel frame. 25-story, three-dimensional buildings were used as a basic model
The evolution from stick-framed steel HSS mullions with infill shear panels, to steel mullions braced with rods, to a full-bay structural mega-panel shows the possible beginning of a fundamental change
Structural design and analysis for the three studies represented here by Simon Shim, PE, Senior Associate at HOK, NY.
Architectural design and renderings for the latest Structural Skin study by Michael Miller, designer at HOK, NY.
Architectural design and renderings for the Structural eXterior Enclosure entry to the Metals In Construction 2017 competition by Michael Miller, Zifan Liu, and Zhenhuan Xu, designers at HOK, NY. LEED analysis and thermal modeling by Apoorv Goyal, Sustainability Specialist at HOK, NY.
Support for the three studies represented here was generously provided by Matt Breidenthal, Regional Leader of Engineering, HOK.
Cossutta, Aldo. “From Precast Concrete to Integral Architecture”. Progressive Architecture, October 1966.
Fuller, Buckminster. Nine Chains to the Moon. Anchor Books. 1971. (Orig. 1938).
Gordon, J. E. Structures, or Why Things Don’t Fall Down. Penguin Books. 1978.
Neary, John. “Structural Skin: Integrating Structure and Cladding,” Facade Tectonics Institute, October 2016.
Neary, John and HOK team, “Structural eXterior Enclosure,” Finalist, Metals In Construction 2017 “Reimagine Structure” competition. Metals In Construction Magazine, Spring 2017.