Structural Skin

Many prominent, recent buildings feature forms suggesting structural surface while their enclosures are really non-load-bearing curtain wall. At the same time, we instinctively read the increasing proportion of opaque area on the facade required by the energy codes as solid, an opportunity to augment the primary building frame with perimeter structure. We have evident aesthetic desire to see structural form, and technical incentive in the form of increased opaque surface area to use the exterior enclosure as structural skin to make buildings more efficient, and more sustainable.

Preliminary modeling of a 24-story braced moment-frame with a 90’ x 90’ floor plate shows that structural cladding occupying the same depth as a conventional curtain wall has the capacity to limit lateral drift and reduce tonnage of the primary steel frame. The study evaluates three different schemes of moment-connected mullions braced by infill plate or diagonal rods. Each version replaces conventional aluminum mullions with stronger and stiffer hybrid mullions of steel and aluminum cassette glazing frames.

The initial expectation was that the skin would allow reduction in tonnage of the primary frame, resulting in reduced embodied CO2. It turns out that the reduction of steel in the primary frame is more than offset by the amount of steel added to the cladding in each case, but the net result in the last iteration is a reduction of embodied CO2 in the frame and skin due to the relatively high embodied CO2 of the replaced aluminum.

Next steps include refining details and exploring the potential of shaped or corrugated surface and structural laminated glass to enhance structural skin. This preliminary investigation shows that integrating structure and cladding, and using steel instead of aluminum in the enclosure framing, can save embodied energy.