A rare tornado struck Johns Island, South Carolina in late September, damaging about 80 homes and destroying one. The localized damage may offer some interesting construction lessons, wind engineering expert David O. Prevatt reported. The tornado also offered an opportunity for an innovative steel structural insulated panel (SIP) structure to demonstrate its toughness in real-life high wind conditions.

Prevatt, a professor in the department of Civil and Coastal Engineering at the University of Florida, examined film footage and reports of the tornado damage and analyzed the damage pattern, looking in particular for reasons why just one house out of dozens in the tornado's path would have suffered complete destruction (see: "The 25 September 2015 Johns Island Tornado," by David O. Prevatt and David B. Roueche). The one destroyed home appears to have lost its entire roof frame from the uplift force of the wind, Prevatt noted.

A still frame from flyover video taken by Charleston County after the tornado shows minor cosmetic damage to nearby homes, including the innovative SIP house at lower left, while one house built in 1999 is completely destroyed.
A still frame from flyover video taken by Charleston County after the tornado shows minor cosmetic damage to nearby homes, including the innovative SIP house at lower left, while one house built in 1999 is completely destroyed.

Constructed in 1999, the house was built just before South Carolina adopted the 2001 International Residential Code, which incorporated wind-resistant framing requirements evolving out of lessons learned from 1994's Hurricane Andrew, which struck Miami, Florida. Prevatt noted what appears to have been a weak link in the destroyed building's uplift load path: "Hurricane straps [were] present between rafters and wall top plate. However, the wall top plate separated cleanly from the wall structure and failed along with the roof, indicating the load path between the wall plate and vertical studs may have been the critical failure point resulting in the observed damage to this home," reported Prevatt. "A far stronger connection for attaching the horizontal top plates to the vertical wall studs is by using through metal straps and/or through overlapping the wall sheathing on to the top plates such that the sheathing and its fasteners form the connection." The roof peak showed another relatively weak connection detail, Prevatt wrote: "Rafters meeting at the peak of the roof were toe-nailed together rather than connected with metal plates or straps." So, while some kind of erratic or random storm pattern could be the reason that just one house in the storm's path was destroyed, concludes Prevatt, it is far more likely that better construction details, required by the more advanced International Code in force when neighboring homes were built, account for the survival of those houses.

One of the surviving houses, however, is different from the others: it's a zero-energy home with SIP walls and solar panels on the roof, built by husband-and-wife team Steve and Tina Bostic, whose company, Amerisips Constructors LLC, builds with an innovative metal-skinned structural insulated panel system the company calls "Insulsteel." The Charleston Post and Courier reports on that story here (see: "Builder’s ‘eco-shell’ cited as saving home from Johns Island tornado," by Jim Parker).

"Dozens of homes sustained damage, and Sonny Boy Lane was hit as hard as any single street," the paper reported. "Blue tarps covered roofs on a number of the sprawling homes on large lots, and at least one house was mostly destroyed. At the same time, an Amerisips-built house, fit together with structural insulated panels 'to withstand 200 mph winds,' absorbed a few broken windows and minor damage to the siding and back stairs, the company says. The house is next door to homes that suffered roof damage and sits across the street from the totaled brick house."