“We often use redundant support systems with long-span structures. Perhaps we should do the same with smaller structures,” he says.

The majority of non-residential buildings in Joplin incorporate structural-steel or load-bearing masonry, says Treece, who was surprised by the number of facilities that sustained minimal structural damage, despite extensive facade damage.

However, pre-engineered metal buildings in a light industrial district were destroyed. “Part of the stability of these buildings is in the cladding, which tornadoes rip off first,” says van de Lindt. “Specifying stronger steel would be costly and reduce the savings associated with pre-engineered metal.”

By comparison, structural damage to a pair of towers at St. John's—one made of reinforced concrete and the other a steel-framed structure with concrete decks—was minimal, though the tornado shattered scores of windows while destroying portions of the 367-bed center's upper floors and exterior walls.

“This was what we call a dirty storm. The debris clouds were so large that virtually everything in the tornado's path was riddled with debris,” says Larry Tanner, a research associate with Texas Tech University's College of Engineering and a member of a FEMA team evaluating damage and issuing recommendations for rebuilding.

As hospital officials contemplate rebuilding, the damage has prompted discussion about blast-resistant glazing, including units equipped with interstitial laminates, to prevent flying glass in hospitals and other critical facilities. “It's an appropriate response, but it's also hard to imagine cladding an entire building with it because of the costs,” says Tanner.

Big-box retailers also are under scrutiny. The Walmart and Home Depot stores, sited at the same intersection, featured long spans, tall walls, metal roofs and large front windows. “Big boxes are built quickly and for short life spans, typically 14 to 19 years,” says Gees. “You build them and hope for the best.”

Eyewitnesses say the tornado lifted the roof off of Walmart and dropped pieces of it on employees and customers. A support beam crushed a couple and their child. Seven bodies were recovered from beneath a collapsed concrete wall at Home Depot.

“The big boxes performed very poorly,” says Tanner. The Home Depot, a tilt-up concrete structure with steel columns, steel joist girders and light-steel joists, “was built for gravity loads but did not appear to exert much resistance to wind loading,” says Tanner. “Virtually all four of its walls collapsed.”

The Walmart, made of non-reinforced concrete walls and light-steel columns, joist girders and decking, “lost the front half of the store and parts of the back half,” says Tanner.