Glazing on the interim facility can withstand 200-mph winds. Once complete, Mercy's permanent replacement facility will likewise reflect the latest thinking in tornado-resistant design. Exteriors, generators, core and stairwells are all designed to withstand an EF-5 event.

The tornado's effects presented engineers with ideas for the hospital's nearby permanent rebuild. As investigators sifted through the debris of St. John's, it became clear certain systems had performed better than others. While nearly all windows were destroyed, laminated units in the behavioral health division remained in place, even though they cracked.

Likewise, roof sections made of metal decks and Styrofoam insulation performed more poorly than areas enclosed by concrete, leaving a quarter of the roof structure destroyed or damaged. As a result, all roofs on the replacement structure will be made of concrete and incorporate a double-roof system featuring a pair of waterproof membranes, one sandwiched between metal deck and concrete and the other atop the roof.

To minimize wind damage and prevent shards from becoming projectiles, the replacement facility will incorporate glazing at least 20% percent stronger than safety glass in place at St. John's. Units in non-critical areas will be rated to withstand 140-mph winds. Critical areas, including intensive care, will be rated to withstand 250 mph. Similar to auto safety glass, units are undergoing testing by West Des Moines-based AWS Wall Systems to ensure they perform as intended.

Select spaces above grade also will function as protective zones. Though a "condition gray" warning provided St. John's staff time to roll patient beds into corridors, patients nonetheless sustained injuries from shattered glass. New hallways will sandwich 2-ft by 4-ft alcoves between patient rooms, their walls supported by metal diagonals. Metal storm doors will terminate the hallways.

Some of the most critical improvements at Mercy involve power. In addition to incorporating redundant primary power, Mercy will locate back-up generators in a central utility plant made of concrete, with a 12-ft by 12-ft below-grade tunnel routing power lines to the hospital. Critical areas in Mercy will be equipped with uninterruptable power as well.

Surprisingly, efforts to tornado-proof Mercy aren't translating into a huge capital investment relative to total construction costs. "It's going to cost about $7 million to harden the facility, or about 2% of the budget," says Wittkop.

The effort continues. "We're still working with various concepts," says Farnen. "We'll continue to refine our glazing in Iowa. There are concerns about the potential for bubbling with our underlying [roof] membrane. We're getting there. It's always a challenge when you find yourself doing something for the first time."