"Years ago, that might be justification for an F5 rating on the original Fujita scale," a writer on the NWS website explains. "These days, the quality of the construction is examined before a rating is assigned. One of the factors determining the rating is the use of anchor bolts."
As shown in the picture, the home to left in the Vilonia (Faulkner County) area had cut nails instead of anchor bolts to fasten the structure to the foundation.
In fact, according to the article, in Vilonia (Faulkner County), there was little indication of anchor bolts where homes once stood.
At a subdivision to the southwest of town, the official survey results read,"Houses completely destroyed; only slabs remaining at several places. No anchor bolts used in foundations."
So what does this mean?
While the article points out using cut nails to secure homes to the foundation is widely practiced and the minimum standard in most of the building codes - Dr. David Prevatt, Associate Professor of Civil and Coastal Engineering at the University of Florida says they may not provide enough security in high winds.
"It is my opinion that cut nails can only be considered a temporary connection while installing a wall," Dr. Prevatt says. "They are in no way, shape or form have the capacity to resist the sliding loads or the uplift loads reduced by high winds that impact the walls of a building."
Which means, without those anchor bolts, it took less wind to sweep houses away in Vilonia. This explains the EF4, and not EF-5, rating the NWS ultimately gave.
And while the NWA says the "odds are remote that any one location is impacted by such a tornado," Dr. Prevatt still believes the construction standards need to be examined to reduce casualty rates.
"Bottom-line," Prevatt says, "we either build them (our houses) stronger or communities must accept that certain number of homes randomly selected by nature's wrath, will be blown away and kill people in every moderately strong tornado for the foreseeable future."
The NWS says not every clean slab that was checked along the tornado path had cut nails. Several did have anchor bolts. "However, there were signs that some of the bolts were improperly installed."
In general, the article says, if survey crews found anchor bolts, and they were used as intended, the slabs were not clean.
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