I saw an article about the devastation of Oklahoma homes on the Yahoo news and the idea of house constructions and the Building code system that is used. i post it here :View Photo
Reuters/Reuters - Members of the National Guard help a homeowner look for items to salvage several days after the building had been destroyed by a tornado in Moore, Oklahoma, May 24, 2013. REUTERS/Lucas Ja …more
By Greg McCune
(Reuters) - In a residential neighbourhood near the centre of a monster tornado that struck Moore, Oklahoma last month, two partially damaged houses stand like an island among others flattened by the storm.
The walls and roofs of the buildings in a new housing development called Featherstone Addition are still upright while there is nothing left but a concrete foundation where other homes once stood nearby.
The two homes were not completely spared but are salvageable, according to David Prevatt, a civil engineer who saw them when he surveyed the damage after Moore took a direct hit from an EF5 tornado, the strongest rating.
He is convinced that the two houses survived because they were built stronger than most in Oklahoma and the rest of "tornado alley" - the region stretching from Texas to Iowa that accounts for roughly a fourth of all U.S. tornadoes.
"This notion that we cannot engineer buildings economically to withstand tornado loads is a fallacy," said Prevatt, who has studied damage from hurricanes and the devastating tornadoes in 2011 in Joplin, Missouri and Tuscaloosa, Alabama.View Photo
Reuters/Reuters - Members of the National Guard help a homeowner look for items to salvage several days after the building had been destroyed by a tornado in Moore, Oklahoma, May 24, 2013. REUTERS/Lucas Ja …more STRONGER HOUSES
Building a house to limit tornado damage involves making strong connections from the foundation to the walls to the roof, said Prevatt.
Nails hammered into the wood at a 90 degree angle provide little resistance to the upward suction of a tornado, several construction and engineering experts said.
"We saw that in many places in Moore," said Randy Shackelford, Research Engineer and Code Specialist for Simpson Strong-Tie Company in McKinney, Texas. "Two nails sticking up where the stud (wall frame) once was."
The next best connection is what is called a "toenail" in construction jargon, which involves driving the nail into the wood at a 45 degree angle. This provides significantly more resistance than nails driven at a 90 degree angle.
The 2012 national code which Oklahoma decided not to adopt immediately calls for three toenails per connection but Oklahoma's code allows two nails, which is significantly weaker, Shackelford said.
The two houses Prevatt saw standing in Moore amid the devastation were built with what are called "hurricane ties" or metal straps to bind the roof to the walls, which are stronger than nails, he said. These are required by the robust Florida building code but not in Oklahoma