Earthquakes, Home Building Codes and Legend’s Seismic Design Procedures
New technology and research provides new opportunities for safety in structures
“A home [only] “built to code” is the worst possible home you can legally build, we go far beyond that.” – Jim Chapman
Oregon home building codes have addressed seismic concerns and the eventuality of earthquakes for many years, gradually increasing seismic design requirements as new technology and research provides new opportunities for safety in structures.
Essentially, the biggest concern with residential homes is the attachment to the concrete foundation. Since earthquakes create lateral shifts, or “shear”, where the foundation moves with the ground one direction then reverses direction, causing the structure above to want to slide off it.
That same concept moves up the structure, as each level tries to catch up with the level below it. When the first-floor level is held tight to the foundation, it reverses direction, and the second floor tries to slide off the first or tip it over. Same with the roof. During a large earthquake event, each level is traveling in the opposite direction from the level below it.
As you can imagine, that is the reason structures only built for gravity are flattened in a large quake. Terrifying, actually.
The home building code has a simple prescriptive seismic design method for meeting the shear force requirements. However, since those of us at Legend Homes feel that a home “built to code” is the worst possible home you can legally build, we go far beyond that.
Each home is structurally engineered by a third party licensed firm, and their full package of seismic design calculations and specifications is reviewed by the Building Department prior to issuing the permit to build.
All those shear components and details are included on the plans for each home, which we supply to our homeowners on a jump drive (along with behind-the-walls photos and documentation).
We then use a specialty hardware supplier that not only provides the required connectors and fasteners, but instructs the framers on correct installation procedures. The final assemblies are inspected by the city Building Inspector, and is the most critical of all inspections they do.
The recent dust-up in the paper about earthquake damage is related more to older structures, especially brick ones. The retrofit programs for homes are for those built prior to the 1980’s, and typically only address the attachment of the structure to the foundation.
The bottom line is that we have great confidence that no Legend home is going to fall down in an earthquake. We have done all that can be done to prepare for it, and there are no additional measures you could do that would make a difference.
However, you should take a good look around your home at its contents. You are far more likely to be injured by falling bookcases, artwork, storage racks, piled boxes, etc. than by walls, floors, or ceilings.
Jim L. Chapman
Legend Homes Corporation