The Anatomy of a Safe Building
Although many considerations of building safety are economic, some structural problems, such as unreinforced masonry construction, are potential life safety issues. Even though structural problems in homes, if they exist, are usually more economic risks than safety issues, some of these risks can be substantial and should be addressed in earthquake country.
A safe building is one that can withstand the sideways push of an earthquake. Buildings are built to withstand the downward pull of gravity. Earthquakes push on a building in all directions -- up and down, but most of all, sideways.
A safe building is built on a firm foundation. The foundation should be solid, with a continuous perimeter. The house should be securely fastened to the foundation with bolts or plates that anchor the sill plate to the foundation.
A safe building is securely connected together. Mortar in brick and masonry fails under even moderate shaking and should not be considered a structural element. A house with a crawl space (not a slab foundation) may have a small, wood-frame wall that surrounds the crawl space between the bottom of the house and the foundation, known as a cripple wall or pony wall. Cripple walls need bracing to resist the sideways push. The opening for a garage door can be a weak part of a wall and other walls must compensate if garage doors are located in the first floor of a multistory building.
A safe building is built of strong materials. Damaged concrete and rotten wood undermine the integrity of the building.
A safe building protects the plumbing. Broken water pipes will cause water damage and broken gas pipes are a great fire hazard. Pipes need some, but not too much, room to sway. Long spans of pipe need to be secured to a wall or floor. Gas connections should be the flexible type to avoid breakage during earthquakes from movement of the appliance.
Many houses in Nevada are not as safe as they could be. The section "Common Structural Problems" below presents some common structural problems and how to recognize them. To fix them, you will need to obtain more information.
Plywood strenghting of cripple walls. If your home has an unbraced cripple wall between the foundation and the first floor it may collapse and shift off its foundation during an earthquake.
Click for larger image
Further Reading:
Yanev (1991) Peace of Mind in Earthquake CountryFor professional contractors and engineers,
Smith and Furukawa (2000) Introduction to Earthquake Retrofitting
Ambrose and Vergun (1999) Desigh for Earthquakes (technical)
Association of Bay Area Governments Website
See the yellow pages of the telephone book under Engineering and General Contracting and Engineers -- Structural.
Common Structural Problems
Inadequate foundations or foundation anchorage. Go into your crawl space and look at your foundation. If the foundation is damaged or built in the "pier and post" style, consult a contractor or engineer about replacing it with a continuous perimeter foundation. Look for bolts in the mudsills. They should be no more than feet apart in a single story and 4 feet apart in a multistory buildings and have large, heavy plate washers. Adding bolts or steel plates to unsecured houses is one of the most important toward earthquake safety. This can be done by a contractor or by someone moderately adept at home maintenance.
Unbraced cripple walls. Go into your crawl space and look for panels of plywood or diagonal wood sheathing connecting the studs of the cripple walls. You or a contractor can strengthen the cripple walls relatively inexpensively (see adjacent figure).
Soft first stories. Look for large openings in the lower floor, such as a garage door or a hillside house built on stilts. Consult a professional to determine if your building is adequately braced.
Unreinforced masonry. If your house is built of brick or rock, and it is an older home, it may be unreinforced. Solid bricks or rocks with mortar between them that lack reinforcement may not have adequate shear resistance to shaking. Chimneys are a common unreinforced masonry element of homes. Consult a professional to determine if your building is adequately reinforced.
What If I Rent
As a renter, you have less control over the structural integrity of your building, but you do control which building you rent. Remember these points as you look for rental housing:
- Apartment buildings have the same structural requirements as houses.
- Structures made of unreinforced masonry and with soft first stories have caused the greatest loss of life in earthquakes.
- Foundation and cripple wall failures have led to expensive damage but less loss of life.
- Objects attached to the sides of buildings, such as staircases and balconies, have often broken off in earthquakes, injuring those below.
Ask your landlord these questions:
- Is this building built with earthquake resistance?
- What retrofitting has been done on this building?
- Have the water heaters been strapped to the wall studs?
- Can I secure furniture to the walls?
When was your house built?
Regulations for safety in building construction are codified in the Uniform Building Code (UBC), first issued in 1927 by a nonprofit, independent organization called the International Conference of Building Officials (ICBO). The UBC is updated and reissued every three years. It is up to local jurisdictions (cities, towns, counties) to adopt and enforce building codes in Nevada.
Back to Top | Previous | Next1927 First Uniform Building Code (UBC) issued, primarily about fire safety.
1935 UBC requires foundation bolting and reinforcement of masonry foundations.
1946 UBC requires some reinforcement of masonry chimneys.
1961 UBC requires all structural members to resist earthquake forces.
1973 UBC requires bracing of cripple walls.
1976 UBC requires 50% increase in earthquake design forces for box-type structures and stronger force loads for hospitals and concrete-moment frame buildings
1988 UBC adds a new soil profile type for considering site resonance for tall or long-period structures.
1991 UBC increases bracing requirements for cripple walls and requires bracing of water heaters.
1997 UBC requires testing of steel welds and increased design forces for concrete tilt-up wall anchorage. Heavy plate washers required for foundation anchor bolts.
