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In North America, the design of houses and other buildings for earthquakes has been primarily to minimize serious injury or death to people, and wood-frame construction has generally net this requirement very well. In Canada, the objective of earthquake-resistant design is to prevent major failure and loss of life. According to the Structural Commentaries to the National Building Code of Canada:

”Structures designed in conformance with these provisions (of the 1995 National Building Code of Canada) should be able to resist moderate earthquakes without significant damage and major earthquakes without collapse.”

A survey of the performance of wood-frame construction in earthquakes shows a remarkably low fatality level. Most of the residential buildings in the survey areas were built according to conventional practice and were not specifically designed by engineers.

The vast majority of wood-frame buildings survived the strong shaking almost unscathed or with various degrees of superficial and structural damage. A few wood-frame buildings with recognized structural deficiencies collapsed. A summary of the deficiencies is given in the next section.

Three recent California earthquakes provide insight into the performance of North American style wood-frame construction.

San Fernando Earthquake, 1971
The earthquake occurred in a northerly suburban area of Los Angeles and consequently affected a large number of single-family homes as well as hospitals and commercial structures. Hospitals and office buildings built of unreinforced masonry and reinforced concrete collapsed, or were severely damaged to the point where they had to be demolished.

The majority of the wood-frame houses performed well, especially from the standpoint of life safety. Damage in older wood-frame houses in the San Fernando area ranged from superficial to partial collapse. Serious damage included houses sliding from foundations, collapse of “cripple walls” in crawl spaces, collapse of add-ons such as porches and collapse of masonry chimneys. Newer two-storey apartment buildings with large ground-level openings were also severely affected. Typically, these apartments were constructed to allow parking at the ground level. Because one wall of the garage storey was open, these flows were “soft storeys.”

The importance of providing strong walls and foundation connections was recognized in this earthquake and building codes were updated accordingly.

Loma Prieta Earthquake, 1989
The epicentre of the earthquake was located 100 km south of San Francisco but its effects were felt on the North Shore of San Francisco Bay. The earthquake caused the collapse of a number of engineered structures including the double-deck freeway in Oakland that resulted in the death of 49 motorists. At the epicentre, housing was subjected to peak ground accelerations as large as 0.5g and possibly larger. Newer housing at the epicentre generally performed well unless they were situated on ground fissures or had large openings in lower storey walls.

Some older housing at the epicentre experienced extensive damage due to inadequate braced walls, cripple wall failures and inadequate connections to the foundations. In the Marina District of San Francisco some older four-storey buildings were particularly susceptible due to ground storey parking that resulted in non-reinforced soft storeys.

Northridge, 1994
The earthquake struck the densely populate San Fernando Valley, in northern Los Angeles on January 7th, 1994. Although moderate in size (M6.7) the peak ground accelerations were amongst the highest ever recorded ad significantly higher than those specified in the building codes at the time. There were numerous building collapses, many in large structures. Amongst the reasons given for the limited death and injuries from the earthquake was the time of the earthquake:

“The earthquake occurred at 4:31 a.m when the majority of people were sleeping in their wood-frame single family dwellings, generally considered to be the safest type of building in an earthquake. If the earthquake had occurred during the day, say at 11:00 a.m., several hundred people would have been killed at the retail store and parking garage of the Northridge Fashion Mall alone, where actually only one person was killed. Also, due to timing of the earthquake, people were not present on sidewalks to be injured from falling debris, particularly from unreinforced masonry and tilt-up buildings or falling facades from other buildings.”

The 1994 Northridge Earthquake has been extensively studied. A very high percentage of wood-frame houses performed well in the earthquake. Most of the damage to such buildings was non-structural in nature and easily repairable.

There were several typical modes of failure that had been experienced in previous earthquakes. Although building codes had addressed there issues in new construction, the failures reflected the need to upgrade existing buildings to remove obvious deficiencies. In the Northridge Earthquake, a vulnerable wood-frame building type was low-rise, multi-storey, apartment structures with a soft first-storey. Such buildings, with large, often continuous openings for parking, did not have enough wall area and strength to withstand the earthquake forces and resulted in several collapses of the ground floor. The most tragic collapse of the earthquake occurred when 16 people died in one such apartment building. Although the building was engineer-designed, the peak ground acceleration at the location was substantially greater than the 0.4 g design value used in the building codes at the time.

Other wood-frame construction performed exceedingly well. In a statistical-based study of the seismic performance of residential construction in the Northridge earthquake, the authors concluded:

“SFD (Single Family Dwelling) homes suffered minimal structural damage to elements that are critical to the safety of occupants. Structural damage was most common in the foundation system. The small percentage of surveyed homes (approximately two percent) that experienced significant foundation damage were located in areas that endured localized ground effects or problems associated with hillside sites.”

Following an earthquake, it is necessary to ensure displaced people can be provided with emergency accommodation. If too many houses are destroyed, emergency shelters will be overwhelmed. In the days after the Northridge Earthquake, many buildings in the Los Angeles area were inspected to determine if they were safe for occupancy. When there was concern about the safety of a building, an initial inspection was carried out and the building was tagged either “Red” for hazardous, “Yellow” for buildings that posed a threat to life, but not so much that an occupant could not re-enter to remove possessions and “Green” for buildings that did not pose a life-safety hazard to the occupants. After 4 days, almost 50,000 residential buildings had been inspected and over 85% of these were “Green Tagged”.

Most of the buildings were inspected because residents and owners were concerned the condition of their homes might be unsafe. Therefore buildings inspected were generally more damaged than the vast majority of wood-frame buildings for which no inspection was requested. Furthermore, many of the red and yellow-tagged buildings were subsequently revised to green.

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