Seismic Hazard Calculations
The damage potential of an earthquake is determined by how the ground moves and how the buildings within the affected region are constructed. Expected ground motion can be calculated on the basis of probability, and the expected ground motions are referred to as seismic hazard.
In Canada, the evaluation of regional seismic hazard for the purposes of the National Building Code (NBC) is the responsibility of the Geological Survey of Canada. The seismic hazard maps prepared by the Geological Survey are derived from statistical analysis of past earthquakes and from advancing knowledge of Canada's tectonic and geological structure. On the maps, seismic hazard is expressed as the most powerful ground motion that is expected to occur in an area for a given probability level. Contours delineate regions likely to experience similarly strong of ground motions. A simplified seismic hazard map indicates the relative seismic hazard across Canada.
The seismic hazard maps and earthquake load guidelines included in the National Building Code are used to design and construct buildings to be as earthquake proof as possible. The provisions of the building code are intended as a minimum standard. They are meant to prevent structural collapse during major earthquakes and thereby to protect human life. The provisions may not, however, prevent serious damage to individual structures.
Seismic Hazard Information in the National Building Code (NBC)
Building design for various earthquake loads is addressed in sections 4.1.8, 18.104.22.168, 22.214.171.124, 126.96.36.199, and 188.8.131.52 of the 2005 NBC. In addition, a table in Appendix C starting on page C-11 of Division B, volume 2 of the Code provides ground motion design values for many of the larger communities across Canada. While the National Building Code is chiefly intended for new buildings (Article 184.108.40.206 of Division A), appendix A (appendix note A-220.127.116.11) outlines the principles by which the code should also be applied to the use and modification of existing buildings.
The seismic hazard is described by spectral-acceleration values at periods of 0.2, 0.5, 1.0 and 2.0 seconds. Spectral acceleration is a measure of ground motion that takes into account the sustained shaking energy at a particular period. It is a better measure of potential damage than the peak measures used by the 1995 code, and thus will improve earthquake-resistant design. Peak Ground Acceleration is still used for foundation design. All parameters are expressed as a fraction of gravity. The four spectral parameters allow the construction of uniform hazard spectra (UHS) for every place in Canada.
Ground motion probability values are given in terms of probable exceedence, that is the likelihood of a given horizontal acceleration or velocity being exceeded during a particular period. The probability used in the 2005 NBC is 0.000404 per annum, equivalent to a 2-per-cent probability of exceedence over 50 years. This means that over a 50-year period there is a 2-per-cent chance of an earthquake causing ground motion greater than the given expected value.
Most buildings are well designed for withstanding vertical forces, but the horizontal component of ground motion is critical to earthquake-resistant building design. In the urban areas of coastal British Columbia, for example, 40-per-cent gravity is a typical seismic load at an appropriate probability for buildings. A building, designed to tolerate a sideward pushing force equal to 40 per cent of its own weight, should prove earthquake resistant.
Calculation of Seismic Hazard
The seismic hazard at a given site is determined from numerous factors. Canada has been divided into earthquake source regions based on past earthquake activity and tectonic structure. The relation between earthquake magnitude and the average rate of occurrence for each region is weighed, along with variations in the attenuation of ground motion with distance. In calculating seismic hazard, scientists consider all earthquake source regions within a relevant distance of the proposed site.
The four spectral acceleration seismic hazard maps show levels of ground shaking at periods of 0.2, 0.5, 1.0 and 2.0 seconds (equivalent to frequencies of 5, 2, 1, and 0.5 Hertz). This is important because different buildings are susceptible to different frequencies of earth motion, and damage is frequently associated with a resonance between earthquake ground motion and the building's own natural frequency. A high-rise of ten stories or more may sway with a natural period of 1 or 2 seconds, whereas in response to the same earthquake a brick bungalow across the street may vibrate at nearly 10 Hertz.
The UHS is a description of the seismic hazard at a site in terms of building height. See sample UHS for 4 Canadian cities.
Consequently, low brick buildings can be severely damaged by a moderate (magnitude 5.5) local earthquake that has most of its energy in the high-frequency range. High-rises may be affected more acutely by larger, more distant events. In the Mexican earthquake of 1985, most of the severe damage in Mexico City, 400 kilometres from the earthquake's epicentre, occurred in high-rise buildings with natural periods near 2 seconds, due to ground amplification by poor foundation conditions.
In building construction and design, not only the size of a probable earthquake should be considered, but also the nature of the ground motion most likely to occur at the site. Seismic hazard calculations provide part of this information. As our understanding of earthquakes and of their effects on engineered structures continues to develop, the seismic provisions of the National Building Code will be revised to enhance public safety and minimize earthquake losses.
Determine National Building Code of Canada seismic hazard values for your site - Use the on-line seismic hazard calculators.
For more information about the National Building Code of Canada, visit the Institute for Research in Construction, National Research Council of Canada.
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