Shaken and Stirred

Research in earthquake engineering over the last four decades has helped drive the evolution of seismic provisions of the National Building Code of Canada (NBCC). In step with this evolution has been an increase in the understanding of Canadian seismicity over the years, resulting in improved seismic hazard values for which buildings are designed.

However, buildings designed and constructed prior to the enactment of modern seismic code provisions may be deficient in resisting earthquakes. This is especially true for buildings constructed before the mid-1970s, which accounts for a large inventory of existing buildings across the country.

These buildings may be at risk when subjected to strong earthquakes. This not only poses a threat to the safety and well-being of Canadians, it also carries potentials for significant seismic damage, disruptions in services and considerable economic losses. The Geological Survey of Canada (GSC) reports that approximately two-thirds of Canada’s population living in urban centres are exposed to three-quarters of its seismic risk (See Figure 1 below).

FIGURE 1

Relative contribution to seismic risk in Canada (Source: Geological Survey of Canada)

Seismic risk associated with building construction can be computed by following a two- or possibly three-tier approach with increasing levels of sophistication in each tier. Tier I assessment forms rapid seismic screening, which helps to establish priorities for further assessment with more detailed and costly analysis techniques. By its very nature, however, seismic screening is not intended to be an accurate tool. What it does is help identify more vulnerable and more critical building infrastructure with higher seismic risk potentials for further assessment.

Seismic Risk

Seismic risk associated with buildings can be computed as the product of building damageability and consequence of failure. As such, the consequence of failure reflects the importance and/or exposure of a building.

A post-disaster building (for example, a hospital or a fire station) is expected to function during and after an earthquake and, as such, is classified as an “important” building with severe consequences of failure. Similarly, a densely populated movie theatre is likely to lead to higher fatalities than a scarcely populated farm building, revealing use and occupancy as a major risk parameter.

For global seismic risk assessment of a city or a geographic region, this factor reflects the exposure of infrastructure in the region to seismic actions, with large and densely populated cities having higher exposures.

Building damageability can be assessed by integrating both site seismic hazard and building vulnerability, the former reflecting the seismicity of a region, including the effects of ground conditions. The GSC has developed seismic hazard maps for Canada, which indicate the intensity of earthquakes in different regions across the country. This information is incorporated in the NBCC for use in building designs.

Seismic maps were developed in 1953, 1970, 1985 and 2005, all in line with the progression of enhanced knowledge and improved statistical analyses associated with the increase in strong motion data that has become available over the years. The 2005 seismic hazard map for Canada (Figure 2, below), forms the basis for the seismic provisions in the current 2010 NBCC).

FIGURE 2

Researchers often identify ground shaking associated with earthquakes as the dominant seismic hazard factor in prompting building damage. Other factors connected with earthquake-induced ground movements, such as landslides and liquefaction (See Figure 4, below), are classified as consequences of ground shaking, and are not considered part of site seismic hazard for the purpose of Tier I seismic screening.

Building collapse due to liquefaction during the Aug. 17, 1999 earthquake in Turkey (Saatcioglu et al, 2001)

These effects can nonetheless have severe consequences on building performance and should be assessed separately. For example, there have been building collapses as a result of liquefaction and associated loss of support at building foundation. However, the site soil condition, through which seismic waves travel, is considered in hazard values.

There is a substantial difference in the magnitude and frequency content of seismic waves that travel through soft soils and hard rock. Soft soils amplify the effects of seismic waves on structures, and may increase risk for buildings on such soils.

This difference in site seismic hazard values is reflected in terms of soil types (classifications). These values can be expressed more conveniently in the form of seismic microzonation maps, based on different soil types and seismicity of the region. Microzonation, in turn, permits site-specific risk analysis. (Figure 3, below, shows the microzonation of Ottawa with areas of different soil types identified on the basis of the 2010 NBCC soil classification).

Figure 3

Micronization of Ottawa (Source: Hunter, J.A. et al. 2012)

Building vulnerability is computed by integrating deficiencies in the structural system. Deficiencies in buildings are primarily attributed to the use of brittle construction materials, lack of proper seismic design and detailing practices, selection of improper structural layout, poor quality of construction and irregularities in the structural system that tend to increase seismic force and deformation demands.

Buildings utilizing brittle construction material – such as unreinforced masonry or conventional reinforced concrete construction (without seismic design and detailing practices) – do not have the ability to deform in the inelastic range of materials to dissipate seismic-induced energy.

Ductility, defined as the ability to develop inelastic deformations without significant strength decay, and associated energy dissipation capacity combine to be one desirable feature of earthquake-resistant construction. Building codes promote ductile design principles for earthquake-resistant construction.

Seismic damage can result from the use of unreinforced masonry construction coupled with a lack of sufficient seismic design and detailing practices in reinforced concrete buildings. Plan irregularity, often resulting from the eccentricity of the centre of building mass and the centre of rigidity – as is the case for elevator shafts with concrete walls located near the end of a building, as opposed to located symmetrically – produce torsional effects. This produces greater seismic force and deformation demands on critical vertical elements.

FIGURE 5

Loss of a storey due to vertical irregularities during the Feb. 27, 2010 earthquake in Chile (Saatcioglu et al, 2001)

(a) Rear view showing setbacks

 

(b) Front view showing damage

 Vertical irregularities in the form of either discontinuous walls or setbacks (See Figure 5, above) also augment seismic demands at locations of discontinuity, thereby increasing building vulnerability. That said, deformation control in buildings is an asset for minimizing damage to brittle structural and non-structural elements, including exterior façades, building cladding and glass windows, which helps to reduce building vulnerability.

Buildings laterally braced by properly designed shear walls and other bracing elements often perform well during earthquakes, controlling potential seismic damage substantially.

Factors that affect building vulnerability are many, necessitating that parameters considered in seismic screening be limited to those that are most important, based on previous experience and knowledge of structural performance. These parameters are often associated with design requirements outlined in building codes, which have progressively improved over the years. That means that the year of construction is an important indicator for vulnerability assessment.

Seismic Screening Manual

A manual was developed in 1992 for seismic screening of buildings in Canada (NRCC 1992). The manual is meant to be used as the first step in a multi-phase seismic assessment process to identify buildings requiring further such assessments, while also providing seismic priority indices to prioritize the assessments. Buildings deemed to be in need of further investigation must be analyzed using more refined techniques before decisions can be made regarding seismic vulnerabilities and needs for seismic risk mitigation strategies.

The 1990 edition of NBCC, which is based on 1985 seismic hazard values, serves as the reference building code in the screening manual. The latest edition of the code, NBCC 2010, not only employs the fourth generation seismic hazard values, it recently was revised to also incorporate new ductility and overstrength-related force modification factors. These changes necessitated modifying the screening manual, which resulted in computer software Screen, detailed in Seismic screening of buildings in Canada based on the Canadian seismicity as per NBCC-2010 (Saatcioglu, M., Shooshtari, M. and Foo, S. 2012).

The method used for seismic screening is based on computing the product of relevant seismic risk factors as indicated below:

• SPI = SI + NSI – Seismic priority index (SPI) consists of structural index (SI) and non-structural index (NSI);

• SI = A * B * C * D * E – Structural index consists of seismicity factor, soil condition factor, structure type factor, building irregularity factor and building importance factor;

• NSI = B * E * F – Non-structural index (NSI) consists of soil condition factor, building importance factor and non-structural hazard factor.

SPI scores of less than 10 may indicate low priority; 10 to 20 may indicate medium priority; and those 20 or higher may indicate high priority for further assessment. Buildings with SPI scores of 30-plus can be considered potentially hazardous.

Note: This article references material from the National Research Council of Canada, including the Manual for Seismic Screening of Buildings for Seismic Investigation; the Ottawa-Gatineau seismic site classification map from combined geological/geophysical data from the Geological Survey of Canada; and a number of research papers (Saatcioglu et al), including Seismic screening of buildings in Canada based on the Canadian seismicity as per NBCC-2010 and Performance of masonry and steel buildings during the February 27, 2010 Maule (Chile) Earthquake.