Building code considerations
Canadian building codes address the need to prevent these components from displacement and severing service lines that may also cause damage during seismic events. As one might expect, codes also address the critical need to ensure occupant health and safety.
Requirements to provide seismic restraints for non-structural components are mandatory in many building codes when the seismic hazard index value (discussed in the following paragraphs) for a building exceeds a given threshold. In Canada, the codes provide formulas for calculating the magnitude of both horizontal and vertical seismic forces on these components, but do not specify how to provide the necessary restraints.
As the requirements are similar for most jurisdictions in North America (and as some readers may have clients on either side of the border), this article discusses some of the relevant code provisions for seismic bracing of non-structural components. While local jurisdictions may have their own codes that somewhat vary from the primary codes, the principal relevant codes are the National Building Code of Canada (NBC) and the International Building Code (IBC) in the United States.
Section 220.127.116.11 (Division B) of Volume 2 of the 2010 NBC provides design criteria for engineers. In the United States, the latest version of IBC (i.e. 2012) has not been widely adopted—consequently, the 2009 edition is the most prevalent code. It references American Society of Civil Engineers (ASCE) 7-05, Minimum Design Loads for Buildings and Other Structures; in particular, that standard’s Chapter 13 is the primary source for U.S. designers in devising seismic bracing of non-structural components.
In both NBC and IBC, there are several important parameters determining the bracing requirements of non-structural components. Some locations have higher risks of seismic activity than others. The building location defines important acceleration values related to anticipated ground motion that are used in calculating the design seismic force for bracing. (The seismic demand spectrum [SDS] factor can be found in ASCE 7-05 Equation 13.3-1, and the Sa(0.2) factor in the Vp equation of NBC Section 18.104.22.168).2
Building use and occupancy
NBC Table 22.214.171.124 assigns “importance categories” to all buildings—low, normal, high, or post-disaster. In a similar manner, ASCE 7-05 assigns a category (I, II, III, or IV) based on the building’s use and occupancy.
For example, storage buildings or facilities with a low direct hazard to occupants in the event of failures are generally rated as having a ‘low’ importance factor. On the other hand, buildings that may be used as temporary shelter in the event of a disaster (e.g. schools or community centres) would be rated as having ‘high’ importance. The code also suggests buildings that are not rated as low, high, or post-disaster should be assigned a category of ‘normal’ importance.
Buildings providing essential services in the event of a disaster are assigned “post-disaster” category in NBC or Category IV–Essential Facility in IBC. Examples include:
- fire and police stations;
- power and water pumping stations;
- buildings enabling communications; and
- fuel and/or energy facilities.
There is also an Importance Factor (IE) that is assigned to the building under both applicable codes. Post-disaster or essential facilities are given the highest importance factor (NBC Article 126.96.36.199), which also manifests itself into higher bracing design forces for non-structural building components in both ASCE 7-05 Equation 13.3-1 and the Vp equation of Section 188.8.131.52 of NBC. When a building is considered vital in the event of a disaster, not only are design bracing forces higher, but the bracing requirements for post-disaster and essential facilities are also considerably more stringent in both Canada and the United States.
Geotechnical soil site class
The building’s location has an impact on its geotechnical soil class. This site-specific attribute is important in determining the seismic requirements for a given site. In the absence of knowledge of soils on a specific site, some codes require the engineer of record to assume a site class (in IBC, for example, Site Class D is assumed which is representative of a stiff soil). Unfortunately, such assumptions can be conservative and result in soil co-efficients and ultimately bracing design forces that may be higher than required. Such assumptions can also lead to bracing requirements that would not otherwise exist.
Soil site class is determined in several ways; in Canada, this work is done by a geotechnical engineer.
One way the geotechnical engineer can determine soil site class is to undertake a standard penetration test. (This is typically done with a drilling rig and a hammer test. Based on blow counts, the building codes provide the appropriate site soil class information.) Another option is for the geotechnical engineer to perform shear wave (S-wave) velocity testing. Earthquake damage is usually considered to be caused primarily by vertically propagating shear waves. The velocity at which these shear waves travel through a given material, such as rock or soil, has a strong influence on the response of the material because shear wave velocity is directly related to shear modulus.3 Based on tested velocity values, the building codes provide detailed site class information.
The third method to determine site soil class in accordance with the building code is by reviewing the average undrained shear strength of the soil. This is the existing condition of the soil, before dissipation of pore water pressure due to consolidation.
There are also other factors determining the bracing force for a specific component. Both Canadian and U.S. codes have tables of values for component amplification factors and response modification factors. Since this is an inertial force, the component operating weight is relevant. Additionally, there are a couple of important heights:
- height of structure at point of attachment of the component with respect to base; and
- average roof height of structure with respect to the base.
Does the equipment need to be certified by the manufacture that it will be operational after the design earthquake in Canada? In California manufacture must certify equipment for a given seismic event and provide a label on the equipment. For post disaster facilities the equipment is tested on a shake table.