Specifying Seismic Restraints: Ensuring safety with non-structural components

by mdoyle | March 23, 2013 9:02 am

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By Edward Kolodziejski, M. Eng., P.Eng., and Brian Burton
One of nature’s most destructive natural disasters, earthquakes cannot be successfully forecasted or prevented. However, it is possible to predict the effects of these seismic events on buildings and infrastructure by analyzing historical data.

Over the last two decades, the possibility of malfunction or catastrophic failure of non-structural building components during earthquakes has received considerably more attention than in the past. In many jurisdictions across Canada, local building codes now include provisions requiring seismic restraint of mechanical, electrical, and safety system components. These issues can apply to almost any building, regardless of occupancy type.

In stronger earthquakes, non-structural components can malfunction, fall, overturn, or become displaced. The result may be direct injuries (and possibly death), along with blocked exits or leaked fluids/gases that create additional risk. (Litigation stemming from failure to provide the required seismic restraints may be a concern for the design architect and engineer.) In other words, even when the building itself remains standing, considerable damage, and harm to occupants can result.

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Non-structural components in a building can include:

• auxiliary framing grids supporting power cables, switchboard, and fibre-optic cables, trays, and contents;
• piping, conduits, and mechanical ductwork;
• suspended and roof/floor-mounted HVAC equipment;
• fuel tanks, fans, and pumps;
• emergency batteries and generators;
• fire-suppression system piping;
• ceiling grids, lighting fixtures, and panelboards;
• chimneys, antennae, and process equipment;
• racking and storage systems;
• partition walls and raised access floors; and
• exterior cladding.

Materials and installation
Experience has shown addressing the installation requirements for seismic restraint of non-structural components early in planning can greatly reduce the cost and complexity of the project. Additionally, comprehensive drawings and details will facilitate proper and cost-effective installation in accordance with existing standards and codes.

There are also ways to simplify installation of restraints when non-structural components are in close proximity to structural components such as walls or floors. Hanger rods are typically stiffened or ‘overlapped’ with steel rods or clamps to resist uplifting or lateral forces that may result from seismic events.

Some of these materials and techniques have been effectively used to isolate vibration and control of noise generated by mechanical equipment in certain applications. These devices can also incorporate steel springs in combination with neoprene gaskets to provide effective seismic restraint.

Additionally, experience has shown installing seismic restraints after most of the components are in place can cause problems that result from interference between various components.¹[3] Planning ahead by all parties the installation of proper seismic restraints of non-structural components can save time and money; it also enables project bids to be more realistic and accurate.

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.

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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 4.1.8.18 (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.

Geographical location
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 [S_DS] factor can be found in ASCE 7-05 Equation 13.3-1, and the Sa(0.2) factor in the V_p equation of NBC Section 4.1.8.18).²[5]

Building use and occupancy
NBC Table 4.1.2.1 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:

• hospitals;
• airports;
• fire and police stations;
• power and water pumping stations;
• buildings enabling communications; and
• fuel and/or energy facilities.

There is also an Importance Factor (I_E) that is assigned to the building under both applicable codes. Post-disaster or essential facilities are given the highest importance factor (NBC Article 4.1.8.5), 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 V_p equation of Section 4.1.8.18 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.

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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[7] 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.

Role of engineers and manufacturers
There are numerous reasons why the engineer of record is being asked to be more involved in the design of seismic restraint of non-structural components. This has not always been the case, as the design of both vertical and horizontal restraint for non-structural components was traditionally delegated to the contractor. The engineer of record’s role was to provide necessary performance criteria for any required bracing, and this was handled via notes on the drawings or through project specifications.

However, owners are now recognizing the value of having the seismic restraint of non-structural components clearly designed and specified by the engineer of record in the contract documents thereby eliminating any uncertainty the contractor(s) may have on interpretation of the owner’s requirements.

Owners have typically learned this as a result of actual experience when healthcare facilities, for example, experienced severe operational problems in spite of the fact the structure itself remained standing during a seismic event.

At the same time, experience has clearly shown the cost and effort in adequately restraining this equipment is much less than the expense involved in replacement, repair, and system downtime that may result from seismic damage. While protecting occupant safety is one of the primary concerns, in many cases, owners are also aware disruption of services may cause additional damage and may result in considerable building damage or significant financial losses.

Experience has also shown improper installations of restraints can result in failure during an actual earthquake. For example, problems have occurred as a result of connection of bracing at improper locations on the structure. Seismic braces are typically diagonal members carrying axial forces to the support. Thus, there is both a horizontal and vertical component of the brace force that must be taken by the support. A common improper installation deficiency is related to attaching bracing to steel joists. Most steel joists are not designed for these vertical forces and, depending on the magnitude of the force, could fail.[8]

Due to heightened awareness and firsthand-observed problems, owners are turning to the engineer of record more than ever before to design bracing. Many engineers have developed specific procedures to assess buildings and equipment components enabling them to determine the best course of action and identify potential life/safety hazards. They have also developed plans to minimize the disruptions to normal building operation.

Manufacturers are becoming more cognizant of seismic bracing requirements, as well as some of the exemptions in the building codes. To that end, HVAC engineers have been developing innovative designs for non-structural equipment that incorporate bracing and connection details sufficiently flexible to withstand building movement without failure or displacement. With such infrastructure pre-designed, the code is much more accommodating without the need for additional seismic bracing.

Conclusion
Experience with earthquakes in Canada has demonstrated some of the country’s building infrastructure (such as non-structural components) may actually be quite vulnerable—failure or loss of functionality can occur at a time when the need is critical. It is possible to mitigate the seismic risk for these components; when restraints are properly designed and installed, negative outcomes can usually be avoided.

Typically, mechanical and electrical equipment supports were designed for gravity loads and did not take into account the horizontal and upward loading caused by earthquakes. Seismic restraints of mechanical and electrical equipment can resist seismic forces when they do occur and ensure systems remain secure. Provision of these restraints can reduce the threat to life and minimize long-term costs that result from equipment damage and associated loss of service. Building owners should have a vested interest in this—aside from the risk to public safety and employees, there are almost certainly other concerns that are not fully addressed by insurance coverage.

Notes
¹ Mechanical, electrical, and other non-structural components may be installed in areas where there may be limited space and, in some cases, the components will be located in close proximity to each other and other building systems. Installing restraints after systems are in place can make both access and restraint installation difficult and time-consuming. (back to top[9])
² The provisions in Part 4 of NBC provide guidance with regard to the structural design of buildings. Structural engineers undertake a number of calculations using the equations provided here to determine the appropriate level of seismic protection. (back to top[10])
³ Shear-wave (S-wave) velocity is highest in bedrock and lowest in soft clay and artificial fill. Seismic shaking amplification depends on the velocity at which the rock or soil transmits S-wave—shaking is stronger where the velocity is lower. (back to top[11])

Edward Kolodziejski, M. Eng., P.Eng.,is a professional structural engineers with exp Services. He has 30 years of experience in institutional, commercial, and industrial buildings, and has worked on many projects that have involved seismic restraint of non-structural components. Kolodziejski can be contacted at ed.kolodziejski@exp.com[12].

Brian Burton is the author of “Fenestration Forum,” published five times a year by Glass Canada magazine. Part of exp, he was also recently appointed to the Canadian Standards Associations (CSA) Fenestration Installation Technician Certification Program Committee. Burton can be contacted at brian.burton@exp.com[13].

Source URL: https://www.constructioncanada.net/specifying-seismic-restraints-ensuring-safety-with-non-structural-components/

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