13 48 00: Sounds, Vibration, and Seismic Control
Last updated: 06-03-2009
The January 1994 earthquake in the Northridge area of Southern California was one of the most destructive experienced in the United States. Thousands of buildings sustained damage including approximately 600 hospital and health care facilities. Much of the damage was to non-structural components including mechanical, electrical, plumbing, and sprinkler systems. Many buildings which did not sustain severe structural damage were rendered unusable because of the damage to non-structural components. The failure of piping and duct systems in particular caused extensive damage by flooding and the loss of necessary services including fire protection. These experiences brought about an increased awareness and enforcement of the requirements for seismic bracing for mechanical systems.
Conceptually, these requirements are straightforward. In order for piping, ductwork and electrical conduits to remain functional after an earthquake, they must be capable of resisting seismic forces through the strength of their attachments to the building. The intent is that these components move with the building and not break away during an earthquake.
Requirements and Design for Seismic Bracing
This section examines seismic bracing guidelines for mechanical/HVAC systems. Specifically not included in this discussion is the bracing of sprinkler system piping as this topic is thoroughly developed in another industry standard.
The requirements for seismic bracing can generally be categorized under two levels of earthquake safety. The first is when the failure of the component poses no hazards itself but the hazard exists if the supports or attachments of the component fail and the movement or failing of the component could pose a hazard to persons in the area. Piping located in ceiling or corridor spaces, in mechanical equipment rooms or in other areas are examples which may be designated as paths of egress.
In the second level, the failure of the component itself poses a hazard. This may include the failure of a piping or duct system which could result in a release of toxic or explosive substances or where a failure could be functional to a requirement that the system remain operational after a seismic event. Examples include fire protection piping, duct systems used in smoke management/control systems, uninterruptible power supplies, heating and cooling supply systems, and piping systems containing medical and life support gases in hospitals.
Mechanical and electrical systems are defined as "non-structural" components and the codes contain specific sections for these components. The technical requirements provide a method to calculate the expected forces to which components may be subject.
The form of the force equation varies among the various codes but the various factors reflect essentially the same information. The codes contain maps and tables where these factors are found and incorporate factors, including distances to known seismic faults, soil conditions and location of the braced components within the building.
The basic premise in the bracing of piping or duct systems is to secure those systems to the building structure such that any movement of the system is in concert with the structure. While piping and duct systems are generally rugged and perform well when subjected to shaking motions, high deflections and movements must be restrained in moderate to severe seismic events. Providing rigidity and secure attachments to the structure are effective in limiting damage to those systems.
A seismic restraint for a piping or duct system has two major considerations; the design of the restraint components and the location of the restraints in the system being braced.
Restraint System Design
A properly designed restraint consists of three components:
- The attachment of the mechanical system to the restraint. The system must be positively attached to the restraint and it must transfer the imposed forces to the restraint.
- The restraint itself must also be capable of carrying the imposed forces and transferring those forces to the structure.
- The attachment of the restraint to the structure is the most critical and the most costly element of the seismic restraint system. A properly designed and installed attachment is essential to providing the rigidity and functionality of the seismic restraint system.
The restraint systems can be divided into two general categories, rigid—metal angles and struts—and non-rigid—generally referred to as cable, and the type of restraint system to be used is generally the decision of the installer. Mixing of rigid and cable braces is not permitted in a system run which is defined as any change in direction except as allowed by offsets.
Cable braces allow more jobsite installation flexibility; however, in a cable bracing system twice as many connections to the structure are required for each brace. Nevertheless, many experienced contractors use cable braces because of the ease of fabrication and the ability to adapt to jobsite conditions. Changes in length requirements are easily accommodated by adjusting the cable length and tension at the end connections. Rigid braces have the economy of requiring fewer attachments to the structure; however, exact sizing and bolt hole alignment can pose problems.
Attachment To The System Being Braced
The attachment of the brace to the pipe or duct system is straightforward. Bent plates with holes drilled to connect to a clevis hanger or trapeze and sized to connect to the bracing member are the most common and least expensive. Hinged connectors, which are pre-drilled for rigid brace connection or which have cable attachment hardware, are available from several manufacturers. While these connectors are more expensive than bent plates, the added installation flexibility can often justify the increased costs.
Attachment to the Structure
Connections generally consist of a connecting element appropriately fastened to the structure. In concrete, structural angles attached with expansion anchors are common. The number of concrete anchors required is a function of the forces anticipated to be experienced in an earthquake and the weight of the element supported. The critical elements, the concrete anchors, are sized for shear and pullout strength with appropriate factors of safety. The types of anchors to be used can vary with the type of concrete and should be reviewed with the structural engineer and the authority having jurisdiction. Some local authorities restrict the types of anchors which may be used. Examples of one-, two-, and four-bolt connections into concrete are illustrated in Figure 1.
Connections into structural steel members can be made by direct bolting, beam clamps, and welded connection tabs. An example is shown in Figure 2.
The combination of these three elements properly selected and installed has proven to be effective in reducing or eliminating earthquake damage to piping and duct systems.
The placement of the seismic braces is as important as the design of the bracing components. The restraints are of two types: Transverse Braces - those designed and installed to restrain movement in the direction perpendicular to the piping or duct run and Longitudinal Braces - those designed and installed to restrain movement in the direction parallel to the pipe or duct run.
The spacing of the transverse and longitudinal braces is determined by structural analysis and is larger for smaller and lighter pipes and ducts. As systems become larger and heavier, the spacing decreases accordingly. Similarly, as the severity of the earthquake threat becomes lower in various locations of the country, the spacing of the restraints is correspondingly greater. Two general rules apply: Each piping or duct run must have a transverse brace at each end of the run and each run must have at least one longitudinal brace.
The spacing of braces within a specific run indicated in commercially available application guides generally varies from 80 ft. (24.4 m) to 10 ft. (3 m).
Under certain conditions, a transverse brace located near a 90-degree turn in the run may act as a longitudinal brace for the adjacent run.
Other considerations come into play when installing seismic braces for piping and duct systems. Some of these are:
A. Piping Systems
- Certain smaller size pipes (1" diameter and smaller) need not be braced.
- Bracing requirements are dependent on the location of the pipes within the building.
- Certain pipes, regardless of size need not be braced if they are located close to the building structure
- Where pipes pass through building seismic or expansion joints, the pipe joints must be capable of accommodating seismic displacements.
- Where rigidly supported pipes are connected to equipment with vibration isolation, those connections must be capable of accommodating seismic displacements. Conversely, when smaller unsupported pipes are connected to rigidly supported equipment (i.e., coils, etc.); those joints must be capable of accommodating movement of the pipes.
- Rigid piping systems may not be braced to dissimilar parts of the building or to dissimilar building systems which may respond differently during an earthquake.
B. Duct Systems
- Brace all ducts with a cross sectional area of 6 square feet or larger.
Exception: No bracing is required if the duct is suspended by hangers 12 inches or less in length as measured from the top of the duct to the supporting structure. Hangers must be positively attached to the duct within two inches of the top of the duct.
- Groups of ducts may be combined in a larger frame. The brace is sized on the combination of the duct weight. A minimum of two sides of each duct must be attached to the frame.
- Under certain conditions, walls that have ducts running through them may replace a transverse brace.
Due to insurance premium incentives, some owners of commercial facilities are requiring seismic bracing equal to or exceeding code requirements even though local jurisdictions might not impose or enforce bracing requirements. Examples are certain high technology manufacturing facilities in the semi-conductor industry and insurance companies whose mandate for office facilities is to remain operational after a seismic event.
How It Happens
The three model codes contain earthquake design requirements which address seismic bracing of piping and duct systems. To the extent that those requirements are adopted and enforced by local jurisdictions will determine if and to what extent seismic bracing for those systems is required. It is the responsibility of the building designer to determine what provisions for earthquake protection will be incorporated into the building design. These requirements can generally be found in the project general conditions, the structural specifications and on structural plans. Seldom, however, do the structural engineers include specific requirements or details for the seismic bracing of non-structural components such as piping, ductwork, or HVAC equipment.
Mechanical/HVAC designers are generally not equipped to provide specific design and detail requirements for the seismic bracing of HVAC systems. The mechanical designer may indicate the use of one or more of the industry guidelines addressing the subject. Or, as is often the case, the choice is left up to the installing contractor. In some instances, when the developer of the guideline also markets the structural elements, they may provide design and application assistance to contractors along with material takeoff and pricing. Other guidelines which are based on nonproprietary products require that the contractor properly apply the information in the guidelines and generate their own material takeoff and pricing. It should be noted that while the structural members and attachment components are similar in the available industry guidelines, the components are not interchangeable between guidelines unless product equivalence is demonstrated to and approved by the jurisdictional authority. In any case, both the engineer and contractor should confirm prior to bidding and construction which industry application guidelines are acceptable to the owner and to the authority having jurisdiction.
Trade Associations and Other Organizations
Sheet Metal and Air Conditioning Contractors' National Association—Located in headquarters outside Washington, D.C., the Sheet Metal and Air Conditioning Contractors' National Association (SMACNA), an international association of union contractors, has 1,965 members in 99 chapters throughout the United States, Canada, Australia and Brazil.
The voluntary technical standards and manuals developed by SMACNA Contractors have found worldwide acceptance by the construction community, as well as foreign government agencies. ANSI, the American National Standards Institute, has accredited SMACNA as a standards-setting organization. SMACNA does not seek to enforce its standards or provide accreditation for compliance. SMACNA standards and manuals address all facets of the sheet metal industry, from duct construction and installation to air pollution control, from energy recovery to roofing.
Sheet Metal Workers' International Association—SMWIA, the Sheet Metal Workers' International Association, represents 150,000 skilled crafts persons in the unionized sheet metal industry throughout the United States and Canada.
American Society of Heating, Refrigerating, and Air Conditioning Engineers—ASHRAE, the American Society of Heating, Refrigerating and Air Conditioning Engineers, seeks to advance the arts and sciences of heating, ventilation, air conditioning, refrigeration and related human factors to serve the evolving needs of the public and its ASHRAE members.
- AISC Specifications for the Design of Cold-Formed Steel Structural Members by the American Iron and Steel Institute. Washington, D.C., 1989.
- ASCE 7-02 Minimum Design Loads for Buildings and Other Structures by the American Society of Civil Engineers. Washington, D.C., 2003.
- ASHRAE HANDBOOK - Heating, Ventilating and Air Conditioning Systems and Equipment, Chapter 54 by the American Society of Heating, Refrigeration and Air Conditioning Engineers, Inc. Atlanta, GA., 2003.
- SMACNA Seismic Restraint Manual - Guidelines for Mechanical Systems by the Sheet Metal and Air Conditioning Contractors' National Association, Inc. Chantilly, VA, 1998.
- Seismic Restraint Guideline by Mason Industries, Inc. Hauppauge, NY, 2002.
- National Uniform Seismic Installation Guidelines by Nusig, Inc. Alamo, CA, 1997.
- International Building Code. 2003.
- NFPA 5000 Building Construction and Safety Code. 2003.
- Title 24 California Code of Regulations, by the California Office of Statewide Health Planning and Development. Sacramento, CA, 2002.
- NEHRP (National Earthquake Hazards Reduction Program) Provisions for Seismic Regulations for New Buildings, Part 1 Provisions, Part 2 Commentary prepared by the Building Seismic Safety Council for the Federal Emergency Management Agency. Washington, D.C., 2000.
- AISC Manual of Steel Construction - Allowable Stress Design, Ninth Edition by the American Institute of Steel Construction. Chicago, IL, 1989.