Seismic codes for nonstructural engineering
Mechanical, electrical, and plumbing components are not always taken into account when the thought of earthquakes comes up, but proper attachment to the structure can be just as important as proper attachment of a beam or column.
While most people know that earthquakes can cause buildings, bridges, and other structures to crack, distort, and even fail, mechanical, electrical, and plumbing system failures often get overlooked. People fear earthquakes because they can be injured from falling structural elements like columns or beams or architectural components like brick facades or windows. But what about the lights overhead, the rooftop units, pipes, or storage tanks? These nonstructural components can injure people when their supports and attachments fail.
The number-one goal of a building code is to protect people. The building code that governs the majority of the United States is the International Building Code (IBC), which is published by the International Code Council (ICC). IBC Chapter 16, as well as Chapters 11-13 and 15-23 of American Society of Civil Engineers (ASCE) 7—Minimum Design Loads for Buildings and Other Structures, address seismic design. Although the main purpose of the IBC is to safeguard against major structural failures and loss of life, this does not imply that damage should be limited or the function of the building be maintained. Buildings and other structures that support the mechanical, electrical, or plumbing (MEP) components are divided into occupancy categories (IBC Table 1604.5), which are used to determine the level of seismic loads and detailing required.
Essential facilities such as hospitals, police and fire stations, power plants, or water treatment facilities are examples of higher level occupancy categories (III or IV), which can require a higher level of analysis, design, and detailing than a lower occupancy category building in the same region of the country. Essential facilities like these require immediate occupancy or continued use after an earthquake, which can require continued function of MEP components after an earthquake as well. Life safety systems such as fire sprinkler systems and essential electrical systems require seismic bracing to stay in service.
To determine the level of analysis, design, and detailing that will be required for the structural, architectural, and MEP components, the structural engineer will need to calculate the seismic design category. This calculation takes into account the location of the building near a fault, the occupancy category of the building (as previously mentioned), and the soil characteristics of the site. Seismic design categories A, B, or C are deemed low to moderate, whereas categories D, E, or F are deemed high to severe. Structures located in California, for example, will typically fall into a high to severe category, while structures located in Wisconsin will fall into low to moderate. Once the seismic design category has been determined, the analysis and design begins. The design of MEP supports and anchorages is covered in ASCE sections 13.3, 13.4, and 13.6.
Below is a partial list of some important items that should be considered and shown by MEP engineers on the construction documents. The list is culled from three very useful documents produced by the Federal Emergency Management Agency (FEMA)—412, 413, 414—as well as from the authors’ personal experience. The FEMA documents are only guides; in all instances local building codes, such as the IBC, control the design.
- The most important thing to remember when detailing and installing seismic restraints for any type of MEP equipment is to make sure building structures are capable of supporting the seismic bracing loads. This should be verified by a trained professional, such as a licensed structural engineer of record.
- The general design concept, including size of bracing and anchorage sizes, should be shown on construction documents. Ask the local building official if the bracing connections and sizes need to be stamped by a licensed engineer.
- Raceways, conduits, ducts, and cable trays: There are a number of different ways to support raceways, but different bracing systems should not be mixed. Each run of conduits or cable trays must have at least one transverse support at each end of the run and at least one longitudinal support anywhere on the run. Seismic bracing is not required per ASCE 7 if the ducts or piping are supported 12 in. on center or less or if the area of the duct is less than 6 sq ft. Figure 1 shows an example of conduit bracing for a larger diameter conduit.
- Rigid floor-mounted equipment: Do not shim, or attach directly to the steel structure or concrete structure. Do not use oversize holes. Where continuous angles are used on either side of the equipment, provide a minimum of four bolts (two to the structure and two the equipment) per side. Where angles are used at each corner, a minimum of three bolts per connection shall be used (two to the structure and one to the equipment or vice versa). Welding to the equipment or existing steel structure is an acceptable connection. Figure 2 shows examples of acceptable and unacceptable rigid floor-mounted attachment configurations.
- Talk to the structural engineer of record to understand the type of construction being used. Certain fasteners cannot be used with certain types of construction.
- Anchors: When choosing an anchor, a designer must take into account the particular application. When using anchors for safety-related applications, such as sprinkler systems, heavy suspended pipes, etc., anchors must be IBC approved and designed per the American Concrete Institute (ACI) 318 Appendix D, and should be designed by a licensed structural engineer of record. Some anchors should not be used with vibratory loads. ASCE 7 does not allow for expansion anchors to be used in mechanical equipment rated over 10 hp unless they have vibration isolators. Contact an anchor supplier to determine the proper product for your application. Approved construction documents may require special inspection or field testing per IBC Chapter 17.
- Special inspection: Special inspection may be required for certain installations. As mentioned above, the installation of anchors will need to be periodically inspected in seismic design categories C, D, E, or F. Some examples of systems that require this inspection are electrical equipment for emergency standby power; piping systems intended to carry flammable, combustible, or highly toxic content; and HVAC ductwork that will contain hazardous materials. Special inspection also will be required for isolator units and energy dissipation devices that are part of the seismic isolation system.
- Power-actuated fasteners: Used to directly fasten equipment for permanent installation, these are typically not used for equipment weighing more than 40 lbs or for equipment not allowed by code for tension applications in seismic design categories D, E, or F per ASCE 7.
- Suspended equipment: Attachment should be located just above the center of gravity to minimize swinging. It should be a rigid attachment with brackets to the equipment using double nuts and washers. Equipment weighing less than 75 lbs does not require special seismic requirements per ASCE 7. See Figure 3 for an example of suspended equipment with seismic bracing.
- Housekeeping pads must be designed for the equipment weight and the seismic loads.
- Isolators: Talk to your equipment representative about whether an isolator is required. The three main types are open, housed, and restrained. Never use housed vibration isolators for seismic applications as they cannot resist uplift. Never use open vibration isolators without snubbers or bumpers. Installation is critical when installing isolators; if an isolator is installed incorrectly, vibration will cause it to malfunction or become a noise problem. See Figure 4 for an installed example of vibration isolators. There are a number of proprietary products on the market today that help with vibration isolation. Ask your equipment representative which product would be most appropriate for your specific job.
Earthquakes are major physical occurrences that can have devastating effects on both existing infrastructure and human life. Due to advancement through research and better code development, structural engineers are better able to design buildings to withstand earthquakes and significantly reduce loss of life and destruction of property. Unfortunately, MEP components are not always taken into account when earthquakes are considered, but properly attaching the components to the structure can be just as important as properly attaching beams or columns. With careful communication and coordination with the structural engineer of record as well as a firm grasp of existing building codes, an MEP engineer can design a system that functions properly before, during, and after an earthquake.
Sanders is a structural project manager for GRAEF. He has 6 years of experience in the design of healthcare, commercial, governmental, industrial, and educational structures. Wolfe is a structural project manager and an associate for GRAEF. He has 12 years of experience in the design of commercial, industrial, and governmental structures.
American Concrete Institute: Building Code Requirements for Structural Concrete, 2005.
American Society of Civil Engineers: Minimum Design Loads for Buildings and Other Structures (ASCE 7), 2005.
Federal Emergency Management Agency: Installing Seismic Restraints for Mechanical Equipment (FEMA 412), 2004.
Federal Emergency Management Agency: Installing Seismic Restraints for Electrical Equipment (FEMA 413), 2004.
Federal Emergency Management Agency: Installing Seismic Restraints for Duct and Pipe (FEMA 414), 2004.
International Code Council: International Building Code, 2006.
Lindeberg, Michael and McMullin, Kurt: Seismic Design of Building Structures—A Professional’s Introduction to Earthquake Forces and Design Details, 2008.