3.2 Estimating Seismic Risk
- 3.2.1 Estimating Seismic Shaking Intensity
- 3.2.2 Estimating Seismic Risk Ratings
- 3.2.3 Assigning Priority Ratings
- 3.2.4 Application of Nonstructural Guidelines
There are two aspects of the estimated seismic risk for a given item:
- What is the seismic shaking intensity that can be expected at the site?
- For a given level of shaking, what is the seismic risk rating of a given nonstructural item in terms of life safety, property loss, and functional loss?
Estimating site specific seismic hazards can be a difficult technical problem, requiring many factors to be taken into account. For the purposes of this nonstructural survey, the shaking intensity is based solely on regional seismicity. For a particular geographic location in the United States, the shaking intensity may be estimated by using the map in Figure 3.2.1-1 that shows the areas that are likely to experience minimal, low, moderate, or high levels of ground shaking during future probable maximum considered earthquake events. The ground shaking has been estimated for a stiff soil site. The information in Figure 3.2.1-1 may be summarized as follows:
Figure 3.2.1-1 Map of probable shaking intensity in the United States.
The above figure (Figure 3.2.1-1) was prepared by Ken Rukstales of the USGS and is based on the Figure R301.2(2) that was developed by the USGS for the 2006 International Residential Code where the Minimal Level = SDC A, Low Level = SDC B, Moderate Level = SDC C and the High Level = SDC D. Figure R301.2(2) was based on the MCE Design Value Maps prepared by the USGS for the 2003 NEHRP Recommended Provisions and presumes that the site soil classification is Site Class D and structure has Normal Occupancy (II).
- High level of shaking: Most of California and Nevada; significant portions of Alaska, Washington, Oregon, Montana, Wyoming, Idaho, and Utah; the areas near New Madrid, Missouri and Charleston, South Carolina; small pocket areas in Arizona, New Mexico, upper New York, and upper Maine; the islands of Hawaii, Puerto Rico, and Guam (not shown).
- Moderate level of shaking: Areas adjacent to the areas of high shaking plus pocket areas in New England, New Mexico, Arizona, West Texas, Colorado, and Oklahoma.
- Low level of shaking: A portion of the western States, a significant portion of the central region of the continental United States east of the Rockies and most of New England.
- Minimal level of shaking: Remaining portions of mid-western, southern continental United States.
Shaking intensity estimates based on the probable shaking intensity map in Figure 3.2.1-1 should be adequate for evaluating components situated at or near the ground in simple, nonessential facilities. For other situations, it may be advisable to choose the next higher shaking intensity or to seek the advice of professional consultants. Note that in areas with minimal shaking, upgrade of nonstructural components generally would not be warranted unless an owner is particularly risk averse or special circumstances exist; the current code would not require many of the protective measures recommended herein, even for new construction.
One reason why the use of professional consultants is recommended for complex facilities is that the generalized shaking intensity map does not take many engineering factors into consideration (see sidebar). Clearly, the complexity and detail of engineering calculations should commensurate with the complexity and importance of the facility and the item in question. It should be noted that current design codes and standards such as the International Building Code (IBC), Minimum Design Loads for Buildings and other Structures (ASCE/SEI 7), and Seismic Rehabilitation of Existing Buildings (ASCE/SEI 41), reference detailed digitized seismic maps of the United States prepared by the U.S. Geological Survey (USGS). These maps consider locations and seismic activity of all known seismic sources and faults which may affect a given site, and the standards provide procedures for adjusting the mapped ground motions for site soil conditions. For designs requiring compliance with building code or national standard requirements, the maps referenced by the code or standard in effect at the time must be used to establish minimum criteria.
In addition, it may be appropriate to consider more than one earthquake scenario for a particular facility, since earthquakes of different magnitudes may occur at different average time intervals. For some facilities, it may be useful to evaluate more probable frequent events, such as those that are likely to occur every 100 years. While new construction projects have to anticipate the most severe shaking, others who are doing voluntary retrofits may find it more economical to plan for a smaller, more frequent event.
Estimating the earthquake forces acting on a particular item in a particular building can be a difficult technical problem. In order to perform engineering calculations, an engineer may have to consider the following factors:
- the proximity of the building site to an active fault
- soil conditions at the site (other than stiff soil)
- the flexibility of the building structure
- the location of the item in the building
- the flexibility of the floor framing or walls in the immediate vicinity of the item
- the flexibility and strength of the item and its attachments
- the weight and configuration of the item
- the characteristics of any connection details between the item and the structure
- the expected relative displacement between two connection points in adjacent stories or across a seismic gap
- the function of the item
- the function of the facility
Refer to the current International Building Code (IBC) and ASCE/SEI 7 for seismic design requirements for nonstructural components and ASCE/SEI 41 for existing construction.
The risk ratings provided in Appendix E are based on a review of damage to nonstructural components in past earthquakes and on the judgment of the authors and their advisory panel. Estimates of future earthquake damage to either the structural or nonstructural components of a building are only that—estimates—and should be used with discretion. The approximations provided in this guide are adequate for the purpose of making an initial determination of the seismic risk of the nonstructural components of a simple facility. For a facility that is more complex, or for one where the potential risk is high, more detailed analyses should be performed by an in-house engineer or a professional consultant. In this document, the seismic risks for life safety, property loss, and functional loss have been rated simply as high, medium, or low for different levels of shaking intensity. Note that these ratings refer to primary losses caused by damage to the item in question; potential consequences or secondary losses are not considered. Appendix E contains more detailed notes concerning the definitions and assumptions used in assigning risk ratings. Stated briefly:
- Life Safety risk is the risk of direct injury by the item.
- Property Loss risk is the risk of incurring a cost to repair or replace the item as a result of damage incurred.
- Functional Loss risk is the risk that the item will not function as a result of the damage incurred.
The estimated risk ratings shown in Appendix E assume that the item is unbraced and unanchored and are intended for buildings with ordinary occupancies, not for essential facilities. The primary purpose of this information is to assist in assigning priority ratings, described below, and to help in identifying the most critical hazards.
Prioritization may be based on budget constraints, risk considerations (i.e., those elements that pose the greatest risks to safety, property or function are retrofitted first), availability of unoccupied space, or to achieve the highest cost to benefit ratio.
A simplified priority rating system might be used to indicate which items are more vulnerable to earthquake damage and to indicate those items whose failure is most likely to have serious consequences. All components could be assigned a high, medium, or low priority, or each item or type of item could be ranked in order from highest to lowest. The highest priority might be assigned to those components for which all three risk ratings are high. If loss of function is not a serious concern, then the highest priority might be assigned to items for which the life safety risk is high and the upgrade cost is lowest, since these hazards could be reduced most cost-effectively. The assignment of priorities may vary widely for different types of facilities, and this document merely provides some guidelines that can be used to establish a ranking system. In assigning the rating priorities, the requirements for new construction should be considered. If it is not required for new construction, then it does not make much sense to do a seismic retrofit of that item in an existing facility.
When estimating seismic risk and assigning priority ratings, it should be noted that current building codes and seismic design standards for new construction do not require seismic design of anchorage and bracing for nonstructural components in every part of the United States.
- In areas denoted as experiencing minimal levels of seismic shaking intensity in Figure 3.2.1-1, no seismic anchorage or bracing of nonstructural components is required.
- For most buildings in areas denoted as experiencing low levels of seismic shaking intensity, only parapets are required to be braced. For essential facilities, all architectural components are required to be anchored and braced.
- In areas denoted as experiencing moderate levels of seismic shaking, all architectural components are required to anchored and braced. However, in most buildings, electrical and mechanical components and systems do not require anchorage and bracing. For essential facilities, mechanical and electrical components are required to be braced.
- In general, in areas denoted as experiencing high levels of seismic shaking intensity, all architectural, mechanical, and electrical components are required to be anchored and braced in all buildings.
In addition, current seismic codes and standards also exempt mechanical and electrical components from bracing or anchoring, regardless of seismic area, in nonessential facilities, if they weigh less than 400 pounds and are mounted at a height 4 feet or less above the floor or, if elevated, weigh less than 20 pounds. Distributed systems in nonessential facilities, such as piping or HVAC ducting, are also exempt from bracing or anchoring if they weigh less than 5 pounds per lineal foot and are provided with flexible connections.
Current seismic codes and standards do not provide much guidance on when seismic anchorage and bracing are required for contents except for cabinets and computer access floors which are treated as architectural components. The reason why they are typically not treated in standards for new construction is that furniture, fixture, equipment and contents are usually installed after the building has been approved for occupancy by the building official; thus, the building official no longer has any control over the installation after occupancy approval has been given.