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- 1. Seismic Design Considerations in Model Codes by S. K. Ghosh, Ph.D., President, S. K. Ghosh Associates, Inc.* Ground motion resulting from earthquakes presents unique challenges to the design of structures. Earthquakes produce large- magnitude forces of short duration that must be resis- ted by a structure without causing collapse and prefer- ably without significant damage to the structural ele- ments. The lateral forces due to earthquakes have a major impact on structural integri- ty. Lessons from past earth- quakes and research have provided technical solutions that will minimize loss of life and property damage associated with earthquakes. Special detailing is required, and for materials without inherent ductility, such as concrete and masonry, a critical part of the solution is to incorporate reinforcement in the design and construction to assure a ductile response to lateral forces. Code Issues Building codes establish minimum requirements for building design and construc- tion with a primary goal of assuring public safety and a secondary goal, much less important than the primary one, of minimizing property damage and maintaining function during and following an earthquake. With respect to earthquake hazards, Reinforced concrete masonry wall. the underlying issues to be considered in the development of code criteria are the levels of seismic risk and the establishment of appropri- ate design requirements commensurate with those levels of risk. Since the risk of severe seismic ground motion varies from place to place, it logically follows that seismic code provisions will vary depending on location. The variable aspect of code provisions for seis- mic design has been accentuated by the fact that local and regional building code jurisdic- tions in the United States have typically based their provisions on one of three model building codes: the Uniform Building Code (predomi- nant in the west), the Standard Building Code (predominant in the southeast), and the BOCA National Building Code (predominant in the northeast).Given the greater frequency and intensity of earthquakes in the west, it is not surprising that the Uniform Building Code (UBC) has traditionally placed more emphasis on seismic design provisions than the Standard Building Code (SBC) or the BOCA National Building Code (BOCA/NBC). However, this situation is changing. Representatives from the three model code sponsoring organizations agreed to form the International Code Council (ICC) in late 1994, and, in April 2000, the ICC published the first edition of the International Building Code (IBC). It is intended that IBC will eventually replace the previous three model codes. The IBC includes significant changes in seismic design requirements from the three existing model building codes, particularly in how the level of detailing requirements for a specific structure is determined. Evolution of Seismic Design Criteria Seismic Zones. Until relatively recently, seis- mic design criteria in building codes depended solely upon the seismic zone in which a struc- ture was located. Zones were regions in which seismic ground motion, corresponding to a certain probability of occurrence, was within Table 1. Basis for Seismic Design Criteria in Model Codes and Standards Seismic Zones Seismic Seismic Design Performance Categories Categories Classifications 0, 1, 2, 3, 4 A, B, C, D, E A, B, C, D, E, F Criteria for Location Location & Location, Building Classification Building Use Use, & Soil Type Used by UBC 1997 SBC 1999 IBC 2000 Model Codes SBC 1991 BOCA/NBC & Standards BOCA/NBC 1999 1990 MSJC 1999 MSJC 1992 Masonry Today / October 2000 3
- 2. Table 2. Determining the Seismic Design Category Step Column 2 Column 3 Consider short-period Consider long-period ground motion ground motion Determine spectral response accelerations At short period, SS At 1 second period, S1 from contour maps or (Site Class B) (Site Class B) CD-ROM Determine Site Class: • If Site Class Do site-specific design Do site-specific design (by IBC criteria) is F • If data available for shear wave velocity, standard penetration Choose from Site Class A–E Choose from Site Class A–E resistance, and undrained shear strength • If no data available Use Site Class D Use Site Class D Determine site coefficient Fa FV for acceleration or Table 1615.1.2(1) Table 1615.1.2(2) velocity Determine soil-modified spectral response SMS = Fa SS SM1 = FV S1 acceleration Calculate the design spectral response SDS = 2/3 SMS SD1 = 2/3 S M1 acceleration Determine Seismic Use SUG I, standard SUG I, standard Group (SUG) occupancy buildings occupancy buildings of structure SUG II, assembly buildings SUG II, assembly buildings SUG III, essential facilities SUG III, essential facilities Determine Seismic A, B, C, or D* as A, B, C, or D* as a Design Category a function of SUG and SDS function of SUG and SD1 from Table 1616.3(1) from Table 1616.3(2) Choose most severe SDC Compare Col. 2 with Col. 3 from previous line *It is possible for E or F to be the Seismic Design Category, per footnotes to Tables 1616.3 (1) and 1616.3 (2). certain ranges. The United States was divided into Seismic Zones 0 through 4, with 0 indicating the weakest earthquake ground motion, and 4 indicating the strongest. The level of seismic detailing (including the amount of reinforcement) for masonry structures was then indexed to the Seismic Zone. Seismic Performance Categories. However, given that public safety is a primary code objective, and that not all buildings in a seismic zone are equally crucial to public safety, a new system of classification called the Seismic Performance Category (SPC) was developed. The SPC classification included not only the seismicity at the site but also the occupancy of the structure. The SPC, rather than the Seismic Zone, became the determinant of seismic design and detailing requirements, thereby dictat- ing that seismic design requirements for a hospital be more restrictive than those for a small business structure constructed on the same site. The detailing requirements under Seismic Performance Categories A & B, C, and D & E were roughly equivalent to those for Seismic Zones 0 & 1, 2, and 3 & 4, respectively. Seismic Design Categories. The most recent development in structural classification has been the establishment of Seismic Design Categories as the determinant of seismic detail- ing requirements. Recognizing that building performance during a seismic event depends not only on the severity of sub-surface rock motion, but also on the type of soil upon which a structure is founded, the SDC is a function of location, building occupancy, and soil type. Table 1 summarizes how building codes and the Masonry Standards Joint Committee’s design standard, Building Code Requirements for Masonry Structures (ACI 530/ASCE 5/TMS 402), have addressed seismic design over the past decade. Dates list the last edition in which a given classification system was used by a model code or standard. As indicated, IBC 2000 is the first model code to use Seismic Design Categories. Impact of Changes Clearly, the procedure for establishing the seis- mic classification of a structure has become more complex. Determining the Seismic Zone simply required establishing the location of the structure on Seismic Zone maps that were con- tained in model codes. Determining the Seismic Performance Category of a structure required: 1) the interpolation of a ground motion parameter on a contour map, based on the location of the structure, 2) determining the use classification of the structure, and 3) con- sulting a table. As shown in Table 2, the process leading to the classification of the Seismic Design Category involves several steps. Site-specific soil data must be gathered to establish the site class; otherwise, the default site class is D. The classification procedure requires evaluation of SDC for a short-period and a long-period ground motion parameter. After working through the calculations, the most severe SDC determined from the two con- ditions is selected. 4 Masonry Today / October 2000
- 3. M A S O N R Y T O D A Y Upcoming Events Seminars to Address the New MSJC Code and Specification The American Concrete Institute (ACI) and the Masonry Society (TMS) are offering a series of 1-day seminars to guide you through the MSJC Code and Specification requirements in detail, highlighting changes from earlier editions of the Code and Specification. Topics include pre- stressed masonry, inspection and quality assur- ance, adhered veneer, hot weather construction, and reformatting of the code. Fall 2000 seminar dates are planned as follows: Albany, NY—October 10 Albuquerque, NM—November 1 Baltimore, MD—November 28 Cincinnati, OH—October 4 Kansas City, MO—September 27 Little Rock, AR—October 25 Oklahoma City, OK—December 11 Orlando, FL—December 6 St. Louis, MO—September 28 San Antonio, TX—December 14 To learn more about designing masonry struc- tures, attend one of the design seminars on everyday masonry problem solutions by ACI and TMS. The resource for these courses is the Masonry Designers’ Guide (MDG-2). The com- prehensive 1-1/2 day seminars focus on design techniques and solutions to common problems. Fall 2000 and Spring 2001 dates are planned as follows: Atlanta, GA—May 15–16 Charlotte, NC—December 11–12 Chicago (Northbrook), IL—May 1–2 Cincinnati, OH—June 19–20 New York, NY—November 9–10 Orlando, FL—May 25–26 Pittsburgh, PA—October 23–24 Washington, DC—November 27–28 ACI is handling reservations for both seminars listed above at 248.848.3815. Table 3. Seismic Design Category of 2000 IBC vs. Seismic Classification under Previous Codes Location Model Code Seismic IBC Site Class Zone or Category A B C D E Seismic Design Category Washington, DC 1997 SPC - A A A A B C BOCA/NBC New York, NY 1997 SPC - C B B B C D BOCA/NBC Philadelphia, PA 1997 SPC - B B B B C C BOCA/NBC Atlanta, GA 1997 SBC SPC - B A B B C D Orlando, FL 1997 SBC SPC - A A A A B B Charlotte, NC 1997 SBC SPC - C B B C D D San Francisco, CA* 1997 UBC Zone 4 D D D D ** Denver, CO 1997 UBC Zone 1 A B B B C Seattle, WA 1997 UBC Zone 3 D D D D ** * Downtown, 4th and Market Streets. ** Site specific geotechnical investigation and dynamic site response analysis must be performed. Table 3 provides a snapshot of the potential impact of the difference in seismic clas- sification of a structure under the new IBC criteria, as compared to prior editions of the BOCA/NBC, the SBC, and the UBC. This table assumes a Seismic Use Group I (standard-occupancy) classification for a structure. Note that Site Class D is the default soil classification, which must be used unless soil testing indicates that a dif- ferent classification is applicable. It is evident that the trend in regions previously under BOCA/NBC or SBC jurisdiction would be to require a higher level of detail- ing and reinforcement under the new IBC provisions unless soil testing indicates a Site Class of A, B, or C. However, regions previously under UBC jurisdiction would tend to remain unchanged in detailing requirements for the default Site Class D and would sometimes be permitted to be designed with a reduced level of detailing and reinforcement for Site Classes A, B, or C. Conclusion SDC ratings can be more or less stringent than the previous ratings. The soil condi- tion at the site is the additional variable that must now be dealt with. While this adds another element to an already complicated procedure, it does incorporate established knowledge about the effect of soil properties during an earthquake into seismic design criteria. There is an associated economic impact to these changes. When a structure is assigned to a higher Seismic Design Category under the IBC than what its Seismic Performance Category would have been under the BOCA/NBC or the SBC, more restrictive code provisions increase the cost of design, materials, and construction. * S.K. Ghosh Associates Inc., 3344 Commercial Ave., Northbrook, IL 60062 Tel: 847.291.9864 E-mail: skghosh@aol.com Masonry Today / October 2000 5