6 Chapter 3 Why Adopt a Seismic Code? 7.1 on the Richter scale and was the strongest to affect a U.S. city since the 1964Alaskan earthquake.' It shook the San Francisco Bay Area and killed sixty-three people. Two-thirds of the deaths were a result of the Cypress viaduct collapse. Although the ground-shaking was intense within the metropolitan area, few buildings collapsed. Most of the damage occurred to unreinforced masonry buildings built before the adoption of seismic codes. Nearly all major reinforced concrete structures built after World War II survived without collapse. Even at the quake's epicen- ter new buildings and buildings located on firm ground suffered little damage. Informed observers attribute the success to the required UBC seismic codes.2 This example illus- The term seismic code refers to the FIGURE 3.1 Most of the butilding damage trates that code requirements reduced seismic design requirements in- in the 1989 Loma Prieta earthquake wvas to the damage and loss of life during olderunreinforcedmasonry buildings cluded within building codes. In the this moderate earthquake. built before the adoption of seismic codes. past, local governments sometimes The 1994 Northridge, California, (Photo: Rob Olshansky) viewed the seismic sections of the earthquake shows similar evidence. model codes as optional, adopted at Almost all the buildings in the local discretion. Now seismic affected area were built during the provisions are fully integrated into past fifty years under one of the UBC all three model codes. Local govern- seismic codes. Virtually all buildings, ments should adopt the latest even in the areas of strongest shaking, version of a model code in its remained standing and allowed for entirety, including the seismic safe evacuation of occupants. Regret- sections, in order to be operating at tably, one apartment building col- the current standard. This point is lapsed on its residents, and two high- very important and is emphasized occupancy concrete-frame buildings throughout this book. collapsed, fortunately with no 3 occupants at the time. Still, these Seismic Codes Are Effective three buildings were built under an Experience with recent earthquakes older version of the UBC code, and in the United States and throughout damage and life loss would have the world shows that seismic codes been immeasurably greater without work. Cities with seismic codes the seismic-resistant construction suffer much less damage than those prevalent in the San Fernando Valley. without such codes. A Kyoto University study of the The Loma Prieta earthquake 1995 earthquake in Kobe, Japan, clearly illustrates the effectiveness of Richter magnitude 6.9, found that seismic codes. Occurring on October damage to reinforced concrete 17, 1989,this earthquake measured buildings closely paralleled improve- WhyAdoptA Seismic Code? 7 ments to seismic provisions in the Japanese building code. More than 55 percent of pre-1970 buildings (old version of code) were severely damaged, compared with no post- 1980buildings (newest version of code). Similarly, steel buildings built before 1970 sustained severe dam- age, compared with little damage in post-1981 buildings. 4 Ohbayashi Corporation studied buildings it had constructed in Kobe and found that 58 percent of pre-1971 buildings were damaged, compared with 28 percent of 1972-80buildings and only 16 percent of post-1981 build- ings. 5 In contrast, a Richter magnitude 6.9 earthquake in Armenia in 1988 destroyed entire communities and tions are as important to building FIGURE 3.2 Lessons about underlying killed 25,000people. This disaster stability as the epicenter location. In soil conditions learned in the 1985 Mexico has been attributed to several City quake can help areas built on fill, response to this new information, factors: design deficiencies; poor such as the Back Bay area of Boston shown ICBO in the 1988 and 1991 UBC quality of construction; and the above, ninitnize damage. (Photo: Greater editions has emphasized soil Boston Convention & Visitors Bureau) earthquake's intensity exceeding conditions by increasing the force 6 that anticipated by the code. Similar requirements according to the type problems exist in much of the of underlying soil. The National United States. Earthquake Hazard Reduction Programn (NEHRP)Reconiended Provisions Even smaller earthquakes can (described on page 8) have also cause extensive damage where taken into account soil conditions in buildings are not designed for the latest edition. seismic shaking. A Magnitude 5.6 earthquake in 1993 at Scotts Mills, Today's Seismic Codes Are Based Oregon, caused significant struc- on More Than Sixty Years of tural damage to a number of Earthquake Experience unreinforced masonry (brick) buildings in the area.7 A high school Seismic codes in use now reflect a building was significantly damaged long history of learning from and vacated, 16 residences and 54 earthquakes and represent the businesses sustained major damage, collective knowledge of hundreds of and the Oregon State Capitol, in design and construction profession- Salem, suffered cracking in the als. The following is a brief account rotunda. The estimated damage cost of that history. See Appendix A for a to public facilities alone was nearly more detailed account. $13 million. This earthquake con- The earliest seismic design firmed the susceptibility of provisions in the U.S. were intro- unreinforced buildings to severe duced in the appendix to the 1927 damage, even in a minor earth- Uniforn Building Code, the first quake. edition of the UBC. By the 1950s, New lessons are learned from some California municipalities had every earthquake and incorporated adopted additional seismic-resistant into U.S. seismic codes. For example, design and material specifications. the 1985 Mexico City earthquake The 1949 edition of the UBC con- confirmed that the local soil condi- tained the first national seismic hazard map. After the 1971San Chapter 3 a Fernando earthquake, revisions were a.. - l. U1 Un U - j i' . ; L " _ made to the 1973 UBC, and new requirements were introduced in the Earthquake magnitude is a measure of the absolute size of an earthquake 1976 edition. 9 so that we may compare earthquakes with one another. Generally speaking, earthquakes that release more energy Early in the 1970s the National Science Foundation (NSF) funded a * shake for a longer amount of time, project, under the guidance of the * affect a wider area, and National Bureau of Standards (NBS, now the National Institute of Stan- * produce more violent shaking near the source. dards and Technology), to evaluate Because we cannot measure the energy released by an earthquake, existing earthquake-resistant design Charles Richter in 1935 devised a substitute measure-the Richter provisions. This extensive multi-year magnitude scale. The scale is based on what a seismograph would project relied on the input of a large measure; it has no inherent meaning of its own. The Richter scale is number of seismic design experts logarithmic, and each whole number increase in the scale represents and resulted in a 1978 report by the approximately a 31.5-fold Applied Technology Council titled increase in energy release: that X V 22'3 121;30 ioros Tentative Provisionsforthe Development is, a magnitude 7 earthquake e "a', vi of Seismic RegulationsforBuildings releases about 31.5 times more . 6 (ATC 3-06). , energy than does a magnitude 6 6 earthquake. Several different Under a contract with the Federal magnitude scales are now in3 Emergency Management Agency common use, and they all v (FEMA), the Building Seismic Safety vce share basic characteristics with Council (BSSC, formed in 1979 within the Richter Scale. i the National Institute for Building Sciences, NIBS) revised ATC 3-06 by Shortly after an earthquake ~ ~~~~ 37$\0' azn% , C S a consensus of its members. In 1985 occurs, the surface wave magnitude or body wave magni- FEMA released the NEHRP Recom- tude is often reported. The mended Provisionsfor the Development 7 scale that most accurately of Seismic Regulationsfor New Build- } I AL JX~~~~~~V represents the energy of an ings, commonly called the NEHRP earthquake is the moment Provisions. Although not a code, the 0 10 20 30MILES magnitude scale. For smaller NEHRP Provisions are designed to earthquakes (less than magni- provide guidance to the writers of FIGURE 3.3 The Loma Prieta, California, tude 6), the scales are nearly building codes. FEMA and BSSC earthquakeof 1989 had a magnitude of identical, but only the moment continue to update the NEHRP 7.1, but intensities in the affected area magnitude scale can distin- rangedfrom MlMI VII to IX. (Source: Provisions every three years, with the guish differences among very USGS Circular1045,1989) latest edition being published in large earthquakes. 1994. The 1997 edition is due out in Earthquake intensity is a measure of the actual shaking experienced at December 1997. a location. The United States uses the Modified Mercalli Intensity Scale, a twelve-point qualitative scale that describes observable effects of AllThree Model Codes Contain earthquakes. For example, Intensity VIII is described, in part, as "dam- State-of-the-Art Seismic age slight in specially designed structures; considerable in ordinary Requirements substantial buildings with partial collapse; great in poorly built struc- The past two decades have seen great tures ... fall of chimneys, factory stacks, columns, monuments, walls. strides in the knowledge of building Heavy furniture overturned." Whereas magnitude is an inherent quality responses to earthquakes. Based on of an earthquake, intensity generally decreases with greater distance the collective efforts of engineers, from the earthquake's center. Intensity is a very useful measure because scientists, and tradespeople, the it describes what is most important to society-the degree of damage to NEHRP Provisions contain seismic structures built by humans. design provisions that are technically advanced and widely accepted. Since 1992 all three model codes require seismic design standards consistent with the NEHRP 9 Why AdoptA Seismic Code? EQ Modified Mercalli Intensity Scale Figure 3.4 Percentage of Buildings Expected in Each Damage State for Various Shaking Intensities: Buildings Designed for Seismic Zone 4 under the I99 I UBC Size of Earthquake Standardized Damage States (Magnitude) A B C D E 6.0-6.5 7.5-8.0 None Slight Moderate Extensive Complete Distance t o Fault 30 mi. 50 mi. 60-90% 10-40% I-5%