EVOLUTION OF UBC AND IBC STATIC LATERAL FORCE PROCEDURE

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1 EVOLUTION OF UBC AND IBC STATIC LATERAL FORCE PROCEDURE Introduction: A model building code is a document containing standardized building requirements applicable throughout the United States. Model building codes set up minimum requirements for building design and construction with a primary goal of assuring public safety, and a secondary goal of minimizing property damage and maintaining function during and following an earthquake. Since the risk of severe seismic ground motion varies from place to place, seismic code provisions vary depending on location. The three model building codes in the United States were: the Uniform Building Code (predominant in the west), the Standard Building Code (predominant in the southeast), and the BOCA National Building Code (predominant in the northeast), were initiated between 1927 and The US Uniform Building Code was the most widely used seismic code in the world, with its last edition published in Up to the year 2000, seismic design in the United States has been based on one these three model building codes. Representatives from the three model codes formed the International Code Council (ICC) in 1994, and in April 2000, the ICC published the first edition of the International Building Code, IBC In 2003, 2006, 2009 and 2012 the second, third, fourth and fifth editions of the IBC followed suit. The IBC was intended to, and has been replacing the three independent codes throughout the United States. Initiation of the Static Lateral Force Procedure: The work done after the 1908 Reggio-Messina Earthquake in Sicily by a committee of nine practicing engineers and five engineering professors appointed by the Italian government may be the origin of the equivalent static lateral force method, in which a seismic coefficient is applied to the mass of the structure, or various coefficients at different levels, to produce the lateral force that is approximately equivalent in effect to the dynamic loading of the expected earthquake. The Japanese engineer Toshikata Sano independently developed in 1915 the idea of a lateral design force V proportional to the building s weight W. This relationship can be written as F = C W, where C is a lateral force coefficient, expressed as some percentage of gravity. The first official implementation of Sano s criterion was the specification C = 10 percent of gravity, issued as a part of the 1924 Japanese Urban Building Law Enforcement Regulations in response to the destruction caused by the great

2 Kanto earthquake. In California, the Santa Barbara earthquake of 1925 motivated several communities to adopt codes with C as high as 20 percent of gravity. Evolution of the Equivalent Static Lateral Force Method: The equivalent lateral seismic force on a structures V was firstly taken as a percentage of the building weight, as stated above. Secondly it was based on the seismic zone factor, building period, building weight and system type. Thirdly, it was based on site specific ground motion maps, building period, importance factors, soil site factors and building response modification factors, as shown in Table (1). The first edition of the U.S. Uniform Building Code (UBC) was published in 1927 by the Pacific Coast building Officials (PCBO), contained an optional seismic appendix, also adopted Sano s criterion, allowing for variations in C depending on the region and foundation material. For building foundations on soft soil in earthquake-prone regions, the UBC s optional provisions corresponded to a lateral force coefficient equal to the Japanese value. For buildings on hard ground, the lateral force coefficient is 7.5 percent. While not the most advanced analytical technique, the equivalent static lateral force analysis method has been and will remain for some considerable time the most often used lateral force analysis method. The 1937 UBC stipulated a lateral force coefficient, which is dependent on soil conditions, applied not only to dead loads but also to 50 % of the live load. The 1943 UBC introduced a lateral force coefficient in terms of number of stories and limited this number to 13. In subsequent code editions the equation was modified for number of stories in excess of 13. UBC 1949 edition contained the first USA seismic hazard map, which was published in 1948 by US Coast and Geodetic Survey and was adopted in 1949 by UBC, as well as subsequent editions until The seismic design provisions remained in an appendix to the UBC until the publication of the 1961 UBC. The 1961 UBC Code introduced the use of four factors to categorize building system types. The 1970 UBC used a zoning map which divided the United States into four zones numbered 0 through 3. The 1973 UBC contained many modern enhancements including the V = ZKCW equation for seismic design, which was revised in the aftermath of San Fernando earthquake. Also, UBC 1973 introduced the impact of irregular parameters in estimating the seismic force levels. The concept of soil factor was first acknowledged by recognizing the importance of local site effects in the 1976 edition of UBC. In addition to this, UBC 1976 Added zone 4 to California, and included new seismic provisions especially those related to 63

3 the importance of local site effects. The lateral force structural factor, Rw was increased to take advantage of ductility of lateral force resisting systems. The 1985 UBC used a Z factor that was roughly indicative of the peak acceleration on rock corresponding to a 475-year return period earthquake. The 1988 UBC introduced the use of twenty- nine response modification factors plus three additional for inverted pendulum systems. Also, the base shear equation was changed from the 1985 UBC edition, and six seismic risk zones 0, 1, 2a, 2b, 3 and 4 are used. Until 1997 edition of UBC, seismic provisions have been based on allowable stress design. In UBC 1997 revised base shear and based it on ultimate strength design. Added to this, a new set of seismic-zone dependent soil profile categories S A through S F, has been adopted and replaced the four site coefficients S 1 to S 4 of the UBC 1994, which are independent of the level of ground shaking. Also, old R w factor has been replaced by a new R factor, which is based on strength design, and two new structural system classifications were introduced: cantilevered column systems and shear wall-frame interaction systems. Moreover, the 1997 edition of UBC included a reliability factor for redundant lateral force systems, and the earthquake load (E) is a function of both the horizontal and vertical components of the ground motion. In response to an appeal for more unified design procedures across regional boundaries, the International Building Code was developed and the first edition introduced in Subsequent IBC code editions were introduced in 2003, 2006 and The 2000 IBC has established the concept of Seismic Design Category (SDC), which is based on the location, the building use and the soil type, as the determinant for seismic detailing requirement. One of the most significant improvements in the 2000 IBC over the 1997 UBC is the ground parameters used for seismic design. In 2000 IBC, the 1997 UBC seismic zones were replaced by contour maps giving MCE spectral response accelerations at short period and 1-second for class B soil. The IBC Code versions 2000, 2003, 2006, 2009 reference to ASCE 7-05, contain up-to-date seismic provisions, including eighty-three building system response modification factors. The 2006 IBC and 2009 IBC reference ASCE 7-05 for virtually all of its seismic design requirements. 64

4 Table (1): Development of seismic base shear formulas based on UBC and IBC codes UBC/IBC Code Editions Lateral Force Specific Notes UBC UBC 1946 F = C W - Seismic design provisions included in an appendix, for optional use. - C, which is dependent on soil bearing capacity, is in % of weight. UBC UBC 1958 F = C W - C is dependent on number of stories. - First USA seismic hazard map included. UBC UBC 1973 V = Z K C W - Seismic design provisions moved to the main body of the code. - Seismic zones introduced. - Lateral force system structural factors included. - Fundamental period of vibration included. UBC UBC 1979 UBC UBC 1985 UBC UBC 1994 UBC 1997 IBC IBC-2012 V = Z I K C S W - Seismic zone 4 introduced. - Soil profiles introduced. - Building importance factors included. V = Z I K C S W - Soil profiles expanded. V = Z I C W / Rw - Soil profiles expanded. - Seismic zones modified. V = Cv I W / RT - Soil profiles expanded, and dependent on soil dynamics. - System redundancy factor introduced. - Additional structural systems introduced. - Vertical component of ground shaking included. - Seismic provisions are based on strength-level design. V = C s W - Spectral accelerations introduced. - Safety concept redefined. - Seismic design categories, SDC introduced. - System response modification factors expanded. 65

5 Seismic Code Provisions Are Based on Earthquake Historical Data: The equations used to determine Seismic Design Forces throughout the United States as well as the rest of the world are based on historical data that has been collected during past earthquakes. The 1925 Santa Barbara earthquake led to the first introduction of simple Newtonian concepts in the 1927 Uniform Building Code. As the level of knowledge and data collected increases, these equations are modified to better represent these forces. In response to the 1985 Mexico City earthquake, a fourth soil profile type, S 4, for very deep soft soils was added to the 1988 UBC, with the factor S 4 equal to 2.0. The heavily instrumented San Francisco (1989-Loma Prieta) and Las Angeles (1994- Northridge) earthquakes increased this knowledge dramatically. The 1994 Northridge Earthquake resulted in addition of near-fault factor to base shear equation, and prohibition on highly irregular structures in near fault regions. Also, redundancy factor added to design forces. The 1997 UBC incorporated a number of important lessons learned from the 1994 Northridge and the 1995 Kobe earthquake, where four site coefficients use in the earlier 1994 UBC has been extended to six soil profiles, which are determined by shear wave velocity, standard penetration test, and undrained shear strength. Safety Concepts: Structures designed in accordance with the UBC provisions should generally be able to: 1. Resist minor earthquakes without damage. 2. Resist moderate earthquakes without structural damage, but possibly some nonstructural damage. 3. Resist major earthquakes without collapse, but possibly some structural and nonstructural damage. The code is intended to safeguard against major failures and loss of life; the protection of property is not its purpose. While it is believed that the code provides reasonably for protection of life, even that cannot be completely assured. The UBC intended that structures be designed for life-safety in the event of an earthquake with a 10-percent probability of being exceeded in 50 years (475-year return period). The IBC intends design for collapse prevention in a much larger earthquake, with a 2-percent probability of being exceeded in 50 years (2,475-year return period). 66

6 Detailing Requirements of ACI : Based on R of ACI , for UBC 1991 through 1997, Seismic Zones 0 and 1 are classified as classified as zones of low seismic risk. Thus, provisions of chapters 1 through 19 and chapter 22 are considered sufficient for structures located in these zones. Seismic Zone 2 is classified as a zone of moderate seismic risk, and zones 3 and 4 are classified as zones of high seismic risk. Structures located in these zones are to be detailed as per chapter 21 of ACI Code. For Seismic Design Categories A and B of IBC 2000 through 2006, detailing is done according to provisions of chapters 1 through 19 and chapter 22 of ACI Seismic Design Categories C, D, E and F are detailed as per the provisions of chapter