THE INTERNATIONAL BUILDING CODE AND ITS IMPLICATIONS

Transcription

1 4th International Conference on Earthquake Engineering Taipei, Taiwan October 12-13, 2006 Paper No. 238 THE INTERNATIONAL BUILDING CODE AND ITS IMPLICATIONS ON SEISMIC DESIGN W.S. Pong 1, Anson Lee 1 and Zu-Hsu Lee 2 ABSTRACT The study begins with a discussion of the history and development in the last few decades of building code for seismic design in the United States. Then, it focuses on a comparison of the Uniform Building Code 1997 (UBC1997) and the International Building Code 2003 (IBC 2003) as they relate to seismic design. The study compares the seismic provisions of UBC 1997 and IBC 2003 for design base shear in various seismic zones and soil types. In conducting this case study, it was generally observed that IBC 2003 takes into account more factors in deriving values for each criterion. It introduced the seismic design category that combines the occupancy or seismic use group with the soil modified seismic risk or the soil characteristics at the site of the structure. The results of this case study show that there are significant differences in some criterion. This point should not be underestimated. It would be wise to remember that building codes serve as a guide to reaching minimum standards. It is, therefore, imperative that the structural engineer professional not only keep up to date with the codes and their differences, but also be aware that what is most important is to have a good working knowledge and understanding of the fundamentals of seismic design principles. Keywords: International Building Code, Uniform Building Code, Design Base Shear, Design Response Spectrum, Importance Factor, Redundancy Factor INTRODUCTION The intent of this case study is to provide a brief evolution of the building code in the United States, concentrating on a comparison of UBC 1997 and IBC 2003 provisions regarding seismic analysis and design. In order to have a common reference for comparison, a hypothetical model of a steel SMRF building is assumed and its data is used in the application of the two codes under varioius defined circumstances. This paper focuses on a comparison of the results and thus will not enumerate equations and procedures that can be found in Chapter 16 of UBC 1997 or Chapter 16 of IBC The reader can refer to the project paper by Lee (Lee 2005) for detailed calculations and derivations of the numerical results. HISTORY Prior to the year 2000, cities and counties across the United States adopted building codes on a regional basis. Breyer (Breyer 2003) explains that local governments then used one of the 3 regional model codes, namely: Uniform Building Code (UBC), the BOCA National Building Code, or the Standard Building Code. In 1994, the International Code Council (ICC) was created to develop a 1 School of Engineering, San Francisco State University, 1600 Holloway Avenue, San Francisco, CA 94132, USA, , (fax), wspong@sfsu.edu 2 Dept. of Management & Information Systems, Montclair State University, Upper Montclair, NJ 07043, USA

2 single comprehensive code without regional limitations. The ICC unified the 3 model codes and produced the International Building Code (IBC) with IBC 2000 as its first publication in the year 2000 (Breyer 2003). The latest version of the IBC is currently being considered for adoption by the State of California to replace UBC It should be noted that there are significant differences between UBC 1997 and IBC 2003 on seismic provisions. The UBC 1997 was based on the Structural Engineer s Association of California s (SEAOC) recommended guidelines for Lateral Force Requirements, more popularly known as the Blue Book (Dowty 2000). On the other hand, IBC 2000 and IBC 2003 are based on the Federal Emergency Management Agency s (FEMA) National Earthquake Hazards Reduction Program (NEHRP) Recommended Provisions for the Development of Seismic Regulations for New Buildings. The IBC frequently references the American Society of Civil Engineers (ASCE) publication ASCE 7-02 for technical provisions (Dowty 2000). CASE STUDY BUILDING This case study assumes a hypothetical five-story steel special moment resisting frame building, with a dimension of 150 feet by 240 feet. The story height for the first floor will be 18 feet and all other upper floors will have a story height of 15 feet. The longer side of building will have 12 bays, each 20 feet wide. The shorter side will have 7 bays with the two exterior bays having a width of 25 feet and all the other interior bays having a width of 20 feet. Only the exterior frames will resist lateral forces. Members for the frames will be selected from wide flange sections of the American Institute of Steel Construction, Inc. (AISC). All of the joints at the base are assumed to be fixed. The building will be studied in four different circumstances, namely: As an office building located at 1600 Holloway Avenue, San Francisco, CA As an essential facility building (hospital) located at 1600 Holloway Avenue, San Francisco, CA As an office building located at 1209 L St, Sacramento, CA As an essential facility building (hospital) located at 1209 L St, Sacramento, CA Soil site conditions are assumed to be unknown in all of the circumstance mentioned above. Figure 1. The Shorter Side of the Building Frame. IMPORTANCE FACTOR Although IBC 2003 revised its table format for an importance factor using a subscript E for seismic application, it gives the value of I E =1, which is the same as that used in UBC 1997 for office buildings. IBC 2003, however, has a higher importance factor of I E =1.5 for essential facilities such as the hospital in our case study, as compared to I=1.25 for UBC 1997.

3 STRUCTURAL PERIOD IBC 2003 gives a structural period of seconds, which is 1% less than the UBC 1997 structural period of seconds. This can be accounted for as the result of the different formulas and parameters used. However, the difference in this case study is insignificant. DESIGN BASE SHEAR IBC 2003 s significant difference from UBC 1997 is its derivation of Design Ground Motion Parameters. It introduced several different parameters that are not in UBC It is, therefore, difficult to directly correlate the parameters used in the two codes. However, by superimposing their graphs, differences between the two codes can easily be seen. The difference in the response spectrum gives a direct clue as to how it affects the design base shear. Figs. 2 and 3 below superimpose the design response spectrum for the two codes and show its variation in relation to the different circumstances for our case study. The two arrows indicate the structural period for each of the code. Figure 2 shows that the design response spectrum for the San Francisco circumstance did not vary significantly between the two codes, while Figure 3 shows the IBC 2003 design response spectrum to be lower than that of UBC 1997 for the Sacramento circumstance. Soil Type D Spectral Acceleration (g) UBC97 IBC2003 Period (seconds) Figure 2. Design Response Spectrum for the San Francisco Circumstance. Soil Type D Spectral Acceleration (g) Period (seconds) UBC97 IBC2003 Figure 3. Design Response Spectrum for the Sacramento circumstance. To investigate the variation in the design base shear between the two codes as it relates to different soil types and UBC 1997 zones, locations for the UBC 1997 Zones 2B, 2A and 1 were added to our list of circumstances mentioned in the case study above. These added locations are used only for comparison of the design base shear and not for the other topics of comparison. The different locations are shown together with the other design base shear factors of each design code in Tables 1 and 2.

4 Only the results for design base shear with the occupancy category of office will be shown, since the occupancy category for essential facility (hospital) will only differ by its importance factor. Tables 1 and 2 shows the factors for the derivation for design base shear for soil type D, while Figure 4 shows graphically the difference in design base shear between the two codes among the different locations with soil type D as an example. Table 1. Design Base Shear Factors for IBC Design Code IBC 2003 Zone (UBC97) 4 3 2B 2A 1 Location San Francisco Sacramento Spokane Cambridge Raleigh State CA CA WA MA NC Zip Code Occupancy Office Importance Factor 1 Soil Type S D / D W (total, kips) S S S F a F v S MS =F a *S S S M1 =F v *S S DS =2/3*S MS S D1 =2/3*S M Seismic Design Category E D C C C T S =S D1 /S DS (sec) T a (sec) R Cs Max Cs Min Cs Min Cs Category E, F N/A N/A N/A N/A Controlling Cs Cs Max Max Max Max V (kips)

5 Table 2. Design Base Shear Factors for UBC Design Code UBC 1997 Zone (UBC97) 4 3 2B 2A 1 Location San Francisco Sacramento Spokane Cambridge Raleigh State CA CA WA MA NC Zip Code Occupancy Office Importance Factor 1 Soil Type S D / D W (total, kips) Z Na 1.35 N/A N/A N/A N/A Nv 1.8 N/A N/A N/A N/A Ca Cv R Ts (sec) Ta (sec) Cs Max Cs Min Cs Min Cs Zone N/A N/A N/A N/A Controlling Cs V (kips) Design Base Shear (kips) 3, , , , , San Francisco Design Base Shear (Soil Type D) Sacramento Spokane Cambridge Raleigh Locations (UBC Zones 4 to 1) IBC 2003 UBC 1997 Figure 4. Design Base Shear for the Different Locations for Soil Type D. Using our case study building, Table 3 below shows the percentage difference of IBC 2003 s design base shear to that of UBC 1997, as calculated for the different combinations of location and soil type, while Figure 5 shows a graphic presentation of the design base shear ratio of IBC 2003 over UBC

6 1997. Table 3 and Fig. 5 illustrate the difference in design base shear between the two codes for the five different locations. Table 3. Tabulated Difference of IBC 2003 Design Base Shear from UBC 1997 Design Base Shear. Zone 4 3 2B 2A 1 City San Francisco Sacramento Spokane Cambridge Raleigh State CA CA WA MA NC Soil Type A 15% -48% -67% -50% -9% Soil Type B 15% -48% -65% -50% -15% Soil Type C 7% -45% -65% -57% -11% Soil Type D 3% -44% -61% -52% -9% Soil Type E -4% -41% -64% -56% -8% Design Base Shear Ratio (various soil types) Ratio IBC2003/UBC San Francisco Sacramento Spokane Cambridge Raleigh A B C D E equal Locations (UBC Zones 4 to 1) Figure 5. Design Base Shear Ratio (IBC 2003/UBC 1997) for Various Soil Types. Summary of Results by Zones: For Zone 4, San Francisco, IBC 2003 generally shows a slightly higher value than UBC 1997 except for soil type E. For Zone 3, Sacramento, IBC 2003 generally shows a lower value than UBC 1997 For Zone 2B, Spokane, IBC 2003 generally shows a lower value than UBC For Zone 2A, Cambridge, IBC 2003 generally shows a lower value than UBC For Zone 1, Raleigh, IBC 2003 generally shows a slightly lower value than UBC The results presented above show that IBC 2003 values are not far from UBC 1997 values for Zones 4 and Zones 1. However, they also show that IBC 2003 values are significantly lower than UBC 1997 for Zones 3, 2B and 2A. For our case study of an office building in San Francisco, IBC 2003 and UBC 1997 design base shear value show little difference. IBC 2003 is slightly higher by 3%. However, for our case study of an office building in Sacramento, the IBC 2003 design base shear is 44% lower than under UBC VERTICAL DISTRIBUTION OF BASE SHEAR Only the results for vertical distribution of design base shear with the occupancy category of office will be shown as an example, since the values for an essential facility (hospital) occupancy category will only differ by its importance factor. For San Francisco, Figure 6 below shows that IBC 2003 s

7 distribution at the roof level is lower than the 5th floor level when compared to the roof and 5th floor level for the UBC 1997, which are almost of equal value. This is mainly due to UBC 1997 s added value of F t at the roof level. For Sacramento, the vertical distribution shows the same pattern as in San Francisco, with IBC 2003 s vertical distribution of design base shear generally being less than UBC 1997, primarily because of its lower Design Base Shear. Vertical Distribution of Base Shear (Office) Lateral Force (kips) 1, Roof 5th 4th 3rd 2nd Floor Level UBC97 SF IBC2003 SF UBC97 Sacramento IBC2003 Sacramento Figure 6. Chart for the Vertical Distribution of Design Base Shear. DRIFT RATIO The different case study circumstances were also compared by their drift ratios. The drift ratio is defined in Eq. 1 as, Drift Ratio = Story Drift / Drift Limit (1) where Story Drift is the height between two story levels and the Drift Limit is the allowable story drift for the code being used. For the office buildings in our case study, Table 4 shows that IBC 2003 results in drift ratios that are slightly lower than under UBC 1997 for the San Francisco area and are generally lower than under UBC 1997 for the Sacramento area (mainly due to the lower design base shear derived using IBC 2003 for the Sacramento area). Both codes have the same drift limit of 2% of the story height for office buildings. The office buildings at the two locations pass the drift ratio criterion for both codes giving drift ratios that are lower than 1. However, for the hospital buildings in the case study, the results vary. For the hospital building in San Francisco, Table 5 shows that IBC 2003 results in drift ratios that are generally higher than under UBC For the IBC 2003 case, most of the floor level drift ratios exceed 1, making it fail for that criterion. This failure is mainly due to the lower drift limit of 1% of the story height that IBC 2003 imposes on hospital buildings (essential facility). The hospital building in Sacramento passes this criterion for IBC 2003 despite the more stringent drift limit, mainly due to its lower design base shear. Table 4. Drift Ratios for Office Buildings. UBC 97 SF IBC2003 SF UBC 97 SAC IBC2003 SAC Roof th th rd nd Remarks All pass All pass All pass All pass

8 Table 5. Drift Ratios for Hospital Buildings. UBC 97 SF IBC2003 SF UBC 97 SAC IBC2003 SAC Roof th th rd nd Remarks All pass Failed All pass All pass REDUNDANCY FACTOR (Ρ) AND Ρ LIMITS For this case study, IBC 2003 s redundancy factor requirement proves to be more stringent than UBC 1997 in two ways: 1. It requires that ρ be computed for the entire structure at all levels in both directions, while UBC 1997 only requires ρ to be computed for the lower two thirds of the structure. 2. For SMRF buildings with seismic design category E, IBC 2003 requires ρ not to exceed 1.1, unlike UBC 1997, which only requires ρ not to exceed 1.25 as a general requirement for all SMRF buildings. Table 6 shows that the San Francisco office building does not pass the redundancy factor requirement for IBC 2003, mainly due to its being categorized under seismic design category E, which imposes a ρ limit of 1.1. The Sacramento office building passes this criterion for IBC 2003 under seismic design category D, which imposes a ρ limit of Table 7 shows the case study for hospital buildings. The end result is noted as Remarks. These results are the same as those for the office buildings, since the seismic design category used for IBC 2003 are consistent with that of the office buildings. Table 6. Redundancy Factor for Office Buildings. UBC1997 IBC2003 UBC1997 IBC2003 SF SF Sacramento Sacramento Limit <1.25 <1.1 <1.25 <1.25 Design Category E D 5 th Not reqd Not reqd th Not reqd Not reqd rd nd st Remarks Ok Failed Ok Ok For IBC2003, the soil type can impact the redundancy factor limit since it influences the seismic design category of the building. The redundancy factor limit for UBC1997 is only dependent on the building type regardless of location or occupancy type. The redundancy factor limit for UBC1997 in our case study is consistently 1.25, since our buildings are categorized as SMRF.

9 Table 7. Redundancy Factor for Hospital Buildings. UBC1997 IBC2003 UBC1997 IBC2003 SF SF Sacramento Sacramento Limit <1.25 <1.1 <1.25 <1.25 Design Category E D Roof Not reqd Not reqd th Not reqd Not reqd th rd nd Remarks Ok Failed Ok Ok CONCLUSIONS This case study provides valuable insights into UBC 1997 and the new IBC 2003 regarding seismic design and analysis of SMRF steel buildings. The comparison of numerical results provided in this study support the conclusion that the differences between UBC 1997 and IBC 2003 are not just superficial for some criteria. In fact, this study has shown that in some instances, buildings modeled and designed using the UBC 1997 code do not pass the IBC 2003 standard regarding the criterion of redundancy factor ρ. This means that the modeled building needs to increase its number of bays or the number of frames that resist lateral load in order to pass the IBC 2003 redundancy factor standard. This study has also shown that for essential facilities such as a hospital, buildings designed using UBC 1997 may in some cases not pass IBC 2003 s stringent requirement for drift limits. This study also shows that there could be significant difference in the seismic design base shear between the two codes depending on the location and occupancy use of the building. In addition, it should be noted that the vertical distribution of the design base shear is different between the two codes especially in the last two upper levels. In conducting this case study, it was observed that generally, IBC 2003 takes into account more factors in deriving values for each criterion. It introduced the seismic design category that combines the occupancy or seismic use group with the soil modified seismic risk or the soil characteristics at the site of the structure. For example, IBC 2003 s restrictions on building heights and structural irregularity, and choice of analysis procedure and level of detailing required, are dependent on seismic design category; whereas UBC 1997 uses only Zone categories for these items. In addition, IBC 2003, Figure 1615 (1-10), which are the contour maps for the 0.2 and 1-second spectral response acceleration, are more detailed when compared to the seismic zone map of UBC 1997, Figure There are still many differences between the two codes that are beyond the scope of this case study. This paper s findings and applications are also limited to its hypothetical case circumstances. In conclusion, the results of this case study have shown that there are significant differences in some criterion and it highlighted the importance of knowing those. This point should not be underestimated. It would be wise to remember that the building codes serve as a guide reaching minimum standards. It is, therefore, imperative that the structural engineer professional not only keep up to date with the codes and their differences, but also be aware that what is most important is to have a good working knowledge and understanding of the fundamentals of seismic design principles.

10 REFERENCES Breyer, Donald E., Kenneth J. Fridley, David G. Pollock, and Kelly E. Cobeen (2003). Design of Wood Structures-ASD, McGraw-Hill, San Francisco, CA. California Department of Conservation Division of Mines and Geology (1998). Maps of Known Active Fault Near-Source Zones in California and Adjacent Portions of Nevada. Dowty, Susan, Philip J. Samblanet, Roger Sharpe, S.K. Ghosh, John R. Henry, Jon Siu and Hank Martin (2000). UBC-IBC Structural Comparison and Cross Reference ( ). International Conference of Building Officials Publication Department, Whittier, CA. FEMA 450-CD-2003 Edition/June 2004 International Conference of Building Officials (1997) Uniform Building Code Volume 2, International Conference of Building Officials Publication Department, Whittier, CA. International Code Council, Inc. (2002) International Building Code, International Code Council, Inc., Country Club Hills, IL. Lee, Anson, Comparison of UBC 1997 and IBC 2003 on Seismic Design of Steel Buildings (SMRF), Research Project, San Francisco State University, San Francisco.