the international building code and its implications on seismic design

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. Pong1, Anson Lee1 and Zu-Hsu Lee2 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 2003. 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, 415-3387738, 415-338-0525 (fax), [email protected] 2 Dept. of Management & Information Systems, Montclair State University, Upper Montclair, NJ 07043, USA

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 1997. 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 94132. As an essential facility building (hospital) located at 1600 Holloway Avenue, San Francisco, CA 94132. As an office building located at 1209 L St, Sacramento, CA 95814. As an essential facility building (hospital) located at 1209 L St, Sacramento, CA 95814. 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 IE=1, which is the same as that used in UBC 1997 for office buildings. IBC 2003, however, has a higher importance factor of IE=1.5 for essential facilities such as the hospital in our case study, as compared to I=1.25 for UBC 1997.

STRUCTURAL PERIOD IBC 2003 gives a structural period of 0.914 seconds, which is 1% less than the UBC 1997 structural period of 0.919 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 1997. 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.

2.0000 1.5000

UBC97

1.0000

IBC2003

0.5000 0.0000 0 0. 39 1 0. 69 1 0. 95 3 1. 25 3 1. 55 3 1. 85 3

Spectral Acceleration (g)

Soil Type D

Period (seconds)

Figure 2. Design Response Spectrum for the San Francisco Circumstance.

1.0000 0.8000 0.6000 0.4000 0.2000 0.0000

UBC97 IBC2003

0 0. 31 3 0. 56 3 0. 86 3 1. 16 3 1. 46 3

Spectral Acceleration (g)

Soil Type D

Period (seconds)

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.

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 2003.

Design Code Zone (UBC97) Location State Zip Code Occupancy Importance Factor

4 3 San Francisco Sacramento CA CA 94132 95814

Soil Type W (total, kips)

IBC 2003 2B 2A 1 Spokane Cambridge Raleigh WA MA NC 99224 02141 27614 Office 1 SD / D 16846.20

SS

1.828

0.563

0.309

0.328

0.201

S1

1.161

0.218

0.092

0.089

0.096

Fa

1.00

1.35

1.55

1.54

1.60

Fv

1.50

1.96

2.40

2.40

2.40

SMS=Fa*SS

1.828

0.760

0.479

0.505

0.322

SM1=Fv*S1

1.742

0.427

0.221

0.214

0.230

SDS=2/3*SMS

1.219

0.507

0.319

0.337

0.214

SD1=2/3*SM1 Seismic Design Category

1.161 E

0.285 D

0.147 C

0.142 C

0.154 C

TS=SD1/SDS (sec)

0.953

0.562

0.461

0.423

0.716

0.914 8 0.152 0.159 0.054 0.073 Cs 2566.24

0.914 8 0.063 0.039 0.022 N/A Max 657.00

0.914 8 0.040 0.020 0.014 N/A Max 339.14

0.914 8 0.042 0.019 0.015 N/A Max 328.08

0.914 8 0.027 0.021 0.009 N/A Max 353.88

Ta (sec) R Cs Max Cs Min Cs Min Cs Category E, F Controlling Cs V (kips)

Table 2. Design Base Shear Factors for UBC 1997.

4 3 San Francisco Sacramento CA CA 94132 95814

Soil Type W (total, kips) Z Na Nv Ca Cv R Ts (sec) Ta (sec) Cs Max Cs Min Cs Min Cs Zone4 Controlling Cs V (kips)

0.3 N/A N/A 0.36 0.54 8.5 0.600 0.919 0.069 0.106 0.040 N/A 0.069 1162.39

SD / D 16846.20 0.2 N/A N/A 0.28 0.4 8.5 0.571 0.919 0.051 0.082 0.031 N/A 0.051 862.64

0.15 N/A N/A 0.22 0.32 8.5 0.582 0.919 0.041 0.065 0.024 N/A 0.041 690.11

0.075 N/A N/A 0.12 0.18 8.5 0.600 0.919 0.023 0.035 0.013 N/A 0.023 388.19

Design Base Shear (Soil Type D) 3,000.00 2,500.00 2,000.00 1,500.00 1,000.00 500.00 0.00

IBC 2003

ig h

Fr an ci sc o Sa cr am en to Sp ok an e C am br id ge

UBC 1997

Sa n

Design Base Shear (kips)

0.4 1.35 1.8 0.594 1.152 8.5 0.776 0.919 0.148 0.175 0.065 0.068 0.148 2493.24

UBC 1997 2B 2A 1 Spokane Cambridge Raleigh WA MA NC 99224 02141 27614 Office 1

R al e

Design Code Zone (UBC97) Location State Zip Code Occupancy Importance Factor

Locations (UBC Zones 4 to 1)

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

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 City State Soil Type A Soil Type B Soil Type C Soil Type D Soil Type E

4 San Francisco CA 15% 15% 7% 3% -4%

3 Sacramento CA -48% -48% -45% -44% -41%

2B Spokane WA -67% -65% -65% -61% -64%

2A Cambridge MA -50% -50% -57% -52% -56%

1 Raleigh NC -9% -15% -11% -9% -8%

A B C D E equal R al ei gh

ge C am br id

e Sp ok an

en to Sa cr am

Fr an ci sc o

1.40 1.20 1.00 0.80 0.60 0.40 0.20 0.00

Sa n

Ratio IBC2003/UBC 1997

Design Base Shear Ratio (various soil types)

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 1997. • For Zone 2A, Cambridge, IBC 2003 generally shows a lower value than UBC 1997. • For Zone 1, Raleigh, IBC 2003 generally shows a slightly lower value than UBC 1997. 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 1997. 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

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 Ft 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.

Lateral Force (kips)

Vertical Distribution of Base Shear (Office) 1,000.00 800.00

UBC97 SF

600.00

IBC2003 SF

400.00

UBC97 Sacramento

200.00

IBC2003 Sacramento

0.00 Roof

5th

4th

3rd

2nd

Floor Level

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 1997. 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 Roof 0.583 0.444 0.478 th 5 0.982 0.953 0.912 4th 0.859 0.847 0.875 rd 3 0.968 0.939 0.994 nd 2 0.944 0.891 0.982 Remarks All pass All pass All pass

IBC2003 SAC 0.196 0.488 0.471 0.528 0.510 All pass

Table 5. Drift Ratios for Hospital Buildings.

UBC 97 SF IBC2003 SF UBC 97 SAC Roof 0.563 0.662 0.494 5th 0.998 1.556 0.942 4th 0.867 1.374 0.900 rd 3 0.992 1.540 1.000 nd 2 0.973 1.469 0.940 Remarks All pass Failed All pass

IBC2003 SAC 0.323 0.809 0.780 0.851 0.784 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 1.25. 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.

Limit Design Category 5th 4th 3rd 2nd 1st Remarks

UBC1997 IBC2003 UBC1997 IBC2003 SF SF Sacramento Sacramento