Vol.10, No.1 EARTHQUAKE ENGINEERING AND ENGINEERING VIBRATION Earthq Eng & Eng Vib (2011) 10: 99-113 March, 2011 DOI: 10.1007/s11803-011-0050-8 A comparison of IBC with 1997 UBC for modal response spectrum analysis in standard-occupancy buildings Tariq M. Nahhas† Umm Al-Qura University, Makkah, Saudi Arabia Abstract: This paper presents a comparison of the seismic forces generated from a Modal Response Spectrum Analysis (MRSA) by applying the provisions of two building codes, the 1997 Uniform Building Code (UBC) and the 2000-2009 International Building Code (IBC), to the most common ordinary residential buildings of standard occupancy. Considering IBC as the state of the art benchmark code, the primary concern is the safety of buildings designed using the UBC as compared to those designed using the IBC. A sample of four buildings with different layouts and heights was used for this comparison. Each of these buildings was assumed to be located at four different geographical sample locations arbitrarily selected to represent various earthquake zones on a seismic map of the USA, and was subjected to code-compliant response spectrum analyses for all sample locations and for five different soil types at each location. Response spectrum analysis was performed using the ETABS software package. For all the cases investigated, the UBC was found to be significantly more conservative than the IBC. The UBC design response spectra have higher spectral accelerations, and as a result, the response spectrum analysis provided a much higher base shear and moment in the structural members as compared to the IBC. The conclusion is that ordinary office and residential buildings designed using UBC 1997 are considered to be overdesigned, and therefore they are quite safe even according to the IBC provisions. Keywords: response spectrum analysis; seismic forces; multi-story buildings; seismic design; building codes; IBC; UBC 1 Introduction1 The 1997 Uniform Building Code (International Conference on Building Codes, 1997) was the first building code that included seismic design provisions that were significantly based on seismic data collected in the early 1990’s. This code is usually referred to as “1997 UBC” and is called “UBC” in this paper. In this code, the design response spectrum to be used for a Modal Response Spectrum Analysis (MRSA) was based on factors such as soil profile and seismic zone based on fault proximity. UBC was adopted in the USA and became the basis of the seismic provisions of the national codes of several developing countries for the seismic design of buildings. The International Building Code, first released in 2000 (International Code Council Inc., 2000), was developed as a collective effort of various independent code bodies in the USA. The International Building Code was meant to replace UBC and all other independent and legacy codes within the USA and to provide guidelines for codes in other Correspondence to: Tariq M. Nahhas, Civil Engineering Department, College of Engineering, Umm Al-Qura University, PO Box 16222, Makkah, Saudi Arabia Tel: 00966505525354 E-mail: tmnahhas@uqu.edu.sa † Associate Professor Received May 19, 2010; Accepted November 8, 2010 countries throughout the world. In various parts of the world including the USA, the IBC has replaced the UBC and is considered to be a benchmark code. The first revision to this code was released in 2003 (International Code Council Inc., 2003), the next in 2006 (International Code Council Inc., 2006) and the latest revised and enhanced version was released in 2009 (International Code Council Inc., 2009). This code referred to as “IBC” in this paper is scheduled to remain in a revision cycle with a new release every three years. Seismic design provisions of the IBC that are significantly different from the UBC and all the previous building codes are based in large part on the recommended provisions for seismic regulations for new buildings and other structures by the 1997 NEHRP “National Earthquake Hazards Reduction Program” (Building Seismic Safety Council, 2004). The ground motion maps of the 1997 NEHRP provisions adopted by IBC are based on the 1996 US Geological Service ground motion maps, which are quite different from 1991 NEHRP provisions used in the previous building codes. One of the most significant changes in the new maps is the use of the Maximum Considered Earthquake (MCE) ground motions to develop design response spectrum. The MCE ground motions are typically defined as the maximum level of earthquake ground shaking that is considered reasonable for typical structures to resist. The basic approach is to provide an approximately 100 EARTHQUAKE ENGINEERING AND ENGINEERING VIBRATION uniform margin against collapse in all regions of the United States. There are several publications and web pages describing the differences between UBC and IBC. The reader may refer to Kaplan AEC Engineering Inc (2000) and Ghosh and Khuntia (1999) and other similar publications. The UBC provides a complete procedure and set of formulae for response spectrum analysis contained in its provisions for seismic design, whereas IBC (2006 IBC and 2009 IBC) refers to ASCE7-05 (American Society of Civil Engineers, 2006). IBC, in accordance with ASCE 7-05, requires that in the ranges of a very long period, the design response spectrum be modified based on work developed by Newmark and Hall (1982). For this reason, separate maps to find the “Long Period Transition Period” called TL have been introduced. However, this branch of the design response spectrum will only affect the response spectrum analysis for buildings with very long fundamental time periods. Other than this, the design response spectrum for the 2003 IBC is identical to the 2006 IBC and 2009 IBCs. Despite the fact that IBC is the prevailing code in the USA, the UBC is still in wide use in developing countries. Also, a large number of existing buildings were designed to satisfy the provisions of UBC. Several countries that have adopted UBC provisions in their national building codes require comparative studies of UBC with IBC. Such comparative studies are necessary to satisfy the engineers and decision makers to switch over to using the IBC guidelines. Comparative studies have been published earlier (Ghosh and Khuntia, 1999; Adem and Ramazan, 2006; Pong et al., 2006 a, b; Pong et al., 2007). Among them, a comparative study (Adem and Ramazan, 2006) between the Turkish Earthquake Code and the Eurocode 8 with UBC based on MRSA using the finite element analysis method for structural analysis was carried out. This is the only publication that compared two codes using MRSA. In Adem’s comparative study, the IBC response spectra were mentioned as being different from UBC and others, but no MRSA data comparing IBC and UBC were presented. The comparative study by Ghosh and Khuntia (1999) is the first publication to compare IBC and UBC. However, the study was based on the Equivalent Lateral Force Procedure (ELFP), and not on MRSA. Another comparative study (Pong et al., 2006 a, b) deals with the issue of comparing IBC and UBC. It is the most important paper on this issue and has important findings. It reports mixed findings for two selected sites in San Francisco and Sacramento. The results presented in Pong’s study were also obtained using ELFP instead of MRSA. This comparative study concluded that UBC is more conservative in some cases, however, the results presented are not conclusive enough for structural designers and officials in various countries who are evaluating the two codes to determine whether to switch over to IBC or not and whether existing buildings designed using UBC are safe or not. Another comparison by Pong et al. (2007) did not specifically deal with UBC Vol.10 and IBC, and instead compared IBC with Mexico’s code and thus is not as relevant to the subject of this paper. The present paper investigates the forces generated in the structure through MRSA according to the provisions of the two codes using finite element analysis to obtain the internal forces rather than ELFP. Thus, this work is different from the previous publications, and fills a gap in the published research on this issue. By investigating MRSA, the paper addresses the trend in structural design of using FEM software packages. In IBC, MRSA is allowed for all cases whereas ELFP is not acceptable in some cases. This paper focuses on ordinary residential buildings of Occupancy Category 2, because they are common throughout the world in much larger numbers than buildings in other categories such as hospitals and school buildings, and almost 100% of the population in developing countries reside in such buildings. In order to carry out a comparative study, a sample of four buildings with different layouts and heights is considered. These buildings are assumed to be located at each of four arbitrary sample locations shown on a map of the USA. The sample locations were chosen to be from the USA spectral acceleration as given in the IBC because maps of other countries are still being developed and data are not readily available. The selected sample locations represent different seismic activities. Since the sample locations were selected on the map, they do not relate to any particular buildings at these locations. For these sample locations, the parameters SS and S1 are taken from the spectral acceleration map of the USA as given in the 2009 IBC. Design response spectra are generated for the four sample points for five site classes giving twenty cases. Thus, twenty response spectra are generated for UBC and another twenty for IBC. For each case, response spectrum analysis is performed for all four buildings representing a total of 160 analyses. A comparison of the magnitudes of base shear and maximum moment at the base of the columns used for column and foundation design for each case is presented. The effect of scaling required by UBC and IBC when the internal forces are based on MRSA and the effect of considering the buildings as hospital buildings is also presented. The results clearly indicate which code provides higher values of internal forces. Since the buildings considered in this paper are not tall with very long fundamental time periods, the discussion and results presented herein apply equally to 2006 and 2009 versions of the IBC. All provisions of the IBC used in this research are the same for both versions of IBC. 2 Modeling and analysis The research presented in this paper required a 3D finite element modeling of multi-story building structures. The software package ETABS (Extended Threedimensional Analysis of Building Systems) (Computers & Structures Inc., 2008) was used for this purpose. No.1 Tariq M. Nahhas: A comparison of IBC with 1997 UBC for modal response spectrum analysis in standard-occupancy buildings The 3D building structures used in this research were modeled as special moment resisting frame systems, which are a requirement of building codes in higher seismic zones and is permitted for all zones. Slabs were modeled using shell elements to represent the real slab behavior, providing stiffness in all directions and transfer mass of slab to beams. A rigid diaphragm was assumed at all floor levels. The modal combination method used for all models was the CQC (complete quadratic combination), which was preferred over SRSS (square root of sum of squares) because the structural models of the sample buildings are all three-dimensional with the possibility of closely spaced modes. It is well known that for structures with closely spaced modes, CQC results are generally much more accurate (Gupta, 1992). Actually, for all the buildings used in this research, the internal forces obtained using CQC were verified to be about the same as SRSS. This is because the closely spaced modes for these structures have very small or negligible modal mass participation. In any case, the use of a modal combination method other than CQC will not affect the comparative results because both IBC and UBC design spectra are applied to a given structure using the same modal combination and the results only indicate the effect of the difference between the design response spectra. 3 Sample locations Four sample locations were selected rather arbitrarily. These locations are located on the IBC map of “maximum considered earthquake accelerations” as shown in Fig. 1, while the same geographical locations 101 are located on the UBC Seismic Zones map as shown in Fig. 2. The exact position of these locations in terms of latitude and longitude are given in Table 1, which also shows the UBC zones and IBC spectral accelerations for short and long periods. Sample Locations 1, 2, 3 and 4 are randomly selected points in UBC Zone 2A, Zone 2B, Zone 3 and Zone 4, respectively. Note that these locations cover four seismic zones but are arbitrary and do not represent any specific buildings in these locations. 4 Design spectra cases For each sample location described in the previous section, all five IBC/UBC site classes representing different soil types are considered. This amounts to 20 different cases for each building. A summary of these twenty cases is shown in Table 2. The first column in this table has the location ID and refers to the sample locations described in the previous section. For all twenty cases, the table shows the UBC zone and the seismic coefficients Ca and Cv as well as the IBC parameters SS, S1 and TL. For each case shown in Table 2, the design response spectrum is generated for the UBC as well as the IBC. Each of these design response spectra is applied to the four different multi-story building structures as described in the next section. Figures 3(a) and 3(b) show a comparison between IBC and UBC design response spectra for all 20 cases, where the darker line indicates UBC and the lighter line indicates IBC. It presents a good comparison of IBC vs. UBC design response spectra. For the selected sample points, it is obvious from the figures that the upper curve is due to the UBC Fig. 1 Sample locations shown on IBC 2009 spectral acceleration map 102 EARTHQUAKE ENGINEERING AND ENGINEERING VIBRATION Vol.10 Fig. 2 Sample locations shown on UBC 1997 seismic zoning map Table 1 Sample geographical locations data Geographical location ID 1 2 3 4 Location UBC 1997 Latitude Longitude 34º 37º 43º 34º 98º 114º 123º 119º 30’ 30’ 00’ 00’ 00’ 30’ 00’ 00’ IBC 2006 Zone Ss % 2A 2B 3 4 40 70 100 200 Table 2 Design spectra cases LOC ID 1 2 3 4 Site class A B C D E A B C D E A B C D E A B C D E Zone 2A 2A 2A 2A 2A 2B 2B 2B 2B 2B 3 3 3 3 3 4 4 4 4 4 UBC 1997 Ca 0.12 0.15 0.18 0.22 0.30 0.16 0.20 0.24 0.28 0.34 0.24 0.30 0.33 0.36 0.36 0.48 0.60 0.60 0.66 0.54 Cv 0.12 0.15 0.25 0.32 0.50 0.16 0.20 0.32 0.40 0.64 0.24 0.30 0.45 0.54 0.84 0.64 0.80 1.12 1.28 1.92 Ss 0.4 0.4 0.4 0.4 0.4 0.7 0.7 0.7 0.7 0.7 1 1 1 1 1 2 2 2 2 2 IBC 2006 S1 0.095 0.095 0.095 0.095 0.095 0.15 0.15 0.15 0.15 0.15 0.4 0.4 0.4 0.4 0.4 1 1 1 1 1 TL 12 12 12 12 12 6 6 6 6 6 16 16 16 16 16 8 8 8 8 8 S1 % 9.5 15 40 100 No.1 Tariq M. Nahhas: A comparison of IBC with 1997 UBC for modal response spectrum analysis in standard-occupancy buildings 0.35 0.30 0.25 Case 1: Location ID: 1 Site class: A 0.20 0.15 0.10 0.05 0 0 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 00 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 1 2 3 4 5 6 7 8 9 10 2 3 4 5 0.4 Case 3: Location ID: 1 Site class: C 6 7 8 9 10 Case 4: Location ID: 1 Site class: D 0.3 0.2 0.1 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 0 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0 0 0 0.5 0.6 0.4 2 3 4 5 6 1 2 3 4 5 6 0.5 Case 7: Location ID: 2 Site class: B 0.3 1 8 9 10 7 8 9 10 Case 8: Location ID: 2 Site class: C 0.4 0.3 0.2 7 Case 6: Location ID: 2 Site class: A 0.7 0.2 0.1 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 00 1 0.5 0.6 0 0 Case 2: Location ID: 1 Site class: B 0.6 Case 5: Location ID: 1 Site class: E 0 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 00 103 0.1 1 2 3 4 5 6 7 8 9 10 00 10 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 Case 9: Location ID: 2 Site class: D 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 10 Case 10: Location ID: 2 Site class: E 1 2 Fig. 3(a) Response spectra cases (Cases 1 to 10) 3 4 5 6 7 8 9 10 104 EARTHQUAKE ENGINEERING AND ENGINEERING VIBRATION 10 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 00 10 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 00 0.7 0.6 0.5 Case 11: Location ID: 3 Site class: A 0.4 0.3 0.2 0.1 0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 2 3 4 5 6 7 8 9 Case 13: Location ID: 3 Site class: C 0 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 00 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0 1 1 2 3 4 5 6 7 8 9 Case 12: Location ID: 3 Site class: B 1 2 3 4 5 6 7 8 9 10 Case 14: Location ID: 3 Site class: D 1 2 3 4 5 6 7 8 9 10 1.4 1.2 1.0 Case 15: Location ID: 3 Site class: E Case 16: Location ID: 4 Site class: A 0.8 0.6 0.4 0.2 1 2 3 4 5 6 7 8 9 10 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 0 10 10 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0 0 Case 17: Location ID: 4 Site class: B 1 0 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0 0 Case 19: Location ID: 4 Site class: D 0 Vol.10 1 2 3 4 5 6 7 8 9 10 Case 18: Location ID: 4 Site class: C 1 2 3 4 5 6 7 8 9 10 Case 20: Location ID: 4 Site class: E 1 2 Fig. 3(b) Response spectra cases (Cases 11 to 20) 3 4 5 6 7 8 9 10 No.1 Tariq M. Nahhas: A comparison of IBC with 1997 UBC for modal response spectrum analysis in standard-occupancy buildings and the lower is due to the IBC, which clearly indicates that the design response spectra represent more severe seismic loads for the UBC. The building structures for each of the four buildings are described as follows. The basic structural data concerning all four buildings including the section sizes are given in Table 4. Note that the effect of the detailed design of an individual building is not an issue in this comparative study. Even if the building structures had been designed with dimensions and cross-sections entirely different from those assumed in this study, it will not affect the comparative results presented in this paper. This is because for a given building, two different design response spectra are applied and then the results are compared. The results show only the difference due to the design response spectra. The actual magnitude of internal forces is not the same for two buildings designed differently but the comparative effect does not alter the results. This point has been proven through testing one of the buildings (Building Structure A) as described below. 5 Sample buildings A sample of four buildings, ranging from four to six floors, is used in this study. These buildings are assumed to be of Occupancy Category II as defined in IBC2009, and Occupancy Category 4 in UBC97, and situated at the four sample locations described in Section 3. Hospital, schools and other critical buildings are excluded. Instead, it includes ordinary residential and office buildings, which according to this occupancy category, have an importance factor equal to 1 in both codes. This occupancy category represents the seismic design based on structural properties and the structural response to seismic design response spectra. In contrast, the other occupancy categories represent seismic design requirements with extra precautions for public safety and are not based on structural considerations alone. All four buildings are assumed to have a special moment resisting frame system. For such frames, the IBC response modification factor (R) is 8.0 and the UBC response modification factor is 8.5. Concrete compressive strength of 42 MPa and modulus of elasticity equal to 30459.4813 MPa was assumed with reinforcing steel having a yield strength of 420 MPa. The fundamental time periods of the four buildings, as shown in Table 3, were obtained by the modal analysis using ETABS software and were also calculated using ELFP. Building Structure A Building Structure A is shown in Fig. 4. This building has greater academic interest and it was used to perform most of the experimentation using ETABS to gain insight on the effect of various parameters and to verify the results using hand calculations. It is a 4story reinforced concrete building with all columns and beams of the same cross-section of 250 mm × 400 mm. The earthquake excitation direction is along the Y-axis. Its fundamental mode of vibration was found to have the time period of 0.779 s. This time period is higher than the ELFP fundamental time period shown in Table 3 (UBC: 0.585, IBC: 0.565). Note that IBC does not allow Table 3 Fundamental time periods Building A UBC (ELFP) 0.585 IBC (ELFP) 0.565 FEM (ETABS) B 0.585 0.565 0.761 C 0.557 0.533 0.558 D 0.639 0.628 0.954 0.779 Table 4 Data for building sample building structures Data item No. of floors Story height (m) Building A 4 Building B 4 Building C 5 Building D 6 4 4 3 3 Beams section (mm) 250 × 400 B1: 200 × 500; B2: 200 × 400 B1: 200 × 750; B2: 200 × 500 Column section (mm) 250 × 450 C1: 200 × 500; C2: 200 × 400 200 × 500 175 B1: 200 × 800; B2: 200 × 600 B3: 200 × 400; B4: 120 × 500 C1: 200 × 600; C2: 200 × 500 C3: 200 × 400 175 175 175 Concrete Concrete Concrete Concrete Y- direction X- direction X- direction X- direction Slab thickness (mm) Material Excitation direction 105 106 EARTHQUAKE ENGINEERING AND ENGINEERING VIBRATION 6000 Beam 250×450 6000 Beam 250×450 2 Beam 250×450 Beam 250×450 Beam 250×450 4000 Beam 250×450 1 C B A Vol.10 Beam 250×450 Y Z X Unit: mm (a) Plan view (b) Three-dimensional model Fig. 4 Building A the beams and columns. The cross section set (columns = 450 mm × 450 mm and beams = 200 mm × 600 mm) was used. For this test, geographical location 3 was assumed. The results are given in Fig. 5 showing the Building A - Sample point 1 (Base shears in Y-direction) 1.4 Base shear (102kN) the use of ELFP for calculating the base shear and other internal forces for buildings in Seismic Design Category D and above if the modal fundamental time period of a structure calculated by FEM is larger than that by ELFP. In such cases, methods like the modal response spectrum analysis, linear response history, nonlinear static procedure or nonlinear response history analysis must be used (Ghosh et al., 2009). In such cases, the most commonly used method is the response spectrum analysis method (ASCE 7-05 Section 162), which is also allowed by IBC and has been adopted for all cases in this paper. Using this method, the time periods and modal participating mass ratios are calculated using FEM as shown in Table 5 for each of the 10 modes. Since more than 98% mass participation occurs in the direction of earthquake excitation, the number of modes considered is sufficient. Since this structure was used to verify several aspects of the problem, the effect of variation in structural design on the comparative results was also investigated. For this purpose, a test was performed with Building A designed in two different ways. One design was as described above and the other design had different dimensions of IBC UBC 1.2 1.0 0.8 0.6 0.4 0.2 0 A B C D E A B C Period(s) 1 2 3 4 5 6 7 8 9 10 0.778839 0.718533 0.531021 0.25301 0.228186 0.168369 0.148673 0.129074 0.1105 0.09514 E Fig. 5 Comparison of base shear for two different designs of Building A Table 5 Modal periods & participating mass for Building Structure A Mode D Sample Point 3 (Cross Sections Set 1) Sample Point 3 (Cross Sections Set 2) Individual participation (%) Along X Along Y Along Z 85.5258 0 0 0 84.0825 0 0 0 0 10.1768 0 0 0 11.1355 0 0 0 0 3.3889 0 0 0 3.7229 0 0.9085 0 0 0 0 0 Cumulative participation (%) Along X Along Y Along Z 85.5258 0 0 85.5258 84.0825 0 85.5258 84.0825 0 95.7026 84.0825 0 95.7026 95.218 0 95.7026 95.218 0 99.0915 95.218 0 99.0915 98.9409 0 100 98.9409 0 100 98.9409 0 No.1 Tariq M. Nahhas: A comparison of IBC with 1997 UBC for modal response spectrum analysis in standard-occupancy buildings base shear comparisons. In Fig. 5, Cross-section Set 1 refers to the original design of Building A and Crosssection Set 2 refers to the second design. Similar results are obtained for the maximum moment comparison. This clearly proves the point that though the structural response itself differs and depends on the structural design, the structural design will not have any significant effect on the comparative results. Building Structure B The floor plan of Building Structure B is shown in Fig. 6. It is a four-story concrete building. Table 6 shows the time periods and modal participating mass ratios for each of the first ten modes. The fundamental time period of this structure is 0.761 s. Again for this building, the fundamental time period is higher than the ELFP time period given in Table 3 (UBC: 0.585, IBC: 0.565) and the same comments made in the discussion of Building Structure A concerning the analysis method are applicable here. The first ten modes considered for the modal analysis give 99% mass participation in the direction of excitation of the earthquake. B4 C2 B4 B4 (a) Plan The floor plan of Building Structure D is shown in Fig. 8. It is a six story concrete building. Table 8 shows the time periods and modal participating mass ratios for each of the first ten modes. The fundamental time period of this structure is 0.945 s and more than 99% mass participation occurs in the direction of excitation of the earthquake. As with the other buildings, the fundamental time period of 0.954 s calculated by modal analysis using B2 B3 C1 B2 B3 B3 C2 B3 Building Structure D B3 C1 B2 B2 C2 The floor plan of Building Structure C is shown in Fig. 7. It is a five story concrete building. Table 7 shows the time periods and modal participating mass ratios for each of the first ten modes. The fundamental time period of this structure is 0.558 s. For this building, the fundamental time period is also higher (though very slightly) than the ELFP time period given in Table 3 (UBC: 0.557, IBC: 0.533). The ten modes considered make the mass participation greater than 98% in the direction of excitation of the earthquake. C1 C1 B3 B2 C2 B3 C2 B4 B3 C1 B3 B3 C3 B4 B3 B4 4000 C2 B3 C2 C3 B4 B3 C2 Building Structure C F 4800 B4 B3 B4 C1 B3 C1 E 2500 B3 C2 C2 B4 B3 B1 4000 B3 B3 B1 1500 D C2 C3 B3 B1 1500 4 B4 C3 B3 4000 5 C2 B2 B4 B4 C1 4000 B4 B3 C2 B4 2 C 5500 B4 B3 C1 4000 3 B 5500 B4 B2 1 B3 A C2 C2 B3 C1 Y Z X B4 Unit: mm (b) Three-dimensional model Fig. 6 Building B Table 6 Modal periods & participating mass for Building Structure B Mode Period (s) 1 2 3 4 5 6 7 8 9 10 0.776387 0.761277 0.652396 0.251660 0.247979 0.211274 0.146275 0.144426 0.122442 0.106966 107 Individual participation (%) Along X Along Y Along Z 85.1174 0.0498 0 0.0670 74.8168 0 0.0031 10.3425 0 10.2617 0.0115 0 0.0149 9.0406 0 0.0004 1.2828 0 3.5142 0.0049 0 0.0064 3.0557 0 0.0002 0.4072 0 1.0134 0.0011 0 Cumulative participation (%) Along X Along Y Along Z 85.1174 0.0498 0 85.1843 74.8665 0 85.1874 85.2091 0 95.4491 85.2206 0 95.4640 94.2612 0 95.4644 95.5440 0 98.9786 95.5490 0 98.9849 98.6046 0 98.9851 99.0118 0 99.9985 99.0129 0 108 EARTHQUAKE ENGINEERING AND ENGINEERING VIBRATION F C 500 C1 B1 C1 B1 C1 B1 C2 B1 C2 B1 B1 C2 B1 C2 B1 C1 5 B1 B1 C1 B1 C1 C2 B1 C2 B1 C1 C2 B1 C2 C2 B2 C2 2500 C1 B2 B1 B1 B1 B2 4 C1 B2 B1 1500 B1 B1 3000 3 C1 3000 B1 C1 2000 B2 B1 2 B1 B2 2500 H G 500 4000 B1 2000 B1 3000 1 E D C1 B1 B1 B B1 A Vol.10 C1 B1 Y Z C1 X Unit: mm (a) Plan (b) Three-dimensional model Fig. 7 Building C Table 7 Cumulative participation (%) Period (s) Along X Along Y Along Z Along X Along Y Along Z 1 0.555816 0 85.4394 0 0 85.4394 0 2 0.488576 83.5009 0 0 83.5009 85.4394 0 3 0.429375 0.96550 0 0 84.4664 85.4394 0 4 0.185058 0 10.0069 0 84.4664 95.4463 0 5 0.159820 10.0356 0 0 94.5019 95.4463 0 6 0.141734 0.09550 0 0 94.5975 95.4463 0 7 0.111193 0 3.1075 0 94.5975 98.5537 0 8 0.093398 3.49090 0 0 98.0883 98.5537 0 9 0.084234 0.02350 0 0 98.1118 98.5537 0 10 0.082548 0 1.1627 0 98.1118 99.7164 0 B2 D 4200 4200 B2 1800 4200 B2 B2 B2 B1 B1 B1 B2 F E G B1 B2 C B2 1800 B2 B 4200 4200 2 Individual participation (%) Mode A 1 Modal participating mass for Building Structure C B2 B2 2600 3 6 B1 B2 B2 B2 B2 B2 B2 B1 B2 B1 (a) Plan B2 B2 B2 B2 B2 B1 B1 B1 5 4200 B2 B1 B2 5200 B2 B1 B1 4 B1 4900 B1 B1 B2 Y Z X B1 Unit: mm (b) Three-dimensional model Fig. 8 Building D No.1 Tariq M. Nahhas: A comparison of IBC with 1997 UBC for modal response spectrum analysis in standard-occupancy buildings 109 Table 8 Modal participating mass for Building Structure D Mode Period (s) 1 0.944924 2 3 Individual participation (%) Cumulative participation (%) Along X Along Y Along Z Along X Along Y Along Z 84.2847 0 0 84.2847 0 0 0.710861 0 84.3473 0 84.2847 84.3473 0 0.690169 0.1712 0 0 84.4559 84.3473 0 4 0.312748 9.7628 0 0 94.2187 84.3473 0 5 0.234799 0 9.8149 0 94.2187 94.1622 0 6 0.228175 0.0147 0 0 94.2334 94.1622 0 7 0.185529 3.3580 0 0 97.5914 94.1622 0 8 0.138858 0 3.4090 0 97.5914 97.5712 0 9 0.135439 0.0872 0 0 97.6786 97.5712 0 10 0.132930 1.4573 0 0 99.136 97.5712 0 6 Scaling of base shear IBC and UBC require that the base shear obtained by code-compliant Modal Response Spectrum Analysis (MRSA) be scaled up by a factor of equivalent static base shear calculated using ELFP. The IBC refers to ASCE7-05 for response spectrum analysis of building systems, where clause 12.9.4 states that the MRSA shall not be less than 85% of the ELFP base shear. Similarly, according to UBC clause 1631.5.4, the MRSA base shear shall not be less than 100% of the ELFP base shear for irregular buildings and not less than 90% for regular buildings. Both codes define their own methods for determining ELFP base shears. To understand the effect of scaling, the base shear is computed in three ways: ELFP, MRSA without scaling and MRSA with scaling. The results are shown in Fig. 9 for Building A, which is assumed to be located at Sample Location 1 for all the five site classes. As seen in the figures, UBC is over conservative. Similar results have been presented (for ordinary residential buildings) by Ghosh and Khuntia (1999) and Pong et al. (2006 a, b). Qualitatively, the results concerning the safety of the design are the same, i.e., UBC generates overconservative designs and therefore the structures are safer. However, for certain conditions, scaling is required by both codes if MRSA is used instead of ELFP. It is obvious from Fig. 9 that though the base shear increases after scaling and affects the design of a building for any of the two codes, the difference between the base shear due to the two codes remains approximately the same in both cases with or without scaling. This proves that scaling (whenever required by the code provisions) does not affect the results of comparison between the two codes. Therefore, all comparisons in this paper are presented without using the scaling. Since the results presented in Fig. 9 assume that the building falls in a standard occupancy category including only the residential and office buildings, the same exercise was repeated for a hospital building. For this purpose, to observe the effect of scaling, building B was assumed to be a hospital building located at geographical Sample Location 4 with Soil Type D. Figure 10 shows the base shear obtained for both UBC and IBC using ELFP, MRSA without scaling and MRSA with scaling. It is obvious and interesting to note that the results are not the same as for ordinary occupancy buildings but instead, IBC and UBC both generate about the same base shear. For MRSA without scaling, UBC generates significantly higher base shear than IBC but Building A - Sample Point 1 (base shears in Y-direction) 1.2 Base shear (102kN) ETABS is higher than the ELFP time period given in Table 3 (UBC: 0.639, IBC: 0.628) for this building. This building has the Modes 8, 9 and 10 closely spaced as is obvious from Table 8. However, the modal participation is insignificant and therefore the results obtained by using modal combination methods CQC or SRSS are expected to be the same. This was verified numerically by performing ETABS analysis using CQC and SRSS. Similar verifications were made for all the buildings and it was found that the use of CQC or SRSS provide almost the same results for all buildings classified as standard occupancy. 1.0 IBC UBC 0.8 0.6 0.4 0.2 0 A B C D E A B C D E A B C D E ELFP base shears MRSA base shears (without scaling) MRSA base shears (with Scaling) Fig. 9 Comparison of ELFP and MRSA base shears (Building A) EARTHQUAKE ENGINEERING AND ENGINEERING VIBRATION 25 Base shear (102kN) 20 Building B - Sample Point 4 - Soil Type D (base shears in X-direction) IBC UBC 15 10 5 0 ELFP base shears MRSA base shears MRSA base shears (without scaling) (with scaling) Fig. 10 Building B considered as hospital - comparison of ELFP and MRSA base shear Base shear (102kN) when the scaling is applied, the results as shown in Fig. 10 indicate that the qualitative comparison for MRSA with scaling is about the same as for ELFP. These results depend on the structural characteristics and the location of the buildings and in some cases for hospital buildings, IBC may generate larger values for the base shear than UBC as reported in Pong et al. (2006 a, b). Building A 2.0 IBC 1.8 UBC 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0A B C D E A B C D E A B C D E A B C D E Sample Point 1 Sample Point 2 Sample Point 3 Sample Point 4 Fig. 11 Comparison of MRSA base shear for Building A 7 8 6 4 2 A B C D E A B C D E A B C D E A B C D E Sample Point 1 Sample Point 2 Sample Point 3 Sample Point 4 Fig. 12 Comparison of MRSA base shear for Building B 6 Building C IBC UBC 5 4 3 2 1 0 A B C D E A B C D E A B C D E A B C D E Sample Point 1 Sample Point 2 Sample Point 3 Sample Point 4 Fig. 13 Comparison of MRSA base shear for Building C IBC UBC 10 0 8 Base shear (102kN) Base shear (102kN) 12 The maximum base shear using MRSA for each case was obtained by the response spectrum analysis runs of ETABS. For Building A, the results are shown in Fig. 11, where the maximum base shears due to IBC and UBC are compared for Soil Types A to E assuming the building located at all four sample geographical locations. The results actually verify the expected results based on the design response spectra comparison shown in Figures 3(a) and 3(b). However, from the design spectra, the large difference that is now obvious between IBC and UBC could have not been guessed. Similar results were obtained for Buildings B, C and D as shown in Figs. 12, 13 and 14, respectively. Quantitatively, the results vary from building to building but qualitatively remain about the same. Note that the maximum base shear varies from Sample Location 1 (area of low seismicity) to Sample Point 4 (area of high seismicity) in a logical manner. Also, the maximum base shear values increase with varying site classes from A to E for each sample location except for Sample Location 4, when for all buildings, the maximum base shear for Site Class E is significantly less than for Site Class D. This is actually related to the modal contribution and the design response spectrum for the Sample Point 4. The design response spectrum Building B 14 Vol.10 7 Results Base shear (102kN) 110 20 18 16 14 12 10 8 6 4 2 0 Building D IBC UBC A B C D E A B C D E A B C D E A B C D E Sample Point 1 Sample Point 2 Sample Point 3 Sample Point 4 Fig. 14 Comparison of MRSA base shear for Building D Tariq M. Nahhas: A comparison of IBC with 1997 UBC for modal response spectrum analysis in standard-occupancy buildings (1) . where, VUBC and VIBC are the maximum base shears in the building due to UBC and IBC, respectively. Note that the results are more quantitative. First, it shows that the difference between the values of maximum base shear are much higher for Sample Locations 1 and 2 as compared to the areas of higher seismicity (Locations 3 and 4), and it also shows that the UBC provides a drastically higher maximum base shear in the range of 40% to 60% for Sample Locations 1 and 2. 60 Percentage difference of base shear Building A Building B Building C Building D 50 40 30 20 10 0 A B C D E A B C D E A B C D E A B C D E Sample Point 1 Sample Point 2 Sample Point 3 Sample Point 4 Maximum moment in columns (102kN.m) Fig. 15 Conservativeness of UBC spectrum base shear over IBC Building A 1.2 1.0 IBC UBC 0.8 0.6 0.4 0.2 0 A B C D E A B C D E A B C D E A B C D E Sample Point 1 Sample Point 2 Sample Point 3 Sample Point 4 Fig. 16 Comparison of maximum moments at base for Building A Maximum moment in columns (102kN.m) D = 100×(VUBC – VIBC) / VIBC 111 This implies that UBC was too conservative for areas of low seismic activity and IBC corrected this. Also, the percentage difference between IBC and UBC base shear is prominently higher for Class E in Sample Location 3. For Sample Location 4, however, the difference between the two codes is lower for Site Class E as compared to Site Class D. Figures 16 to 19 summarize the corresponding results of the maximum moment at a column-base Building B 2.5 2.0 IBC UBC 1.5 1.0 0.5 0 A B C D E A B C D E A B C D E A B C D E Sample Point 1 Sample Point 2 Sample Point 3 Sample Point 4 Fig. 17 Comparison of maximum moments at base for Building B Maximum moment in columns (102kN.m) for Soil Types D and E at Sample Point 4 have a large difference in the peak spectral acceleration. For Site Class D it is close to 1.7 g, where for Site Class E it is about 1.4 g. This discrepancy does not exist for other sample locations and is related to how the code has been developed for this area of high seismicity. However, it does not affect the main issue addressed in this paper and the comparative results between IBC and UBC even for this case are the same, i.e., UBC gives higher base shear than IBC. To show the comparison in a more quantitative way for all four buildings at a glance, the percentage difference in base shear between the IBC and UBC is plotted in Fig. 15 and is defined as follows: Building C 1.0 IBC 0.9 UBC 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0A B C D E A B C D E A B C D E A B C D E Sample Point 1 Sample Point 2 Sample Point 3 Sample Point 4 Fig. 18 Comparison of maximum moments at base for Building C Maximum moment in columns (102kN.m) No.1 Building D 3.0 2.5 IBC UBC 2.0 1.5 1.0 0.5 0A B C D E A B C D E A B C D E A B C D E Sample Point 1 Sample Point 2 Sample Point 3 Sample Point 4 Fig. 19 Comparison of maximum moments at base for Building D 112 EARTHQUAKE ENGINEERING AND ENGINEERING VIBRATION 60 Percentage difference of maximum moments Building A Building B Building C Building D 50 40 30 20 10 0 A B C D E A B C D E A B C D E A B C D E Sample Point 1 Sample Point 2 Sample Point 3 Sample Point 4 Fig. 20 Conservativeness of UBC spectrum moments over IBC junction. Figure 20 shows the percentage difference of maximum moments. Again, this shows the quantitative nature of the difference between IBC and UBC and the results are about the same as for similar plots of the maximum base shear shown in Fig. 15. 8 Conclusions A comparison of the 2009 IBC with the 1997 UBC has been presented focusing on the specific provisions of the two codes for Modal Response Spectrum Analysis (MRSA) of residential and office buildings in the standard occupancy category. The results are needed by structural engineers and code development authorities in developing countries throughout the world that have a large number of existing buildings designed according to the 1997 UBC and need to evaluate their seismic safety. To make valid comparisons in general terms, four geographical sample locations in four different seismic activity regions of the USA have been arbitrarily selected. The locations are not associated with particular buildings. Four buildings ranging from four to six floors were chosen to represent the bulk of residential and office buildings in developing countries and were used for comparison purposes. The design response spectra for all five site classes at all four geographical locations were generated both for the IBC and the UBC. The design response spectra clearly indicate that the UBC design response spectra are always more conservative than the IBC. The structural models for the sample buildings were created using ETABS software and the results for maximum base shear and maximum bending moment were obtained using MRSA. The effect of scaling of the base shear according to the two codes was shown for both ordinary residential and hospital buildings. It was shown that scaling as required by both codes does not affect the results and in all cases, the UBC was found to be over conservative as compared to the IBC. It was found that the maximum base shear and the maximum internal moments generated by the UBC in areas of lower seismic activity are much higher than IBC when compared to Vol.10 the more seismically activity areas. It seems the IBC is successful in correcting the over-conservativeness of the UBC in areas of lower seismic activity. Furthermore, the UBC yields higher base shear and internal moments for all cases. Therefore, the buildings designed using the 1997 UBC can be considered safer than the buildings designed using the IBC. The results presented apply to residential and office buildings. For hospital buildings, the two codes produce very close results. It is possible that a hospital building designed using the UBC may not satisfy all the provisions of the IBC as demonstrated in a previous publication. The study presented in this paper increases the understanding of an important earthquake engineering research issue concerning the IBC and the UBC dealing with the safety of structural designs using the UBC. Since the UBC is still widely used among structural designers in developing countries, the question about comparing the two codes and obtaining conclusive results is an important research issue and a design concern. The results presented herein will help structural designers as well as the authorities responsible for the development of building codes in various countries throughout the world. References Adem D and Ramazan L (2006), “A Comparative Study of the Design Spectra Defined by Eurocode 8, UBC, IBC and Turkish Earthquake Code on R/C sample buildings,” Journal of Seismology, 10(3): 335–351. 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