A Comparative Study of the Seismic Provisions of Iranian Seismic Code(Standard No.2800) and International Building Code 2003

ASIAN JOURNAL OF CIVIL ENGINEERING (BUILDING AND HOUSING) VOL. 12, NO. 5 (2011) PAGES 579-596

A COMPARATIVE STUDY OF THE SEISMIC PROVISIONS OF

IRANIAN SEISMIC CODE (STANDARD NO. 2800) AND INTERNATIONAL BUILDING CODE 2003

N. Imashi and A. Massumi* Department of Civil Engineering, Tarbiat Moallem University, Tehran, Iran

Received: 15 September 2010 Accepted: 2 March 2011

ABSTRACT

This article provides a comparison process on how to calculate seismic forces by the static analysis method stated both in the international Building Code (IBC) 2003 and in the Iranian Seismic Code (IS 2800-05). The seismic coefficient for the equivalent lateral force is specified by the following factors: fundamental period, importance factor, spectral response acceleration, and building response modification factor. In this article the above-mentioned parameters are obtained through the IBC 2003 and are compared against those covered in the IS 2800-05. Studies and comparison of factors would lead to significant differences in the results obtained using the two codes. In order to clarify the problem, design base shear of a building with combined system (special moment steel frames + eccentric bracings) in four different soil types and vertical distribution of base shear at story level is obtained, in accordance with both codes; and the results are compared with diagrams and tables. The results prove the need to review the IS 2800-05 and develop more appropriate relations towards achieving economic and functional objectives.

Keywords: Iranian seismic code (IS 2800-05); international building code 2003 (IBC 2003); seismic forces; static analysis method; equivalent lateral force

1. INTRODUCTION

The seismic prone plateau of Iran has registered frequent earthquake occurrences across the land in its thousands-year-old history. Approval of and the requirement to apply the first edition of the Iranian code of practice for seismic resistant design of buildings (Standard No. 2800) was practically enacted in 1987 and 1988. Regulations available in this Code were translations of some chapters of basic building regulations, issued by the US Building Officials and Code Administrators (BOCA), and also certain building regulations of National Building Code of Canada (NBC), 1970, Building Standard Law (BSL) of Japan,

* E-mail address of the corresponding author: [email protected] (A. Massumi)

N. Imashi and A. Massumi

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1960 and France. The second edition, incorporating criteria of the Uniform Building Code (UBC 1994), was developed in 1997 which enjoyed greater safety level. Reviewing the second edition started in 2000 leading to the third edition, approved and officially announced and imparted and for design, control and inspection of buildings in 2005 [1].

Before 2000, three regional model Codes prevailed in the United States; the UBC Code in west, the BOCA Code in north and the Standard Building Code (SBC) was prevalent in the south of that country. The International Council of Codes was established in 1994 to develop the unique comprehensive code not bound by regional limitations; and it ultimately formulated the International Building Code (IBC 2000) as the first publication. IBC 2003 was the next version, which was developed based on the Federal Emergency Management Agency (FEMA) instructions in the framework of National Earthquake Hazards Reduction Program (NEHRP) recommending certain precautions to improve seismic regulations for new buildings [2].

Since the IS 2800-05 is derived from UBC 1994 and BOCA 1978, which have undergone major changes over the years, this study aims to compare factors effective in specifying seismic force by the static method covered in both the IBC 2003 and the IS 2800-05 and to examine strengths and weaknesses of the IS 2800-05.

2. DESIGN BASE SHEAR

According to IS 2800-05, seismic lateral force for regular buildings to 50 meters high and irregular buildings to 18 meters high or five stories above the base level may be obtained by the equivalent static analysis method. In this analysis approach, base shear is obtained from Eq. (1): WCV = (1)

where W is the effective weight of building (all dead load + a percentage of live load) and C is the seismic response coefficient. This factor is, in accordance with Eq. (2):

RABIC = (2)

where A is the function of design baseline acceleration, B is the reflection coefficient of the building, I is the importance factor and R is the response modification factor. The minimum value of V is Vmin= 0.1AIW [3].

IBC 2003, the earthquake lateral force, effective on the structure may be calculated by static analysis methods, including Index Force Analysis Procedure, Simplified Analysis Procedure, and Equivalent Lateral Force Procedure. Index Force Analysis Procedure may be applied for either regular or irregular structures assigned to seismic design category A.

Simplified Analysis Procedure may be applied for all structures assigned to seismic design category A, B, and C, both regular and irregular and Equivalent Lateral Force Analysis, in addition to the above cases, is also applicable to some of the regular and irregular structures assigned to seismic design category D, E, and F with period smaller of

COMPARATIVE STUDY OF SEISMIC PROVISIONS BETWEEN IRANIAN...

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3.5 TS (TS = SD1/SDS). In the Index Force Analysis structures designed to resist the minimum lateral force, Fx, applied at each level given by Eq. (3): in which Wx is the portion of the total building weight at story level x.

xx WF = 01.0 (3)

In the two other procedures, seismic base Shear, V, will be determined in accordance with

Eq. (4): WCV S = (4)

where W, the effective seismic weight of the structure, includes the total dead loads and some percentage of live and snow load [4]. The values considered in the code are slightly different from those of IS 2800-05.

Cs is the seismic response coefficient which is, in simplified analysis procedure, the function of response modification factor, R, and design spectral response acceleration at short periods, SDS, and is given by Eq. (5).

RS

C DsS= 2.1 (5)

In the equivalent lateral force procedure, Cs is the function of response modification

factor, R, fundamental period, T, design spectral response acceleration at period of 1 second, SD1, and importance factor of the structure, which is obtained from Eq. (6).

TIRSC

E

DS

)(1= (6)

The minimum value for Cs equals 0.44 SDS IE and shall not exceed SDS /(R / IE ). For

structures located in seismic design category E or F, and for those located in areas where SI 0.6g, CS shall not be taken less than 0.5S1 /(R / IE ).

In order to incorporate the vertical component impacts of seismic force, the code takes into account a combination of the conjugate impacts of horizontal and vertical components of seismic force. The seismic force that must be considered in the combination of structural load design is given by Eq. (7). DSQE DSE += 2.0 (7)

The vertical component of seismic force is equal to 0.2 SDS D and includes up and down

pointing impacts of earthquake. In this equation, D is the effect of dead load, E is the combined effect of horizontal and vertical earthquake-induced forces, QE is the effect of horizontal seismic forces, SDS is the design spectral response acceleration at short periods and to account for structural redundancy scale, the code offers a factor named redundancy coefficient . where is a redundancy coefficient obtained in accordance with Eq. (8) [4].


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XArmax

1.62= (8) is a scalar between 1.0 and 1.5 and shall in no case be taken less than 1.0. rmax is the ratio of design story shear resisted by the single element carrying the most shear force in the story to the total story shear, for a given direction of loading. Ax is the floor area in m2 of the diaphragm level immediately above the story [4]. This is also another difference between the two codes. The IS 2800-05 incorporates the vertical earthquake-induced forces in few special cases and it normally considers only the effect of horizontal seismic force in structural calculations, except for the above cases; also, structural redundancy effect is not explicitly foreseen by the code.

2.1 Soil classification In IS 2800-05, soil is categorized into four groups I through IV and only the shear wave velocity parameter is taken into consideration in this classification [3]. On the other hand, there are six seismic site soil classifications, A through F in IBC 2003 and in addition to considering shear wave velocity for soil classification, Standard Penetration Resistance N (or NCH) and undrained Shear Strength of soil (Su) parameters are also taken into account, such that having available one of the specifications, classification may often be done easily. In case of uncertainty about soil type, IBC 2003 recommends that type D profile is selected and profile E is chosen only if there is proof of such soil in the area [4]. However, it is stated in IS 2800-05 that if there is any doubt on conformity of building site with soil type specifications given in table, the soil profile offering greater reflection factor should be selected [3]. Taking shear wave velocity in the ground as the base criterion, relation between the two codes with respect to soil classification is given in Table 1.

Table 1: Comparison of soil profile classification in IS 2800-05 and IBC 2003

Average soil properties for top (30.480 m) of soil profileSoil profile name/generic

description Shear wave velocity Vs(m/s)

Soil type in IBC

Soil type in 2800

Hard rock 1500 Vs A I-a

Rock 750 Vs 1500 B I-a

Very dense soil and soft rock 375 Vs 750 C I-b, II

Stiff soil profile 175 Vs 375 D III

Soft soil profile Vs 175 E IV

Soil requiring site-specific evaluation --- F ---

2.2 Site ground motion In IS 2800-05, cities and important places in Iran are divided into four regions in terms of relative seismic hazard such that it may be very high (Base design acceleration = 0.35g), high


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(Base design acceleration = 0.3g), medium (Base design acceleration = 0.25g) or low (Base design acceleration = 0.2g) for different regions [3]. Considering earthquake occurrence risk in different regions of each province and sectional division maps, six pro-seismic sections are introduced in IBC 2003 and the mapped maximum considered earthquake spectral response acceleration for the short period and 1-second period respectively denoted by Ss and S1 forms. Two factors Fa and F are also defined in the code, which express nonlinear property of the soil profile. Fa is the site coefficients for short periods and F is the site coefficients for the 1-second periods the site coefficients and were multiplied by Ss and S1 respectively for each site class, and is specified in Tables 2 and 3. They collectively incorporate the combined regional seismicity impact and soil profile type [4].

Table 2: Values of site coefficient (Fa) as a function of site class, and mapped spectral response

acceleration at the short period (Ss)

Mapped maximum considered earthquake spectral response acceleration at short period

Site class

Ss0.25 Ss=0.5 Ss=0.75 Ss=1.00 Ss=1.25

A 0.8 0.8 0.8 0.8 0.8

B 1.0 1.0 1.0 1.0 1.0

C 1.2 1.2 1.1 1.0 1.0

D 1.6 1.4 1.2 1.1 1.0

E 2.5 1.7 1.2 0.9 0.9

F b b b b B

Table 3: Values of site coefficient (Fv) as a function of site class, and mapped spectral response

acceleration at 1-second period (S1)

Mapped maximum considered earthquake spectral response acceleration at 1-second period

Site class

S10.1 S1=0.2 S1=0.3 S1=0.4 S10.5

A 0.8 0.8 0.8 0.8 0.8

B 1.0 1.0 1.0 1.0 1.0

C 1.7 1.6 1.5 1.4 1.3

D 2.4 2.0 1.8 1.6 1.5

E 3.5 3.2 2.8 2.4 2.4

F b b b b b

b: in specifying proper values for Fa and F, geological research and dynamic analyses should be carried out, except for structures with periods less than or equal to 0.5s [4].


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2.3 Occupancy importance factor From importance perspective, IS 2800-05 divides buildings in four groups and defines the importance factor for very high importance buildings as 1.4, for high importance buildings as 1.2, for average important buildings as 1.0, and for lesser importance buildings as 0.8 [3]. In IBC 2003 also, buildings were divided in four groups with the difference that importance factor for the essential buildings (Seismic Use Group (SUG) III) is 1.5, for high importance buildings (SUG II) 1.25 and for average and low importance buildings (SUG I) is 1.0 [4].

2.4 Fundamental period of building The most common method to estimate vibration period of a building is from empirical relations, considering building specifications (structure type) and its height from base level. Empirical relations to calculate the fundamental period of the building is given in Table 4 for both codes.

Table 4: Comparison of empirical period in IS 2800-05 and IBC 2003

Structural system IS 2800-05 (m) IBC 2003 (ft) IBC 2003 (m)

Steel moment-resisting frames 0.08H0.75 0.028H0.80 0.0724H0.80

Concrete moment-resisting frames 0.07H0.75 0.016H0.90 0.0466H0.90

Eccentrically braced steel frames --- 0.030H0.75 0.0731H0.75

All other structural systems 0.05H0.75 0.020H0.75 0.0488H0.75

Figure 1(a, b, c and d) shows periods calculated by two codes for 5, 10, 15 and 20 story

buildings, respectively (all stories height is equal to 3.40 m). In the third edition of IS 2800-05 and in calculation of period, frames with concentric and

eccentric bracing are in one group whereas in IBC 2003, frames with eccentric bracing are separated from concentric bracing group, with respect to period calculation relations. In fact there is a distinction between the two codes in this regard. It seems that grouping the two bracing systems in one setting to determine the fundamental period of building would be problematic.

According to IS 2800-05, the fundamental period of building may be calculated using analytical methods; in which case, the specified value shall not exceed 1.25 times the period obtained by empirical relation [3].

In IBC 2003, the maximum period value for design purposes depends on design acceleration response spectrum at 1-s period shall not be taken larger than CuTa, where Ta is the approximate fundamental period of building with concrete and Steel moment frame structure obtained from relation 0.1N (N number of stories) provided that building height does not exceed 12 floors and that minimum height of each floor is 10 feet (3 m). It may be noted that larger value of Cu are permitted as the soil-dependent seismic risk of a location decreases (Table 5) [4].


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0.0

0.5

1.0

1.5

2.0

2.5

5 10 15 20

T(s)

No. of Stories

IBC 2003

IS 2800-05

0.0

0.5

1.0

1.5

2.0

2.5

5 10 15 20

T(s)

No. of Stories

IBC 2003

IS 2800-05

(b) Concrete moment frames (a) Steel moment frames

0.0

0.5

1.0

1.5

2.0

2.5

5 10 15 20

T(s)

No. of Stories

IBC 2003

IS 2800-05

0.0

0.5

1.0

1.5

2.0

2.5

5 10 15 20

T(s)

No. of Stories

IBC 2003

IS 2800-05

(d) Other buildings (c) Concentric braced steel frames

Figure 1. Calculated periods by IBC 2003 and IS 2800-05 codes for 5, 10, 15 and 20 story buildings, (all stories height is equal to 3.40 m)

Table 5: Coefficient for upper limit on calculated period

SD1 Cu

>0.4 1.4

0.3 1.4

0.2 1.5

0.15 1.6

0.1 1.7


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)(1 0TTSB += 00 TT SB +=1 TsTT 0

32))(1( TTSB S+= TsT (9)

where Ts is a scalar and it represents the ground period. T is the fundamental period of building in terms of seconds and T0 indicates the boundary between very stiff structures acceleration and the constant acceleration range from acceleration spectrum [3]. Also, S is considered to account of the resonating effect of soft soil on ground movement at bedrock; its value increases as the soil gets softer and is specified in Table 6.

Table 6: Values of S in IS 2800-05

Low and medium relative hazard area

High and very high relative hazard area Soil type T0 Ts

S S

I 0.1 0.4 1.5 1.5

II 0.1 0.5 1.5 1.5

III 0.15 0.7 1.75 1.75

IV 0.15 1.0 2.25 1.75

In order to obtain design acceleration spectrum through IBC 2003, the following

measures are taken: the design ground motion parameters can be derived from the table and contour maps of IBC 2003. The mapped maximum considered earthquake spectral response acceleration for 0.2s (short), Ss, and 1.0s (long) periods, S1, are first obtained from the IBC 2003 seismic maps. The contours represent the spectral response acceleration as a percent of gravity, assuming 5% damping and soil condition classified under site class B. The spectrum is based on Maximum Considered Earthquake (MCE) with 2 percent probability of reoccurrence in 50 years (2500-year return period). IBC 2003 goal is to provide Design Based Earthquake (DBE) level design with 10 percent probability of reoccurrence in 50 years (475-year return period) [4].

In order to convert Maximum Considered Earthquake spectral response acceleration (MCE) to Design Based Earthquake (DBE), the 2/3 ratio is used. Considering the soil type and acceleration response spectrum on bedrock, applying Fa (Acceleration-Related Soil Factor) and F (Velocity-Related Soil Factor) factors, maximum acceleration response spectrum parameters (SMI and SMS) and then their corresponding design values (SDI and SDS) that are 2/3 parameters for maximum acceleration response spectrum are obtained, in this code [4]. saMSDS SFSS == 3

2.32 (10)


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111 32.

32 SFSS vMD == (11)

Where Fa, site coefficient is the peak response part of fixed acceleration (equivalent to B = 1 + S in the IS 2800-05). Acceleration magnification factor in addition to soil type depends also on sectional acceleration of earthquake and as it decreases, magnification increases. This rule is observed in IS 2800-05 in a way that for 0.20g to 0.25g accelerations, the maximum B factor that is equal to (1 + S) is increased up to 3.25 times but for 0.30g to 0.35g accelerations, maximum B value has become 2.75 [5]. At the beginning of the diagram, (T


588

c) In calculating spectral acceleration, the exponent (power) for fundamental period of structure in IS 2800-05 is taken to be 2/3, while in IBC 2003 it is 1; consequently, the descending part of the spectrum in IBC 2003 is steeper than that of the IS 2800-05.

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.6 1.2 1.8 2.4 3.0 3.6

S a

T

Very High Risk - IS 2800-05

High Risk - IS 2800-05

Medium Risk - IS 2800-05

Low Risk - IS 2800-05

Very High Risk - IBC 2003

High Risk - IBC 2003

Medium Risk - IBC 2003

Low Risk - IBC 2003

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.6 1.2 1.8 2.4 3.0 3.6

S a

T








Low Risk - IBC 2003

(b) Soil type II or C with 5 percent attenuation (a) Soil type I or B with 5 percent attenuation

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.6 1.2 1.8 2.4 3.0 3.6

S a

T








Low Risk - IBC 2003

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.6 1.2 1.8 2.4 3.0 3.6

S a

T








Low Risk - IBC 2003

(d) Soil type IV or E with 5 percent attenuation (c) Soil type III or D with 5 percent attenuation

Figure 2. Design acceleration spectra for different soil profiles with 5 percent attenuation in IBC 2003 and IS 2800-05

2.6 Response modification factor Response modification factor, proposed in IS 2800-05 is for structures that are designed by permissible stress method whereas in IBC 2003 ultimate strength design method is applied. Since ultimate limit response modification factor (RU) and ultimate allowable stress factor (RW) are approximately related through RW=1.4RU [6], response modification factor values in IBC are multiplied by the scalar 1.4 and are compared in Table 7 for several systems.

Comparison of response modification factors in IBC 2003 and IS 2800-05 shows that IBC 2003 has assumed greater response modification factors for bearing walls and building frame systems. The values for intermediate reinforced concrete moment frames are equal in both


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codes. Also, intermediate and ordinary steel moment frames in IBC 2003 assumes smaller response modification factors relative to IS 2800-05. Intermediate concrete moment frames system + ordinary reinforced concrete shear walls, and intermediate steel moment frames + concentrically steel bracings assume smaller response modification factors relative to IS 2800-05. Special moment frames (concrete or steel) + Special reinforced concrete shear walls have approximately the same values in both codes. Other lateral resistant systems in the IBC 2003 have greater response modification factors in comparison with IS 2800-05.

Table 7: Comparison of response modification factors in IS 2800-05 and IBC 2003 Standards

Structural system Lateral force resisting system

2800 (Rw)

IBC (Ru)

IBC (Rw)

Difference based on IBC (%)

Special reinforced masonry shear walls 7 5.5 7.7 9.0 Intermediate reinforced concrete shear walls 6 4 5.6 -7.1 Ordinary reinforced concrete shear walls 5 3 6.3 20.6

Bearing walls

system Shear walls with reinforced masonry 4 3.5 4.9 8.4 Special reinforced concrete shear walls 8 6 8.4 4.7 Intermediate reinforced concrete shear walls 7 -- -- - Ordinary reinforced concrete shear walls 5 5 7.0 28.6 Shear walls with reinforced masonry 4 4 5.6 28.6 Steel eccentrically braced frames 7 7-8 11.2 37.5

Building frames system

Steel concentrically braced frames 6 5-6 7.0 14.3

Special reinforced concrete moment frames 10 8 11.2 10.7 Intermediate reinforced concrete moment frames 7 5 7.0 0.0 Ordinary reinforced concrete moment frames 4 3 4.2 4.7 Special steel moment frames 10 8 11.2 10.7 Intermediate steel moment frames 7 4.5 6.3 -11.1

Moment resisting frames systems

Ordinary steel moment frames 5 3.5 4.9 -2.0 Special moment frames (concrete or steel) + Special reinforced concrete shear walls 11 8 11.2 1.7

Intermediate concrete moment frames + Ordinary reinforced concrete shear walls 8 5.5 7.7 -3.8

Intermediate steel moment frames + Ordinary reinforced concrete shear walls 8 5.5 7.7 -3.8

Special steel moment frames + Special steel eccentrically braced frames 10 7-8 9.8-11.2 10.7

Special steel moment frames + Special steel concentrically braced frames 9 8 11.2 19.6

Intermediate steel moment frames + Eccentrically steel bracings 7 - - -

Dual system with

moment frames

Intermediate steel moment frames + Concentrically steel bracings 7 4.5 6.3 -11.1

IBC 2003 considers eccentrically braced frames in both moment-resisting and nonmoment-

resisting connections at columns away from links conditions, but for the latter connection, it


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considers a 15 percent greater response modification factor. Also, IBC 2003 considers three special, intermediate and ordinary states for reinforced masonry shear walls, intermediate and ordinary steel concentrically braced frames. IBC 2003 considers each of the above cases in its own place, while in IS 2800-05, they are all covered within a unique system.

3. VERTICAL DISTRIBUTION OF BASE SHEAR

Force distribution through the height is linear for all structures and all periods, in IS 2800-05 and is calculated by Eq. (14).

=

= ni

ii

xxtx

hW

hWFVF

1

)( (14)

For long period buildings an extra force Ft=0.07TV is applied to the top floor, in IS 2800-

05. If the building period is less than or equal 0.7 sec, Ft value may be considered zero [3]. The distribution of force over the height of building, in IBC 2003 is complex and

depends on the period of vibration of the building, and the characteristic shape of the vibration modes, and is obtained from Eq. (15).

=

= ni

kii

nxx

x

hW

hWVF

1

(15)

where Fx = the lateral seismic force at story level x; wi(wx) = the portion of the total building weight at story level i (or x); hi(hx) = the hight from the ground floor to story level i (or x); k = an exponent related to the period of structure [4].

IBC 2003 prescribes three types of distribution of the entire base shear: A triangular distribution for buildings having a fundamental period not exceeding 0.5

seconds, k is equal to 1. A parabolic distribution for buildings having an elastic period in exceeding 2.5 seconds,

k is equal to 2. A linear interpolation between linear and parabolic distribution for buildings with

periods between 0.5 and 2.5 seconds [7]. Unlike IS 2800-05, additional force Ft is not considered here. When period is greater than

2.5s, the impact of higher modes is important and that's why instead of linear distribution of shear the height, some parabolic distribution is used.


591

4. STORY DRIFT

In IS 2800-05, the design story drift is obtained by multiplying the lateral deflections at the floor level resulted from elastic analysis under design base shear, by 0.7R factor (Mi=0.7RWi), after applying P- effects. IS 2800-05 has limited design story drift for structures with period less than 0.7 seconds to 0.025 times the floor height and for structures with period greater than or equal to 0.7 seconds to 0.020 times the floor height [3].

The IBC 2003 offers the design story drift limitation in accordance with the importance factor value. This tries to provide safety through applying restrictions. For instance, by applying more restrictions on story drift of the likely sensitive structures it thrives to reduce probable failure of filler walls, partitions and other non-structural elements and consequently provide more safety. The code considers different structural systems; for example, with regard to buildings less than 4 stories high with shear wall or masonry wall and partitioning, it assumes 0.015, 0.020 and 0.025 times floor height restrictions respectively to buildings of III, II and I importance. Also, for higher than 4 storey buildings, 0.01, 0.015 and 0.02 times storey height restrictions are respectively applied to buildings of III, II and I importance. For masonry shear wall structures, a more severe restriction, as low as 0.007 times the storey height is applied, as these structures have low ductility. The adjusted design earthquake displacement at floor level x, is obtained from Eq. (16) [4].

I

C xedx

= (16) Instead of 0.7R factor, other parameters such as importance factor and structural system

type are termed, in this code. Cd is the deflection amplification factor and is a functional of response modification factor and represents displacement increase in nonlinear phase; its specific value varies in accordance with structure type. xe is the lateral deflection at floor level x resulted from elastic analysis.

The design story drift, x, for story x, is obtained from Eq. (17).

1= xxx (17)

5. CASE STUDY

To better show the difference between two codes, a 12-story building, located on the soil profile type D (IBC 2003) or type III (IS 2800-05), is selected and its base shear force, in linear static form is obtained by both codes and are compared to each other. The building is located in a high relative hazard area with 0.3g base design acceleration (according to IS 2800-05).The system is equipped with special steel moment frame + eccentric steel bracing and is residential. Its total weight is 100788 kN and its height from base level is 49.2 m. Computer analysis specifies 2.0 second as the fundamental period of structure. Specifications of the building are shown in Figure 3 [7].

The base shear force obtained by both IS 2800-05 and IBC 2003, are summarized in Table 8.


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Figure 3. Specifications of the case study building

Table 8: Base shear force according to IS 2800-05 and IBC 2003 (kN)

IS 2800-05 IBC 2003 Relation Scalar value Relation Scalar value

Steel moment frame system T=0.08H0.75 1.47 T=0.08H0.75 1.62 Eccentric bracing system T=0.08H0.75 0.92 T=0.08H0.75 1.35

Approximate period - - Ta=0.1N 1.2 Tave 1.2 1.39 Tmax 1.25T 1.50 CuTa=1.4Ta 1.95

S1 0.3 A 0.30 Ss 0.75 F 1.8 S 1.75 Fa 1.2

SM1=FS1 0.54 SMS=FaSs 0.90

SDS=2/3 SM1 0.60

Sa

32))(1( TTSB S+= 1.65 SD1=2/3 SM1 0.36

R Ru=Rw/1.4 7.14 Ru 8 I residential 1 residential 1 C C=ABI/R 0.069 SD1/(R.T) 0.023

Csmin 0.1AI 0.03 0.44SDSIE 0.024 Csmax - - SDS/(R/IE) 0.075

V=C.W C.W 6985 CsW 2666 T>0.7s Ft=0.07TV 587 - -


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The base shear force values for the building studied on the different soils are obtained based on both codes and compared with each other in Figure 4, shows the ratio of base shear force in IS 2800-05 to the base shear force in IBC 2003 over different soil profiles.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

B C D E

V 280

0/V I

BC

Soil Profile

0

2000

4000

6000

8000

10000

B C D E

Shea

r (k

N)

Soil Profile

IS 2800-05IBC 2003

(b) Ratio of base shear force in IS 2800-05

relative to IBC 2003 (a) Base shear force vs. soil type

Figure 4. Base shear force vs. soil type and ratio of base shear force in IS 2800-05 relative to IBC 2003

5.1 Vertical distribution of base shear for the building studied After specifying shear force, seismic lateral force exerted on each floor is obtained and are presented in Table 9. (k is calculated from linear interpolation as 1.73).

Table 9: Vertical distribution of base shear (kN)

Stories IS 2800-05 IBC 2003 12 1676.8 531.8 11 866.9 459.2 10 790.3 391.3 9 713.6 327.9 8 636.9 269.4 7 560.3 215.8 6 483.6 167.3 5 406.9 124.1 4 330.3 54.7 3 253.6 29.4 2 176.9 29.4 1 88.5 8.9


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Figure 5(a, b, c and d) shows vertical distribution of base shear for the buildings studied on different soil profiles and Figure 6 shows the ratio of lateral force, calculated based on IS 2800-05, to lateral force of the same stories based on IBC 2003.

0

2

4

6

8

10

12

0 400 800 1200 1600 2000 2400

Stor

y No.

Lateral Force (kN)

IS 2800-05

IBC 2003

0

2

4

6

8

10

12

0 400 800 1200 1600 2000 2400

Stor

y No.

Lateral Force (kN)

IS 2800-05

IBC 2003

(b) Vertical distribution of base shear on

soil types C or II (a) Vertical distribution of base shear on soil

types B or I

0

2

4

6

8

10

12

0 400 800 1200 1600 2000 2400

Stor

y No.

Lateral Force (kN)

IS 2800-05

IBC 2003

0

2

4

6

8

10

12

0 400 800 1200 1600 2000 2400

Stor

y No.

Lateral Force (kN)

IS 2800-05

IBC 2003

(d) Vertical distribution of base shear on

soil types E or IV (c) Vertical distribution of base shear on

soil types D or III

Figure 5. Vertical distribution of base shear for the buildings studied on different soil profiles


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0

2

4

6

8

10

12

0 2 4 6 8 10

Stor

y No.

F2800/FIBC

Soil Profile B

Soil Profile C

Soil Profile D

Soil Profile E

Figure 6. Ratio of stories lateral force in IS 2800-05 to lateral force of same stories in IBC 2003

5.2 Determining the story drift for the building studied Both codes have restricted the maximum inelastic story drift for the building studied to 0.02 times the story height. The Cd factor in the IBC 2003 for this structural system is 4.5 and the actual story drift at x level is 4.5 times story drift resulted from elastic analysis. IS 2800-05 assumes the actual story drift as the product of story drift obtained from elastic analysis of design earthquake and the 0.7R factor. Thus, the conversion factor, in this case study example, to translate story drift from elastic analysis into actual story drift is taken to be 5.

6. CONCLUSIONS

This study signifies the considerable differences in the factors effective on determining shear force in the two codes. These differences are especially pronounced in response modification and spectral acceleration factors and eventually lead to major differences in the shear force value from both codes.

Shear force values assume greater quantity in IS 2800-05 as compared to the IBC 2003, for all soil profiles and all seismically active areas. Regarding structural systems studied, the least difference in shear force value is seen for the soil type B and the greatest difference in soil type D. Also, increases in relative seismic hazard would lead to greater percentage difference for shear force values between the two codes.

Lateral force distribution in the building height shows that distribution pattern is different among the two codes. In IS 2800-05, force distribution in the height is linear for all structures and all periods but an additional force is applied to the top floor of long period buildings. In IBC 2003, however, the additional force Ft is not considered and vertical force distribution for all structures with period greater than 0.5s is parabolic.

The IBC 2003 offers the story drift limitation in accordance with structural system type


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and importance factor value. By applying more restrictions on relative displacement of the likely sensitive structures, the code has reduced failure probability for filler walls, partitions and other non-structural elements. In IS 2800-05, however, the story drift limitation is dependent only on fundamental period of the structure.

In order to incorporate the vertical component impacts of seismic force, in IBC 2003 the earthquake load effect, E, considered as a combination of horizontal effect and a vertical component force; also, to account for structural redundancy scale, the code offers a factor named redundancy coefficient . This factor is directly multiplied by the seismic force, but no such measure is taken in the IS 2800-05.

REFERENCES

1. Building and Housing Research Center (BHRC), Iranian Code of Practice for Seismic Resistant Design of Buildings, Standard No. 2800-94, 1st edition, Building and Housing Research Center: Tehran, Iran, 1994.

2. Pong W, Lee ZH, Lee A. A comparative study of seismic provisions between International Building Code 2003 and Uniform Building Code 1997, Earthquake Engineering and Engineering Vibration, No.1, 5(2006) 49-60.

3. Building and Housing Research Center (BHRC). Iranian Code of Practice for Seismic Resistant Design of Buildings, Standard No. 2800-05, 3rd edition, Building and Housing Research Center, Tehran, Iran, 2005.

4. International Code Council, Inc. International Building Code (IBC 2003), 2003. 5. Faroughi A. A review on starting point of acceleration response spectrum in 3rd edition

of standard no. 2800, www.civilica.com, 2005, (in Persian). 6. Taheri Behbahani AA. A Philosophical Approach to Seismic Codes for Buildings,

Building and Housing Research Center, Tehran, Iran, 1997, (in Persian). 7. Taranath BS. Wind and Earthquake Resistant Buildings: Structural Analysis and

Design, Marcel Dekker, New York, 2005.

Documents

A Comparative Study of the Seismic Provisions of Iranian Seismic Code(Standard No.2800) and International Building Code 2003