Comparison of the USA, China and Japan Seismic Design Procedures

Guangren Yu1, M. ASCE and Gary Y.K. Chock2, F. ASCE

1Structural Engineer, Martin & Chock, Inc., 1132 Bishop Street, Suite 1550, Honolulu, HI 96813 2President, Martin & Chock, Inc., 1132 Bishop Street, Suite 1550, Honolulu, HI 96813

ABSTRACT

This paper presents a comparison of the current seismic design procedures in the United States, China and Japan. The seismic design practices in the three countries are compared by focusing on issues such as (1) design ground motion; (2) classification of building structures; (3) soil/site classification; (4) design response spectrum; (5) base shear calculation; (5) analysis procedures; and (6) drift control and deflection. Tables and diagrams are presented to illustrate the differences and similarities of methodologies utilized by the three countries in dealing with these common seismic design considerations.

INTRODUCTION

In the U.S., the 2012 International Building Code (IBC) is the currently adopted building code for most states. The IBC references American Society of Civil Engineering Standard 7, Minimum Design Loads for Building and Other Structures (ASCE, 2010) and other material-specific codes as its seismic design provisions including some amendments or modifications.

Unlike the IBC that references other standards as seismic provisions, the Chinese Code for Seismic Design of Buildings is a self-contained document that includes almost all the necessary seismic design requirements for building structures. The current seismic code in China is the Code for Seismic Design of Buildings GB 50011-2010.

In Japan the seismic design requirements are specified in the Building Standard Law of Japan which applies to all buildings throughout the country. Since the first seismic code was introduced in Japan in 1924, the Japanese seismic design provisions were substantially revised in 1981 and 2000 (Kuramoto, 2006). The major revision in

1981 was the introduction of a two-phase seismic design procedure. Performance-based seismic methodologies and requirements were included in the 2000 version Japanese seismic code, but at the same time the previous seismic design provisions were kept as an alternative. Since the prescriptive seismic provisions revised in 1981 are still normally used in design offices in Japan, the paper will compare the 1981 Japanese seismic design provisions.

DESIGN GROUND MOTION

In the ASCE 7-10 seismic provisions, ground motion hazards are defined in terms of the risk-targeted Maximum Considered Earthquake (MCER). For most regions of the U.S., the MCE is defined with a uniform hazard probability of exceedance of 2 percent in 50 years (return period of about 2500 years). In regions of highest seismicity, such as coastal California (where the seismic hazard is typically controlled by characteristic large-magnitude events occurring on a limited number of well-defined fault systems), the MCE is calculated by multiplying the median estimate of ground motion resulting from the characteristic event by 1.5. The MCE ground motion parameters are then adjusted such that a 1% of risk of collapse in a 50-year period is provided for a generic building. The MCER is mapped in terms of the spectral acceleration at short period (0.2 second), Ss and at 1 second, S1, for Class B sites, which are firm rock sites. For sites other than Site B, two coefficients, Fa and Fv are used to modify the Ss and S1 values. The MCER spectral response accelerations adjusted for Site Class effects are designated SMS (=FaSS) and SM1 (=FvS1), respectively, for short-period and 1-second-period response. The design ground motion was selected at a ground shaking level that is 2/3 of the MCER ground motion. Accordingly, two additional parameters, SDS (= 2/3SMS) and SD1 (=2/3SM1), are used to define the acceleration spectrum for the design level event (Chock, 2010).

In the Chinese seismic code, the expected performance of a structure is conceptually defined at three levels of seismic hazard: frequent earthquakes, moderate earthquakes and rare earthquakes. Table 1 gives the probabilistic definitions of these three levels of seismic hazard. Seismic hazards in China are defined for the “moderate earthquake” at a uniform 10 percent probability of exceedance in 50 years. The seismic hazards, in terms of Seismic Peak Ground Acceleration and Characteristic Period of Response Spectrum (Tg) are specified in Seismic Ground Motion Parameter Zoning Maps of China GB 18306-2001 (CEA, 2001). This standard includes two maps. The first map is the zoning map of peak ground acceleration with 10 percent probability of exceedance in 50 years using a seven-level grading system, i.e., < 0.05g, 0.05g, 0.1g, 0.15g, 0.20g, 0.30g, and ≥0.40g. The other map is the Characteristic Period (Tg) zoning map which has three zones or three Seismic Groups in which the first group is defined as Tg = 0.35s, the second group as Tg = 0.40s, and the third group as Tg = 0.45s, all based on a reference Site Class II (medium firm). The Characteristic Period (Tg) has to be adjusted based on the actual site class.

Instead of the peak ground acceleration map, Seismic Intensity is used for design purposes, and it is determined by the peak ground acceleration as shown in Table 2. The Seismic Intensity level used for design is to be adjusted based on building types

and site classes (Yu et al. 2010). The Appendix of the Chinese Seismic Design Code GB50011-2010 also provides a Tabulation of Seismic Intensity, PGA and Seismic Group for all administrative districts (county or above).

Table 1 Quantified Definition of the Three-Level Hazards - China

Level of seismic hazard Quantified definition of the hazard

Probability of exceedance during 50 years Return period (years)

Frequent earthquake 63% 50 Moderate earthquake 10% 475

Rare earthquake 2% - 3% 1640 - 2475

Table 2 Seismic Intensity Designation and Peak Ground Acceleration (Return period =475yrs) - China

Peak Ground Acceleration (g) ≤ 0.05 0.10 0.15 0.20 0.30 ≥ 0.40

Seismic Intensity 6 7 8 9 In the Japanese seismic code, the expected performance of a structure is

conceptually defined at two levels of seismic hazard: medium-level earthquakes and severe earthquakes with return periods of around 50 years and 500 years, respectively. Seismic hazard characteristics are established on the seismic zone (Z) and the characteristic period (Tc) of the site soil. In Japan the entire country is seismically active. Although the country is divided into three zones (A, B and C), there is minor differences among these zones with seismic zone factor ranging from 1.0 to 0.8. For most areas of Japan, seismic zone factor is either 1.0 or 0.9. The characteristic period (Tc) is shown in Table 3.

Table 3 Soil Type Definitions and Characteristic Periods (Tc) - Japan

Soil type

Definition Tc

Type I

Rock, stiff sand or gravel, and other soils mainly from the Tertiary Era or earlier, or any other soil that has been shown by surveys or studies to have a natural period similar to soils mentioned above

0.4

Type II Other than Type I and Type III 0.6

Type III

Alluvium mainly consisting of organic or other soft soil (including fill if any) with a depth of 30m or greater, reclaimed land from swamps or muddy shoal with a depth of 3m or greater and less than 30 years have passed since the reclamation, or any other soil that has been shown by surveys or studies to have a natural period similar to soils above

0.8

CLASSIFICATION OF BUILDINGS

The IBC classifies buildings and other structures as Risk Category from I to IV based on the nature of occupancy (Table 4). Importance factors used in the calculation of snow loads and seismic load effects are assigned to each structure based on its Risk Category. For wind loads, rather than importance factors, there are separate maps of wind speed with different hazard levels that are assigned to each Risk Category. Generally the value of importance factor increases with the importance of the facility. Risk Categories I and II have a seismic importance factor of 1.0. The seismic importance factors for Risk Category III and IV are 1.25 and 1.50, respectively. Structures assigned a greater seismic occupancy must be designed for larger seismic forces. As a result, these structures are expected to experience lower ductility demands than structures with lower occupancy importance factors and, thus sustain less damage. Risk Categories are also a basis for determining Seismic Design Categories, which are keys for establishing the detailed seismic design requirements for any structure.

Table 4 The IBC Classification of Buildings and Other Structures - USA

Risk Category Nature of Occupancy I Representing a low hazard to human life in the event of failure II Except those listed in other categories III Represent a substantial hazard to human life in the event of

failure IV Designed as essential facilities

The Chinese Standard for Classification of Seismic Protection of Building

Constructions GB 50223-2008 (MHURDC 2008) classifies building structures as Building Type A to D with Type A as most important and Type D as less important (Table 5). There is no occupancy importance factor in Chinese code. Instead, the seismic design intensity should be adjusted according to the function of the building structure (Yu et al. 2010).

Table 5 Chinese Standard for Classification of Seismic Protection of Building Constructions - China

Building Type Description A (Extremely

important) Special facilities related to national security or causing secondary disasters in the event of failure.

B (Very important) Facilities designed to be functional during and after earthquake or building representing a substantial hazard to human life.

C (Important) Those not listed in other types.

D (Less important) Representing a low hazard to human life or causing no secondary hazard.

There is no classification of buildings based on occupancy in the Japanese seismic

code. A building does have to be classified as one of the four groups based on

building height. The purpose of this classification is for the selection of the seismic design procedure.

SOIL OR SITE CLASSIFICATION

The 2012 IBC classifies each site as one of six site classes from A to F based on one of three soil properties measured over the top 100 feet. The three properties are soil shear wave velocity, standard penetration resistance and soil undrained shear strength. Site Class A is hard rock which is typically found in the eastern United States. Site Class B is softer rock typical of the western parts of the country. Site Class C, D or E indicates progressively softer soils. Site Class F indicates soil so poor that a site-specific evaluation is needed to determine appropriate site coefficients.

The 2010 Chinese seismic code first classifies a site as Favorable, Common, Unfavorable and Hazardous (Table 6). Some restrictions apply at unfavorable or hazardous sites as shown in Table 6. The code also classifies a site as one of four classes from I to IV depending on the equivalent shear wave velocity and the effective soil depth which is generally measured from ground surface to the soil layer with shear wave velocity greater than 500m/s.

Table 6 Site Classification of Chinese Seismic Code

Site Classification (I)

Definition Restrictions

Favorable Rock or stiff soil

Common Except those listed in other categories

Unfavorable Soft soil, liquefiable soil and other unfavorable conditions

Be avoided for all construction except where appropriate measures are being taken

Hazardous

Areas with hazards of landslide, avalanche, subsidence, ground fissure or debris flow

Prohibited for Building Types A and B. Not suggested for Building Type C

The Japanese seismic code classifies a site as one of three types (per Table 3).

Type I is rock or other hard soil and Type III defines soft soil. Type II soils are those other than Type I or Type III. There is no soil factors in the Japanese seismic code, but instead the soil effects are implicitly considered in the design response spectrum, which indicates that for a building with natural period greater than 1.5 seconds, the factor can be calibrated as 1.0 for Type I (hard soil), 1.5 for Type II (medium soil) and 2.0 for Type III (soft soil).

DESIGN RESPONSE SPECTRUM

Figure 1 illustrates the design response spectrum specified by the 2012 IBC. The point TS (TS = SD1/SDS) corresponds to the period which divides the short-period range from the long-period range. The point T0 equals 20% of the value of TS. TL is long-period transition period which marks the transition between long period and very long period. Relatively few structures have such a long period to fall into this range. The TL maps are also included in the code.

The Chinese seismic code specifies the seismic influence coefficient curve (α) which is comparable to a response spectrum (Figure 2). In fact, the curve expresses two levels of seismic hazard, the frequent earthquake or rare earthquake depending on the value of αmax (maximum seismic influence coefficient). Table 7 gives the values of αmax for the three levels of seismic hazards. Seismic design forces are determined based on the αmax for the frequent earthquake multiplied by a load factor of 1.3. Characteristic Period of Response Spectrum, Tg, is the period where the transition to long-period range occurs. T= 5Tg is the point corresponding to the period which divides the nonlinear relationship (between α and T) and linear relationship. The parameters, γ, η1, η2, in Figure 2 are determined by Equations 1-3, respectively.

ζζ

γ63.0

05.09.0+−

+= Eq. 1

ζζ

η32405.002.01 +−

+= Eq. 2

ζζ

η6.108.0

05.012 +−

+= Eq. 3

where ζ is the damping ratio and η2 ≥ 0.55.

Table 7 Maximum Horizontal Seismic Influence Coefficient (αmax) - China

Seismic Intensity 6 7 8 9

PGA Zone < 0.10g 0.10g 0.15g 0.20g 0.30g ≥ 0.40g

Frequent earthquake1 0.04 0.08 0.12 0.16 0.24 0.32 Moderate earthquake2 0.12 0.22 0.32 0.42 0.60 0.80

Rare earthquake3 (elasto-plastic drift

check) 0.28 0.50 0.72 0.90 1.20 1.40

1, 2, 3 See Table 1 for definitions of “Frequent earthquake”, “Moderate earthquake” and “Rare earthquake”.

Based on Table 7, as return period lengthens, the hazard (i.e. peak ground acceleration) range deviation gets larger (Yu et al. 2010). For the moderate earthquake with a return period of 475 years, the PGA of Seismic Intensity 6 is about 3 times that of frequent earthquake, and the PGA of Seismic Intensity 9 is about 2.5 times that of frequent earthquake. However, for a rare earthquake with a return period of around 2000 years, the PGA of Seismic Intensity 6 is about 7 times that of the

frequent earthquake, and the PGA of Seismic Intensity 9 is only about 4.4 times that of the frequent earthquake. For areas designated with low or moderate seismicity, designs based on hazards of the shorter return period of frequent earthquakes could have inadequate protection if a severe earthquake occurs (Chock, 2010).

In the Japanese seismic code, the design spectral coefficient (Rt) curve is comparable to a response spectrum. A design response spectrum as shown in Figure 3 can be constructed by multiplying design spectral coefficient Rt by the seismic zone factor Z and the standard shear coefficient C0. Depending on the value of C0, the curve in Figure specifies two levels of seismic hazard, moderate earthquakes (when C0 = 0.2 or 0.3 for wood construction at a site with soft soil) or severe earthquakes (when C0 = 1.0).

Figure 4 shows a comparison of the U.S. and Japanese design spectra. The parameters used for constructing the spectra are given in Figure 4. While the U.S. design spectral acceleration for the short period is similar to the Japanese spectra for severe earthquake, for the long period the Japanese design spectral acceleration is higher.

Figure 1 USA Design Response Spectrum, Sa

max2αηαγ

⎟⎟⎠

⎞⎜⎜⎝

⎛=TTg

max12 )]5(2.0[ αηηα γgTT −−=

Tg 5Tg 6.0 T(s)

η2αmax

α

0.45αmax

00.1

Figure 2 China Seismic Influence Coefficient, α

Figure 3 Japan Design Response Spectrum, Sa

Figure 4 The U.S. and Japan Design Spectra

SEISMIC BASE SHEAR

For the equivalent lateral force procedure, ASCE-7, GB 50011 and the Japanese seismic code multiply seismic weight by a factor and use a similar equation to calculate base shear (Table 8). However, there are some essential differences in determining the factors CS, α1 and Ci.

The base shear in ASCE -7 is determined based on the design ground motion which is about two-thirds of the soil-modified and risk-adjusted maximum considered ground motion (MCER). In contrast, two levels of seismic load effects (frequent/moderate earthquake and rare/severe earthquake) are given both in the Chinese seismic code and the Japanese seismic code.

ASCE-7 uses the response modification factor, R, to account for the inherent ductility and overstrength of seismic lateral resisting systems. Basically the R-factor decreases the design base shear for those systems with good earthquake performance. For example, the R-factor for a special concrete moment frame system is 8 and a ordinary concrete moment frame (with much less stringent detailing and other requirements) has a R-factor of 3.5. Seismic load effects for the special moment frame may have less than half of the ordinary moment frame. The R-factor ranges from 1.5 to 8.

No response modification factor (R) appears in the GB 50011 base shear equation. A lateral system and its components are required to meet the code required detailing

so that only minor damage would occur during a moderate earthquake. Adjustment factors (γRE) ranging between 0.75 and 1.0 are used to modify seismic capacity of structural components based on the type of material and type of structural components (generally seismic capacity is increased by dividing the nominal capacity by this γRE factor).

As stated earlier, the Japanese seismic code adopts a two-phase design procedure. In the first-phase design for moderate earthquake, allowable stress design is employed to determine structural configuration and dimensions. The structure is required to respond elastically to seismic loads specified for moderate earthquake by the Japanese seismic code. The second-phase design is to explicitly check lateral system overstrength given that system ductility is sufficient during severe earthquake. Ds is a structural characteristics factor to account for the contribution of ductility. The factor ranges from 0.25 to 0.55 which means seismic load effects are reduced in a factor of 4.0 to 1.8 due to the contribution of ductility. Thus the required overstrength factor ranges from 1.0 for a ductile system to 2.2 for a non-ductile system.

The Japanese seismic code implicitly uses a constant equivalent R factor (= 4) for all building systems when the building’s ultimate strength has to be checked explicitly (Uang, 1991). This value is only approximately half the highest values used by ASCE-7. Even smaller equivalent R factors are adopted if a simpler design procedure is used.

In ASCE -7 seismic importance factor (I) increases the base shear for an Risk Category III or IV structure with a factor of 1.25 or 1.5, respectively. The Chinese code does not explicitly include such an importance factor and the issue is addressed by adjusting the seismic fortification intensity according to the building type. The Japanese code does not adjust the base shear for occupancy categories.

The Chinese Seismic Code requires that besides dead loads, seismic weight have to include 50% uniform live load for residential or office buildings and 80% uniform live load for library stack rooms or archive rooms. In ASCE-7 no live load is required to be included in seismic weight except a minimum of 25% of floor live load for storage area plus partition weight assuming partitions can be rearranged.

The seismic load effect (E) has a load factor of 1.0 when combining factored loads using strength design according to ASCE-7 or the IBC. The Chinese seismic code assigns a load factor of 1.30 to the horizontal seismic load effect (SEhk) when combining the load effect of the frequent earthquake (50-year return period) with other loads to perform strength design. No load factor is assigned to seismic load effect in the Japanese seismic code.

Table 8 Base Shear Calculations

Subject The U. S.

code (ASCE – 7)

The Chinese code (GB

50011)

The Japanese code (BSL) Moderate

earthquake Severe earthquake

Base shear V = CSW FEk = α1 Geq ∑=

=n

ijjii WCQ udessun QFDQ =

Seismic design

coefficient

CS= SDS/(R/I)

α1 seismic influence coefficient determined by Figure 2

Ci = Z Rt Ai C0

Qun = Required ultimate lateral strength

Qud =

∑=

=n

ijjiti WCAZRQ 0

Ds = structural characteristics factor

Fes = a factor accounting for eccentricity and vertical stiffness distribution

Upper limit

CS ≤ SD1/(T)(R/I) for T ≤ TL CS ≤ SD1 TL /(T2)(R/I) for T ≥ TL

Governed by Figure 2

Governed by Figure 3

Lower limit

CS ≥ 0.01 CS ≥ 0.5S1/(R/I) for S1 ≥ 0.6g

Governed by Figure 2

Governed by Figure 3

Seismic weight W Geq

Wi = weight of the j-th story

STRUCTURAL ANALYSIS PROCEDURE

Approaches to analyze a structure subject to seismic load effects includes static methods (linear or nonlinear) and dynamic methods (response spectrum analysis or time history analysis).The more complex the building is and the greater the seismic hazard, the more rigorous analysis needs to be performed. Table 9 summarizes the requirements of ASCE-7, GB 50011, and the Japanese seismic code when selecting the structural analysis procedure. The analysis procedure required by ASCE-7 is based on seismic design category and building configuration. GB 50011 requires that the analysis procedure be selected based on dynamic properties, building height, regularity and seismic hazard.

In Japanese seismic code, usually two-phase design needs to be performed. At first, a building is classified as Group from 1 to 4 based on building height. Then a design route is selected based on the building group. The first-phase design is allowable stress design and the structure responds elastically so any linear analysis is acceptable. For the second-phase design, when it is needed to check lateral system overstrength, usually nonlinear static or dynamic analysis (pushover or nonlinear time history analysis) is required. The second-phase design is not required for a building up to 31m in height, provided the building is of regular shape (Trembley et al. 1996). A structure taller than 60m is required to be designed by special study and usually

nonlinear time history analysis needs to be employed. The design is subject to technical peer review and the Japanese Ministry of Land, Infrastructure and Transport has final authority.

Table 9 Permitted Analytic Procedures

Analysis procedure

ASCE 7-10 GB 50011-2010

Japanese Code

Equivalent static

approach

1. Structures assigned to Seismic Design Category A, B or C 2. Structures assigned to Seismic Design Category D, E or F with the following characteristics: a) light-frame construction; or b) Regular structures with T<3.5Ts or c) Irregular structures with T<3.5Ts and not having torsional irregularities and not having vertical stiffness, weight or geometric irregulaties.

Structures with a linear first mode shape or structures not higher than 40m and with dominant shear deflection and vertically uniformly distributed stiffness

Applicable to first-phase design

Modal response spectrum analysis

All structures

Structures not permitted for equivalent static procedure

Applicable to first-phase design. For second-phase design, nonlinear pushover or nonlinear time-history analysis usually is required to check ultimate lateral strength

Time history analysis

Extremely irregular structures, Type A structures or those high-rise structures listed in Table 10.

Table 10 Criteria for when the Time History Analysis Should be Selected for the High-Rise Buildings in China

Intensity or Site class Building height limit (m) Intensity 8 and Site Class I or II

Intensity 7 >100

Intensity 8 and Site Class III or IV >80 Intensity 9 >60

DRIFT AND DEFLECTION

The design story drift (Δ) shall be computed as the difference of the deflections at the top and bottom of the story under consideration. ASCE-7 uses Eq. 4 to calculate the deflection of Level x.

IC xed

xδ

δ = Eq. 4

where Cd = the deflection amplification factor depending on type of seismic lateral

resisting system; δxe = the deflection determined by an elastic analysis using design seismic load

effect; and I = the importance factor.

Table 11 Allowable Story Drift, Δa (ASCE-7) - USA

Structure Occupancy Category I or II III IV

Structures, other than masonry shear wall structures, 4 stories or less with interior walls, partitions, ceilings and exterior wall that have been designed to accommodate the story drift

0.025hsx* 0.020hsx 0.015hsx

Masonry cantilever shear wall structures 0.010hsx 0.010hsx 0.010hsx Other cantilever shear wall structures 0.007hsx 0.007hsx 0.007hsx

All other structures 0.020hsx 0.015hsx 0.010hsx * hsx = the story height below Level x

The Chinese seismic code GB 50011 requires that the elastic story drift (Δue) is determined using seismic load effects of the frequent earthquake level with load factor of 1.0. Δue should not be greater than the allowable story drift in Table 12.

The Chinese code also requires that some structures have to be checked for elasto-plastic story drift criteria under seismic load effects of the rare earthquake event (Table 13). Nonlinear static or nonlinear time history analysis has to be performed to compute the elasto-plastic drift except that a simplified method may be used for concrete moment frames or concrete columns in a single story industrial building with no stiffness irregularity and not higher than 12-stories. Using the simplified method, the elasto-plastic story drift may be calculated from the following equations:

epp uu Δ=Δ η Eq. 5

or yy

pyp uuu Δ=Δ=Δ

ξ

ηµ Eq. 6

where Δup = elasto-plastic story drift Δue = elastic story drift under rare earthquake event ηp = Amplification factor.

The elasto-plastic story drift, Δup, should not be greater than the allowable elasto-plastic story drift as shown in Table 14.

Table 12 Allowable Elastic Story Drift (GB 50011) - China

Structure Allowable story drift (θeh) Concrete moment frame 0.00182h

Concrete moment frame –shear wall interactive system

Concrete shear wall with flat plate floor Moment frame with a tube core

0.00125h

Concrete shear wall or tube within a tube 0.001h Moment frame supporting discontinuous walls 0.001h

Mid-rise or high-rise steel structure 0.0033h

Table 13 Structures Required or Suggested to Check for Elasto-Plastic Story Drift - China

Required Suggested 1. Tall single-story concrete bent frame with Seismic Intensity 9 or Seismic Intensity 8 with site class III or IV. 2. Concrete moment frame with story yielding ratio less than 0.5 and with Intensity 7 , 8 or 9 3. Steel structures higher than 150m 4. All Building Type A structures; and Building Type B: concrete or steel structures 5. Seismically isolated structures or structures with damping systems.

1. High-rise steel structures with height of less than 150m 2. Masonry structures with concrete moment frame supporting structures above 3. Shear wall structure with flat plate floor 4. Building Type B: concrete or Steel structures with Intensity 8 or Intensity 7 with site class III or IV 5. Structures listed in Table 10 and with vertical irregularity, i.e., beams, slabs or trusses supporting discontinuous walls or frames above.

Table 14 Allowable Elasto-Plastic Story Drift (GB 50011) - China

Structure Allowable story drift (θph)

Single story concrete bent frame column 0.0333h Concrete moment frame 0.02h

Ground-level Moment frame or shear wall supporting masonry structure above

0.01h

Concrete moment frame –shear wall interactive system; Concrete shear wall with flat plate floor;

Moment frame with a tube core

0.01h

Concrete shear wall or tube within a tube 0.0083h Mid-rise or high-rise steel structure 0.02h

The Japanese seismic code calculates the story drift (δi) using seismic load effects

of the moderate earthquake level. The story drift angle is then calculated as

i

ii hR δ= Eq. 7

Where hi is the story height. Ri should not exceed 1/200. This limit may be exceeded if nonstructural members are designed to sustain the story drift, but in no case should Ri exceed 1/120. The Japanese code has no difference in the drift control requirement for different occupancies.

The allowable story drifts in Table 11 specified by ASCE-7 are corresponding to maximum inelastic deformation. Based on Equation 1, the allowable story drift (Δa’) corresponding to design seismic force level can be calculated as

ICda

aΔ

=ʹΔ Eq. 8

where Cd varies from 1.0 to 6.5. For a special steel moment frame structure, with standard occupancy and nonstructural elements not designed to accommodate the story drift, the allowable story drift corresponding to design seismic force is

( ) sxsx

d

aa hhI

C0036.00.1

5.5020.0

==Δ

=ʹΔ

The allowable story drift specified by the Japanese seismic code is 0.005 hsx. However, considering the Japanese seismic code uses a constant equivalent R factor (= 4) for all building systems and the US code uses R = 8.0 for a special steel moment frame, resulting a higher base shear the Japanese comparable allowable story drift would be equivalent to

sxsxa hh 0025.0)8/4(005.0 ==ʺΔ which is more stringent than the U.S. seismic code. However, for a less ductile

lateral system with R less than 5.5, the U.S code will become more stringent.

CONCLUSIONS

The main features of the U. S. and Japanese seismic design practices have been discussed and compared. While they have essential common features, there are many considerations on which the U. S., China and Japanese seismic designs have significant differences.

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