General Comparison of Seismic Design between the Chinese Code and the European code

. To promote overseas projects, it is necessary for designers to understand and distinguish the similarities and differences between the Chinese standard GB50011(Edition 2016) and the European standard EN1998. By referring to relevant papers, comparing the ground types, response spectrum, structural importance factors, seismic precaution level and seismic zoning between the GB50011(Edition 2016) and EN1998, it can be concluded that the overall seismic design concepts in the Chinese and European codes are similar but there are some small differences in ground type classification, impact of ground type on seismic action, response spectrum, importance factor, seismic precautionary criterion, seismic precautionary measures, and seismic zone.


GB50011-2010 (Edition 2016) Code for Seismic Design of Buildings
(hereinafter referred as "GB50011 Edition 2016") implemented since August 1, 2016, including provision modification on one appendix and 10 articles compared with the 2010 edition. On the other hand, European standard seismic design code EN1998, is divided into six parts from EN 1998-1 to EN1998-6. Among those, EN 1998-1 applies to the design of buildings and civil engineering works in seismic regions [2]; EN 1998-2 contains the particular requirements and rules applicable to the design of earthquake resistant bridges [3]; EN 1998-3 provides criteria for the evaluation of seismic performance and the design of retrofitting of existing building [4]; EN 1998-4 specifies the design of silos, tanks and pipelines [5]; EN 1998-5 provides design principle and rules for foundation, retaining structure and geotechnical aspects under seismic actions [6] and EN 1998-6 settles requirements, criteria and rules for the design of towers, masts, and chimneys [7].
Comparison studies between GB50011  Besides, with the proposal and promotion of "One Belt and Road Initiatives", Chinese engineering companies are participating in a growing number of international projects and the domestic designers are inevitably facing the challenge of understanding of the international design standards. Increasing attention is paid to the seismic resistance standard which is very important for structural design.
Thus, in order to make clear and comprehensive understanding of the two codes' difference as per the design sequence, this article will be focusing on comparison between the GB50011 Edition 2016 and EN1998 regarding to contents including and not limited to site category division, response spectrum, structure importance factor, seismic categories and criterion, and the division of seismic zones. It will greatly improve the designer's understanding of similarities and differences between the two codes and provide reference for the designer who will do projects as per EN1998. Specific studies for ground type S 1 or S 2 in determining the ground type is compulsory, and possibility of soil failure under seismic action shall also be considered particularly for S 2 . The analysis depth for the average shear wave velocity is 30m in the EN1998.
On the other hand, the GB50011 Edition 2016 has taken both the average shear wave velocity and the thickness of soil layer covering the site into account when defining the ground types. The analysis depth for the average shear wave velocity is taken at the depth less than 20m. The GB50011 Edition 2016 has defined the ground types into 5 categories as follows:

Table2. Ground types in GB50011
The major difference between the GB50011 Edition 2016 and EN1998 is that the GB 50011 has taken account of both average shear wave velocity and embedment thickness in the definition of ground types, while the EN1998 defines the ground type by a single factor of average shear wave velocity and does not consider the impact of soil 30m underneath; The EN 1998 has considered a deeper analysis depth for the average shear wave velocity and it's dividing the ground types into more detailed categories than that in GB50011 Edition 2016 .

The impact of different ground types in designing seismic actions
The ground types defined in GB50011 Edition 2016 is the only factor contributing to the value of characteristic period for response spectrum and does not contribute to the seismic intensity, hence the value of the seismic influence coefficient is consistent for different ground types, except when designing the medium-long period response spectrums. On the contrary, seismic intensity and characteristic period defined in the EN1998 are controlled by the type of ground. The approximate trend as suggested in EN1998 is that values of effective peak ground acceleration and characteristic period increases as the ground hardness decreases.

Comparison in response spectrum
Both standards adopt the response spectrum to describe the action of seismic. The GB50011 Edition 2016 introduces the seismic influence factor in modifying the response spectrum as in Figure 1, while the EN1998 introduces acceleration in the design of elastic response spectrum and with the introduction of behaviour factor q, the EN1998 is able to convert the elastic response spectrum into design response spectrum as in Figure 2 . The value of T B , T C and T D and the soil factor S describing the shape of the elastic response spectrum depend upon the ground type as shown in Table 3: Table3. Values of the parameters describing the recommended Type 1 and Type 2 elastic response spectra Response spectrum shapes defined in the GB 50011 Edition 2016 and the EN1998 are similar as showed above, whereas the EN1998 is using the modification coefficients of T B , T C , T D and S to associate the design response spectrum with its ground types and seismic characteristics.

Importance factors
The GB50068-2001 Unified Standard For Reliability Design Of Building Structures [11] defines the importance factors according to its safety classes, design life and the engineering experience was adaptively applied. The categories were determined as shown in Table 4a and  Table 4b below: EN1998 classifies buildings into 4 importance classes depending on the consequences of collapse for human life, on their importance for public safety and civil protection in the immediate post-earthquake period, and on the social and economic consequences of collapse [2] as shown in Table 5:

Table5. EN1998-1 Importance factors for buildings
It can be told from Table 4 and Table 5 that the importance factors are approximately divided into 4 categories. The importance factor for class I as defined in EN 1998 (which is 0.8) is 11.1% lower than that for minor importance building defined in GB50068 Edition 2016, and the EN1998-1 has taken 9.1% and 21.4% higher value of importance factors in class III and class IV respectively than that in GB50068 Edition 2016.

Comparison in seismic precaution criterion and seismic zone classification between the Chinese standard and
Eurocode.

Seismic precaution criterion
The GB50011-2010 (Edition 2016) has introduced three levels of seismic precaution of building as follows: Level 1 requires that the building to remain undamaged and preserve integrity during and after an earthquake incident with 63% exceedance probability within 50 years (return period equals to 50 years); Level 2 requires the building to maintain repairable during and after an earthquake incident with exceedance probability of 10% (return period of 475 years); and the Level 3 is the criterion for a building to achieve the requirement for Non-collapse during and after an earthquake incident with exceedance probability of 2%~3% (return period of 2000 years).
EN 1998-1 introduces the performance requirements and compliance criteria as two categories: No-collapse requirement is that the structure shall be capable to withstand the seismic events with a exceedance probability of 10% within 50 years (return period of 475 years) without local or global collapse; and the Damage limitation requirement indicates that the structure to be capable to withstand the seismic events with a exceedance probability of 10% within 10 years (return period of 95 years) without the occurrence of damage and associated limitations of use [2].
Following is a graphic comparison between Chinese standard and European standard in seismic precaution levels: It is straight and clear from the comparison above that Chinese seismic design standard implements three levels of seismic precaution of the building while the European standard has two. Minimum requirement defined in EN 1998 (damage limitation) is higher than that in GB50011 (Level 1), and the No-Collapse requirement demands a similar precaution conditions to the Level 2 precaution criterion. In addition, GB50011 Edition 2016 provides criterion designed for seismic event with a return period of 2000 years.
In practice, engineers following GB50011 would perform strength check and displacement check for each component of the structure under LEVEL 1 condition. The LEVEL 2 and LEVEL 3 seismic precaution would be achieved by relative detailing measurements for most regular structure, and additional elastic-plastic analysis is required for the weak story, non-regular structure and building with a specified occupation.
Likewise, the EN1998 imposes strength check for components under "No-Collapse" condition and introduces behaviour factor to adjust considering the ductility of different structural types.

Seismic precautionary measures
To consider the hysteretic dissipation capacity and allow the structure to develop stable mechanism under corresponding seismic loading, the EN1998 and GB50011 Edition 2016 provide seismic resistant provision in detailing design and construction measures in building design and component dimension. Depending on the ductile behaviours and energy dissipation capacity under seismic actions, the EN1998 has classified material in two ductility classes DCM (medium ductility) and DCH (high ductility), while the GB50011 divides from class 1 to 4 according to the structure types and height. The differences between the two codes are listed as follows: 1. Axial compressive force ratio. Axial compressive force is a crucial factor for the local ductility and dissipation capacity for column. The EN1998 points out that the value of normalised axial force Vd shall not exceed 0.65 and 0.55 for DCM and DCH respectively. By contrast, the GB50011 Edition 2016 adopts 0.65 to 0.9 for class 1 to 4. As such, looser requirement is desired by the GB50011 that may lead to a smaller cross section and more vulnerable to intensive earthquake. 2. Longitudinal reinforcement ratio. Longitudinal reinforcement contributes to the major seismic performance for beam and column. Therefore, it is indispensable to apply a minimum ratio of longitudinal reinforcement to ensure strength and ductility for the structure. Comparison between the GB50011 and EN1998 is illustrated in Table 6 and  Table 7: Table6. Minimum longitudinal reinforcement ratio of beam

Table7. Minimum longitudinal reinforcement ratio of column
In a word, the EN1998 demands a slightly higher ratio of longitudinal reinforcement ratio to the GB50011. 3. Critical regions strengthening measures. Critical region is one of the primary parameters in seismic design to prevent brittle failure. Properly enhancement of confinement steel (stirrup) shall be provided in critical regions. Table 8 and Table 9 highlights the comparison in critical regions strengthening measures for beam and column: The comparison indicates that GB50011 Edition 2016 employs slightly stricter requirements than the EN1998. It is noteworthy that the lowest grade of reinforcement used in the EN1998 has a strength of fyk=400MPa which is higher than that in GB50011, providing a better shear capacity and it is beneficial to the development of plastic hinge.

Seismic zones
The GB50011 Edition 2016 classifies seismic zone and describes the characters of seismic spectrum according to peak ground acceleration and characteristic period of acceleration response spectrum as specified in GB18306 Seismic Ground Motion Parameters Zonation Map of China. The revised 2016 version of GB50011 had increased the lowest level of seismic intensity class to VI and expanded the range of each classes to abolish the non-