SOME ATTENTIONS ON STRUCTURAL DESIGN STANDARDS IN CURRENT CONDITIONS OF VIETNAM
Ха Ву
Инженер, Архитектурный университет Ханои, Вьетнам
НЕКОТОРЫЕ ОСОБЕННОСТИ СТАНДАРТОВ СТРУКТУРНОГО ПРОЕКТИРОВАНИЯ В СОВРЕМЕННЫХ УСЛОВИЯХ ВЬЕТНАМА
Аннотация
Проектирование, учитывающее надежность работ, является популярной точкой зрения на проектирование в мире. Статья анализирует метод, который берет за основу надежность конструкции. Рассматриваются системы стандартов Соединенных Штатов Америки, России, Европы и Вьетнама. Автор разъясняет необходимость добавить фактор надежности конструкции при вычислении и разработки структуры в целом. Наконец, автор дает 2 предположения о том, как использовать фактор надежности в практическом структурном проектировании во Вьетнаме.
Ключевые слова: надежность, стандарт, структура, сейсмическая нагрузка, ветровая нагрузка.Hа Vu
Engineer, Hanoi Architectural University, Vietnam
SOME ATTENTIONS ON STRUCTURAL DESIGN STANDARDS IN CURRENT CONDITIONS OF VIETNAM
Abstract
Design taking into consideration the "importance" of the works is a popular design viewpoint in the world. The article analyses the method which takes the importance of the construction extracted from the Standard Systems of the United States of America, Russia, Europe and Vietnam into account. In which, the author clarifies the need to supplement the important factors of the construction when calculating and designing the structure in general. Finally, the author gives two notices on how to use the important factor in practical structural design in Vietnam.
Keywords: importance, standard, structure, seismic load, wind load.
1.Introduction
The taking into consideration of the importance of the works requires both types of information: Classification, decentralization of works based on their importance; and respective importance factor values. Standard systems of the US, Russia, Europe have this information. However, the norm and standard system of Vietnam only has QCVN 03/2012/BXD "Principles for classification, decentralization of civil, industrial works and urban infrastructure", without respective importance factor values. Therefore, the research and consideration on the work importance method in structural design in current conditions of Vietnam has an important role, in the context Vietnam is strongly developing its social infrastructure.
2.The method of taking into consideration importance of works in standard systems
2.1. American Standard ASCE 7-2010
ASCE 7-2010 decentralizes works into 04 risk categories from I to IV, in which category I is the lowest, category IV is the highest[1].
Table 1 - Risk category of Buildings and Other Structures for Flood, Wind, Snow, Earthquake, and Ice Loads
| Use or Occupancy of Buildings and Structures | Risk Category |
| Buildings and other structures that represent a low risk to human life in the event of failure | I |
| All buildings and other structures except those listed in Risk Categories I, III, and IV | II |
| Buildings and other structures, the failure of which could pose a substantial risk to human life. | III |
| Buildings and other structures, not included in Risk Category IV, with potential to cause a substantial economic impact and/or mass disruption of day-to-day civilian life in the event of failure. | |
| Buildings and other structures not included in Risk Category IV (including, but not limited to, facilities that manufacture, process, handle, store, use, or dispose of such substances as hazardous fuels, hazardous chemicals, hazardous waste, or explosives) containing toxic or explosive substances where their quantity exceeds a threshold quantity established by the authority having jurisdiction and is suffi cient to pose a threat to the public if released. | |
| Buildings and other structures designated as essential facilities. | IV |
| Buildings and other structures, the failure of which could pose a substantial hazard to the community. | |
| Buildings and other structures (including, but not limited to, facilities that manufacture, process, handle, store, use, or dispose of such substances as hazardous fuels, hazardous chemicals, or hazardous waste) containing sufficient quantities of highly toxic substances where the quantity exceeds a threshold quantity established by the authority having jurisdiction to be dangerous to the public if released and is sufficient to pose a threat to the public if released. | |
| Buildings and other structures required to maintain the functionality of other Risk Category IV structures. |
(Source: ASCE 7-2010 Minimum design loads for buildings and other structures)
Based on the guideline on decentralization of buildings, ASCE 7-2010 gives importance factor corresponding to categories as follows:
1) For seismic load
Table 2 - Importance Factors by risk Category of Building and Other Structures for Earthquake Load
| Risk Category | Seismic Importance Factor |
| Ie | |
| I | 1.00 |
| II | 1.00 |
| III | 1.25 |
| IV | 1.50 |
(Source: ASCE 7-2010 Minimum design loads for buildings and other structures)
For equivalent horizontal static method, Ie value is multiplied directly into the bottom shear force V. For response spectrum method, under Article 12.9.2, Ie value is multiplied into the response spectrum, and then such response spectrum is used to calculate design parameters of interest such as floor eccentricity, internal force in structures, ...Thus, the use of importance factor for seismic load according to ASCE 7-2010 of the US is similar to ISO 9386/2012 of Vietnam.
2) For wind load
In ASCE 7-2005 , the wind pressure qz at height z is calculated by the formula (1):
qz = 0.613×Kz×Kzt×Kd×V2×I (N/m2) (1)
In which:
Kd - wind direction factor;
Kz – velocity pressure exposure coefficient according to the height and land forms;
Kzt – topographic factor;
V - basic wind speed (3-second gust, measured at 10 meter height from ground, the standard terrain is C, recurrence interval: 50 years), in m/s;
I - importance factor for wind load.
Table 3 - Importance Factor for Wind Load according to ASCE 7-2005 of the US
| Category | Importance Factor for Wind Load | |
| Non-Hurricane Prone Regions and Hurricane Prone Regions with V = 85-100 mph and Alaska | Hurricane Prone Regions with V > 100 mph | |
| I | 0.87 | 0.77 |
| II | 1.00 | 1.00 |
| III | 1.15 | 1.15 |
| IV | 1.15 | 1.15 |
(Source: ASCE 7-2005 Minimum design loads for buildings and other structures)
In addition, in basic combinations for calculation under extreme conditions where wind load is dominant, the wind load is multiplied by the combination factor of 1.6 (article 2.3.2, ASCE 7-2005 of the US). In ASCE 7-2010 of the US, wind pressure qz at height z is calculated using the formula:
qz = 0.613×Kz×Kzt×Kd×V2 (N/m2) (2)
In which, factors Kd, Kz and Kzt still take the same as ASCE 7-2005
The importance factor does not appear in the formula (2). Instead, ASCE 7-2010 gives three wind velocity maps A, B, C (with base wind speed of 3 second gust, measured at 10 meter height from the ground, standard terrain is C) for different categories of works. In details:
- Figure A: For category II buildings and other structures. The excess probability is 7% within 50 years (Mean Recurrence Interval-MRI =700 years).
- Figure B: For category III and IV buildings and other structures. The excess probability is 3% within 50 years (MRI= 1700 years).
- Figure C: For category I buildings and other structures. The excess probability is 15% within 50 years (MRI= 300 years).
Thus, ASCE 7-2010 of the US divides three types of figure for different categories of buildings. In ASCE 7-2010 the wind load combination factor in basic combinations where wind load is dominant is 1, because the recurrence interval of wind load takes the maximum prescribed for each category rather than the conventional recurrence interval of 50 years for wind load as in ASCE 7-2005. In addition, when calculating flood, snow, ice loads, the American Standard ASCE 7 also considers the importance factor.
2.2. Russian Standard System
Under Article 4 of the Russian Federation Law No. 384-FZ signed on 30/12/2009 "Technical regulations on the safety of buildings and other structures,"[11] buildings and other structures are divided into three levels of importance: high, medium and low. Clause 7, Article 26, of this law stipulates that: Safety calculations of structural solutions of buildings and other structures must be made taking into account the importance of the buildings and other structures which are being designed. With this purpose, the calculated value of internal forces in structural components and foundations of buildings and other structures must be determined taking into account the importance factor which should not be less than the following values:
- 1.1 for buildings and other structures of high importance;
- 1.0 for buildings and other structures of medium importance;
- 0.8 for buildings and other structures of low importance.
Thus, the taking into consideration of the importance of works through decentralization and use of importance factor in structural calculation is required under the Russian Federation Law. On that basis, GOST R 54257-2010 standard "Reliability of construction structures and background: basic principles and requirements"[9] gives more detailed provisions on work decentralization based on their importance. Accordingly, depending on social, economic and environmental consequences caused by damaged or demolished buildings and other structures are decentralized into categories 1a (special importance), 1b (high importance), 2 (normal importance) and 3 (low importance). 1a and 1b are smaller grades of category 1 according to the Russian Federation Law 384-FZ dated 20.12.2009.
Based on the work category, GOST R 54257-2010 of the Russian Federation gives the minimum value of importance factor in the following table:
Table 4 - Minimum values of importance factor
| Category | Minimum values of importance factor |
| 1a | 1,2 |
| 1b | 1,1 |
| 2 | 1,0 |
| 3 | 0,8 |
(Source: TCVN 9386: 2012. Earthquake resistant work design)
Specific categories according to the importance and importance factor value are determined by the designer and approved by the investor, but not lower than the values listed in Table 4. Different structures of the buildings allow different use of importance factors. The importance factor is multiplied directly with impact consequences determined by calculating basic load combinations according to the first limit state. When the calculation is based on the second limit state, the allowed importance factor is 1. For special load combinations (for example, combinations containing seismic load), the rules mentioning the importance of works are specified separately in design standards or design tasks. It is understood that, if the specialized design standard (eg shock-resistant design standard) has its own regulations on the importance, these regulations should be followed and the importance factor above is unused. Former versions of shock-resistant design standards (SNiP II-7-81 and SNiP II-7-81*) gave no guideline on classification of works by the importance.
The new standard SP 14.13330.2011 [10] has added new guidelines on classification of works by the importance and had respective importance factors (Table 3) separately for earthquake resistant design. The new shock-resistant design draft of Vietnam has been compiled on the basis of this standard of Russia.
Table 5 - Factor K0 is determined according to the work function
| Function of buildings and other structures | Value K0 |
| 1. Memorial buildings and other structures; large-scale theaters, sport centers and concert spaces of over 2000 seats, governmental buildings of high importance, radio stations with a total generating capacity of over 500 W | 2.0 |
| 2. Buildings and other structures: - Their function should be maintained upon earthquake and when remedying consequences (governmental information house, house for emergency rescue services and police; water and energy supply systems; fire prevention and gas supply works, works containing large amounts of poisons and explosives, possibly causing great damages to residential areas, health authorities with equipment to be used in accidents); - In which appearing danger to people inside (hospitals, schools, kindergartens, railway stations, airports, museums, theaters, circuses, concerts and sports spaces, roofed markets, commercial complexes with presence of more than 300 people at the same time, more than 16 floor buildings); - House and other buildings, if suspended, can lead to severe damage on economy, society and environment. | 1.5 |
| 3. Buildings and other structures not listed in 1 and 2 | 1.0 |
| 4. Temporary works with shelf life of less than 3 years | 0.75 |
(Source: TCVN Draft "Work design in earthquake areas", 2014)
2.3. European standards
Appendix B of the European Standard EN 1990 [3] has instructions on how to classify works based on the consequences caused by damaged works (Table 6) as follows:
Table 6 - Definition of consequences classes
| Consequences class | Description | Examples of buildings and civil engineering works |
| CC3 | High consequence for loss of human life, or economic, social or environmental consequences very great | Grandstans, public buildings where consequences of failure are high ( e.g. a concert hall) |
| CC2 | Medium consequences for loss of human life, social or environmental consequences considerable | Residential and office buildings, public buildings where consequences of failure are medium (e.g. an office building) |
| CC1 | Low consequences for loss of human life, and economic, social or environmental consequences small or negligible | Agricultural buildings where people do not normally enter (e.g. storage buildings), greenhouses |
(Source: Table B1 - EN 1990-2002 Basis of structural design)
Section B.2 and B.3 of European standard EN 1990 refers to classification of reliability of works (Reliability class - RC). Accordingly, works are divided into three types of reliability RC1, RC2 and RC3. However, the relationship between concepts RC1, RC2, RC3 with CC1, CC2, CC3 is mentioned generally that they can suitable with each other In addition, EN 1990 also gives classification by the design life and applicable region to determine the effect live load. According to the author, the guidance in Appendix B of EN 1990 is quite complicated and difficult to be applied in practiced design, due to lack of specific indications as other standard systems. Eurocode 8 [4] divides works into 04 categories from I to IV (Table 7).
Table 7 - Importance classes for buildings
| Buildings | Importance class (Category) |
| Buildings of minor importance for public safety, e.g agricultural building, etc. | I |
| Ordinary building, not belonging in the order categories. | II |
| Buildings whose seismic resistance is importance in view of the consequences associated with a collapse, e.g school, assembly halls, cultural institutions etc. | III |
| Building whose integrity during earthquakes is of vital importance for civil protection, e.g. hospitals, fire stations, power plants, etc. | IV |
(Source: BS EN 1998-1: 2004 Design of structures for earthquake resistance. Part 1 - General rules, seismic actions and rules for buildings)
Importance factors for categories are 0.8, 1, 1.2, 1.4 respectively and directly multiplied in reference ground acceleration value agr to be turned into the ground acceleration value used in design.
2.4. Vietnam standard
The regulation system (QC), standard system (TC) of Vietnam has QCVN 03/2012 "Principles for classification, decentralization of civil, industrial works and urban infrastructure”[6] gives method for classification, decentralization of works, aimed at determining economic – technological solutions and approving construction design, investment projects. Accordingly, works are divided into five categories: special category, categories I, II, III, IV. In which the special category is the highest, category IV is the lowest. However, in regulation and standard system of Vietnam, except shock-resistant design standard ISO 9386:2012 [7], the importance factor of buildings and other structures corresponding to the decentralization methods stated in NTR 03:2012/BXD is not mentioned to. In means that in structural calculations with basic load combinations, there is no difference between the special category works and normal category works. Particularly, the shock-resistant design standard TCVN9386:2012 refers quite specifically to the work decentralization and its respective importance factor as follows [8]:
- Works are decentralized into 5 categories according to their importance, similar to QCVN 03:2012;
- Special category: required to be calculated with the greatest possible acceleration value without stating the importance factor value γI, in particularly:
- Category I: γI = 1.25;
- Category II: γI = 1;
- Category III: γI = 0.75;
- Category IV: Unnecessary to calculate earthquake;
- Method: Like Eurocode 8, multiplied directly in the reference ground acceleration agr. The Vietnam standard system for purpose of calculating structure is primarily based on Russian standard system, but absolutely without concept of the importance factor, although there has had specific provisions on decentralization of works. Only shock-resistant design standard TCVN 9386:2012 based on Eurocode 8 has importance factor.
Based on the analysis above, a comparison between systems can be given as follows (Table 8):
Table 8 - Summary and comparison of methods taking into consideration importance factor in structural calculations
3.Conclusion
Through studies and analysis of a number of standard systems commonly used in Vietnam, we can see that the decentralization of works by importance and taking it into consideration in structural calculations are necessary. Especially with the wind load, one of regularly appeared loads when designing civil and industrial projects. However, standard systems of Vietnam have no specific regulations. In addition, although in Vietnam, snow load is less common, it may still occur in the northern mountainous regions of Vietnam, so adding this type of load into the standard system of Vietnam is necessary
The method of taking into consideration the importance of structural design and practice is to use "importance factor". Looking through structural design standards mentioned in this article, there are two attentions on using “importance factor”
Firstly, applying separate "importance factor" for some certain loads. For example, wind, earthquake, snow, ice, flood loads, such as the American standard ASCE 7-2010). The remaining loads (dead load, live load) are used in designs not depending on work categories.
Secondly, using an "importance factor" for all basic loads (frequent load, temporary load), and a separate importance factor for special loads (eg seismic load), as mentioned in the Russian standard.
For basic loads, Vietnam standard system does not mention to the importance factor, although main design standards of Vietnam are compiled on the basis of Russian standard system. This fact requires an addition of the method taking into consideration “the importance” of works in general structural design calculations to Vietnam standard system. In addition, there should have studies on how to determine importance factor in countries and how to apply this importance factor in conditions of Vietnam on the basis of the design criteria for safety and economy, suitable with conditions of Vietnam.
References
- 1. ASCE 7-2005 Minimum design loads for buildings and other structure
- 2. ASCE 7-2010 Minimum design loads for buildings and other structure
- 3. BS EN 1990:2002 Basis of structural desig
- 4. BS EN 1998-1:2004 Design of structures for earthquake resistance. Part 1 – General rules, seismic actions and rules for building
- Vietnam Standard Draft "Work design in earthquake areas", 2014.
- Nguyen Xuan Chinh. Translation draft EN 1990-2002.
- QCVN 03/2012 «Principles for classification, decentralization of civil, industrial works and urban infrastructure».
- TCVN 9386: 2012. Earthquake resistant work design.
- GOST R 54257-2010 «Reliability of constructions and foundations. Вasic principles and requirements».
- SP 14.13330.2011 Seismic Building Design Code. Updated Version of SniP II-7-81*.
- Russian Federation Federal law N384-FZ of December 30, 2009. «Technical regulation on safety of buildings and structures».