Foreword

QCVN 02:2009/BXD was compiled by the Institute of Construction Science and Technology, submitted for appraisal by the Department of Science, Technology and Environment, and promulgated by the Ministry of Construction in Circular No.: ……./2009/TT-BXD dated ….. …… 2009 by the Minister of Construction. This regulation replaces the Appendices: 2.1; 2.2; 2.3; 2.5; 2.6; 2.8; 2.12 of Volume III, Vietnam Construction Regulations issued by the Ministry of Construction under Decision No. 439/BXD-CSXD dated September 25, 1997.

CHAPTER 1 GENERAL PROVISIONS

1.1 Scope of Application

The natural environmental data for construction in this Regulation is applicable to the formulation, appraisal, approval of construction activities including construction planning, preparation of construction investment projects, construction design, construction execution, construction supervision, and management of construction investment projects in Vietnam.

1.2 Terminology

In this Regulation, the following terms are defined as follows:

1) Typhoon is a tropical cyclone with maximum winds of grade 8 or higher and may include gusts. Grade 10 to 11 typhoons are called strong typhoons; grade 12 and above are called super typhoons.

Note: Wind grades are calculated using the Beaufort scale (Table 3.2, Appendix of Chapter 3)

2) Tornado is a high-velocity rotating column of air formed within a narrow range and dissipates in a short time.

3) Flood is the phenomenon of river water rising to a high level over a certain period of time, then receding.

4) Flash flood (or waterspout) is a flood that occurs in mountainous areas during high-intensity rains, creating torrential flows. Flash floods have great destructive power and occur suddenly.

5) Storm surge is the phenomenon where sea level rises higher than normal tidal levels due to the influence of a typhoon.

6) Thunderstorm is a strong convective phenomenon of the atmosphere causing sudden electrical discharges accompanied by thunder and lightning.

7) Lightning is the phenomenon of electrical discharge in the atmosphere during a thunderstorm.

8) Lightning strike density is the number of electrical discharges to the ground per 1 km² in a year.

9) Earthquake (also called seismic activity) is the shaking of the earth’s surface, caused by sudden movements of geological blocks along geological faults within the earth (called tectonic earthquakes), volcanic eruptions (called volcanic earthquakes), collapses of caves, landslides, meteorite impacts and man-made explosions.

10) Hypocenter is the point where an earthquake originates, where earthquake energy is released and transmitted to the surrounding space in the form of elastic waves, causing shaking of the earth’s surface. The epicenter is the vertical projection of the hypocenter on the earth’s surface.

11) Earthquake magnitude is a quantity representing the amount of energy that an earthquake releases in the form of elastic waves. Earthquake magnitude is assessed using the Richter scale, with a value equal to the base 10 logarithm of the maximum amplitude (microns) of the horizontal component of the seismic wave on the record of a standard Wood Anderson short-period seismograph at a distance of 100 km from the epicenter.

12) Earthquake intensity is a quantity representing the intensity of shaking it causes on the ground surface and is assessed using scales that categorize the degree of impact of an earthquake on buildings, structures, objects, animals, people and ground deformation. Earthquake intensity is commonly assessed using the MSK-64 (Medvedev-Sponheuer-Karnik) scale.

13) Atmospheric salinity is the total amount of chloride salts in the air in terms of the number of milligrams of Cl^– ions deposited on 1m² of structure surface in one day and night (mg Cl^–/m².day)

1.3 Data in the Regulation

1.3.1. The data in this Regulation includes: Meteorological data; weather data and adverse natural phenomena; wind pressure and velocity data for design; lightning strike density data and earthquake data for design.

1.3.2. Data on geotechnical engineering, hydrogeology, hydrology, soil mineralization, apply according to Appendices 2.9 ÷ 2.11, Volume III, Vietnam Construction Regulations issued by the Ministry of Construction under Decision No. 439/BXD-CSXD dated September 25, 1997.

1.4 Data Sources

1.4.1 Data sources for Chapter 2

The meteorological data in Chapter 2 is taken from “Climate Data for Construction Design” provided in TCVN 4088:1985 after being reviewed in 2008.

1.4.2 Data sources for Chapter 3

• Weather data and adverse natural phenomena data are compiled from published statistical data of the National Center for Hydro-Meteorological Forecasting – Ministry of Natural Resources and Environment.
• Tidal data in the East Sea and the distribution of storm surge heights with a 5% assurance frequency are extracted from the “Vietnamese Hydro-Meteorological Atlas” issued in 1994.
• Atmospheric salinity data is kept the same as Appendix 2.12 of Vietnam Construction Regulations Volume III, issued in 1997.
• Data on earthquake epicenters, seismogenic faults, and earthquake-generating zones are provided by the Institute of Geophysics – Vietnam Academy of Science and Technology.

1.4.3 Data sources for Chapter 4

Wind pressure data for the territory of Vietnam is taken from TCVN 2737:1995 “Actions and influences – Design standards”.

1.4.4 Data sources for Chapter 5

Data on lightning strike density over the territory of Vietnam is established based on the map of average annual lightning strike density over the territory of Vietnam provided by the Institute of Geophysics.

1.4.5 Data sources for Chapter 6

Earthquake data for the territory of Vietnam is established based on the 1:1,000,000 scale zoning map of peak ground acceleration for the territory of Vietnam provided by the Institute of Geophysics.

CHAPTER 2 METEOROLOGICAL DATA

2.1 Characteristics of Vietnam’s Climate

2.1.1 Climatic Seasons

Vietnam belongs to the humid tropical monsoon climate zone. The territory of Vietnam is divided into two regions, North and South, with different climates.

a) The North (from 16 degrees north latitude, level with the Hai Van Pass, northwards): Has a cold winter. In the lowlands, the coldest month has an average temperature of 10 – 150C;

b) The South (from 16 degrees north latitude, level with the Hai Van Pass, southwards): Does not have a cold winter. In the lowlands, it is hot year-round and clearly divided into two seasons: the rainy season and the dry season. The rainy season is from May to October, the dry season is from November to April.

2.1.2 Sunshine, Air Temperature, Solar Radiation

Sunshine: Throughout the territory, sunshine duration is long. Average annual sunshine hours: North < 2000 hours, South > 2000 hours;

Air temperature: The North has an average annual temperature below 240C; the South has an average annual temperature of 24 – 280C;

Solar radiation: Abundant radiation. The average annual total radiation in the North is < 586 KJ/cm²; in the South is > 586 KJ/cm².

2.1.3 Air Humidity and Weather Seasons

Throughout the territory, the relative humidity of the air is high year-round: 77 – 87%. In some places, during certain periods, there can be large changes in air humidity, leading to special periods and weather conditions.

a) Drizzle, cold and damp period

In the North, during the winter monsoon period (Northeast monsoon), there is often wet and damp drizzle, the relative humidity of the air is high, sometimes saturated.

b) Muggy weather

In the eastern part of the North and the coastal areas of the Central region, around the end of winter and beginning of spring (February to April), there is often muggy weather: The air has a temperature of 20 – 250C and very high relative humidity, over 95%, sometimes saturated.

c) Hot and dry weather

In the low-lying areas east of the Truong Son mountain range and the valleys of the Northwest region, in the summer, there are hot and dry westerly, northwesterly, and southwesterly winds that are active for 10 – 30 days in a year. The weather becomes hot and dry, with temperatures above 350C and relative humidity below 55%.

2.1.4 Rain, Snow

Throughout the territory, the amount of rain and the number of rainy days per year are relatively large, with an average of 1,100 – 4,800 mm and 67 – 223 days. Rain is unevenly distributed and concentrated in the rainy months. Many rainstorms have high intensity, many continuous and prolonged rainy spells, causing floods.

Throughout the territory, there is no snow except for a few times a year and on a few high mountains in the North.

2.1.5 Construction Climate Zones

The territory of Vietnam is divided into two climatic regions: the northern construction climate and the southern construction climate. Each climatic region has different climatic zones (see the map in Figure 2.2).

2.1.5.1 Northern Construction Climate

The northern construction climate includes 4 zones.

Zone IA – Climate of the Northwest mountains and the Truong Son range:

• Includes the provinces of Lai Chau, Dien Bien, Son La, the western parts of Hoa Binh, Thanh Hoa, Nghe An, Ha Tinh and Quang Binh, Quang Tri, Thua Thien Hue provinces;
• Most of this region has a cold winter. The lowest temperature can drop below 00C in the North and below 50C in the South of the region. In the high mountainous areas of the North, there is a possibility of frost and snowfall;
• This region is affected by hot and dry weather. In low valleys, the highest temperature can exceed 400C. The Northwest region is not influenced by the sea, the climate has many continental characteristics, with large daily temperature amplitudes. Except for some low areas in the North and the southern tail, in this region, the requirements for cold protection are equal to those for heat protection. The heating period can last 60 – 90 days;
• Over most of this region, there is one dry season each year that nearly coincides with the cold period. There is no period of drizzle, cold dampness or muggy weather. Rain has high intensity and is unevenly distributed;
• This region is less affected by typhoon winds but tornadoes may occur.

Zone IB – Climate of the Northeast and Viet Bac mountains:

• Includes the provinces of Cao Bang, Lang Son, Ha Giang, Tuyen Quang, Bac Kan, Thai Nguyen, Lao Cai, Yen Bai, the northeastern part of Hoa Binh province, the western part of Hanoi, Quang Ninh, Phu Tho, the northern part of Vinh Phuc, Bac Giang;
• This is the region with the coldest winter. The terrain height is an important factor in the formation of the climate. Low temperatures can drop below 00C, with the possibility of frost and snowfall in high mountains. Summer is less hot than in the lowlands, but in low valleys, the highest temperature can exceed 400C. In this region, cold protection is more necessary than heat protection, especially at night and in high mountainous areas. The heating period can last over 120 days;
• Except for a short dry period, the climate is generally wet. There is a lot of rain, flash floods can occur in many areas.
• The coastal part of this region is directly affected by typhoons.

Zone IC – Climate of the Red River Delta:

• Includes the entire plains and midlands of the northern half of the provinces and cities: Bac Giang, Bac Ninh, Vinh Phuc, the western part of Hoa Binh, Hanoi, Quang Ninh, Hai Phong, Hai Duong, Hung Yen, Thai Binh, Ha Nam, Nam Dinh, Ninh Binh;
• This region has a cold winter but being near the sea, it is less cold than zone IB. The temperature and humidity amplitudes are lower than those in zones IA and IB. The lowest temperature is less likely to drop below 00C in the North and 50C in the South. The highest temperature can reach 400C. There is a lot of rain with quite high intensity.
• Typhoons directly affect the coastal provinces.

Zone ID – Climate of South North and North Central Vietnam:

• Includes Thanh Hoa, Nghe An, Ha Tinh, Quang Binh, Quang Tri, Thua Thien Hue.
• The highest temperature can reach 420C to 430C due to the direct influence of hot and dry weather. In this region, heat protection is important but it is also necessary to provide shelter from cold winds in winter. There is a lot of rain with quite high intensity. The wet and dry seasons are not homogeneous.
• Typhoons have a direct impact on the entire region, strongest in the coastal areas.

2.1.5.2 Southern Construction Climate

The southern construction climate includes 3 zones.

Zone II_A – Climate of the South Central Coast:

• Includes the entire plains and low hills below 100m in the provinces and cities: Quang Nam, Quang Ngai, Binh Dinh, Phu Yen, Khanh Hoa, Ninh Thuan, Binh Thuan
• The climate is basically tropical monsoon, without a cold winter (except for the northern part which still has a slightly cold winter). The lowest temperature is usually not below 100C. The highest temperature can exceed 400C. Due to the influence of the sea, both the daily and annual temperature amplitudes are small. Cold protection is not needed in the region;
• The coastal part is directly affected by typhoons.

Zone II_B – Climate of the Central Highlands:

• Includes the entire mountainous area above 100m in the southern half of the provinces and cities: Gia Lai, Kon Tum, Dak Lak, Dak Nong, Lam Dong, the western part of Quang Nam, Da Nang, Quang Ngai, Binh Dinh, Phu Yen, Khanh Hoa, Ninh Thuan, Binh Thuan, the northern part of Dong Nai, Binh Duong and Binh Phuoc;
• Mountainous, tropical climate. The northern part in winter is slightly affected by the Northeast monsoon, the degree of cold depends on the terrain elevation;
• In high areas, it is less cold, the temperature in winter months is 4 to 50C higher than zone IB, in other areas over 50C. The lowest temperature on high mountain belts is from 00C to 50C, in other areas above 50C;
• Below the low mountain belt, summer is hot, in valleys the highest temperature can reach 400C. At altitudes above 1500m, there is no hot season. The western part has some characteristics of continental climate, with large daily temperature amplitudes similar to the Northwest region. Except for high mountainous areas, the main requirement here is heat protection;
• The rainy and dry seasons are in sharp contrast. Rain intensity is quite high. The dry season often lacks water;
• This region is less affected by typhoon winds.

Zone II_C – Climate of the South:

• Includes the entire plains of the provinces and cities: Dong Nai, Binh Duong, Binh Phuoc, Tay Ninh, Ho Chi Minh City, Ba Ria Vung Tau, Vinh Long, Tra Vinh, Dong Thap, Ben Tre, Long An, Tien Giang, An Giang, Hau Giang, Can Tho, Soc Trang, Kien Giang, Bac Lieu and Ca Mau;
• Each year there are only dry and wet seasons that contrast sharply with each other, corresponding to the two wind seasons and are not homogeneous in the region, the rain intensity is quite high.

2.2 Maps and Data Tables

The meteorological maps and data tables are provided in the Appendix of Chapter 2, including:

2.2.1 Meteorological stations and construction climate zoning map

Table 2.1 Coordinates of meteorological station locations, page 3;

Figure 2.1 Map of meteorological station locations, page 3.

Figure 2.2 Map of construction climate zones, page 3.

2.2.2 Air temperature and relative humidity

Table 2.2 Average monthly and annual air temperature (0C), page 3;

Table 2.3 Average monthly and annual highest air temperature (0C), page 3;

Table 2.4 Average monthly and annual lowest air temperature (0C), page 3;

Table 2.5 Absolute monthly and annual highest air temperature (0C), page 3;

Table 2.6 Absolute monthly and annual lowest air temperature (0C), page 3;

Table 2.7 Average monthly and annual air temperature daily amplitude (0C), page 3;

Table 2.8 Daily variation of air temperature (0C), page 3;

Table 2.9 Average monthly and annual absolute air humidity (mbar), page 3;

Table 2.10 Average monthly and annual relative air humidity (%), page 3;

Table 2.11 Average monthly and annual lowest relative air humidity (%), page 3;

Table 2.12 Absolute monthly and annual lowest relative air humidity (%), page 3;

Table 2.13 Daily variation of relative air humidity (%), page 3;

Table 2.14 Frequency of occurrence of temperature-humidity levels (‰), page 3.

2.2.3 Wind data

Table 2.15 Average monthly and annual wind velocity (m/s), page 3;

Table 2.16 Calm wind frequency (PL %) frequency (P %) and average wind velocity (V m/s) in 8 directions, page 3.

2.2.4 Solar radiation

Table 2.17 Altitude (H) and azimuth angle (A) of the sun (degrees), page 3;

Table 2.18 Total radiation on a horizontal surface (W/m²/day), page 3;

Table 2.19 Diffuse radiation on a horizontal surface (W/m²/day), page 3;

Table 2.20 Direct radiation intensity on 8 vertical surface orientations (W/m²), page 3;

Table 2.21 Diffuse radiation intensity on 8 vertical surface orientations (W/m²), page 3;

Table 2.22 Average monthly and annual total sunshine duration (hours), page 3;

Table 2.23 Daily variation of sunshine duration (hours), page 3;

Table 2.24 Illuminance on a horizontal plane (klx), page 3.

2.2.5 Rain, fog and thunderstorms

Table 2.25 Average monthly and annual rainfall (mm), page 3;

Table 2.26 Maximum daily rainfall (mm), page 3;

Table 2.27 Maximum average rainfall duration (mm), page 3;

Table 2.28 Average monthly and annual number of rainy days (days), page 3;

Table 2.29 Average number of rainy days by level (days), page 3;

Table 2.30 Monthly and annual rainfall (mm) according to assurance frequencies (%), page 3;

Table 2.31 Average monthly and annual number of days with drizzle (days), page 3;

Table 2.32 Average monthly and annual number of days with fog (days), page 3;

Table 2.33 Average monthly and annual number of days with thunderstorms (days), page 3;

Table 2.34 Average monthly and annual total cloud cover (days), page 3;

Table 2.35 Average monthly and annual low cloud cover (days), page 3.

2.3 Use of Meteorological Data

2.3.1 Chapter 2 provides measurement data at meteorological stations. When using, data from the meteorological station closest to the construction site should be used. In addition, it is necessary to further consult actual meteorological data available at the construction site, especially in the following cases:

a) The construction site is located at an elevation that differs greatly from the elevation of the meteorological station;

b) The construction site is located in a terrain with factors that affect meteorological data such as: Mountains, hills, rivers, streams.

If the actual data falls outside the values provided in this Regulation, it is necessary to work with the National Center for Hydro-Meteorological Forecasting to obtain official data.

2.3.2 When designing structures or construction methods for structures subjected to wind effects, the wind data in this chapter should not be applied, but the wind pressure values provided in Chapter 4 must be used for calculations.

CHAPTER 3 WEATHER DATA AND ADVERSE NATURAL PHENOMENA

3.1 Characteristics of Adverse Weather and Natural Phenomena

3.1.1 Typhoons

a) Typhoons are particularly dangerous weather phenomena, causing strong winds that blow off roofs, can topple houses, accompanied by heavy rains that cause serious flooding. The wind grade and their degree of hazard refer to Table 3.2 of this Regulation.

b) In Vietnam, the annual typhoon season is from June to November, the typhoon frequency decreases from North to South. The usual occurrence times are as follows (Table 3.1):

June – September, typhoons frequently make landfall in the Quang Ninh – Thanh Hoa sea area;

July – October, typhoons frequently make landfall in the Nghe An – Quang Binh sea area;

August – November, typhoons frequently make landfall in the Quang Tri – Quang Ngai sea area;

October – December, typhoons frequently make landfall in the Binh Thuan – Ninh Thuan, Binh Thuan – Ca Mau sea area.

Statistics on typhoons making landfall on the coastal regions of Vietnam (1961÷2008) are given in Table 3.3. According to statistical data, typhoons have not been observed to make landfall in Vietnam in February.

3.1.2 Tornadoes

a) Tornadoes are dangerous weather phenomena, causing swirling winds that rise high, blowing off roofs, toppling houses, especially simple houses.

b) According to statistics from the National Center for Hydro-Meteorological Forecasting (Table 3.4): In the North, tornadoes often occur during the transitional periods from winter to summer (April, May), whenever a cold air mass appears; In the South, the number of tornado occurrences is less than in the North and Central regions.

3.1.3 Floods

a) Floods occur during the rainy season, when heavy rainstorms strongly pour water into rivers and streams, causing dike breaches or overflows, resulting in widespread flooding of houses and construction sites.

b) The flood season is the time when floods usually occur, as stipulated in the Regulation on Tropical Depression, Typhoon, and Flood Reporting promulgated together with Decision No. 245/2006/QD-TTg dated October 27, 2006 of the Prime Minister as follows:

• On rivers in the North from June 15 to October 15;
• On rivers from Thanh Hoa to Ha Tinh from July 15 to November 15;
• On rivers from Quang Binh to Ninh Thuan from September 1 to December 15;
• On rivers in Binh Thuan, the South and the Central Highlands from June 15 to November 30.

Historical flood events from 1945 to the present are statistically recorded by the National Center for Hydro-Meteorological Forecasting in Table 3.5.

On the Red River, two exceptionally large flood events occurred in August 1945 and August 1971, causing dike breaches in many places.

3.1.4 Flash Floods

a) Flash floods are a particularly dangerous hydrological phenomenon. In some cases, they have tremendous destructive power and become a natural disaster. Flash floods often occur in mountainous areas, where there are high mountains interspersed with valleys and low rivers and streams, the stability of the soil layer on the surface of the basin is weak due to strong weathering processes, and the vegetation cover is destroyed.

b) Flash flood events in the years 1958 – 2007 are statistically recorded by the National Center for Hydro-Meteorological Forecasting in Table 3.6.

c) Localities where flash floods often occur are: Bac Kan, Cao Bang, Ha Giang, Lai Chau, Lang Son, Lao Cai, Phu Tho, Quang Ninh, Son La, Thai Nguyen, Tuyen Quang, Yen Bai; Binh Thuan, Dak Lak, Quang Binh, Quang Nam.

3.1.5 Thunderstorms

a) Thunderstorms are classified as dangerous weather because lightning in thunderstorms can kill people, cause forest fires, house fires, damage machinery and equipment, especially electronic devices. In Vietnam, thunderstorms occur year-round, but are usually more frequent in the summer. Especially in mountainous or lake areas during hot and humid months.

b) The average monthly and annual number of days with thunderstorms at meteorological stations is given in Table 2.33. The map of average annual lightning strike density over the territory of Vietnam established by the Institute of Geophysics is presented in Chapter 5 of this Regulation.

3.1.6 Earthquakes

a) Large earthquakes can destroy houses and structures, causing serious damage to people and property.

b) Earthquake epicenters and seismogenic faults in the territory of Vietnam are shown on the map in Figure 3.3.

3.1.7 Marine Hydrology

a) Marine hydrological phenomena that may adversely affect houses and structures in the sea areas of Vietnam are the tidal regime, storm surge, wave height during typhoons.

b) The tidal map in the East Sea, the map of storm surge height distribution with a 5% assurance frequency are shown in Figure 3.1 and Figure 3.2. The average wave height corresponding to wind grades refer to Table 3.2.

3.1.8 Atmospheric Salinity

a) Salts in the marine atmosphere combined with high humidity cause strong corrosion of steel structures and reinforced concrete structures in Vietnam.

b) The map of atmospheric salinity zoning is given in Figure 3.4. The distribution of atmospheric salinity for parts of the territory of Vietnam is as follows:

• North (from 16 degrees north latitude and above): [Cl^–] = 0.9854 X^-0.17 , error ±16%
• South (from 16 degrees north latitude and below): [Cl^–] = 3.9156 X^-0.22 , error ±23%

Where:

[Cl^–] – atmospheric salinity, mgcl^–/m².day

X: Distance from the sea to the shore, km.

3.2 Maps and Data Tables

The maps and data tables of adverse weather and natural conditions are provided in the Appendix of Chapter 3, including:

Table 3.1 Frequency of typhoons making landfall on the coastal regions of Vietnam (1961÷2008), page 3;

Table 3.2 Wind grade according to Beaufort scale and wave height, page 3;

Table 3.3 Statistics on typhoons making landfall on the coastal regions of Vietnam (1961÷2008), page 3;

Table 3.4 Distribution of tornado occurrences in some years throughout the country (1971 ÷ 2007), page 3;

Table 3.5 Statistics on historical flood events from 1945 to present (1945 ÷ 2006), page 3;

Table 3.6 Statistics on flash flood events (1958 ÷ 2007), page 3;

Figure 3.1 Tidal map in the East Sea, page 3;

Figure 3.2 Map of storm surge height distribution with a 5% assurance frequency, page 3;

Figure 3.3 Map of earthquake epicenters and seismogenic faults in the territory of Vietnam, page 3;

Figure 3.4 Map of atmospheric salinity zoning, page 3.

CHAPTER 4 WIND PRESSURE AND VELOCITY DATA FOR DESIGN

4.1 Scope of Application

The wind pressure and velocity data provided in Chapter 4 is used for the design calculations of structures and construction methods for structures subjected to wind loads.

4.2 Characteristics of Wind Pressure and Velocity Data

4.2.1 The wind pressure zoning map of the territory of Vietnam (Figure 4.1) is established for a return period of 20 years. The data in the map has been processed from the data of meteorological stations as follows:

a) Wind velocity V0 (m/s) is averaged over a period of 3 seconds, at a height of 10 m above the reference datum, corresponding to terrain category B (which is relatively open terrain, with a few sparse obstructions not higher than 10m).

b) Wind velocity V0 is averaged probabilistically with a return period of 20 years. It is not the maximum velocity in the data set, but it can be exceeded once in 20 years.

c) The value of wind pressure W0 (kN/m2) is determined from wind velocity V0 (m/s) according to the formula:

W₀ = 0,613.V₀².10 (4.1)

4.2.2 On the map, wind pressure is divided into zones IA, IIA, IIB, IIIA, IIIB, IVB and VB (Figure 4.1) by isolines and the boundary lines of the zoning for typhoon influence, where the symbol A denotes areas less affected by typhoons, and B denotes areas affected by typhoons.

4.2.3 The zoning of wind pressure W0 according to administrative localities is established in a table (Table 4.1) based on the wind pressure zoning map at a scale of 1:1,000,000 provided by the National Center for Hydro-Meteorological Forecasting. In addition to the wind pressure value W0, Table 4.1 also provides the wind velocity value V0 with an averaging period of 10 minutes and a return period of 50 years to apply to standards that require wind velocity as input.

4.3 Maps and Data Tables

The maps and data tables on wind pressure are provided in the Appendix of Chapter 4, including:

Figure 4.1 Wind pressure zoning map, page 3;

Table 4.1 Wind pressure zoning according to administrative localities, page 3;

Table 4.2 Wind pressure at meteorological observation stations in mountainous and island areas, page 3;

Table 4.3 Conversion factors for wind pressure from a 20-year return period to other return periods, page 3;

Table 4.4 Conversion factors for wind velocity from a 50-year return period to other return periods, page 3

4.4 Use of Wind Pressure Data

4.4.1 The wind pressure at the construction site is determined in one of two ways: using the wind pressure zoning table according to administrative localities (Table 4.1) or using the wind pressure zoning map of the territory of Vietnam (when higher accuracy is required). Due to the scope of the document, the map in Figure 4.1 is a reduced version, the large-scale map of 1:1,000,000 is stored at the National Center for Hydro-Meteorological Forecasting.

4.4.2 For structures in mountainous and island areas at the same elevation, terrain type, and adjacent to the meteorological observation stations listed in Table 4.2, the design wind pressure value is taken according to the independent values of those stations.

4.4.3 For standards that use wind pressure based on a return period other than 20 years, it is necessary to convert the wind pressure based on the 20-year return period provided in this Regulation to the wind pressure with the return period specified in that standard through the conversion factors given in Table 4.3.

4.4.4 For standards that use wind velocity based on a return period other than 50 years, it is necessary to convert the wind velocity based on the 50-year return period provided in this Regulation to the wind velocity with the return period specified in that standard through the conversion factors given in Table 4.4.

CHAPTER 5 LIGHTNING STRIKE DENSITY

5.1 Characteristics of Lightning Strike Density Data

5.1.1 The lightning strike density data for the territory of Vietnam is provided in the form of a map and according to administrative localities.

5.1.2 The map of average annual lightning strike density over the territory of Vietnam is established by the Institute of Geophysics. On the map, the lightning data is divided into zones according to the lightning strike density (times/km2/year) as follows: Less than 1.4; from 1.4 to 3.4; from 3.4 to 5.7; from 5.7 to 8.2; from 8.2 to 10.9; from 10.9 to 13.7 and greater than 13.7 by isolines of lightning strike density.

5.1.3 The lightning strike density data according to administrative localities is established based on the map of average annual lightning strike density of Vietnam according to the following principle:

a) For localities that fall entirely within a zone with the same lightning strike density on the map, the lightning strike density of that map zone is used.

b) For localities that lie in two zones with different lightning strike densities, the lightning strike density of the locality is taken according to the zone with the higher value.

5.2 Maps and Data Tables

The map and data table on lightning strike density are provided in the Appendix of Chapter 5, including:

Figure 5.1 Map of average annual lightning strike density over the territory of Vietnam, page 3.

Table 5.1 Lightning strike density according to administrative localities of the territory of Vietnam, page 3.

5.3 Use of Lightning Strike Density Data in Design

5.3.1 When designing lightning protection for structures, it is necessary to base on the lightning strike density at the location of the structure according to the data of this Regulation.

In addition, it must be based on other factors including the lightning protection requirements of the structure, the characteristics of the structure (type of structure, height of the structure, function of use), the terrain type where the structure is located, the separation distance to trees or other structures.

5.3.2 The lightning strike density on islands is recommended by the Institute of Geophysics to be taken from 2.5 to 7.0 times/km2/year.

CHAPTER 6 EARTHQUAKE DATA

6.1 Characteristics of Earthquake Data

6.1.1 Earthquake data for the territory of Vietnam is provided in the form of a peak ground acceleration zoning map and according to administrative localities.

6.1.2 The 1:1,000,000 scale peak ground acceleration zoning map of the territory of Vietnam is established and legally responsible by the Institute of Geophysics. The map is established for a return period of 500 years corresponding to ground type A (rock or rock-like formations). The map is divided into zones by isolines of the reference peak ground acceleration value (agR): 0.04 g; 0.08 g; 0.12 g and 0.16 g (g – gravitational acceleration). The agR value at points between two isolines is determined according to the principle of linear interpolation.

6.1.3 The zoning of reference peak ground acceleration according to administrative localities down to the district level is established in a table (Table 6.1) based on the 1:1,000,000 scale peak ground acceleration zoning map of the territory of Vietnam according to the following principle:

a) For each locality, a representative point is selected. This point has defined coordinates listed in Table 6.1 belonging to a ward of a district (for example, Cong Vi ward of Ba Dinh district in Table 6.1) or a town, township of a district (for example, TT An Lac of Binh Chanh district in Table 6.1).

The peak ground acceleration of the representative point in this locality is its own value but is considered the peak ground acceleration value of the entire locality.

b) The peak ground acceleration is given in Table 6.1.

6.1.4 The earthquake intensity on the MSK-64 scale is determined by converting from the peak ground acceleration value through Table 6.2.

6.2 Maps and Data Tables

The map and data tables on earthquakes used in design are provided in the Appendix of Chapter 6, including:

Figure 6.1 Map of 500-year return period peak ground acceleration over the territory of Vietnam (for ground type A), page 3;

Table 6.1 Zoning of peak ground acceleration according to administrative localities, page 3;

Table 6.2 Table for converting peak ground acceleration to earthquake intensity on the MSK-64 scale, page 3.

6.3 Use of Earthquake Data in Design

6.3.1 When designing seismic resistance of structures according to peak ground acceleration, the reference peak ground acceleration agR of the construction site is determined in one of two ways: According to the table of peak ground acceleration zoning by administrative locality (Table 6.1) or, when higher accuracy is required, according to the peak ground acceleration zoning map of the territory of Vietnam (Figure 6.1).

Note: The map in Figure 6.1 is a reduced version, when it is necessary to look up, the large-scale map of 1:1,000,000 stored at the Institute of Geophysics and the Institute of Construction Science and Technology should be used.

6.3.2 When the design standard applied for seismic resistance calculations of structures requires the use of a peak ground acceleration value with a return period other than 500 years or for ground types other than type A as stated in this Regulation, the above values are converted by the peak ground acceleration with a 500-year return period for ground type A of this Regulation multiplied by the corresponding factors. These factors are taken according to the provisions of the design standard.

6.3.3 When designing seismic resistance of structures according to earthquake intensity, it is necessary to convert the peak ground acceleration value agR in Table 6.1 to earthquake intensity on the MSK-64 scale given in Table 6.2.

6.3.4 For special structures that do not allow damage due to earthquakes such as concrete dams with a height of over 100 m; nuclear power plants; column and tower structures higher than 300 m; high-rise buildings over 60 stories; offshore structures … when designing, earthquake data from specific studies must be used but not less than the earthquake data of this Regulation.