Which Law Mandates Earthquake-Resistant Design?
The main law is the Ministerial Regulation Prescribing the Load Bearing, Resistance and Durability of Buildings and the Ground Supporting Buildings in Resisting Earthquake Vibrations B.E. 2564 (2021), published in the Royal Gazette on 4 March 2021. It replaced the previous 2007 regulation, expanded the controlled area to 43 provinces, and divided risk into 3 zones.
Paired with the Ministerial Regulation is the technical standard DPT 1301/1302-61, Standard for Seismic Resistant Design of Buildings, issued by the Department of Public Works and Town & Country Planning. The current edition is Revision 1 (December 2021), which sets out how to calculate seismic forces, classify soil (Site Class), and detail structural reinforcement. Put simply, the Ministerial Regulation says "where and which buildings must comply", while the DPT standard says "how to do it".
Both work alongside the building control law and the usual foundation requirements — buildings that qualify still need structural calculations certified by an engineer, with a seismic force calculation section added to the report.
Which Areas Does the Law Cover? (43 Provinces, 3 Zones)
The 2021 Ministerial Regulation divides the 43 controlled provinces into 3 zones by risk level. The higher the zone number, the greater the risk and the wider the range of buildings covered by the requirements.
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| Zone | Risk level | Provinces |
|---|---|---|
| Zone 1 (14 provinces) |
Watch — may be affected by earthquakes | Krabi, Chumphon, Trang, Nakhon Phanom, Nakhon Si Thammarat, Bueng Kan, Prachuap Khiri Khan, Phitsanulok, Phetchaburi, Loei, Songkhla, Satun, Surat Thani, Nong Khai |
| Zone 2 (17 provinces) |
Moderate — includes soft soil areas that amplify shaking | Bangkok, Kamphaeng Phet, Chai Nat, Nakhon Pathom, Nakhon Sawan, Nonthaburi, Pathum Thani, Phra Nakhon Si Ayutthaya, Phang Nga, Phuket, Ranong, Ratchaburi, Samut Prakan, Samut Songkhram, Samut Sakhon, Suphan Buri, Uthai Thani |
| Zone 3 (12 provinces) |
High — near active faults in the North and West | Kanchanaburi, Chiang Rai, Chiang Mai, Tak, Nan, Phayao, Phrae, Mae Hong Son, Lampang, Lamphun, Sukhothai, Uttaradit |
What surprises many people is that Bangkok and its surrounding provinces are in Zone 2, even though they are hundreds of kilometers from the faults. The reason is that the area sits on a soft clay basin (Bangkok Soft Clay), which can amplify shaking from distant earthquakes. Tall buildings in Bangkok can therefore feel strong swaying even when the epicenter is in a neighboring country.
Which Building Types Must Be Designed for Earthquakes?
Which building types qualify depends on the zone — Zones 1 and 2 focus on essential and large buildings, while Zone 3 covers the widest range of buildings. The main groups specified by the Ministerial Regulation in Zones 1 and 2 are:
- Buildings essential to public welfare after a disaster: healthcare facilities with overnight inpatients, fire stations, disaster relief centers, communication centers, airports, power plants, and water production or storage plants — these buildings must remain usable immediately after an earthquake
- Buildings storing hazardous materials: warehouses or buildings storing explosives, hazardous chemicals or flammable materials, whose damage would have wide-ranging effects
- Public and large buildings meeting the height/size criteria in the Ministerial Regulation: such as assembly buildings, schools and high-rise buildings. The size and height thresholds differ by zone, so check the numerical criteria in the full Ministerial Regulation or ask the design engineer
In Zone 3, the highest-risk zone, the requirements cover a wider variety of building types and smaller sizes than other zones. Anyone planning to build in the North or West should therefore have an engineer check at the planning stage whether the building qualifies.
How Does Earthquake-Resistant Design Differ from Ordinary Building Design?
Ordinary buildings are designed mainly for vertical loads, while earthquake-resistant buildings must also be designed for horizontal shaking forces, with emphasis on the ductility of the structure. The main differences come down to 3 points.
- Additional horizontal forces: an earthquake shakes a building mainly horizontally. A structure that previously carried only downward loads must also resist lateral forces that reverse back and forth, so columns, beams and joints are checked against a different set of forces than usual.
- Structural ductility: the key idea is to let the structure sway and deform without sudden collapse. This is achieved by placing stirrups/ties more closely at critical locations and detailing beam-column joints so they are well confined, allowing the building to "absorb" energy from the shaking.
- Regular building form: buildings with a symmetrical form and evenly distributed weight twist less when shaken, while buildings with offset shapes, floating columns or an open ground floor (soft storey) are at greater risk of damage and need special compensating design.
In practice, the engineer selects an analysis method permitted by DPT 1301/1302-61 to suit the building's height and regularity, from the equivalent static force method for ordinary buildings to dynamic analysis for tall or irregular buildings. For an overview of the whole structural design process, read A guide to RC structural design and civil engineering
Do Ordinary Houses Need Earthquake-Resistant Design?
Most small houses in Zones 1 and 2 are not yet subject to the Ministerial Regulation, but in Zone 3 the requirements cover a wider range of buildings. Homeowners in the North and West should therefore always have an engineer check whether their house qualifies.
Even if a house is not required to comply, designing for seismic forces is an option homeowners in risk areas can choose for themselves. The extra cost lies mainly in the amount of ties and joint detailing, which is usually worthwhile compared with the damage it can prevent, especially for houses on soft soil or near fault lines. For guidance on the foundation side in soft soil, read How to design earthquake-resistant foundations on soft soil
The size, height and types of buildings that qualify in each zone have specific numerical criteria in the Ministerial Regulation, and local officers have discretion in their review. This article gives an overview for understanding only. Deciding whether a particular building qualifies and how it must be designed must be done directly by the engineer responsible for that project.
How Is the Soil Related to Seismic Forces? (Site Class and Soil Investigation)
The seismic force a building receives depends not only on its distance from the fault but also on the type of soil beneath it — soft soil amplifies shaking more than stiff soil. DPT 1301/1302-61 therefore requires the soil to be classified into a Site Class before calculating forces, because two identical buildings on different soils will receive different design forces.
This is why Bangkok is a controlled area even though it is far from the faults — the Bangkok soft clay basin acts like jelly in a cup, amplifying shaking from distant earthquakes. Classifying the Site Class correctly requires actual soil layer data from a soil investigation, such as the thickness of the soft clay, SPT N-values and the properties of each layer, which also go into the design criteria of the structural calculations
SPN's soil investigation fee for residential houses starts at approximately THB 20,000–40,000 (2–3 boreholes, including basic lab tests and a report certified by an engineer). This price excludes travel and equipment mobilization, which are charged extra according to the distance to the site. For the specific risk of liquefaction in saturated sand, read Earthquakes and foundations: which soils are at risk of liquefaction
This content summarizes criteria from the 2021 Ministerial Regulation and DPT 1301/1302-61 for general understanding only and is not design advice for any particular project. The earthquake-resistant design of each building must be carried out by a licensed engineer (COE license) directly responsible for that project.