TBDY 2018 Seismic Design Guide: Complete Technical Reference

Introduction to TBDY 2018

Turkey's Building Earthquake Code 2018 (Türkiye Bina Antidepreman Yönetmeliği 2018, or TBDY 2018) became effective on January 1, 2019, marking a watershed moment in Turkish seismic design practice. This comprehensive replacement of the 2007 Building Earthquake Code represents a paradigm shift from empirical, zone-based design to a modern, performance-based engineering framework. TBDY 2018 incorporates two decades of earthquake research, including lessons learned from devastating seismic events in Turkey and globally, and aligns Turkish standards with international best practices found in ASCE 7, Eurocode 8, and other leading seismic design codes.

The code's primary objective is to ensure that buildings and infrastructure withstand earthquakes with controlled damage, protecting human life while enabling recovery. Unlike prescriptive codes that dictate specific solutions, TBDY 2018 defines performance objectives that designers can meet through multiple approaches, fostering innovation and allowing economy in design.

Turkey Earthquake Hazard Map (TDTH): Moving Beyond Zones

Perhaps the most significant advance in TBDY 2018 is the replacement of the old seismic zone system with the Turkey Earthquake Hazard Map (Türkiye Deprem Tehlike Haritası, or TDTH). Developed by the Disaster and Emergency Management Authority (AFAD) in collaboration with earthquake engineering experts, the TDTH provides site-specific seismic hazard parameters for any coordinate location in Turkey, rather than assigning buildings to broad geographic zones.

The TDTH defines four earthquake design levels, each with assigned return periods:

• DD-1 (Frequent Earthquakes): 72-year return period; used for operational level checks
• DD-2 (Design Basis Earthquake, DBE): 475-year return period; primary design level for most buildings
• DD-3 (Rare Earthquakes): 975-year return period; used for some performance verifications
• DD-4 (Maximum Considered Earthquake, MCE): 2475-year return period; used for collapse prevention evaluation and isolation design

For each earthquake level and any geographic coordinate, AFAD provides two spectral acceleration parameters measured at the ground surface (after accounting for soil amplification):

• SS: Spectral acceleration at short periods (0.2 seconds)
• S1: Spectral acceleration at 1-second period

These values are obtained from probabilistic seismic hazard analysis (PSHA) using the latest ground motion prediction equations, specifically the Akkar & Bommer (2010) model calibrated to Turkish earthquake data. Engineers can access the interactive AFAD TDTH web map to retrieve parameters for any building location instantly, eliminating the uncertainties inherent in broad zone assignments.

Soil Classification System (ZA Through ZF)

TBDY 2018 classifies soils into six categories based on the shear wave velocity of the top 30 meters (VS30). This follows the framework established in international codes but refined for Turkish soil characteristics:

• ZA (Very Stiff Soil/Rock): VS30 > 1500 m/s; typically bedrock or highly competent rock
• ZB (Stiff Soil): VS30 = 760–1500 m/s; dense sands, gravels, or stiff clays
• ZC (Medium Soil): VS30 = 360–760 m/s; dense sand and gravel mixtures, medium stiff clay
• ZD (Soft Soil): VS30 = 180–360 m/s; loose to medium sand, soft to medium clay
• ZE (Very Soft Soil): VS30 < 180 m/s; soft clay, organic soil, silt; also applies to soft soil >36 m thick
• ZF (Special Soils): Requires site-specific evaluation; includes liquefiable soils, plastic clays, and other problematic materials

The spectral acceleration values from TDTH are already adjusted for the average soil conditions of Turkey. However, engineers performing site-specific studies must account for local soil effects through amplification coefficients. These include:

• FS: Short-period amplification factor (amplifies SS for soil classes ZB–ZE)
• F1: 1-second period amplification factor (amplifies S1 for soil classes ZB–ZE)

Softer soils (ZD, ZE) have larger amplification factors, while stiffer soils (ZA, ZB) have factors near 1.0. For ZF soils, geotechnical investigation and often dynamic analysis are mandatory.

Building Importance Classes (BKS)

TBDY 2018 recognizes that not all buildings require the same level of seismic protection. Four building importance classes (Bina Kullanimi Sinifi, or BKS) assign different seismic design demands:

• BKS = 1 (Critical Buildings): Hospitals, fire and police stations, emergency command centers, power generation facilities, water treatment plants. These must remain operational or suffer minimal damage after earthquakes. Importance factor γ_I = 1.4 (highest)
• BKS = 2 (Essential Buildings): Schools, dormitories, assembly buildings, child care facilities. These have many occupants and societal importance. γ_I = 1.2
• BKS = 3 (Standard Buildings): Residential buildings, offices, retail, hotels, restaurants, and most other structures. γ_I = 1.0 (baseline)
• BKS = 4 (Low-Risk Buildings): Agricultural structures, temporary buildings, and other structures with minimal occupancy. γ_I = 0.8

The importance factor γ_I scales the design spectral acceleration, such that critical buildings experience stronger equivalent ground motions in design. For example, a BKS=1 hospital in a given location experiences 40% higher design forces than a BKS=3 office building at the same site—requiring stronger, more costly construction befitting their post-earthquake role.

Structural Analysis Methods

TBDY 2018 permits engineers to choose from three primary analysis approaches, with requirements depending on building height, regularity, and importance:

1. Equivalent Lateral Force (ELF) Method

The simplest approach, in which the entire ground motion is represented as a static lateral force distribution proportional to height. This method is permitted for regular buildings up to 20 meters (roughly 5–6 stories) in seismic zones of moderate hazard, and up to 10 meters in high-hazard zones. The ELF method rapidly provides initial design estimates and is common in preliminary design and small projects. The base shear is calculated as V = (SDBA / g) × W, where SDBA is the design spectral acceleration, g is gravitational acceleration, and W is the building weight.

2. Modal Response Spectrum (MRS) Analysis

A more rigorous approach suitable for most buildings. MRS analysis calculates the natural frequencies and mode shapes of the structure, then applies the design response spectrum to each mode and combines results statistically (using SRSS or CQC combination). This method accounts for multiple dynamic degrees of freedom and captures frequency-dependent effects of the ground motion. TBDY 2018 requires MRS for buildings taller than 20 meters or with significant irregularities, and for all buildings in the highest seismic hazard regions.

3. Nonlinear Time History (NTH) Analysis

The most sophisticated and realistic approach, in which a detailed nonlinear structural model is subjected to recorded or synthetic earthquake ground motion time series. NTH analysis directly computes deformations, nonlinear behavior, and damage progression. TBDY 2018 mandates NTH analysis for all buildings assigned to BKS=1 (critical buildings) and for BKS=2/3 buildings over 50 meters or with significant vertical or plan irregularities. A minimum of three ground motion records must be used; seven or more is recommended for important buildings. Results are reported as mean or mean-plus-one-standard-deviation values.

Performance Levels and Verification

A defining feature of TBDY 2018 is its explicit performance-level framework. Rather than designing for a single "code-level" earthquake, engineers verify that buildings meet specific performance targets at different hazard levels. Four performance levels are recognized:

• Immediate Occupancy (SH - Sınırlı Hasar): Minimal structural and nonstructural damage; building is immediately usable; permanent drift <0.5%; intended for post-DD-1 (72-year event)
• Life Safety (CAN - Can Güvenliği): Moderate damage but no collapse risk; life safety is ensured; permanent drift <1%; intended for post-DD-2 (475-year event)
• Collapse Prevention (GÖ - Göçme Öncesi): Severe damage, significant permanent drift (1–2%), but structure does not collapse; life safety is marginal; intended for post-DD-4 (2475-year MCE)
• Continued Operation (KK): Building remains fully functional with minimal or no damage; typically required only for critical systems (hospitals, power plants)

TBDY 2018 specifies which performance level is required for each building importance class at each earthquake design level. For a typical BKS=3 building, for example:

• At DD-1 (72-year): Immediate Occupancy (SH) is target
• At DD-2 (475-year, the design basis): Life Safety (CAN) is target
• At DD-4 (2475-year): Collapse Prevention (GÖ) must be verified

Critical buildings (BKS=1) require more stringent targets: Continued Operation at DD-2 and Life Safety at DD-4. This two-level (or three-level) verification ensures that even the largest credible earthquakes do not cause widespread collapse while ordinary design earthquakes cause controlled, repairable damage.

Structural System Types and Response Modification Factors

TBDY 2018 recognizes that different structural systems dissipate earthquake energy differently. A frame system, which relies on ductile bending of beams and columns, can withstand larger deformations before collapse than a brittle shear wall. To reflect this, the code assigns response modification factors (or behavior factors) R to different structural systems:

• Frame Systems (Moment-Resisting Frames): R = 8; high ductility, suitable for tall buildings; requires capacity design and ductile detailing
• Shear Wall Systems (Coupled or Uncoupled): R = 6; moderate ductility; walls must be properly reinforced and anchored
• Dual Systems (Frames + Shear Walls): R = 7; combines frame ductility with wall stiffness; must transfer lateral loads through both subsystems
• Cantilever Wall Systems (Walls Without Coupling): R = 4; lower ductility; used for shorter buildings or where walls are isolated
• Braced Frame Systems (Steel or Concrete): R = 5–7 depending on brace configuration and steel grade; concentrically braced frames lower, eccentrically braced frames higher
• Pendu lum or Inverted Pendulum (Single-Column Supported Tanks, etc.): R = 2; very limited ductility capacity

The design spectral acceleration applied to the structure is reduced by dividing by R: S_D = (S_DA × γ_I) / R. A frame with R=8 experiences only 12.5% of the elastic spectral acceleration; this reduction reflects the structure's capacity to yield, dissipate energy, and deform without collapse. However, this reduction comes with a design cost: the code requires ductile detailing, capacity design, and inspection to ensure the assumed behavior is achieved.

Key Advances from TBDY 2007

Understanding how TBDY 2018 differs from its predecessor (TBDY 2007) clarifies the magnitude of change in Turkish seismic practice:

• Site-Specific Parameters vs. Broad Zones: TBDY 2007 assigned buildings to five seismic zones (1–5) based on maps; TBDY 2018 retrieves unique spectral parameters for each location via TDTH, accounting for local geology and hazard
• Probabilistic Hazard Analysis: TBDY 2018 uses modern PSHA methods and updated earthquake catalogs; TBDY 2007 was based on older, more empirical approaches
• Four Earthquake Levels vs. One: TBDY 2007 designed for a single earthquake; TBDY 2018 explicitly verifies performance at multiple hazard levels (DD-1, DD-2, DD-3, DD-4)
• Performance-Based Design Framework: TBDY 2007 was force-based; TBDY 2018 permits and encourages displacement-based and performance-based design
• Updated R Factors: Response modification factors have been adjusted based on recent research and have different values for different systems
• Soil Amplification Factors (FS, F1): TBDY 2007 used simpler site classes; TBDY 2018's soil classification and amplification factors are more detailed and flexible
• Nonlinear Time History Requirements: TBDY 2007 rarely required NTH; TBDY 2018 mandates it for critical and tall buildings
• Seismic Isolation Recognition: While seismic isolation was mentioned in 2007, TBDY 2018 devotes an entire Section 5B with detailed isolation design requirements, reflecting the growing adoption of isolation in Turkey
• Nonlinear Static Pushover (NSP): TBDY 2018 permits NSP analysis (not available in 2007) as an alternative for certain buildings, allowing engineers to directly compute capacity and demand

Practical Application: Using TDTH and AFAD Data

To apply TBDY 2018 to a specific project, an engineer follows these steps:

1. Locate the Building: Determine the latitude and longitude of the building site.
2. Access AFAD TDTH Interactive Map: Visit the AFAD TDTH web portal (deprem.afad.gov.tr) and enter the coordinates. The map displays SS and S1 values for all four earthquake levels (DD-1 to DD-4) plus default soil amplification factors (FS and F1).
3. Classify the Building: Assign BKS (importance class) based on building use and occupancy.
4. Perform Geotechnical Investigation: Conduct VS30 testing or literature review to classify soil conditions (ZA–ZF). If VS30 data are unavailable, default to ZC (average conditions).
5. Calculate Design Spectral Acceleration: S_D = (S_DA × γ_I × FS or F1) / R, where S_DA is from TDTH, γ_I is the importance factor, FS/F1 account for soil, and R is the response modification factor for the chosen system.
6. Perform Structural Analysis: Use ELF (for simple buildings), MRS (standard approach), or NTH (for critical/tall buildings) to compute forces, stresses, and drifts.
7. Design Elements and Verify Performance: Ensure all members are adequately sized and detailed. Check permanent drifts and damage indices against TBDY 2018 performance targets for each earthquake level.
8. Document Compliance: Prepare calculations and reports demonstrating compliance with TBDY 2018 for submission to building authorities.

Implementation Challenges and Best Practices

Despite its sophistication, TBDY 2018 implementation presents challenges. Many older buildings predate the code and do not meet its requirements; retrofit design is complex and costly. Geotechnical data (VS30) are often unavailable, forcing engineers to use conservative default values. Nonlinear analysis and performance-level verification require advanced software and expertise, raising project costs. Training of engineers, architects, and building officials is ongoing but not yet universal across Turkey.

Best practices for TBDY 2018 compliance include: early site investigation and AFAD data retrieval; consultation with experienced seismic engineers; use of validated structural analysis software; detailed ductile detailing and construction inspection; and peer review for critical buildings. Performance-based design, while more complex, can yield more economical and resilient buildings than force-based approaches.

Conclusion

TBDY 2018 represents a transformative evolution in Turkish seismic design, moving from zone-based prescriptive rules to a modern, performance-level-driven framework. By providing site-specific hazard parameters, multiple analysis methods, and explicit performance targets, the code enables engineers to design buildings tailored to local conditions and societal needs. While implementation requires investment in expertise and analysis, the result is a built environment that is more resilient, better understood, and capable of protecting lives and enabling recovery after earthquakes.

Implement TBDY 2018 for your project: Our analysis tool automatically retrieves site-specific TDTH parameters for any location in Turkey and guides you through the design process.

Also available in Turkish: TBDY 2018 Deprem Yönetmeliği Rehberi on sismikizolasyon.com