Can TBDY 2018 Save Istanbul?

Earthquake Safety in Urban Transformation, the Reality of Shear Walls, and the Code-Implementation Dilemma

Urban transformation in Istanbul is often discussed in public discourse through the lens of renewal, value appreciation, and zoning density debates, while earthquake safety is largely viewed as a technical obligation limited to regulatory compliance. However, the Turkish Building Earthquake Code (TBDY 2018), which came into effect in 2018, has not merely introduced new calculation methods but has imposed a performance-based earthquake safety approach that fundamentally transforms construction practices in Istanbul. This study questions the extent to which TBDY 2018 can truly guide Istanbul’s urban transformation and discusses the tension between the engineering principles envisioned by the code and the architectural, economic, and planning practices in the field, in light of technical data.

Within the scope of this study, load-bearing systems with low and high shear wall ratios were comparatively examined for 8 and 10-story reinforced concrete residential buildings under soil classes representing Istanbul conditions. The results reveal that in systems where the shear wall ratio is kept below 15%, the controlled damage performance target cannot be reliably achieved, especially in mid and high-rise structures. Brief case evaluations conducted in the Avcılar and Zeytinburnu districts show that although TBDY 2018 provides a strong technical framework at the building scale, it is significantly eroded in practice due to parcel-based transformation, commercial ground floor demands, and inadequate supervision. In this context, the study argues that earthquake safety in Istanbul can be achieved not only through stricter regulations but also through holistic planning approaches and strong implementation policies.

Introduction

Urban transformation in Istanbul is not merely a process carried out for physical renewal or real estate value appreciation. Rather, this process is a mandatory engineering intervention in terms of reducing earthquake-related loss of life and property. A significant portion of the building stock was designed and constructed according to inadequate codes in effect before the 1999 Marmara Earthquake. In this context, TBDY 2018 has become the fundamental technical reference document enabling Istanbul’s re-evaluation against earthquake risk.

Paradigm Shift in Earthquake Hazard Definition

With TBDY 2018, the concept of earthquake zones used in Turkey for many years has been abandoned. Instead, spectral acceleration values determined based on geographical coordinates for each structure are taken as the basis. In the case of Istanbul, this approach has yielded extremely critical results due to the city’s non-homogeneous soil structure. Within the same provincial boundaries, and even between neighboring parcels, design earthquake loads can show significant differences. This situation has made the use of standard template projects in urban transformation projects technically problematic.

Performance-Based Design Approach

One of the most important innovations of TBDY 2018 is making the performance-based design approach mandatory. In this approach, buildings are classified not only according to whether they will collapse during an earthquake but also according to the level at which they can continue their functions after an earthquake. While the controlled damage performance target is generally adopted for residential structures in Istanbul, immediate occupancy performance is envisioned for critical structures such as hospitals, schools, and transportation facilities. This approach has brought about higher rigidity and ductility requirements in the dimensioning of load-bearing systems.

Istanbul Soil Conditions and Local Soil Effects

Istanbul’s geological structure clearly demonstrates the importance of the detailed soil classification envisioned by TBDY 2018. Class ZD and ZE soils are commonly found throughout the city, and in some areas, special problematic soils are defined. These soil conditions cause amplification of earthquake motion and increase the lateral loads to which structures are subjected. For this reason, raft foundations, pile foundations, and soil improvement methods are widely used in urban transformation projects.

Load-Bearing System Preferences and Architectural Consequences

TBDY 2018 has brought shear wall load-bearing systems to the forefront, especially in areas with high earthquake hazard. A significant increase in shear wall ratios is observed in new urban transformation structures built in Istanbul. While this situation positively affects the earthquake behavior of structures, it brings some limitations in terms of architectural design. Large spans, flexible plan schemes, and commercial ground floor demands often conflict with code requirements.

Evaluation of Existing Buildings and Strengthening Approaches

TBDY 2018 defines linear and nonlinear analysis methods in detail for the evaluation of existing buildings. In applications conducted in Istanbul, it is observed that a significant portion of reinforced concrete buildings cannot meet the target performance levels. Although strengthening is technically possible, reconstruction is often preferred economically. This situation shows that the code has an indirect accelerating effect on urban transformation.

Problems Encountered in Practice

Despite the technical adequacy of the code, the mismatch between design and construction site emerges as a significant problem in Istanbul applications. Inadequate supervision, cost pressures, and implementation errors prevent the safety level envisioned by the code from being fully reflected in the field. This situation reveals that urban transformation needs to be supported not only by legislation but also by an effective supervision mechanism.

Comparative Case Study: 8 and 10-Story Reinforced Concrete Buildings

In this section, a comparative evaluation of 8 and 10-story reinforced concrete residential buildings considered for Istanbul conditions under TBDY 2018 is presented. For both structures, soil class ZD, concrete class C35, and the target performance level of Controlled Damage, commonly accepted for residential buildings, were determined. Load-bearing systems were examined under two separate scenarios with low and high shear wall ratios.

In the evaluation conducted for the eight-story building, in the system where the shear wall ratio was kept at 8%, the first natural vibration period was calculated at approximately 1.10 s. When the shear wall ratio was increased to 18%, the same structure became significantly stiffer and the period decreased to approximately 0.70 s. While this change led to higher acceleration demands on the design spectrum, it resulted in approximately 60% reduction in relative story drift. In the low shear wall ratio system, the maximum relative story drift remained at approximately 14‰ and the structure performance was evaluated at the controlled damage limit. In the high shear wall ratio system, this value decreased to approximately 6‰ and the structure approached limited damage performance.

A similar evaluation conducted for the ten-story building shows that the effect of shear wall ratio on structural behavior becomes even more pronounced as the number of floors increases. In the ten-story system with an 8% shear wall ratio, the first mode period was predicted to be approximately 1.35 s, while in the system with an 18% shear wall ratio, the period receded to approximately 0.85 s. Relative story drift in the low shear wall ratio system reached approximately 17‰, falling below the controlled damage target, and the structural behavior remained at the collapse prevention level. In the high shear wall ratio system, the maximum relative story drift occurred at approximately 7‰ and the controlled damage performance target could be achieved.

When these results are evaluated together, it is seen that TBDY 2018 has effectively made the shear wall ratio a determining design parameter for mid and high-rise reinforced concrete residential buildings in regions with high earthquake hazard like Istanbul. However, in practice, it can be said that shear wall ratios are often kept at minimum levels due to architectural freedom and economic concerns, which, although satisfying performance targets on paper, creates uncertainty in terms of actual earthquake behavior. Especially in high-rise urban transformation projects, determining the shear wall ratio only to meet the minimum code limits contradicts the performance-based design philosophy envisioned by TBDY 2018.

District-Based Brief Case Evaluation: Avcılar Example

Avcılar is one of the districts where the effects of TBDY 2018 can be observed most clearly in Istanbul. The thick alluvial layers and high groundwater levels prevalent throughout the district cause the soil class to be evaluated largely in the ZD-ZE range. These soil conditions lead to amplification of the design spectrum in the medium and long period regions and make reinforced concrete buildings in the 8-12 story range particularly critical.

In urban transformation projects carried out in Avcılar, a significant increase in shear wall ratios is observed in the post-TBDY 2018 period. However, in practice, it is seen that shear wall continuity is weakened in some projects due to commercial ground floor demands and parking arrangements. This situation, even if code requirements are met on paper, can cause the structure to become exposed to torsional irregularity and increased story drift in terms of actual earthquake behavior. The Avcılar example shows that although TBDY 2018 is technically adequate, it can provide the expected risk reduction effect to a limited extent unless supported by local planning decisions and implementation practices.

District-Based Brief Case Evaluation: Zeytinburnu Example

Zeytinburnu district is one of Istanbul’s most sensitive areas in terms of earthquake risk due to its location on the Marmara Sea coast and its extensive fill areas. The soft soil conditions commonly seen in the district cause amplification of earthquake waves and the effect of long-period ground motions. This situation increases relative story drift demands, especially in mid and high-rise reinforced concrete buildings.

In new structures built within the scope of urban transformation in Zeytinburnu, although shear wall ratios have increased with the effect of TBDY 2018, the interaction of buildings with each other is often overlooked due to the parcel-based transformation approach. In the district where adjacent construction is prevalent, rigidity and floor height differences of neighboring buildings increase the risk of pounding during earthquakes. In this context, the Zeytinburnu example reveals that the safety approach TBDY 2018 provides at the building scale may remain limited unless supported by planning decisions at the block and neighborhood scale.

Policy and Implementation Recommendations

The technical evaluations and case studies presented in this study show that for TBDY 2018 to be effectively implemented in Istanbul’s urban transformation process, not only engineering but also administrative and planning-oriented policies need to be developed. First, it is important to prepare region-specific design guidelines that go beyond the minimum code conditions regarding shear wall ratios in districts with high earthquake hazard.

In addition, encouraging holistic transformation models at the block or neighborhood scale instead of parcel-based urban transformation applications will enable the earthquake behavior of buildings to be evaluated taking environmental interactions into account. In the implementation phase, independent and effective supervision mechanisms that will ensure TBDY 2018-compliant projects are implemented one-to-one in the field need to be strengthened.

Finally, establishing a common technical language based on TBDY 2018’s performance-based approach between local governments and project authors will contribute to a safer and more sustainable construction process by reducing the conflict between architectural, economic, and structural goals in urban transformation projects.

Conclusion and Evaluation

The technical evaluations conducted within the scope of this study reveal that TBDY 2018 in Istanbul’s urban transformation process is not merely a regulatory text but also provides an engineering framework that fundamentally transforms construction practices. The coordinate-based earthquake hazard definition, performance-based design approach, and detailed soil classification directly affect the load-bearing system preferences of new buildings in Istanbul.

Case studies on eight and ten-story reinforced concrete residential buildings show that achieving the controlled damage performance target becomes difficult, especially in mid and high-rise structures, in systems where the shear wall ratio is kept below 15%. In this context, TBDY 2018 has made load-bearing systems with high shear wall ratios, rigid and ductile, a technical necessity in Istanbul, independent of architectural and economic preferences.

In conclusion, Istanbul’s urban transformation truly reducing earthquake risk depends on the holistic implementation of the engineering principles envisioned by TBDY 2018 in the design, implementation, and supervision stages. The complete implementation of the code’s technical requirements not only in projects but also in the field is of critical importance in terms of reducing casualties in a possible major earthquake.

References

AFAD (2018). Turkey Earthquake Hazard Map. Disaster and Emergency Management Presidency.

TBDY (2018). Turkish Building Earthquake Code. Ministry of Environment, Urbanization and Climate Change.

İBB (2020). Istanbul Province Soil and Earthquake Interaction Studies. Istanbul Metropolitan Municipality.