By Mubashir · Senior Structural Engineer · May 2026
Why Antalya Is a Seismically Demanding Location
Antalya sits on the southwestern coast of Anatolia, where the African plate's northward movement against the Eurasian plate creates a tectonically complex environment. The region has experienced destructive earthquakes throughout recorded history, and the catastrophic 1999 İzmit and Düzce events — though centred further north — fundamentally reshaped structural engineering practice across all of Türkiye. For project P-2023-091, our entertainment steel support structures in Antalya were designed from the outset with seismic performance as the primary governing design consideration.
What makes entertainment structures particularly challenging in seismic zones is the compounding of dynamic loads. Beyond seismic ground motion, these structures carry crowd-induced vibrations, rotating or reciprocating mechanical equipment, and dynamic wind excitation — all simultaneously. The designer must ensure that none of these dynamic load sources interact resonantly with the structure's natural frequency, while also ensuring the structure can survive rare, high-intensity seismic events with controlled, ductile inelastic behaviour.
ASCE 7-22 Seismic Provisions: The Design Framework
For the Antalya project, we applied the seismic design methodology from ASCE 7-22 Chapter 12 as the analytical framework, calibrated with site-specific seismic hazard data from Türkiye's AFAD (Disaster and Emergency Management Authority). ASCE 7-22 organises seismic design around four key parameters that cascade into every design decision.
The site class — determined from the geotechnical report — characterises the soil's amplification behaviour. Antalya's site was classified as Site Class D (stiff soil), which carries the highest amplification factors for short-period spectral accelerations compared to rock sites. Site amplification factors Fa and Fv modify the mapped reference spectral accelerations to produce the design spectral acceleration parameters SDS and SD1 that drive all subsequent seismic force calculations.
The Seismic Design Category (SDC) is assigned based on SDS, SD1, and occupancy category. With the Antalya site's high spectral accelerations and the entertainment-assembly occupancy of the structure, the project was assigned Seismic Design Category D — the most demanding classification for ordinary structures. SDC D triggers the full suite of AISC 341 seismic provisions.
The response modification factor R determines how much the code permits you to reduce seismic forces based on the structure's anticipated ductility. An ordinary steel moment frame (OMF) has R = 3.5, while a special moment frame (SMF) has R = 8. The higher the R, the lower the design base shear — but in exchange, the structure must meet progressively more stringent ductility detailing requirements. Choosing R is therefore not just a numbers decision; it is a commitment to a specific detailing philosophy throughout every connection in the structure.
The importance factor Ie reflects the consequence of failure. Assembly and entertainment occupancy — where large numbers of people gather — typically attracts Risk Category III or IV designation, elevating Ie to 1.25 or 1.5 respectively. A higher Ie directly increases design base shear: the structure must be designed for larger forces precisely because the human life safety consequences of failure are greater.
How Entertainment Occupancy Elevates the Design Standard
The ASCE 7-22 Risk Category system is often underappreciated by engineers who focus only on member forces. When an entertainment or assembly structure is classified Risk Category III, the entire seismic hazard level shifts: design-level ground motions are checked at a 2/3 × MCE (Maximum Considered Earthquake) level rather than using simplified default assumptions. The resulting SDS and SD1 values are higher, meaning larger design base shear, larger overturning moments, and larger seismic demands on connections and foundations.
For the Antalya supports, this meant that even before we selected a structural system, the elevated importance factor had already committed us to design forces roughly 25% higher than would apply to an ordinary office or industrial structure at the same site. Combined with the Site Class D amplification, the total seismic design base shear was significantly higher than a naive first estimate might suggest.
AISC 341 Seismic Provisions: Ductility by Design
Once SDC D is established, AISC 341 — the Seismic Provisions for Structural Steel Buildings — governs the detailing of the structural steel system. The core philosophy is capacity design: the structure is engineered so that inelastic deformation (energy-absorbing yielding) occurs in designated ductile elements — beam plastic hinges, brace yielding segments — and not in the connections or columns where brittle failure would compromise the whole structure.
This produces a hierarchy of strength requirements. If a beam is designed to yield first, the connection must be stronger than the beam's expected plastic moment capacity: not just the nominal Fy × Zx, but 1.1 × Ry × Fy × Zx, where Ry is the ratio of expected-to-nominal yield stress. For A992 wide-flange sections, Ry = 1.1. This means the actual connection must be sized to remain elastic when the beam has already yielded and strain-hardened — often governing the connection design more severely than the elastic demand forces from the structural analysis.
Compactness requirements for seismic systems are more stringent than the standard AISC 360 compactness limits. For highly ductile members in Special Moment Frames, the limiting width-to-thickness ratio for flanges is λhd = 0.32√(E/Fy), compared to the standard compactness limit of 0.38√(E/Fy). This ensures the member can sustain multiple cycles of large plastic rotation without local buckling degrading its capacity. Section selection must explicitly verify these seismic compactness ratios.
Protected zones — regions of anticipated plastic hinging — must remain free from attachments, notches, weld attachments, and other geometric discontinuities that could initiate premature fracture. In practice, this means coordinating with the architectural and MEP teams to ensure that no equipment support brackets, cladding anchors, or conduit penetrations are placed within the protected zone, which typically extends one beam depth from the face of the column.
Connection Detailing in Seismic Zones
Seismic connection design for the Antalya seismic supports required explicit attention to four areas that would not arise in non-seismic connection design.
Demand-critical welds are complete-joint-penetration (CJP) groove welds whose failure could result in partial or total collapse of the structure. AISC 341 Section A3.4 requires that filler metals used in demand-critical welds achieve minimum Charpy V-Notch (CVN) toughness of 20 ft-lb at −20°F. Standard E70 electrodes may or may not meet this criterion depending on the classification — the project welding specification must explicitly call out the CVN requirement, and the fabricator must provide mill certificates confirming compliance.
Panel zone checks at beam-column connections verify that the column web does not yield prematurely under the moment transferred from the beam flanges, prior to the beam itself reaching its plastic moment. If the panel zone is too thin, it becomes the energy-dissipating element — which is acceptable only if it is specifically detailed for ductile panel zone yielding. Doubler plates are the typical remedy when panel zone shear capacity is insufficient.
Continuity plates (column stiffeners) are required when the column flanges are insufficiently stiff to distribute the concentrated beam flange forces. AISC 341 and AISC Design Guide 13 provide explicit criteria for when continuity plates are required, their thickness, and their weld requirements. In seismic design, continuity plate welds are also classified as demand-critical when they connect to the column flanges in the joint panel zone.
Working in an Unfamiliar Seismic Jurisdiction
One of the most instructive aspects of the Antalya project was navigating the translation between Türkiye's national seismic code (TBDY 2018) and the AISC/ASCE-7 framework that our design methodology is built around. Türkiye's seismic hazard maps — published by AFAD — are based on probabilistic seismic hazard analysis calibrated to local tectonic conditions and are more detailed and accurate for Turkish sites than any approximation from US maps.
Our approach in such jurisdictions: use the local hazard data (AFAD spectral accelerations) as input to the ASCE-7 seismic design procedure, and treat the local code as a floor rather than a ceiling. Where TBDY 2018 requirements were more stringent than AISC/ASCE-7 provisions, we applied the local requirement. Where our AISC-based approach was more conservative, we documented the basis and proceeded with the higher standard.
The lesson for any structural engineer working internationally is that seismic design in an unfamiliar jurisdiction requires active research into the local hazard data sources, not just application of a familiar code framework. A conservative, well-documented approach — with explicit acknowledgement of what assumptions were made and why — is both better engineering and better legal protection for the engineer of record.
Redundancy is not optional in seismic design. The ASCE 7-22 redundancy factor ρ penalises structures where a single element failure would cause significant loss of story shear resistance. Designing in redundancy — multiple load paths, distributed lateral resistance — is both a code requirement and sound engineering practice.
Practical Lessons for Seismic Steel Design
The Antalya project reinforced several principles that apply to any seismic steel support structure. First, the structural system selection (moment frame, braced frame, or hybrid) must precede any member sizing — because the R factor and detailing category determine the design forces, not vice versa. Starting with member sizing and then working backwards to select R is a common error that produces unconservative designs.
Second, the geotechnical report is not a formality for seismic design. Site class, liquefaction potential, and soil amplification factors can change the design base shear by a factor of two or more between a rock site and a soft soil site at the same geographic location. Engaging with the geotechnical data early in the design process avoids costly late-stage revisions.
Third, seismic connection design cannot be delegated to a detailer working from standard connection tables. The capacity design requirements, expected strength calculations, and demand-critical weld specifications require explicit engineering judgment on every moment connection in the structure. At Sixteens, structural design and connection design are treated as an integrated scope — because in seismic design, they are inseparable.
Frequently Asked Questions
What seismic code applies in Türkiye?
The primary Turkish seismic code is TBDY 2018 (Turkish Building Earthquake Code 2018), which replaced the earlier TEC 2007. Seismic hazard data is published by AFAD at afad.gov.tr and provides probabilistic spectral accelerations at 10% and 2% probability of exceedance in 50 years for any location in Türkiye. For international projects using AISC design methodology, the AFAD spectral accelerations can be used as input to the ASCE 7-22 site-specific procedure, provided the site class and amplification factors are applied correctly.
How does entertainment occupancy affect seismic design category?
Entertainment and assembly occupancies — where 300 or more people can gather — are typically classified as Risk Category III under ASCE 7-22 Table 1.5-1. This elevates the importance factor Ie to 1.25, which directly increases the design seismic forces. It can also push the Seismic Design Category one level higher: a site that would be SDC C for a Risk Category II structure may become SDC D for a Risk Category III structure, triggering the full AISC 341 seismic provisions.
What makes seismic connection design different from standard connections?
Standard connection design sizes connections to resist the elastic demand forces from the structural analysis. Seismic connection design applies a capacity design philosophy: connections must be stronger than the yielding elements they connect, sized to remain elastic while the beam or brace undergoes ductile inelastic deformation. This typically means connections are designed to the expected plastic capacity of the connected member (1.1 × Ry × Mp) rather than to the analysis demand — and it imposes specific weld toughness requirements (CVN testing for demand-critical welds) that do not apply in non-seismic design.
What analysis method was used for the Antalya project?
The seismic analysis used response spectrum analysis in ETABS, with a site-specific design response spectrum derived from AFAD hazard data and ASCE 7-22 site amplification factors. Response spectrum analysis captures higher mode contributions that equivalent lateral force (ELF) methods miss — important for irregular structures or structures with significant higher-mode mass participation. The analysis produced story shear, overturning moment, and member force envelopes for combination with gravity load cases.