Antalya Seismic-Zone Steel Supports

Project Overview

Project P-2023-091 involved the structural design of entertainment steel support structures at a facility in Antalya, on Türkiye's Mediterranean coast. Antalya sits on the southwestern coast of Anatolia, within a seismically active region where the African plate's interaction with the Eurasian plate produces regular significant earthquakes. The 1999 İzmit and Düzce earthquakes, which caused catastrophic damage elsewhere in Türkiye, fundamentally changed the structural engineering landscape for new construction throughout the country.

The entertainment supports in this project required design to AISC seismic provisions — specifically AISC 341, Seismic Provisions for Structural Steel Buildings — alongside the AISC 360 steel design specification, to ensure the structures could survive the seismic demands of the site without collapse.

Seismic Hazard in Antalya

The seismic hazard assessment for this project used Turkish seismic hazard maps as published by the Disaster and Emergency Management Authority (AFAD) of Türkiye, combined with AISC/ASCE-7 methodology for spectral acceleration determination and seismic design category classification. The Antalya region is classified in a high-hazard seismic zone, with design spectral accelerations at short periods (Ss) and 1-second period (S1) that place the project structures in Seismic Design Category D under the ASCE-7 framework — the most demanding category for common occupancy structures.

Seismic Design Category D triggers the most stringent requirements of AISC 341 for steel structures: ductile detailing, protected zones at plastic hinge locations, continuity plate requirements, and enhanced weld toughness requirements to prevent brittle fracture under cyclic seismic loading.

AISC Seismic Design Approach

The fundamental philosophy of seismic design for steel structures is capacity design: the structure is designed so that inelastic behaviour (yielding) occurs in ductile elements (beam plastic hinges, brace yielding) rather than in brittle connection failures. This requires that connections be designed stronger than the capacity of the yielding elements they connect — not just to the demand forces from the elastic analysis, but to the expected yield strength of the connected members.

For the Antalya supports, ETABS was used for the seismic analysis. A response spectrum analysis was performed using the site-specific response spectrum derived from the Turkish seismic hazard data and site class determination from the project geotechnical report. The analysis produced seismic demands at every node in the structure, which were combined with gravity load demands using the ASCE-7 seismic load combinations (including the 0.2SDS vertical seismic load effect where required).

Connection Design for Seismic Loading

Seismic connection design for the Antalya supports addressed the specific requirements of AISC 341 Section E for ordinary moment frames and concentrically braced frames, depending on the structural configuration of each support type. For moment-resisting connections, the connection strength was required to exceed the expected plastic moment capacity of the connected beam — 1.1 × Ry × Mp, where Ry is the ratio of expected to nominal yield stress for the steel grade used (1.1 for A36 steel, 1.1 for A992 W-shapes). This requirement often governs connection design in high seismic zones and results in larger welds and heavier connection plates than would be required for the elastic force demands alone.

Weld toughness requirements per AISC 341 Section A3.4 specified CVN impact testing for weld filler metals used in demand-critical welds — those where fracture could lead to partial or complete collapse. The specification of CVN-tested electrode material was included in the project welding schedule.

Foundations Under Seismic Loading

Seismic overturning moments at the base of the entertainment supports imposed significant tension demand on the upwind anchor rods. Foundation design for these uplift forces required careful detailing of the anchor rod embedment, concrete breakout cone analysis per ACI 318, and verification that the pad footing mass and weight were sufficient to resist net uplift under the governing seismic load combination without supplementary tie-down elements.

The soft soil conditions present at the Antalya site (classified as site class D — stiff soil) amplify seismic ground motions relative to rock conditions. The site amplification factors Fa and Fv were applied to modify the reference spectral accelerations, resulting in higher design forces than would apply at an equivalent rock site — a critical consideration often overlooked when design teams rely on simplified seismic calculations without proper site-specific hazard adjustment.