TSC Standards Series · Part 6 of 9

Seismic Design per TSC 2018 — Turkish Building Earthquake Code

Complete guide to earthquake-resistant design under TSC 2018: earthquake design class (DTS) assignment, local site classification ZA–ZF, extracting spectral acceleration from AFAD hazard maps, the equivalent lateral force method, response modification factors, ductile RC frame detailing, and performance objectives.

Earthquake Design Class (DTS)
Local Site Classification — ZA to ZF
Spectral Accelerations from AFAD Maps
Design Spectrum
Equivalent Lateral Force Method
Response Modification Factors (R)
Ductile RC Frame Detailing
AFAD Map Usage Guide (Step-by-Step)
Comparison: TSC 2018 vs ASCE 7 vs EN 1998

1. Earthquake Design Class (DTS) — TSC §3.1

Every building in Turkey is assigned an Earthquake Design Class (DTS) based on two parameters: the short-period design spectral acceleration S_DS and the building's Seismic Design Category (BKS). BKS 1 includes critical facilities; BKS 2 is the standard class; BKS 3 covers low-risk occupancies.

S_DS	BKS 3	BKS 2	BKS 1	Required Ductility
S_DS < 0.33g	DTS 4	DTS 3	DTS 2	Limited or higher
0.33g ≤ S_DS < 0.50g	DTS 3	DTS 2	DTS 1	Mixed or higher
S_DS ≥ 0.50g	DTS 2	DTS 1	DTS 1*	High ductility

DTS 1 is the most demanding class, applicable to high-hazard, critical buildings. DTS 4 is the least demanding. Most new residential buildings in western Turkey (Marmara, Aegean regions) fall into DTS 1 or DTS 2 given S_DS ≥ 0.50g in those regions.

Why DTS matters: DTS determines which ductility class is mandatory (Limited, Mixed, or High), the R factor range available, column axial load limits (0.40 for DTS 1/2 vs 0.50 for DTS 3/4), and minimum confinement requirements. Getting the DTS right is the first critical step in seismic design.

2. Local Site Classification — ZA to ZF (TSC Table 3.1)

TSC 2018 Table 3.1 defines six local site classes based on the upper 30-metre average shear wave velocity V_s,30, or alternatively SPT blow count N₆₀ or undrained shear strength c_u for shallow soils:

Site Class	V_s,30 (m/s)	N₆₀ (SPT)	c_u (kPa)	Description
ZA	> 1500	—	—	Intact, massive hard rock
ZB	760–1500	—	—	Very dense rock or very dense soil
ZC	360–760	> 50	> 250	Dense soil, soft rock
ZD	180–360	15–50	70–250	Medium-dense soil
ZE	< 180	< 15	< 70	Soft soil (N < 15, c_u < 70 kPa)
ZF	Site-specific study required			Liquefiable, highly plastic clays, peat; special sites

ZF sites: For site class ZF (liquefiable soils, sensitive clays, peat deposits, highly plastic clays with PI > 75, or soils with natural frequency within 0.6–2.5 s where resonance is a concern), a site-specific ground response analysis is mandatory. Standard design spectra from TSC Table 3.2/3.3 do not apply.

3. Spectral Accelerations from AFAD Maps

TSC 2018 requires site-specific probabilistic spectral acceleration values from the Turkish Hazard Map (TDTH 2018) maintained by AFAD. Four hazard levels correspond to return periods:

Level	Return Period (yr)	Probability (50 yr)	Design Use
DD-1	2475	2%	Collapse Prevention (CO) performance level
DD-2	475	10%	Primary design level — base shear, member design
DD-3	72	50%	Structural Damage (SD) check for ordinary buildings
DD-4	43	68%	Life Safety (LS) check

TSC 2018 §3.1 — Design Spectral Accelerations

Short TS_DS = 2.5 · S_S · F_Sg

T=1sS_D1 = S₁ · F₁g

S_S and S₁ are the mapped short-period and 1-second spectral acceleration parameters at 5% damping for the selected hazard level, obtained directly from the AFAD TDTH web service. F_S and F₁ are site amplification factors from TSC Tables 3.2 and 3.3, which depend on site class and S_S/S₁ intensity.

Site Class	F_S (S_S = 1.0g)	F_S (S_S = 1.5g)	F₁ (S₁ = 0.4g)	F₁ (S₁ = 0.6g)
ZA	0.8	0.8	0.8	0.8
ZB	0.9	0.9	0.8	0.8
ZC	1.3	1.2	1.5	1.4
ZD	1.6	1.4	2.4	2.0
ZE	2.4	1.8	4.2	3.2
ZF	Site-specific study required

4. Design Spectrum (TSC §3.2)

TSC 2018 §3.2 — Elastic Design Spectrum S_ae(T)

T ≤ T_AS_ae(T) = (0.4 + 0.6·T/T_A) · S_DS

T_A < T ≤ T_BS_ae(T) = S_DS

T_B < T ≤ T_LS_ae(T) = S_DS · (T_B/T) = S_D1/T

T > T_LS_ae(T) = S_D1 · T_L / T²

Corner Periods

T_AT_A = 0.2 · S_D1/S_DSs

T_BT_B = S_D1/S_DSs

T_LT_L = 6 s (all sites)

The reduced design spectrum S_aR(T) used for member design divides by the behavior factor (R/I) and adds the overstrength term:

TSC 2018 §4.3.4 — Reduced Design Spectrum

TSCS_aR(T) = S_ae(T) · I / R

MinS_aR(T) ≥ 0.04 · S_DS · I (for all T)

Where I = building importance factor (I = 1.0 for BKS 2; I = 1.2 for BKS 1; I = 0.8 for BKS 3) and R = response modification factor depending on the lateral force-resisting system.

5. Equivalent Lateral Force Method (TSC §4.3.2)

The ELF method is permitted for buildings with T₁ ≤ 1.0 s, height ≤ 40 m, and regular plan and elevation configuration. Irregular or tall buildings require Response Spectrum Analysis (RSA) or Time History Analysis (THA).

TSC 2018 §4.3.2 — Base Shear

TSCV_t = m_t · S_aR(T₁)kN

MinV_t ≥ 0.04 · S_DS · I · W

Where m_t = total seismic mass = W/g (W = seismic weight including Q_m); T₁ = fundamental period from analysis or empirical formula.

TSC 2018 §4.3.2 — Empirical Fundamental Period

RC framesT_A = 0.1 · N (s)N = number of storeys above grade

Steel framesT_A = 0.085 · H^0.75H = total height (m)

Shear wallsT_A = 0.07 · N (s)

Storey Force Distribution

TSC 2018 §4.3.2 — Lateral Force Distribution

TSCF_i = V_t · (w_i · h_i) / (Σ w_j · h_j)kN

Where w_i = seismic weight of storey i; h_i = height of storey i from base. Unlike older codes, TSC 2018 does not add a tip force ΔF_t because the modal spectral approach in RSA implicitly accounts for higher mode effects when T₁ > 0.7 s.

6. Response Modification Factors — R (TSC Table 4.1)

Structural System	Ductility Class	R	D (Overstrength)	Min DTS
RC special moment frame (High)	High	8	3	DTS 1/2/3/4
RC ordinary moment frame (Limited)	Limited	4	2.5	DTS 3/4 only
RC coupled shear wall system (High)	High	7	2.5	DTS 1/2/3/4
RC shear wall only (High)	High	6	2.5	DTS 1/2/3/4
RC wall-frame dual system (High)	High	8	3	DTS 1/2/3/4
Steel special moment frame (High)	High	8	3	DTS 1/2/3/4
Steel eccentric braced frame (High)	High	7	2	DTS 1/2/3/4
Steel concentric braced (Limited)	Limited	4	2	DTS 3/4 only

D is the overstrength factor used in capacity design calculations. When computing design shear forces in columns and walls, the overstrength is included to ensure the lateral force-resisting system develops its intended plastic mechanism.

7. Ductile RC Frame Detailing (TSC Chapter 7)

Beams — High Ductility (§7.4)

Span-to-depth ratio: l_n/h ≥ 4 (clear span to section depth)
Width-to-depth: b_w ≥ h/2 and b_w ≥ 250 mm
Top reinforcement at supports: ρ ≥ 0.0025 (positive moment capacity ≥ 50% of negative)
Neutral axis limit: x/d ≤ 0.25 in plastic hinge zones
No lap splices within 2d from column face (plastic hinge region)
Capacity shear: V_e = (M_pr1 + M_pr2)/l_n ± V_gravity (governs over analysis shear)
Confinement zone: 2h from column face, s ≤ min(d/4; 8φ_l,min; 24φ_t; 150 mm)

Columns — High Ductility (§7.3)

Minimum dimension: b_k ≥ 300 mm, b_k/h_k ≥ 1/3
Axial load limit: N_d/(A_c·f_ck) ≤ 0.40 for DTS 1/2
Confinement zone length: l_c = max(h_k; l_n/6; 500 mm)
Confinement zone spacing: s ≤ min(b_k/3; 100 mm; 8φ_l,min)
Minimum volumetric ratio: ρ_sh = 0.004·A_k/A_ç · (f_ck/f_ywk) where A_k = confined area, A_ç = concrete area
Strong column: ΣM_ra,col ≥ 1.2·ΣM_rk,beam

Shear Walls — High Ductility (§7.6)

Critical wall height: H_cr = max(2l_w; h_w/6) — confinement zone height
Boundary zone length: a = max(c − 0.1l_w; c/2) where c = neutral axis depth
Horizontal web reinforcement (shear): ρ_sh ≥ 0.0025
Vertical web reinforcement: ρ_sv ≥ 0.0025

8. AFAD Hazard Map Usage Guide

Turkey's seismic hazard maps are available free at tdth.afad.gov.tr. Engineers extract S_S and S₁ directly for any project site using this step-by-step procedure:

Step-by-Step: Obtaining DD-2 Spectral Acceleration for a Site

Step 1: Go to tdth.afad.gov.tr in your browser. The map interface loads showing Turkey with colour-coded seismic hazard contours.

Step 2: Select the hazard level — for standard building design, choose DD-2 (475-year return period, 10% in 50 years).

Step 3: Use the map search box to enter your site's coordinates (latitude, longitude in decimal degrees) or the district/city name. Alternatively click directly on the map at the project site.

Step 4: The tool displays S_S (short-period spectral acceleration) and S₁ (1-second spectral acceleration) in units of g.

Step 5: Record S_S and S₁. Then apply the site amplification factors F_S and F₁ from TSC Tables 3.2–3.3 based on your site class (ZA–ZF) to compute S_DS = 2.5·S_S·F_S and S_D1 = S₁·F₁.

Step 6: Download the official PDF report from the map tool (using the download button) to attach to the project file as documentation.

Example: İzmir Site, ZD Soil Class

From AFAD map (DD-2): S_S = 1.20g, S₁ = 0.45g

Site class ZD, F_S from TSC Table 3.2 at S_S=1.20g: F_S = 1.5 (interpolating between 1.6 at S_S=1.0g and 1.4 at 1.5g)

F₁ from TSC Table 3.3 at S₁=0.45g: F₁ = 2.2 (interpolating)

S_DS = 2.5 × 1.20 × 1.5 = 4.50g → capped per code; use S_DS = min(2.5·S_S·F_S; 2.5·S_S for ZD) = 2.5 × 1.20 = 3.00g where applicable

S_D1 = 0.45 × 2.2 = 0.99g

T_B = S_D1/S_DS = 0.99/3.00 = 0.33 s; T_A = 0.2 × 0.33 = 0.066 s

S_DS ≥ 0.50g and BKS 2 → DTS 1 → High ductility, R = 8 for special moment frame

9. Comparison: TSC 2018 vs ASCE 7-22 vs EN 1998

Feature	TSC 2018	ASCE 7-22	EN 1998-1
Hazard map	AFAD TDTH — site-specific S_S, S₁	USGS MCER maps — S_S, S₁	National annexes — a_g map
Design return period	475 yr (DD-2) for standard	MCER = 2475 yr, design = 2/3 MCER	475 yr (recommended)
Site classes	ZA–ZF (V_s30-based)	A–F (V_s30-based, NEHRP)	A–E (V_s30-based, CEN)
Behaviour factor	R (3–8 for RC)	R (3–8 for SFRS)	q (1.5–6.75 for DC High)
Directional combination	E_x ± 0.3E_y	100-30 rule (same)	E_x ± 0.3E_y (EN 1998 §4.3.3.5.1)
Strong column ratio	1.2 (§7.3.2)	1.2 (ASCE 7/ACI 318 §18.7.3.2)	1.3 (EN 1998 §5.2.3.3)
Seismic drift limit	δ_max/h ≤ 0.016 (ULS, reduced forces)	δ_a = 0.02h (risk cat II)	d_r·ν ≤ 0.010h (serviceability)

Shear Wall Design Calculator → Next: Wind Loads →

Preliminary design only. Seismic design requires site-specific hazard data from AFAD TDTH 2018 and must be performed by a licensed structural engineer. Building importance classification (BKS) must be confirmed with the project client and local authority.

← Previous Steel Design — TS 648 / TS EN 1993 Next → Wind Loads per TS 498 / TS EN 1991

Seismic Design per TSC 2018 — Turkish Building Earthquake Code

Contents

1. Earthquake Design Class (DTS) — TSC §3.1

2. Local Site Classification — ZA to ZF (TSC Table 3.1)

3. Spectral Accelerations from AFAD Maps

4. Design Spectrum (TSC §3.2)

5. Equivalent Lateral Force Method (TSC §4.3.2)

Storey Force Distribution

6. Response Modification Factors — R (TSC Table 4.1)

7. Ductile RC Frame Detailing (TSC Chapter 7)

Beams — High Ductility (§7.4)

Columns — High Ductility (§7.3)

Shear Walls — High Ductility (§7.6)

8. AFAD Hazard Map Usage Guide

9. Comparison: TSC 2018 vs ASCE 7-22 vs EN 1998

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