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TSC Standards Series · Part 5 of 9

Steel Structure Design — TS 648 / TS EN 1993

Steel design for buildings in Turkey: transition from legacy TS 648 to the current TS EN 1993-1-1 (Eurocode 3), section classification, tension and compression design with European buckling curves, beam lateral-torsional buckling, connection design, and TSC 2018 seismic steel requirements.

Contents

  1. TS 648 vs TS EN 1993 — The Transition
  2. Section Classification
  3. Tension Members
  4. Compression Members — Flexural Buckling
  5. Beams — Flexure and LTB
  6. Connections — Bolts and Welds
  7. TSC 2018 Seismic Steel Requirements
  8. Worked Example — Compression Column

1. TS 648 vs TS EN 1993 — The Transition

Turkey's legacy steel design standard TS 648:1980 used an allowable stress design (ASD) philosophy. Structural steel design in Turkey has largely transitioned to TS EN 1993-1-1 (Eurocode 3 as adopted by TSE), which uses the Limit State Design approach consistent with TS 500 for reinforced concrete.

AspectTS 648:1980 (Legacy)TS EN 1993-1-1 (Current)
Design basisAllowable Stress Design (ASD)Limit State Design (LSD)
Safety formatSingle safety factor (ν = 1.5–2.0)Partial material factors γM0, γM1, γM2
Section classesCompact / non-compactClasses 1, 2, 3, 4 (EN 1993 Table 5.2)
BucklingPerry-Robertson formulaEuropean buckling curves a0/a/b/c/d
LTBJohnson formulaGeneral method or specific method
SeismicForce reduction factors onlyTSC 2018 Chapter 9 ductility classes
StatusStill legally referenced in some contractsMandatory for new design per TSE
Practice note: Many existing buildings in Turkey were designed per TS 648. When assessing existing structures, TS 648 criteria may still apply. For all new design, TS EN 1993 is the correct standard. TSC 2018 explicitly references TS EN 1993 for steel member design.

2. Section Classification (TS EN 1993-1-1 Table 5.2)

Section classification determines whether local buckling prevents full plastic moment development. The class depends on the slenderness of compression elements (flanges and webs):

ClassBehaviourDesign StrengthTypical Flange Limit (ε = √(235/fy))
Class 1Full plastic moment, rotation capacity for plastic hingesWpl·fyc/tf ≤ 9ε
Class 2Full plastic moment, limited rotation capacityWpl·fyc/tf ≤ 10ε
Class 3Elastic moment only, no local buckling before yieldWel·fyc/tf ≤ 14ε
Class 4Local buckling before yield — effective section requiredWeff·fyc/tf > 14ε

For S355 steel: ε = √(235/355) = 0.814. The Class 1 outstand flange limit is c/tf ≤ 9 × 0.814 = 7.3. Standard HEA and HEB sections in S355 are generally Class 1 or 2 for bending. For seismic applications, TSC 2018 §9.2 requires Class 1 sections in potential plastic hinge zones.

3. Tension Members (TS EN 1993-1-1 §6.2.3)

TS EN 1993-1-1 §6.2.3 — Tension Capacity
PlasticNpl,Rd = A · fy / γM0N (gross section yielding)
FractureNu,Rd = 0.9 · Anet · fu / γM2N (net section at holes)
GovernsNt,Rd = min(Npl,Rd ; Nu,Rd)

Block shear failure (Ant/Anv combination) must also be checked when bolt groups transfer load in shear/tension. For a single row of bolts, net area Anet = A − n · d0 · t, where d0 is the hole diameter (typically bolt diameter + 2 mm for standard clearance holes).

4. Compression Members — Flexural Buckling (TS EN 1993-1-1 §6.3.1)

TS EN 1993-1-1 §6.3.1 — Column Buckling Capacity
EC3Nb,Rd = χ · A · fy / γM1N
χχ = 1 / (Φ + √(Φ² − λ̄²)) ≤ 1.0
ΦΦ = 0.5[1 + α(λ̄ − 0.2) + λ̄²]
λ̄λ̄ = √(A·fy/Ncr) = (Lcr/i) / (93.9ε) for Class 1–3

Where α is the imperfection factor from the European buckling curve assigned to the section:

Buckling CurveαTypical Section (S355, t ≤ 40 mm)
a00.13CHS hot-finished, RHS hot-finished
a0.21I-section (h/b > 1.2), tf ≤ 40 mm, y-y axis
b0.34I-section (h/b > 1.2), z-z axis; HEA/HEB/IPE
c0.49I-section (h/b ≤ 1.2), both axes; Welded I
d0.76Welded box (t > 40 mm), thick plates

Effective length Lcr = k·L where k is the effective length factor (1.0 for pin-pin; 0.5 for fixed-fixed; 0.7 for pin-fixed in practice, including connection flexibility). TSC 2018 §9.4 requires that in seismic frames, columns are designed with the amplified axial forces from capacity design.

5. Beams — Flexure and Lateral-Torsional Buckling

TS EN 1993-1-1 §6.2.5 — Cross-Section Bending Capacity
Class 1/2Mc,Rd = Wpl · fy / γM0
Class 3Mc,Rd = Wel,min · fy / γM0
TS EN 1993-1-1 §6.3.2 — Lateral-Torsional Buckling
LTBMb,Rd = χLT · Wy · fy / γM1
χLTχLT = 1 / (ΦLT + √(ΦLT² − λ̄LT²)) ≤ 1.0
λ̄LTλ̄LT = √(Wy·fy/Mcr)

Mcr is the elastic critical moment for lateral-torsional buckling, which depends on the unbraced length Lb, section geometry (Iz, IT, Iw), loading pattern (C1 factor), and end conditions. When the beam's compression flange is continuously restrained by a composite slab or bracing, LTB need not be checked.

Imperfection Factors for LTB

SectionLTB CurveαLT
Rolled I-sections, h/b ≤ 2a0.21
Rolled I-sections, h/b > 2b0.34
Welded I-sections, h/b ≤ 2c0.49
Welded I-sections, h/b > 2d0.76

6. Connections — Bolts and Welds (TS EN 1993-1-8)

TS EN 1993-1-8 governs connections for steel structures in Turkey. Key bolt capacity checks:

TS EN 1993-1-8 §3.6 — Bolt Shear and Bearing Capacity
ShearFv,Rd = αv · fub · A / γM2per bolt per shear plane
BearingFb,Rd = k1 · αb · fu · d · t / γM2
TensionFt,Rd = 0.9 · fub · As / γM2

Common bolt grades in Turkey: 8.8 (fub = 800 MPa) for standard structural connections, 10.9 (fub = 1000 MPa) for high-strength preloaded connections (friction-type, HSFG). For seismic connections in TSC 2018 §9 ductile frames, preloaded bolts are required at beam-column connections.

Weld Capacity

TS EN 1993-1-8 §4.5 — Fillet Weld Capacity (Simplified)
EC3Fw,Rd = fvw,d · a · lw
EC3fvw,d = fu / (√3 · βw · γM2)

Where a = throat thickness; βw = correlation factor (0.80 for S235; 0.85 for S275; 0.90 for S355). Minimum weld throat a = 0.7 × tmin for tmin ≤ 40 mm per TS EN 1993-1-8 §4.5.2.

7. TSC 2018 Seismic Steel Requirements (Chapter 9)

TSC 2018 Chapter 9 covers seismic design of steel buildings, defining two ductility classes:

ParameterHigh DuctilityLimited Ductility
Ductility classHigh DuctilityLimited Ductility
R factor (moment frame)84
Section class in plastic zonesClass 1 mandatoryClass 1 or 2
Beam-column jointsRBS or equivalent prequalifiedStandard welded or bolted
Connection yield mechanismBeam ends must yield before connectionsConnection strength ≥ 1.1Ry·Mp
Panel zoneMust be designed for capacity shearElastic design permitted
fy/fu limit≤ 0.80 (adequate ductility reserve)≤ 0.85
Reduced Beam Section (RBS): TSC 2018 §9.4.2 recommends or requires RBS (dog-bone) connections for high-ductility moment frames. The beam web or flange is locally reduced near the column face to ensure plastic hinging occurs within the reduced section, protecting the connection. Proportioning follows EN 1998-1 or AISC 341 geometric limits.

8. Worked Example — Compression Column (TS EN 1993-1-1)

Example: HEB 240 Column in S355 Steel — Axial Compression Check
Given: HEB 240, S355 (fy = 355 MPa for t ≤ 16 mm, but flanges tf = 17 mm > 16 mm → fy = 345 MPa, tf ≤ 40 mm). Effective length Lcr = 4.5 m (both axes, pinned-pinned). Design compression NEd = 1450 kN.
Section properties (HEB 240):
A = 10600 mm², iz = 60.1 mm, iy = 103 mm, tf = 17 mm → fy = 345 MPa (t > 16 mm)
Step 1 — Slenderness (weak axis, z-z governs):
λ̄z = (Lcr/iz) / (93.9·ε) = (4500/60.1) / (93.9 · √(235/345)) = 74.9 / 77.6 = 0.965
Step 2 — Buckling curve (HEB, z-z axis, tf ≤ 40 mm):
Use curve b → α = 0.34
Φ = 0.5[1 + 0.34(0.965 − 0.2) + 0.965²] = 0.5[1 + 0.260 + 0.931] = 1.096
χ = 1/(1.096 + √(1.096² − 0.965²)) = 1/(1.096 + √(0.272)) = 1/(1.096 + 0.522) = 0.617
Step 3 — Buckling capacity:
Nb,Rd = 0.617 × 10600 × 345 / 1.00 = 2258 kN
Step 4 — Check:
NEd/Nb,Rd = 1450/2258 = 0.64 ≤ 1.0 ✓
HEB 240 in S355 is adequate for 1450 kN compression over 4.5 m effective length.
Steel Column Calculator → Steel Beam Calculator → Next: Seismic Design →
Preliminary design only. Verify section classification, buckling curves, and effective lengths with the specific structural system. Consult TS EN 1993-1-1 and TSC 2018 Chapter 9 with a licensed structural engineer.
← Previous RC Design per TS 500:2000 Next → Seismic Design per TSC 2018

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