1. CHAPTER 1: INTRODUCTION part C
BFC Structural Steel and Timber Design
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2. Course content
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3. Outline of this Lecture
Notations and symbols in EC3
Recap of common structural steel sections
Local buckling
Understanding failure modes
Section classification
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4. Recognizing important notations and symbols used in EC3
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5. Section
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6. Elastic section modulus
Area
Elastic section modulus
Plastic section modulus
Second moment of area major axis
Second moment of area minor axis
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7. Subscripts
Ed = design value of an effect eg: MEd = design bending moment NEd = axial force Rd = deign resistance eg: MRd = design resistance for bending NRd = resistance to axial force el = elastic property eg: Wel = elastic section modulus pl = plastic property eg: Wpl = plastic section modulus
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8. ULS – Equation 6.10
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9. SLS
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10. Recap of common structural steel section
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11. Rolled and formed sections
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12. Built-up sections
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13. Cold rolled sections
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14. Local buckling failure
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15. Some basic knowledge when designing with structural steel
Structural steel members are made of thin elements
These thin elements are slender, as a result causing local instabilities (known as local buckling in flange and web) even before achieving yield strength.
As such, cross section classification is important to account for such local instabilities.
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17. Understanding failure modes
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18. Fundamental modes of failure in Universal Column under compression
Overall yielding in compression
Overall buckling as a strut
Local buckling in flange toes
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19. Overall yielding in compression
Fundamental modes of failure in Universal Beam under compression (UB as column)
Overall yielding in compression
Overall buckling as a strut (about minor axis)
Local buckling in flange toes and web
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20. Fundamental modes of failure in Universal Beam under flexure (UB as beam)
Overall yielding in flexure
Overall lateral / torsional buckling
Local buckling in compression flange under high strains
Local buckling in web
UB in flexure behaves differently to UB in axial compression
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21. Section Classification
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22. What? CLASS 1 CLASS 2 CLASS 3 CLASS 4 Assoc Prof Dr DYeoh

23. What does it mean? Think of plasticity Think of ductility
Think of rotational capacity in the whole section
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27. Why classify? What is the implication of different CLASS?
CLASS 1 PLASTIC – able to sustain high strain and achieve plastic moment capacity and full plastic hinge in the whole cross-section; no restrictions in normal design.
CLASS 2 COMPACT – achieve plastic moment capacity but with limited plastic hinge; apply plastic modulus under bending for design; elastic analysis only.
CLASS 3 SEMI-COMPACT – plastic capacity not achieved; only elastic capacity; apply elastic modulus for design.
CLASS 4 SLENDER – local buckling failure appear even before yield strength is achieved; apply effective section properties.
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28. EC3 – 6.2.4 Compression The design resistance for Class 1, 2 and 3:
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29. EC3 – 6.2.5 Bending moment The design resistance for Class 1 & 2:
We will revisit this later.
The design resistance for Class 4:
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30. How? Table 5.2 EC3 What is the type of cross-section?
What kind of stress is the section subjected to?
For UB, UC or built-up sections, classify each part separately (ie. Flange and web)
Calculate e factor
Choose worst classification as CLASS for the section.
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31. Example – pure compression
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32. 3. Substitute ε into the class limits in Table 5.2
1. Determine fy
fy = 355N/mm2
2. Determine ε
3. Substitute ε into the class limits in Table 5.2
Outstand flanges (Table 5.2, sheet 2):
cf = (b – tw – 2r)/2 = mm
cf / tf = / =
9ε = 7.29, 10ε = 8.10, 14ε = 11.34
Limit for Class 2 flange = 10  = >
Hence flanges are Class 2
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33. Web – internal compression part (Table 5.2, sheet 1):
cw = h – 2tf – 2r = mm
cw / tw = 200.3/ =
33ε = 26.73, 38ε = 30.78, 42ε = 34.02
Limit for Class 1 web = 33  = > ,
Hence web is Class 1
4. Therefore, overall cross-section classification is Class 2.
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34. Cross-section compression resistance (clause 6.2.4):
Therefore, the design compression resistance of the cross-section is:
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35. Classification under combined bending and axial force
Classify based on the actual stress distribution of the combined loadings. STEP 1 For simplicity, classify using most severe loading condition of pure axial compression. If the section classified as Class 1 or 2, no further works are needed. STEP 2 But if the resulting section classification is Class 3 or 4, it is advisable for economy to conduct a more precise classification under the combined loading.
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36. Example – combined bending and axial force
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40. Comparison Separate load condition Combined load condition
Flange = CLASS 1
Web = CLASS 4
Overall = CLASS 4
Resisting capacity uses effective sectional area
Flange = CLASS 1
Web = CLASS 2
Overall = CLASS 2
Resisting capacity uses whole sectional area
MORE ECONOMICAL DESIGN
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41. End of Introduction part C
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42. Recap what have we learned?
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43. Recap what have we learned?
Notations and symbols in EC3
Recap of common structural steel sections
Local buckling
Understanding failure modes
Section classification
Assoc Prof Dr DYeoh