Local buckling of thin-plate elements - The Behaviour and Design of Steel Structures to EC3 - page 143

Section moment resistance.
The cross-section is Class 2 and therefore unsuitable for plastic design.

M c , Rd = 775 × 10 3 × 355 / 1.0 Nmm = 275.1 kNm. 6.2.5(2)

4.9.3 Example 3 - section moment resistance of
a Class 4 box beam
Problem. Determine the section moment resistance of the welded-box section
beam of S355 steel shown in Figure 4.31c.The weld size is 6 mm.
Classifying the section-plate elements.

t f = 10 mm, t w = 8mm, f y = 355 N / mm 2 EN10025-2

ε = √ ( 235 / 355 ) = 0.814 T5.2

c f /( t f ε) = 410 /( 10 × 0.814 ) T5.2

= 50.4 > 42 and so the flange is Class 4. T5.2

c w /( t w ε) = ( 430 − 2 × 10 − 2 × 6 )/( 8 × 0.814 ) T5.2

= 61.1 < 72 and so the web is Class 1. T5.2

The cross-section is therefore Class 4 since the flange is Class 4.
Effective cross-section.

k σ = 4.0 EC3-1-5T4.2

410
10 × 28.4 × 0.814 ×

λ p = √ = 0.887 > 0.673

4.0

EC3-1-5 4.4(2)
ψ = 1 EC3-1-5T4.1
ρ = ( 0.887 − 0.055 × ( 3 + 1 ))/ 0.887 2 = 0.848 EC3-1-5 4.4(2)
b c , eff = 0.848 × 410 = 347.5 mm
A eff = ( 450 − 410 + 347.5 ) × 10 + ( 450 × 10 )
+ 2 × ( 430 − 2 × 10 ) × 8
= 14935 mm 2
14935 × z c = ( 450 − 410 + 347.5 ) × 10 × ( 430 − 10 / 2 )
+ 450 × 10 × 10 / 2 + 2 × ( 430 − 2 × 10 ) × 8 × 430 / 2
z c = 206.2 mm
I eff = ( 450 − 410 + 347.5 ) × 10 × ( 430 − 10 / 2 − 206.2 ) 2
+ 450 × 10 × ( 206.2 − 10 / 2 ) 2 + 2 × ( 430 − 2 × 10 ) 3 × 8 / 12
+ 2 × ( 430 − 2 × 10 ) × 8 × ( 430 / 2 − 206.2 ) 2 mm 4
= 460.1 × 10 6 mm 4