Applied Loads and Stability of Steel and Steel-Concrete Composite Bridges - Finite Element Analysis and Design of Steel and Steel-Concrete Composite Bridges - page 182

compression or tension should be derived from the bilinear diagram given
in EC3 [ 3.5 ] and should take account of the effects of the method of con-
struction (e.g., propped or unpropped). For class 1 and 2 composite cross
sections with the concrete flange in compression, the nonlinear resistance
to bending M Rd may be determined as a function of the compressive force
in the concrete N c using the following simplified expressions, as shown in
Figure 3.30 :

N c
N c , el

M Rd ¼M a , Ed + M el , Rd M a , Ed for N c N c , el ð 3 : 72 Þ

N c N c , el
N c , f N c , el

M Rd ¼M el , Rd + M pl , Rd M el , Rd

for N c , el N c N c , f

ð 3 : 73 Þ

74 Þ
where M a,Ed is the design bending moment applied to structural steel section
before composite behavior, M c,Ed is the part of the design bending moment
acting on the composite section, and k is the lowest factor such that a stress
limit is reached, where unpropped construction is used, the sequence of
construction should be taken into account, and N c,el is the compressive force
in the concrete flange corresponding to moment M el,Rd .

M el , Rd ¼M a , Ed + kM c , Ed ð 3 :

M , M ,

Rd Rd

1 2

M M pl Rd pl Rd

1.0 1.0

M M

el , Rd el , Rd

M M pl , Rd pl , Rd

M

a , Ed

M pl , Rd

N , N , c c

N N

N , N ,

1.0 1.0

c , el c f c

N

, el c f

N

c f c f

Key:
1 Propped construction
2 Unpropped construction

Figure 3.30 Simplified relationship between M Rd and N c for sections with the concrete
slab in compression recommended by EC4 [ 3.6 ] .