Applied Loads and Stability of Steel and Steel-Concrete Composite Bridges - Finite Element Analysis and Design of Steel and Steel-Concrete Composite Bridges - page 180

due to shear lag may be taken in accordance with EC3 [ 3.6 ] for both con-
crete and steel flanges. For cross sections with bending moments resulting
from the main girder system and from a local system (for example, in com-
posite trusses with direct actions on the chord between nodes), the relevant
effective widths for the main girder system and the local system should be
used for the relevant bending moments.
3.8.6.2 Bending Resistance of Composite Plate Girders
Let us now calculate the bending resistance of composite plate girders,
according to EC4 [ 3.6 ] . The design bending resistance shall be determined
by rigid plastic theory only where the effective composite cross section is in
class 1 or 2. On the other hand, elastic analysis and nonlinear theory for
bending resistance may be applied to cross sections of any class. For elastic
analysis and nonlinear theory, it may be assumed that the composite cross
section remains plane if the shear connection and the transverse reinforce-
ment are designed considering appropriate distributions of design longitudi-
nal shear force. The tensile strength of concrete shall be neglected.
According to EC4 [ 3.6 ] , the calculation of plastic moment resistance M pl,
Rd can be performed assuming there is full interaction between structural
steel, reinforcement, and concrete; the effective area of the structural steel
member is stressed to its design yield strength f yd in tension or compression;
and the effective areas of longitudinal reinforcement in tension and in com-
pression are stressed to their design yield strength f sd in tension or compres-
sion. Alternatively, reinforcement in compression in a concrete slab may be
neglected; the effective area of concrete in compression resists a stress of 0.85
f cd , constant over the whole depth between the plastic neutral axis and the
most compressed fiber of the concrete, where f cd is the design cylinder com-
pressive strength of concrete. Typical plastic stress distributions are shown in
Figure 3.28 as given in EC4 [ 3.6 ] . For composite cross sections with struc-
tural steel grade S420 or S460, where the distance x pl between the plastic
neutral axis and the extreme fiber of the concrete slab in compression
exceeds 15% of the overall depth h of the member, the design resistance
moment M Rd should be taken as bM pl,Rd where b is the reduction factor
as shown in Figure 3.29 given by EC4. For values of x pl / h greater than
0.4, the resistance to bending should be determined from nonlinear or elastic
resistance to bending.
Where the bending resistance of a composite cross section is determined
by nonlinear theory, EC4 [ 3.6 ] recommends the stress-strain relationships of
the materials shall be taken into account. It should be assumed that the