Civil Engineering Reference
In-Depth Information
Figure 3.13
Use of effective width to allow for shear lag
provided that a width of slab
L
e
/8 is present on each side of the shear
connectors.
Where profiled sheeting spans at right angles to the span of the beam
(as in the worked example here), only the concrete above its ribs can
resist longitudinal compression (e.g., its effective thickness in Fig. 3.9 is
80 mm). Where ribs run parallel to the span of the beam, the concrete
within ribs can be included, though it is rarely necessary to do so.
Longitudinal reinforcement within the slab is usually neglected in regions
of sagging bending.
3.5.2
Classification of steel elements in compression
Because of local buckling, the ability of a steel flange or web to resist
compression depends on its slenderness, represented by its width/thickness
ratio,
c
/
t
. In design to EN 1994-1-1, as in EN 1993-1-1, each flange or
web in compression is placed in one of four classes. The highest (least
slender) class is Class 1 (plastic). The class of a cross-section of a com-
posite beam is the lower of the classes of its web and compression flange,
and this class determines the design procedures that are available.
This well-established system is summarised in Table 3.1. The Eurocodes
allow several methods of plastic global analysis, of which rigid-plastic
analysis (plastic hinge analysis) is the simplest. This is considered further
in Section 4.3.3.
The Eurocodes give several idealised stress-strain curves for use in
plastic section analysis, of which only the simplest (rectangular stress
blocks) are used in this topic.
The class boundaries are defined by limiting slenderness ratios that are
proportional to (
f
y
)
− 0.5
, where
f
y
is the nominal yield strength of the steel.
This allows for the influence of yielding on loss of resistance during
