Civil Engineering Reference
In-Depth Information
is convenient to design the beams as simply-supported. Such beams have
the following advantages over beams designed as continuous at supports:
•
very little of the steel web is in compression, and the steel top flange
is restrained by the slab, so the resistance of the beam is not limited by
buckling of steel;
•
webs are less highly stressed, so it is easier to provide holes in them
for the passage of services;
•
bending moments and vertical shear forces are statically determinate,
and are not influenced by cracking, creep, or shrinkage of concrete;
•
there is no interaction between the behaviour of adjacent spans;
•
bending moments in columns are lower, provided that the frame is
braced against sidesway;
•
no concrete at the top of the slab is in tension, except over supports;
•
global analyses are simpler and design is quicker.
The disadvantages are that deflection at mid-span or crack width at
supports may be excessive, and structural depth is greater than for a
continuous beam.
The behaviour and design of mid-span regions of continuous beams are
similar to those of simply-supported beams, considered in this chapter.
The other aspects of continuous beams are treated in Chapter 4.
3.5.1
Effective cross-section
The presence of profiled steel sheeting in a slab is normally ignored when
the slab is considered as part of the top flange of a composite beam.
Longitudinal shear in the slab (explained in Section 1.6) causes shear
strain in its plane, with the result that vertical cross-sections through the
composite T-beam do not remain plane when it is loaded. At a cross-
section, the mean longitudinal bending stress through the thickness of the
slab varies across the width of the flange, as sketched in Fig. 3.13.
Simple bending theory can still give the correct value of the maximum
stress (at point D) if the true flange width,
B
, is replaced by an effective
width,
b
(or
b
eff
),
such that the area GHJK equals the area ACDEF. Research
based on elastic theory has shown that the ratio
b
/
B
depends in a complex
way on the ratio of
B
to the span
L
, the type of loading, the boundary
conditions at the supports, and other variables.
For simply-supported beams in buildings, EN 1994-1-1 gives the effect-
ive width as
L
e
/8 on each side of the steel web, where
L
e
is the distance
between points of zero bending moment. The width of steel flange occupied
by shear connectors,
b
0
, can be added, so
b
=
L
e
/4
+
b
0
(3.55)
