Civil Engineering Reference
In-Depth Information
(4)
With guidance from typical span-to-depth ratios for composite
beams, guess the overall depth of the beam. Assuming that the
floor slab has already been designed, this gives the depth
h
a
of the
steel section.
(5)
Guess the weight of the beam, and hence estimate the design mid-
span bending moment,
M
Ed
.
(6)
Assume the lever arm to be (in the notation of Fig. 3.15)
h
c
/2)
and find the required area of steel,
A
a
, if full shear connection is to
be used, from
(
h
a
/2
+
h
t
−
A
a
f
yd
(
h
a
/2
+
h
t
−
h
c
/2)
≥
M
Ed
(3.107)
For partial shear connection,
A
a
should be increased.
(7)
If full shear connection is to be used, check that the yield force in
the steel,
A
a
f
yd
, is less than the compressive resistance of the con-
crete slab,
b
eff
h
c
(0.85
f
cd
)
.
If it is not, the plastic neutral axis will be
in the steel - unusual in buildings - and
A
a
as found above will be
too small.
(8)
Knowing
h
a
and
A
a
, select a rolled steel section. Check that its web
can resist the design vertical shear at an end of the beam.
(9)
Design the shear connection to provide the required bending resist-
ance at mid-span.
(10)
Check deflections and vibration in service.
(11)
Design for fire resistance.
3.11.1
Composite beam - full-interaction flexure and verticalshear
From Section 3.4, the uniform characteristic loads from a 4.0-m width of
floor are:
•
permanent,
g
k
1
=
2.54 × 4 = 10.2 kN/m
on steel alone
g
k
2
=
1.3
×
4
=
5.2 kN/m
on the composite beam
•
variable,
q
k
=
6.2
×
4
=
24.8 kN/m
on the composite beam
The weight of the beam and its fire protection is estimated to be 2.2
kN/m, so the design ultimate loads are:
g
d
=
1.35(15.4
+
2.2)
=
23.7 kN/m
q
d
=
1.5
×
24.8
=
37.2 kN/m
