Civil Engineering Reference
In-Depth Information
materials given in Section 4.6.1 and Equation 5.13, the design properties
are:
f
yd
=
355 N/mm
2
f
sd
=
435 N/mm
2
,
0.85
f
cd
=
14.2 N/mm
2
E
cm
=
31.0 kN/mm
2
The assumed concrete cover to the reinforcement, 30 mm, and to the
structural steel, 57 mm, satisfy the requirements for 90 minutes' fire
resistance. From EN 1992-1-1, 30 mm cover should be sufficient if the
external face of the column is protected; but if it is exposed to rain and/
or freeze/thaw, it would be necessary to increase either the cover or the
grade of the concrete.
The cross-sectional areas of the three materials are:
A
a
=
6640 mm
2
A
s
=
804 mm
2
A
c
= 94 950 mm
2
The ratio
A
s
/
A
c
is 0.0085, which satisfies Expression 5.17.
From Equation 5.24, the design plastic resistance to axial load is
N
pl
,
Rd
=
6640
×
0.355
+
804
×
0.435
+
94.95
×
14.2
=
2357
+
350
+
1345
=
4052 kN
(5.41)
With the partial factors taken as 1.0, from Equation 5.26,
N
pl,Rk
=
2357
+
350
×
1.15
+
1345
×
1.5
=
4776 kN
(5.42)
From Equation 5.18,
δ
=
2357/4052
=
0.582
which is within the permitted range.
Second moments of area of the uncracked section are needed for the
calculation of the elastic critical load,
N
cr
.
For the steel section, from tables, 10
− 6
I
a
=
52.6 mm
4
For the reinforcement, 10
−6
I
s
=
804
×
0.115
2
=
10.6 mm
4
For the concrete, 10
−6
I
c
=
320
2
×
0.32
2
/12
−
52.6
−
10.6
=
811 mm
4
), is needed for
Equation 5.21. It depends on the relative humidity, taken as 50% for a
centrally-heated building, on the cross-section of the concrete, and on the
'age at first loading',
t
0
. There is, of course, no single age for the bottom
The long-term creep coefficient for the column,
ϕ
(
t
0
,
∞
