Civil Engineering Reference
In-Depth Information
Figure 3.1
Design example - structure for a typical floor
Properties of concrete
Lightweight-aggregate concrete grade LC25/28 will be used for the floor
slabs and normal-density concrete grade C25/30 for the encasement of the
columns and the webs of the beams. The properties of these concretes are
given in Table 1.4. The design compressive strengths for both of these
concretes are:
16.7 N/mm
2
f
cd
=
f
ck
/
γ
C
=
25/1.5
=
The short-term elastic moduli and modular ratios are:
20.7 kN/mm
2
;
n
0
LC25/28:
E
cm
=
=
210/20.7
=
10.1
31.0 kN/mm
2
;
n
0
C25/30:
E
cm
=
=
210/31
=
6.8
The long-term compressive strain of these concretes under permanent
loads is about three times the initial elastic strain, due to creep. In elastic
analysis, this would require separate calculations for permanent and vari-
able loads. For buildings, EN 1994 permits the simplification that all
strains may be assumed to be twice their short-term value. This is done by
using modular ratios
n
=
2
n
0
. Hence,
for grade LC25/28,
n
=
20.2;
for grade C25/30,
n
=
13.6
Properties of other materials
The partial factors
γ
M
to be used are those recommended in the Eurocodes.
A national annex may prescribe other values. The following grades of the
materials are those widely used:
