Civil Engineering Reference
In-Depth Information
which infl uence the cracking and therefore the results are only
as accurate as the input data. Eurocode 2 offers some simple
rules: either the bar size is limited, or the spacing of the bars is
limited (see
Tables 17.10
and
17.11
). It is not necessary to meet
both criteria. The stress in the reinforcement should be the stress
under quasi-permanent loading and can be estimated from:
scheduled, ready for fi xing on site. The minimum areas of
steel should have been calculated to resist bending and shear.
Checks should have been carried out to ensure that defl ection
is within acceptable limits, but reinforcement is provided for a
number of other reasons:
■
to control cracking, which occurs due to fl exure, shrinkage and
=
thermal effects;
to support the top layer of reinforcement in slabs and beams;
f
yk
A
f
yk
Q
k k
QG
k
1
δ
ψ
QG
QG
yk
ψψ
k
+
QG
QG
k
k
sreq
s
sr
σ
s
1
5
Q
13
5
G
A
■
γ
+
Q
k
k
k
+
13
5
G
k
k
k
The note in
Table 17.10
(which is identical to that in
Table 17.11)
gives the basis on which the maximum bar diam-
eter or bar spacing are calculated, but it is generally accepted
that the tables can be used for typical reinforced concrete elem-
ents. Where the element under consideration differs signifi cantly
from these values direct calculation should be carried out.
1
.
15
k
Q
Q
Q
k
k
k
+
13
13
.
sprov
sp
sp
to distribute forces into the designed reinforcement, i.e. secondary
■
reinforcement in slabs and walls;
to prevent buckling of bars in compression, i.e. links in columns.
■
■
This additional reinforcement is usually determined through
empirical rules, which vary slightly from code to code but are
intended to achieve the same end. The empirical rules are usu-
ally presented for particular elements, for example, a column,
and therefore for some elements that do not fi t into these cat-
egories, some interpretation is required.
Another important aspect of detailing is determining the
anchorage and lap lengths for bars and the position of the laps.
A bar should be anchored so that it will not pull out of the
concrete under tension or compression. The anchorage can be
in the form of a straight length of bar or a through a bend at
the end of the bar (subject to limiting rules). Laps are required
to transmit tension or compression from one bar to another
through the concrete. It is good practice to place the laps at a
position of relatively low stress in the bar. For practical pur-
poses the contractor will also want to place day joints around
the position of the lap locations.
The Eurocode 2 approach to determining the anchorage and
lap lengths is to provide factors that consider the parameters
which affect their strength. The bond strength is then multi-
plied by the factors to determine the appropriate lap or anchor-
age length. This is at odds with previous practice of using a
simple multiple of the bar diameter to give a suitable length.
The advantage is that economies of materials can be made by
considering each particular situation. The disadvantage is that
it makes detailing and fi xing more complex. It is, therefore,
appropriate to make simplifying assumptions so that standard
lap lengths can be used throughout a project. One approach is
presented in
Table 17.12
, where a conservative value is given
for most of the factors (see the notes), and an appropriate length
can be read for a bar diameter for various conditions.
One requirement to be aware of is that the laps should be 'stag-
gered', meaning that the position of laps in adjacent bars should
be offset so that high local stresses in the concrete are limited.
17.5.8 Detailing
Having determined all the element sizes and the reinforce-
ment requirements, the reinforcement needs to be drawn and
Steel stress
(MPa)
Maximum bar size (mm)
w
k
= 0.4 mm
w
k
= 0.3 mm
w
k
= 0.2 mm
160
40
32
25
200
32
25
16
240
20
16
12
280
16
12
8
320
12
10
6
360
10
8
5
Notes: The values in this table are based on the following assumptions:
c
= 25 mm,
f
ct,eff
= 2.9 MPa,
h
cr
= 0.5
h
, (
h-d
) = 0.1
h
,
k
1
= 0.8,
k
2
= 0.5,
k
c
= 0.4,
k
4
= 1.0,
k
t
= 0.4 and
k
' = 1.0.
Table 17.10 Maximum bar size to control cracking (data taken from
BSI, 2004)
Steel stress
(MPa)
Maximum bar spacing: mm
w
k
= 0.4 mm
w
k
= 0.3 mm
w
k
= 0.2 mm
160
300
300
200
200
300
250
150
240
250
200
100
280
200
150
50
320
150
100
-
360
100
50
-
17.5.8.1 Information for detailers
The detailing of the reinforcement is usually undertaken by
experienced 'detailers' who take the design intention of
the engineer and produce the reinforcement drawings and
accompanying bar bending schedules. The detailing may be
Notes: The values in this table are based on the following assumptions:
c
= 25 mm,
f
ct,eff
= 2.9 MPa,
h
cr
= 0.5
h
, (
h-d
) = 0.1
h
,
k
1
= 0.8,
k
2
= 0.5,
k
c
= 0.4,
k
4
= 1.0,
k
t
= 0.4 and
k
' = 1.0.
Table 17.11 Maximum bar spacing to control cracking (data taken
from BSI, 2004)
