Civil Engineering Reference
In-Depth Information
4.2.5.1
No control of crack width
'No control' is relevant to serviceability limit states. It is still necessary to
ensure that the concrete retains sufficient integrity to resist shear at ultimate
limit states, by acting as a continuum. It is required that the minimum longi-
tudinal reinforcement in a concrete flange in tension shall be not less than:
•
0.4% of the area of concrete, for propped construction, or
(4.29)
•
0.2% of the area of concrete, for unpropped construction.
(4.30)
These bars are likely to yield at cracks, which may be about 0.5 mm wide,
but they ensure that several cracks form rather than just one, which could
be much wider. The presence of profiled steel sheeting is usually ignored,
which may be conservative in some situations.
4.2.5.2
Control of restraint-induced cracking
Uncontrolled cracking between widely-spaced bars is avoided, and crack
widths are limited, by:
•
using small-diameter bars, which have better bond properties and have
to be more closely spaced than larger bars;
•
using 'high bond' (ribbed) bars or welded mesh;
•
ensuring that the reinforcement remains elastic when cracking first
occurs.
The last of these requirements is relevant to restraint cracking and leads
to a design rule for minimum reinforcement, irrespective of the loading,
as follows.
Let us assume that an area of concrete in uniform tension,
A
ct
, with an
effective tensile strength
f
ct
,
eff
, has an area
A
s
of reinforcement with char-
acteristic yield strength
f
sk
. Just before the concrete cracks, the force in it
is approximately
A
ct
f
ct
,
eff
. Cracking transfers the whole of the force to the
reinforcement, which will not yield if
A
s
f
sk
≥
A
ct
f
ct
,
eff
(4.31)
This condition is modified, in EN 1994-1-1, by a factor 0.8 that takes
account of self-equilibrating stresses within the member (that disappear
on cracking), and by a factor
1
k
c
=
+
0.3
≤
1.0
(4.32)
1
(/ )
+
hz
c
2
0
