Civil Engineering Reference
In-Depth Information
Figure 4.8
Maximum bar spacing for high bond bars
Crack control is achieved by limiting the spacing of the longitudinal
reinforcing bars to the values shown in Fig. 4.8, which depend on
σ
s
and
w
k
, or by limiting the bar diameter in accordance with Fig. 4.7. It is not
necessary to satisfy both requirements, for they have a common basis.
This is that crack-width control relies on the bond-stress/slip property of
the surface of the reinforcement, which is almost independent of bar
diameter. The higher the stress
σ
s
, the greater is the bar perimeter required
to limit the bond slip at cracks to an acceptable level.
For bars of total area
A
s
per unit width of slab, of diameter
φ
and at
spacing
s
, the total bar perimeter is
u
=
πφ
/
s
, and
A
s
=
πφ
2
/(4
s
). From
these equations,
u
=
4
A
s
/
φ
=
2(
π
A
s
/
s
)
1/2
Thus, for a given area
A
s
, limiting either
φ
or
s
will give the required
value of
u
. The limits to
φ
and
s
become more severe as
σ
s
is increased
and as
w
k
is reduced, as shown in Figs 4.7 and 4.8.
Fuller explanation and discussion of crack-width control for concrete
flanges of composite beams is available [17, 42].
4.3
Global analysis of continuous beams
4.3.1
General
The subject of this Section is the determination of design values of
bending moment and vertical shear for 'continuous beams' as defined in
Section 4.1, caused by the actions specified for both serviceability and
ultimate limit states.
