Civil Engineering Reference
In-Depth Information
(1.8b)
(1.9)
V
u
=
bh
a
v
u
where the
x
e
is the effective clear shear span calculated from step 1,
v
u
is as defined for Eqn (1.7) and
v
c
is the shear stress value taken
from Table 5 of CP1 10 (1972) for normal weight concrete and
Table 25 for lightweight concrete (see also BS 8110: Part 1: clause
3.4.5.4 and Part 2: clause 5.4).
Step 3:
Check that the applied shear force
V
does not exceed the shear
capacity
V
u
calculated in step 2.
Step 4:
Provide nominal web reinforcement in the form of a rectangular
mesh in each face. The amount of this nominal reinforcement
should not be less than that required for a wall by clauses 3.11 and
5.5 of CP1 10 (1972); this in effect means at least 0.25% of
deformed bars in each direction (see also BS 8110: Part 1: clauses
3.12.5.3 and 3.12.11.2.9). The vertical bars should be anchored
round the main bars at the bottom; the horizontal bars should be
anchored as links round vertical bars at the edges of the beam. The
bar spacings and minimum percentage should also meet the
requirements of section 1.4.1.5.
1.4.1.4 Bearing strength
For deeper beams (
l/h
<1.5), the bearing
capacity may well be the governing design criterion, particularly for
those having shorter shear spans. To estimate the bearing stress at
the support, the reaction may be considered uniformly distributed over
an area equal to (the beam width
b
)×(the effective support length)
where the effective support length is to be taken as the actual support
length
c
or 0.2
l
o
whichever is the lesser. The bearing stress so
calculated should not exceed 0.4
f
cu.
1.4.1.5
Crack control
The minimum percentage of reinforcement, in the
horizontal or vertical direction, should comply with the requirements for
a wall, as given in clauses 3.11 and 5.5 of CP110 (1972) (see also BS
8110: Part 1: clauses 3.12.5.3 and 3.12.11.2.9). The maximum bar spacing
should not exceed 250 mm. In a tension zone, the steel ratio ρ, calculated
as the ratio of the total steel area to the local area of the concrete in which
it is embedded, should satisfy the condition
(1.10)
The maximum crack width should not be allowed to exceed 0.3 mm in
a normal environment; in a more aggressive environment, the
maximum crack width may have to be limited to 0.1mm. To control
maximum crack widths to within 0.3 and 0.1mm, bar spacings should
not exceed those given in Tables 2 and 3, respectively, of the CIRIA
Guide.
