Civil Engineering Reference
In-Depth Information
anchorage contributes to the confinement of the concrete at the
supports and improves the bearing strength.
1.4.1.2 Shear strength: bottom-loaded beams
Step 1:
Calculate the concrete shear capacity:
V
u
=0.75
bh
a
v
u
where
v
u
is the maximum shear stress taken from Table 6 of CPI 10
(1972) for normal weight concrete and Table 26 for lightweight
concrete (see also BS 8110: Part 1: clause 3.4.5.2 and Part 2: clause
5.4)
Step 2:
Check that the applied shear force
V
does not exceed
V
u
of Eqn (1.7)
Step 3:
Provide hanger bars in both faces to support the bottom loads, using
a design stress of 0.87
f
y
. The hanger bars should be anchored by a
full bond length above the active height
h
a
or, alternatively,
anchored as links around longitudinal bars at the top.
Step 4:
Provide nominal horizontal web reinforcement over the lower
half of the active height
h
a
and over a length of the span equal to
0.4ha measured from each support. The area of this web
reinforcement should not be less than 80% that of the uniformly
distributed hanger steel, per unit length. The bar spacing and
reinforcement percentage should also meet the requirements of
Section 1.4.1.5.
(1.7)
1.4.1.3 Shear strength: top-loaded beams.
The proven concept of the
clear
shear span x
, as used by Kong
et al.
(1972b and 1975) has been adopted by
the CIRIA Guide. The CIRIA Guide has also accepted the Kong
et al.
(1972b) proposal that, for uniformly distributed loading, the
effective clear
shear span
xe may be taken as
l/
4.
Step 1:
With reference to
Figure 1.3,
calculate the effective clear shear span
x
e
which is to be taken as the least of:
i)
The clear shear span for a load which contributes more than 50% of
the total shear force at the support.
ii)
l/
4 for a load uniformly distributed over the whole span.
iii)
The weighted average of the clear shear spans where more than one
load acts and none contributes more than 50% of the shear force at
the support. The weighted average will be calculated as
S
(
V
r
x
r
)/
S
V
r
where
V
r
=
V
is the total shear force at the face of the support,
V
r
=an
individual shear force and
x
r
=clear shear span of
V
r
.
S
Step 2:
Calculate the shear capacity
V
u
to be taken as the value given by
Eqns (1.8) and (1.9)
(1.8a)
