Civil Engineering Reference
In-Depth Information
which is to say that additional stirrups would not yield until shear failure of the beam. In
such case, the retrofit strategy of enlarging section should be selected.
However, the restriction condition should be more rigorous for such members with more
severe diagonal cracks such as thin-web girders. Therefore, the Eq. (3.4) is written as
V
0
.
2
f
c
bh
0
(3
.
5)
b. The minimal stirrup ratio could be given as
ρ
sv,
min
=
nA
sv
1
bh
0
=0
.
02
f
c
f
yv
(3
.
6)
min
When stirrup ratio is less than the requirement of Eq. (3.6), stirrups yield immediately
after cracking and can barely restrain inclined crack development, which leads to diagonal
tension failure abruptly. Accordingly, the retrofit technique of adding stirrups should be
givenfirstpriorityonsuchbeams.
Whereas stirrup ratio lies between maximum value and minimal value, the technique of
adding web reinforcements may be appropriate for lower value in the range and the technique
of enlarging sections with additional stirrups could be favorable provided that the ratio is
close to maximum value.
3.2.2
Section Enlarging Method
1. Introduction
The section enlarging method is effective when bearing capacity and member stiffness are
far below the code-specified requirement. Furthermore, format of enlarging sections (see Fig.
3.2) may be selected from one-side thickening, two-side thickening or three-side thickening
in accordance with loading types, detail features and construction conditions.
2
1
2
1
1
2
(a) Thickening (b) Deepening
(c) Adding reinforced
concrete at tensile side
Fig. 3.2 Section enlarging method:
original member;
additional concrete.
When additional concrete is cast above the original concrete surfaces of continuous beams,
concrete added at mid-span of beams lies in compression zone, while the concrete is subjected
to tensile force at supports. However, thickening underneath is the contrary case.
The concrete added in the tension zone prevents additional reinforcement against erosion,
whereas the concrete added in the compression zone enhances effective depth of the section,
and increases stiffness and strength of the member. The retrofit strategy of adding concrete
layer is definitely feasible.
Generally, the concrete is often added at the tension zone in practice. It is regarded as
an effective method to improve flexural capacity especially for a T-shaped beam with low
reinforcement ratio and short compression depth by means of adding tensile reinforcements
