Civil Engineering Reference
In-Depth Information
greatly increased along with a little increase of section size. For square or rectangle columns,
rolled angles are generally wrapped on four corners and linked with horizontal batten plates
to form a whole body. For circular members such as circular column and chimney, flat-rolled
steel hoops with skeins are often used (Fig. 3.41).
3
2
4
1
5
Fig. 3.41 Encased steel methods
1. original column; 2. rolled angle;
3. batten plate; 4. concrete or mortar;
5. agglomerate.
Retrofitting by bonding steel and column together by filling latex cement or epoxy mortar
or fine concrete in space between them is called wet-enclosing steel method.
This method improves capacity of concrete columns and also improves ductility because
of the restraints of profiled steel and batten plates.
2. Retrofitting design using wet-enclosing steel method
The lateral deformation of a column is restricted by profiled steel bonded on the column
while profiled steel bears compression and moment because of lateral compression induced
by concrete lateral deformation, which may result in decrease of compression strength of
steel. Moreover, as with section-enlarging mentioned above, there is also stress hysteresis in
profiled steel, which prevents the profiled steel to play its role completely. So, the factored
strength of profiled steel should be reduced.
The normal section bearing capacity of wet-enclosing steel reinforcement column could be
computed as a whole section, while the factored strength of profiled steel should be reduced.
According to
Specification for Retrofitting of Concrete Structures
, the reduction coe
cient
of a compression rolled angle is 0.9.
Because of the small thickness of new poured concrete (or mortar) and the existence of
rolled angle and batten plate, the bonding of column and new concrete is weakened. Under
ultimate state, the new poured concrete is possibly flaked off and its contribution is neglected
in the process of retrofitting design.
Calculation methods of section stiffness and bearing capacity of wet-enclosing steel rein-
forcement column are introduced below:
(1) Section stiffness
Section stiffness
EI
of wet-enclosing steel reinforcement column can be approximately
computed by:
EI
=
E
c
I
c
+
E
a
A
a
a
2
(3
.
64)
where
E
c
and
I
c
are respectively for the Young's modulus of concrete in original column
and the inertia moment of the original column;
E
a
is the Young's modulus of profiled steel;
A
a
is section area of profiled steel in each column side;
a
is the distance between centroids
of area of the tensile and compressive profile steel section.
(2) Bearing capacity of axial compression columns
Bearing capacity of wet-enclosing steel reinforcement column is given as follows:
φ
f
c
A
c
+
f
y
A
s
+0
.
9
f
a
A
a
N
(3
.
65)
