Civil Engineering Reference
In-Depth Information
The distance and quantity of buttress columns that the brick wall demands should be de-
termined by calculation.
2. Bearing-capacity checking for retrofitted wall with brick buttress columns
Considering stress-lagging in post-build buttress columns, factored value
f
1
of compression
strength of buttress columns should be multiplied by discount coecient 0.9 when calculating
the bearing capacity of a retrofitted brick wall. Compressive bearing capacity can be checked
by the following formula:
N
ϕ
(
fA
+0
.
9
f
1
A
1
)
(4.1)
where
N
is the axial force induced by load factored value;
ϕ
is the influence coecient for
load-bearing capacity of compressive members engendered by depth-thickness ratio
β
and
eccentricity of axial force
e
, and its value can be obtained from
Code for Design of Masonry
Structures
;
f
and
f
1
are design values of compression strength for original wall and buttress
columns respectively;
A
and
A
1
are the areas of original wall and buttress columns.
It is not necessary to consider the stress-lagging of buttress columns when checking depth-
thickness ratio of a retrofitted wall and requirements for serviceability limit state. Combina-
tion section after adding buttress columns should be adopted to calculate influence coecient
for bearing capacity of compression members.
Example 4.1
An oce building has a cross wall thickness of 240 mm, a spacing of 4
m between the cross walls, depth of 6 m, story height of 3 m, and the reinforced concrete
floor thickness of 120 mm. The pressure transverse walls bear is obtained by calculation,
and its value is 188 kN/m. Site inspection results showed that the strength grade of bricks
is about MU7.5 and the mortar is M0.4.
a. Check the original brick wall capacity. Design value of masonry compression strength
in masonry structure:
f
=0
.
79MPa of brick compression strength can be obtained from
Code for Design of Masonry Structures
.
The calculated height of the wall
H
0
: According to rigidity plans, because of
s
=2
H
,
where s is the spacing of the walls or pilasters which could act as lateral bracing of the
analyzed walls, here it refers to the depth of house.
By consulting code list 4.1.3, we
obtained
H
0
=0
.
4
s
+0
.
2
H
=0
.
4
×
6+0
.
2
×
3=3
.
0m.
β
=
H
h
3000
240
=
=12
.
5
<
[
β
]
ϕ
=0
.
59 can be obtained by code appendix list 5-4, and the design value
N
0
for bearing
capacity of original brick wall according to the following formula;
N
0
=
ϕf A
=0
.
59
×
0
.
79
×
240
×
1000 = 111
.
8kN
<N
= 188 kN
Result indicates that the brick wall must be retrofitted.
b. Retrofitting design. Buttress columns are built with bricks of MU10 and mixed mortar
of M10. Compression strength
f
1
=1
.
99 MPa by consulting code. Establish buttress
columns every 1.5 m on both sides of the original wall. The width of the buttress column
of one side is 240 mm, and the thickness is 125 mm (thickness direction is along the depth
of wall), as shown in Fig. 4.2.
