Civil Engineering Reference
In-Depth Information
material property of the wooden clamps should exceed those of the original beam. The
clamps should be made of straight grained and air-dried timber without knag or boxed
heart. Under no circumstance can humid timber be used. The damaged end of the beam
should be cut even and be connected with the substitute timber tightly and straightly. The
interface between clamps and beam should be smooth and close when fastened by bolts. For
a round-section beam, the clamps and the newly processed plane should be connected well.
Specification and quantity of bolts and length of clamps should be calculated according
to current codes.
The force on bolts can be calculated by the following formula:
R
1
=
M
S
M
S
,
R
2
=
(5.1)
where
S
is the distance between force
R
1
and
R
2
on the bolt;
M
1
is the moment at the
R
2
point (the moment within the wooden clamps);
M
2
is the moment at the
R
2
point (the
moment within the beam).
(2) Haunched connection
Beams can be strengthened with channel steel or other material support at the bottom
(Fig. 5.3)
. The tension bolts connect channel steel and timber beam, and their cushion
plate should be calculated for checking. Tensile force within the bolt can be calculated by
the following formula:
R
1
=
M
S
M
S
,
R
2
=
(5.2)
Fixing bolt
Tension bolt
S
100
Temporary support
Channel steel
R
2
R
1
Fig. 5.3
Strengthening ends of wooden beam with haunched connection (unit: mm).
where
S
is the distance between reaction force
R
1
and
R
2
;
M
1
is the moment in the channel
steel (equal to the moment at beam section );
M
2
is the moment at beam section ;
R
1
is the force within the tension bolt;
R
2
is the extrusion force between channel steel and end
across-grain plane where the bolt is fixed.
It is reliable to use channel steel as a haunched connection because its configuration is
easy to handle. This method can be adopted when a beam is dicult to strengthen by
wooden clamps.
2. Strengthening with bottom-bracing steel tension rods
There are diverse strengthening forms with bottom-bracing steel tension rods. One simple
form is shown in Fig. 5.4. It can be applied to strengthen a shaky beam with small cross
section, which has deficient bearing capacity or excessive deflection. The beam and the
bottom-bracing steel tension rod constitute a new bearing member.
