Civil Engineering Reference
In-Depth Information
AREMA (2008) recommends that the lateral bracing of compression chords and
flanges be designed for a transverse shear force,
R
F
, equal to 2.5% of the total axial
force in both members in the panel. This “notional” force is recommended to ensure
that intermediate knee brace/transverse frames are designed with adequate stiffness
to prevent buckling failure. The bracing members may also have to be designed
considering the shear force in the panel from lateral wind loads. The transverse shear
force due to restraint of compression flange buckling,
R
F
, can then be determined for
through plate girder (
Figure 5.29)
or pony spans, as
R
F
=
0.025
A
f
f
c
,
(5.44)
where
A
f
is the area of the compression chord or flange and
f
c
is the compressive
stress in the chord or flange.
Example 5.17
Determine the AREMA-recommended bracing design force for the knee
braces (at a 3:1 slope) of the through plate girder span of Figure 5.29 with
the following data:
h
=
100 in.
h
k
=
75 in.
S
=
260 in.
150 in.
3
I
c
=
10,000 in.
3
I
b
=
45 in.
2
A
f
=
50 in.
2
(the web plate area)
A
w
=
f
c
=
20 ksi
L
=
100 ft
=
1200 in.
N
p
is the number of panels and is equal to 10
l
=
1200
/
10
=
120 in.
R
F
=
0.025
A
f
f
c
=
0.025
(
45
)(
20
)
=
22.5 kips
Column load on the knee brace
=
(
22.5
)(
3
)
=
67.5 kips
Bending moment at the floorbeam
=
22.5
(
75
/
12
)
=
140.6 ft-kips.
Example 5.18
Determine the transverse stiffness of the through plate girder span
compression flange bracing of Example 5.17 using the Engesser approach and
