Civil Engineering Reference
In-Depth Information
H
e
=
20.4
/
2
=
10.2 kips (a small shear force that is generally neglected)
M
e
=
20.4
[
(
25.0
)/
2
]
/
2
=
127.5
ft-kips
(bendingmomentatthebottomofthe
portal frame end post).
The portal is of the triangular type (Example 5.14) and member forces are
P
A
=−
P
B
=
20.4
(
1.62
)
=
33.0 kips
P
C
=
20.4
(
0.74
)
=
15.1 kips
P
D
=−
20.4
(
1.74
)
=−
35.5 kips
The members shown as dotted lines may be designed for 2.5% of the
compressive force in members A and B. However, as this force will be small
(825 lb in this example), design based on compression member slenderness
ratio criteria (
r
min
≤
member length/120) will likely govern.
Other portal arrangement member forces may be determined in a similar
approximate manner or by rigorous frame analysis. AREMA (2008) indicates
that through truss spans should have portal bracing with knee braces (e.g.,
members A and B in
Figure E5.15)
as deep as clearances (see Chapter 3) will
allow.
Cross frame members at the end of deck spans must transfer the reaction of
the top lateral truss to the bearings and substructure. AREMA (2008) indicates that
diaphragms may be used in lieu of cross frames for closely spaced shallow girders.
Example 5.16 shows the analysis of a typical DPG vertical end brace frame.
Example 5.16
Determine the forces in the members of the end brace frame shown in
Figure E5.16. The force
P
L
is given as 35.5 kips.
P
L
=
35.5 kips(toplateraltrusswindforceandnosingreactionstransferred
to end portal frame).
P
L
A
B
12'
H
1
H
2
10'
V
2
V
1
FIGURE E5.16
Deck span end frame.
