Civil Engineering Reference
In-Depth Information
TABLE E5.3
Shear
Total Panel
Force in
Shear (Wind)
(Top Chord
Shear
Each Diagonal
Panel
(kips)
Compression) (kips)
(kips)
(kips)
U1-U2
(
6.8
)(
5
+
0.5
+
0.5
)/
2
=
20.4
9.3
29.7
+
41.6
U2-U3
20.4
−
6.8
=
13.6
16.0
29.6
+
41.4
U3-U4
13.6
−
6.8
=
6.8
20.0
26.8
+
37.5
U4-U3
0
21.3
21.3
+
29.8
TABLE E5.4
Shear (Wind)
Shear (Nosing)
Total Panel
Force in Each
Panel
(kips)
(kips)
Shear (kips)
Diagonal (kips)
L0-L1
9.8
(
8
)/
2
=
39.2
22.5
61.7
+
/
−
43.1
L1-L2
39.2
−
9.8
=
29.4
22.5
51.9
+
/
−
36.3
L2-L3
29.4
−
9.8
=
19.6
22.5
42.1
+
/
−
29.4
L3-L4
19.6
−
9.8
=
9.8
22.5
32.3
+
/
−
22.6
L4-L3
0
22.5
22.5
+
/
−
15.7
and compression. The bottom lateral bracing member forces are shown in
Table E5.4.
5.2.2.1.2 End Vertical and Portal Bracing
A supplemental structural system is required to transfer lateral forces from horizontal
bracing members that are not located in the plane of the bearings to the horizontal
lateral systems that are in the plane of the bearings. In through spans the lateral forces
may be transferred through a system of knee braces or via sway and end portals. In
deck spans a system of vertical cross frames or diaphragms may serve this purpose.
∗
The vertical end bracing and portal bracing are required to carry the entire reaction
from lateral loads to the substructures via the bearings.
Through truss end portal frames must be designed to transfer the total wind force
reaction of the top lateral truss system,
P
L
, through flexure of end posts.The end posts
are also required to resist additional axial forces from the portal action. In order to
estimate the end portal effects on the end posts, it is assumed that horizontal reactions
are equal at the bottom of the end posts and an inflection point is located midway
between the bottom of the end post and the bottom of the portal bracing frame (often
cross-braced or knee-braced in modern steel trusses) at a distance,
(h
e
−
h
p
)/
2, as
∗
In deck truss spans supported at the top chord end portal bracing is used in the plane of the end diagonal
member.
