Civil Engineering Reference
In-Depth Information
The chord force along grid line A, starting at line 1, is as follows:
367 5
.
+
307 5
.
F
2
=
()
15
=
5062 5
.
lb compression
at grid lineA
2
A
2
This is equal to the chord force at grid line 2B.
300
+
200
F
3
=
5062 5
.
+
(
20
)
=
10 062 5
,
.
lb compres
sion
A
2
This is equal to the chord force at grid line 3B.
F
4
=
10 062 5832 146 210 000
,
.
+
.
-
.
=
,
lb compressi
on
A
(added areas of positive and negative shear diagrams, triangles—see figure)
100
+
0
F
CL
=
10 000
,
+
(
25
)
=
11 250
,
lb compression
2
The chord force along grid line C, starting at grid line 3, is as follows:
696 7
.
+
636 7
.
F
4
=
()
15
=
10 000
,
lb tension
C
2
This force is equal to the chord force at grid line 4A.
100
+
0
F
CL
=
10 000
,
+
(
25
)
=
11 250
,
lb tension
2
This force is equal to the chord force at grid line 4A.
Notice that the direction of all the transfer shears indicates that the members are in
tension, as expected. The force diagram at grid line C closes to zero.
Determination of transverse collector/chord forces (see Fig. 4.4):
The transfer shears for the
transverse collectors are shown on the plan. The net shears on each side of grid line 2
between A and B are acting in opposite directions and must be subtracted from each
other. The collector along grid line 2 from A to B has a uniform net shear of 307.5 - 300 =
7.5 plf. The force at grid line 2B is equal to
F
2
=
7540
.( )
=
300
lb tension
B
The direction of the net transfer shear shows that the member is in tension. The transfer
shears between B and C act in the opposite direction, which also indicates the member
is in tension. The force in the collector on the opposite side of grid line 2B is equal to
F
2
=
30 10
()
=
300
lb
B
Therefore the diagram closes.
The transfer shears applied to the collector along grid line 3 from A to B act in opposite
directions and must be subtracted from each other for a net shear of 200 - 25.8 = 174.2 plf.
The force in the collector at grid line 3B is equal to
F
3
=
174 240
.( )
=
6968
lb tension
B
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