Civil Engineering Reference
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i g u r e
3.11 Longitudinal chord force diagrams.
Determination of longitudinal collector/chord forces (see Fig. 3.11):
The collector/chord forces
are determined by averaging the shears along the member segment being reviewed
and then multiplying them by the member segment length, which is the same thing as
taking the area of the shear diagram along the length of the member being analyzed. It
is important that all the force diagrams be calculated and placed on the plan, as a visual
verification of the analysis. Starting with the forces along grid line A, the chord force at
grid line 2A is equal to
357 1
.
+
214 3
.
F
2
=
()
25
=
7142 9
.
lb
A
2
Note that this is equal to the chord force at grid line 2B that was previously calculated, as it
should be. The direction of the shears transferred into the chord shows that the chord is in
compression. The net shear at grid line 2 changes from +42.9 plf to -37.1 plf at grid
line 3. From similar triangles, the shear becomes zero at 10.725
1
from grid line 2. The net shear
diagram is drawn near grid line A for reference. It can be seen that the area of the positive
shear represents a positive change in the chord force of +230 lb. The area of the negative shear
decreases the chord force by -172.1 lb. Therefore, the chord force at grid line 3A is equal to
F
3
=
7142 9
.
+
230
-
172 1
.
=
7200
lb compression
A
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