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c. If the plumb of the center of runout is out of tolerance, calculate the thickness of shims
for bridge legs to plumb center of runout using either graphical or analytical methods
below. To prevent distortion, shims must be added to all except one leg of the bridge
so that a wedge shape of shims is maintained.
below. To prevent distortion, shims must be added to all except one leg of the bridge
so that a wedge shape of shims is maintained.
Graphical Procedure for Shim Calculation (Figure 19)
(i) Draw two circles and plot bridge legs. One circle will be used for the North-South
orientation and one for East-West.
orientation and one for East-West.
(ii) Plot pivot axis on each circle. The pivot axis will line up with the plumb wire
locations.
locations.
(iii) Plot change in bridge elevation point from appropriate end of pivot axis and
connect by line to opposite end of pivot axis.
connect by line to opposite end of pivot axis.
(iv) Project a line from the end of each leg perpendicular to the pivot line. Count and
tabulate the number of divisions from the shim line to the pivot line along the projected
line.
tabulate the number of divisions from the shim line to the pivot line along the projected
line.
(v) Total divisions of both circles for each bridge leg and subtract the smallest total
value from all the total values to determine amount of shim to add to the legs.
value from all the total values to determine amount of shim to add to the legs.
Analytical Procedure for Shim Calculation (Figure 20)
(i) Draw two circles and plot bridge legs. One circle will be used for the North-South
orientation and one the East-West.
orientation and one the East-West.
(ii) Plot the pivot axis on each circle. The pivot axis will line up with the plumb wire
locations.
locations.
(iii) Project a line from the end of each leg perpendicular to the pivot line. Calculate
the distance along the pivot line from the pivot point to the projected lines.
the distance along the pivot line from the pivot point to the projected lines.
(iv) Calculate and tabulate the shims required for each bridge leg. Change in elevation=
(Distance from Pivot Point)*(Out of Plumb)/Length of Shaft.
(Distance from Pivot Point)*(Out of Plumb)/Length of Shaft.
(v) Total shims north-south and east-west for each bridge leg, subtract the smallest
total value from all the total values to determine the thickness of shims to add to legs.
total value from all the total values to determine the thickness of shims to add to legs.
d. After shims are installed, repeat steps a and b. If the plumb of the center of runout is
still out of tolerance, repeat step c.