The rules limit the sway of the bridges during loading.
Means must be provided for measuring the sway to the limits given in the current
Plumb bobs are very common sway devices. The trick is to get the
point of the plumb bob as close to the round mounted target as possible
without it actually touching the target as the bridge deflects.
There is usually some judgment involved in determining exactly when the
plumb bob moves outside the target. The other problem associated
with all plumb bobs is developing a means to prevent swaying of the
plumb bob--slow movement is key to solving this problem.
Occasionally, competition organizers have
experimented with a variety of "spacer' used to position the plumb bob
at the start of the load sequence.
|Laser Plumb Bob:
This is an improvement on the straight plumb bob as
it is easier to trace the movement of the bridge on the target.
Note that the target in this image is mounted on a flat piece of metal.
This makes the target much easier to position and eliminates the need to
tape the target to the floor.
|Ground Mounted Laser:
For the 2011 ASCE Pacific Northwest Student Conference
garage parking lasers where purchased on sale ($13 each) at a large department
store and mounted on a board to sit on the ground, pointing upward. The
target was placed at the decking level and consisted of thin plywood with a 2"
diameter hole covered by a paper target (with concentric circles of the
appropriate diameter). An edge of the laser beam targets the center of the
target before loading, painting a dot on the paper target. This is
monitored during loading.
||Target at Deck Level