# How accurately are moveable span bridges balanced?

Movable span bridges (bascule and lift) typically employ a counterweight to help reduce the amount of energy and size of motor required to move the span. Due to variations in the the construction of the bridge and counterweight, the theoretical balance of both the span and the counterweight may be off.

After a movable span is complete, how close to perfectly balanced do the span and counterweight need to be?

This paper from the Heavy Movable Structures group goes into detail about how roadway movable spans are balanced. It references AASHTO (American Association of State Highway and Transportation Officials) manuals such as those below:

The general guidelines are as follows:

• The spans are balanced so that the dead load reaction is greater than 1,000 kips per girder when the span is down. This keeps the span down even if the locking mechanism isn't working.
• The balance is calculated multiple times by both the contractor and the engineer based on theoretical and fabricated weights.
• The counterweight is designed so that the weight can be adjusted from the theoretical. It must accommodate a 5% heavier or 3.5% lighter span.
• The counterweight can be adjusted by adding or removing individual 1 cu. ft. blocks. These blocks are typically concrete but can also be steel or lead.
• The final check of weight can be done with strain gauges.

However close the motor & drivetrain can compensate for. The more torque it can output the more you can get away with.

As an example an elevator's counterweight is tuned to the mass of the elevator filled to half capacity and the motor will compensate for the rest.

Also during the operation of the bridge the mass and geometry can change; thermal expansion may move the center of mass; rain, dew, frost and grime will add mass to the span as will a new coat of paint.

• You gave the answer for an elevator. I would bet that bridges have a specific value or percentage as well. That is what I am looking for.
– hazzey
Commented Feb 11, 2015 at 12:10
• merriam-webster.com/dictionary/rime
– user16
Commented Feb 11, 2015 at 18:25
• I would imagine that there isn't a set standard and it would be near entirely at the discretion of the Engineer of Record for the project and the desires of the project owner. Commented Feb 13, 2015 at 17:15