A swing bridge rotates horizontally about a vertical pivot to clear a navigation channel. Transportation inspection manuals distinguish center-bearing and rim-bearing swing spans and describe rack-and-pinion or hydraulic operating machinery. The calculator below models only ideal constant-speed rotation of a symmetric span.
Swing Bridge Interactive Calculator
Vary total symmetric span length, opening angle and constant slew speed. Results report ideal time and half-span tip kinematics; they do not size machinery or reproduce a bridge operating procedure.
Equation Used
- The span is symmetric about a fixed center pivot, so tip radius is L/2.
- Slew speed is constant from start to finish.
- The entered opening angle is horizontal rotation about the pivot.
- Locks, wedges, interlocks, drive torque, friction, wind, acceleration, structural response and clearance are not calculated.
How a Swing Bridge Opens
The Connecticut DOT railroad bridge inspection manual describes swing bridges as two-span trusses or continuous girders rotating horizontally about a center pivot. Symmetric spans are common; unequal bob-tailed spans use counterweights for balance. The manual notes that swing bridges commonly operate 90 degrees or to a waterway skew.
Center-bearing spans carry weight on a central pivot with balance wheels for tipping stability. Rim-bearing spans carry weight through a circular girder, tapered rollers and track while the center pivot maintains concentric position.
What This Explanatory Model Is Useful For
The calculator converts entered rpm to angular speed and shows ideal opening time and half-span tip motion for a symmetric center-pivot span.
Real bridge operation follows bridge-specific plans and operations and maintenance procedures. The FDOT movable-bridge manual describes traffic control, lock release, motion and automatic stopping as a sequence unique to each bridge.
Ideal Constant-speed Opening
One rpm is 6 degrees per second. For opening angle theta and constant slew rate n:
Tip speed equals arc length divided by ideal opening time. This is kinematics only, not drive or structural analysis.
Worked Example: Entered Symmetric Span
For L = 80 m, theta = 90 degrees and n = 0.5 rpm, angular speed is 3.00 degrees/s and ideal opening time is 30.0 s. The half-span tip follows a 62.8 m arc at 2.09 m/s.
Actual operation includes acceleration, deceleration, lock and wedge sequencing and control delays that this model omits.
Model Boundary
- Bridge arrangement: horizontal rotation about a center pivot; center-bearing and rim-bearing support types; commonly a 90-degree opening.
- Calculated: ideal constant angular speed, opening time, half-span tip arc and average tip speed.
- Not calculated: motor or hydraulic sizing, gearing, rack and pinion, friction, wind, inertia, acceleration, stopping, locks, wedges, balance, bearings, structure, navigation clearance or safety interlocks.
Frequently Asked Questions
Why is tip radius half the entered span?
The model assumes a symmetric span whose pivot is at its center.
Do all swing bridges rotate exactly 90 degrees?
No. Official guidance says many operate 90 degrees or to a waterway skew; use the actual bridge geometry.
Why can actual opening time differ?
The model omits acceleration, deceleration and the lock, wedge and control sequence.
Does tip speed size the drive?
No. Drive sizing requires torque, inertia, friction, wind, gearing and duty information.
What is a rim-bearing swing span?
Its weight is carried through a circular girder and tapered rollers on a circular track.
Can this calculator approve bridge operation?
No. Bridge-specific plans, operating procedures, interlocks and inspection requirements govern operation.
References
- Connecticut DOT, Railroad Bridge Inspection Manual, section 6.2.6.1: swing-bridge layout, operation, center-bearing and rim-bearing types.
- Louisiana DOTD, Movable Bridge Inspection Manual: swing-span drive machinery and rack-pinion or hydraulic arrangements.
- Florida DOT, Bridge Maintenance Reference Manual, chapter 17: swing-span supports and bridge-specific operating sequence.
- Federal Railroad Administration Safety Advisory 2013-01: movable-span alignment, locking and operating safeguards.
Building or designing a mechanism like this?
Explore the precision-engineered motion control hardware used by mechanical engineers, makers, and product designers.
