BRIDGE REPAIR/REHABILITATION FEASIBILITY STUDY

additional force effects, the specified force in the operating ropes would have been larger
than the specified imbalanced force in the operating ropes.
• The existing operating ropes do not conform to the type of construction and minimum
size of operating ropes specified in AASHTO Movable (Article 6.8.3.3), which requires
that the operating ropes utilize improved plow steel (IPS) or extra improved plow steel
(EIPS) wire that is preformed and fabricated to 6x19 class wire rope of 6x25 filler wire
construction with hard fiber (polypropylene) core. The wire rope construction specified
in AASHTO Movable is specifically engineered for use in bridge applications, which
considers the tension strength, bending fatigue resistance, wear resistance, and resistance
to crushing. The wire rope specified in the 1980 Plans (Mil. Spec W-83420B, Type 304
stainless steel) consist of 7x7 class wire rope around a wire rope strand core. This wire
rope was engineered for a different application (i.e. aircraft flight control requiring
moderate flexibility) and the wire rope construction does not adequately consider all the
above factors. The 7x7 wire rope construction utilizes larger diameter outer wires, which
are more susceptible to fatigue failure than smaller diameter outer wires. Furthermore,
stainless steel wire rope is more susceptible to fatigue than carbon steel rope. As fatigue
of wire ropes is a critical factor in wire rope for movable bridges, this type of wire rope
construction and this material is not recommended. Without a hard fiber core, the wire
rope is susceptible to crushing failure and flattening, which leads to premature wear.
Lastly, the current 5/8” operating ropes are smaller than the minimum wire rope size of
3/4” specified in AASHTO Movable.
• The sizes of the pulleys, deflector sheaves and winch drums do not conform to the
minimum sizes specified in AASHTO Movable (Article 6.8.3.1.3), which requires a
minimum sheave diameter of 45 times the wire rope diameter and preferably 48 times the
wire rope diameter. The existing 15” deflector sheaves are only 17 times the existing
7/8” wire ropes and the existing pulleys and winch drum 24 times the 5/8” wire ropes.
As such, the undersized sheaves, pulleys and drums result in excessive bending in the
wire rope and corresponding premature wear and fatigue of the wire rope. Although the
existing wire ropes have adequate capacity in direct tension to resist the specified loads,
they do not provide the specified factor of safety of 3.33 per AASHTO Movable (Article
6.6.5) for combined bending and tension. Even if the wire rope were replaced with the
proper wire rope, using the AASHTO design loads, and the current wire rope and sheave
sizes, the current factor of safety for combined bending and tension is 1.75 for the 7/8”
operating ropes and 2.35 for the 5/8” operating ropes. This indicates that the current
design does not provide the specified margin of safety. Without the specified factor of
safety, there is greater risk that an operating rope can fail. Furthermore, low factors of
safety and lack of redundancy in the current bascule span and operating equipment design
could result in the catastrophic collapse of the bascule span should one of the two wire
ropes fail with the bridge raised. The bascule span timber framing lacks the lateral
strength and stiffness required to permit the span to be supported from one side only.
Furthermore, with the current low factors of safety in the operating ropes, a single
operating rope lacks sufficient strength to support the span during operation and thus if
one operating rope were to fail it is likely that the other operating rope will also fail. The
factor of safety for supporting the span with a single operating rope would be only 0.87.
A factor of safety less than 1.00 indicates that failure is likely.
Repair/Rehab. Feasibility Study March 10, 2011
Bridge No. C-07-001 (437) 32 Final Report