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Seismic Effects on Movable Bridge Machinery
SEISMIC EFFECTS ON MOVABLE BRIDGE MACHINERY
by

Dr. Charles Birnstiel
Dr. Gunnar A. Harstead

Hardesty & Hanover, LLP
Consulting Engineer

New York, NY
Park Ridge, NJ


ABSTRACT

The operatability and stability of movable railroad bridges depends on the integrity of machinery.
This holds true for all positions of the movable span. An earthquake that damages the structure
may also damage the machinery causing the span to become inoperable until it is repaired. In
this paper locations of potential machinery damage on three common types of movable railway
bridges are described. The requirements of Chapter 9 of the AREMA Manual for seismic
analysis are discussed in light of the difficulty of modelling machinery. A simple method for
approximate earthquake analyses of some movable bridges is postulated and the results of
such an analysis of a rolling bascule railroad bridge are presented.

INTRODUCTION
A characteristic of movable bridges is the interaction between the machinery and the structure.
Hence actions that affect the structural components also affect some components of the
machinery. Just as wind forces applied to the superstructure have to find a path to the
foundation through stabilizing machinery, ground motion effects from earthquakes are
transmitted from the earth to the movable superstructure via machinery.

Analytical models for earthquake analysis of fixed structures have been extensively developed,
from response spectra analyses using planar single degree of freedom lumped mass models to
time-history analyses of three-dimensional multidegree of freedom systems including nonlinear effects. In contrast, few earthquake analyses have been presented in which the machinery and
the structure are treated in combination as they exist in a real movable bridge. One reason is
that a rigorous sophisticated model of the movable bridge would, for consistency if nothing else,
cope with all the gaps inherent in gear trains and other movable bridge machinery components.
Another reason is the perception that machinery failures due to earthquakes are not important,
perhaps because reporting of such failures has been deficient.

In what follows the three major types of movable railroad bridges are described including typical
span drive machinery and stabilizing machinery. The emphasis is on features of construction
that may be influenced by earthquakes. Potential effects of earthquakes on the these types of
movable bridges are described. After a brief description of the nature and properties of
earthquakes an approximate earthquake analysis targeted to the rack force (force that creates
torque in the pinion shaft of a rolling bascule) is postulated. Results of an approximate analysis
of a railroad bridge in the USA are presented.

TYPES OF MOVABLE RAILROAD BRIDGES
The motions of all movable spans are a combination of rotation and translation (1).

Movable/spans may be categorized on the basis of the selected displacements and axes of
displacements. The four combinations of displacements used for almost all movable railroad
bridges and the corresponding bridge types are:

Rotation about a fixed horizontal axis
Trunnion Bascule

Rotation about a horizontal axis that

simultaneously translates
Rolling Bascule

Rotation about a fixed vertical axis
Swing

Translation along a fixed vertical axis
Vertical Lift Each of the types listed above has sub-types; only those common on American railroads are
described subsequently. More variations of these bridge types are shown, with detailed
descriptions, elsewhere (1, 2, 3, 4).

Simple Trunnion Bascules
Figure 1 illustrates a common form of railroad bascule bridge, a single-leaf bascule with a
counterweight below the deck so that the movable span is nearby balanced about the main
trunnion axis. The trunnion is fixed into the bascule girder web and, hence, rotates with it in the
trunnion bearings which are located on each side of the bascule girder. The bearings are
mounted on towers which transmit the gravity loads and the wind moment applied to the open
bridge to the bascule pier. Note that the rear floor break (the joint between the fixed and
movable deck) is well forward (toward toe of the leaf) of the trunnion axis. This arrangement
has the advantage that live load on the movable deck will always create a moment about the
trunnion that presses the span onto the rest pier bearing. Motor driven lockbars at the toe of
leaf engage sockets mounted on the rest pier when the leaf is seated. The function of the span
locks is to prevent the span from bouncing upward as the moving wheel loads leave the span.

As shown in Figure 1, a pinion engages a circular rack mounted on the bottom flange of each
bascule girder. The center of the rack coincides with the trunnion axis and rotation of the pinion
causes the leaf to rotate in the trunnion bearings. The pinion is the final output of the span drive
machinery which is usually located between the trunnion towers, under the bascule leaf.
Incorporated in the span drive are one or more brakes. When the span is in any open position it
is held stationary by the pinions. In this case the span drive machinery not only operates the
span but stabilizes it as well against any action that would cause it to rotate about the trunnions, including earthquake. The single leaf bascule with its counterweight as part of the leaf, below
deck, is a simple solution for a short span movable railroad bridge.

Heel-Trunnion Bascules
At bridge sites where the necessary span length is such that the bascule pier of a simple
trunnion bascule cannot be accommodated, resort may be had to a trunnion bascule with the
counterweight located above the deck. There are many forms of such articulated counterweight
bascules. J.B. Strauss received a US patent for a subtype and vigorously promoted it among
the railroads. Most had a single leaf, as shown in Figure 2. A single leaf bascule of about 260-
foot span, built about 1919, is still in railway service in Chicago. However, some double-leaf
Strauss bascules were also built for heavy-rail service. A double leaf version having a span of
336 feet serves railway traffic at Sault St. Marie, Michigan.

Referring to Figure 2, the leaf rotates about trunnions at its heel which are mounted on a
triangular tower erected on two piers, denoted as the bascule pier and the counterweight pier.
At the apex of the tower, counterweight trunnions support rocking trusses, of which the
counterweight forms a part. The rocking trusses are connected to the bascule by the links b - d.
The link pins and trunnions are located such that the figure a - c - d - b forms a parallelogram,
a necessary but not sufficient condition to maintain balance of the moving masses, as the leaf
rotates.

The leaf is rotated about the trunnions by forces applied to the leaf at top chord joints b, the
second link pins, by means of the operating struts. Each operating strut is moved by a rack
pinion which engages a rack mounted on its underside. A span drive located between the
towers provides output torque to the rack pinions for moving the leaf. The rack pinions also restrain the leaf against rotation due to wind and ice when it is not seated. As for other types of
mechanically driven bascules, the span drive also acts as stabilizing machinery. Besides the
brakes that are part of the span drive, emergency brakes that clamp directly onto the operating
struts were installed on large heel trunnion bascules. Although not shown in Figure 2, these
railway bridges are usually provided with a lateral centering device and one or two span locks at
the rest pier.

Rolling Bascules
The motion of rolling bascules is due to rotation about a horizontal axis which simultaneously
translates in the horizontal direction. Although rolling bascules are an old form of movable
bridge, William Scherzer received a U.S. Patent in 1893, calling it a rolling lift bridge, a term that
is still used. There are subtypes, depending on the configuration of the spanning member and
the location of the counterweight. Common to all is that the counterweight is rigidly part of the
movable leaf. Figure 3 depicts a single leaf half-through configuration built by some railroads
for spans less than 110 feet. Double-leaf Scherzer railroad bridges of through truss
configuration were also built, that for the Chicago Terminal Railroad having a span of 275 feet.

Rolling bascules of the Scherzer type are characterized by cylindrically curved parts of the
bascule girders or trusses at their ends. Because of their large size, these cylindrical parts of
the girders were originally built in segments and they are called segmental girders. As the
curved ends of the girders roll away from the channel the leaf tilts open to clear it. Slippage
between the segmental girder treads and the tracks on which they roll is prevented by lugs or
teeth that protrude from the track and mechanically engage sockets in the treads. In the figure,
the rack pinion is shown at the center of roll and the rack is straight and located below the rack.
This is the usual situation, but it need not be so. Railroads have built rolling lifts with the pinions located above the ce