Florida East Coast Railway's Strauss Trunnion Bascule Bridge Over the St. Johns River at Jacksonville

Unless otherwise noted, all illustrations and text are copyright 1997, E W Wall.

History

In 1922, the Florida East Coast Railway contracted with the Strauss Bascule Bridge Company of Chicago for the construction of a twin-track bridge to replace the single-track swing-span bridge connecting Jacksonville with South Jacksonville. A new highway bridge (the St. Elmo Acosta bridge, now gone, from which these photos were taken) next to the FEC made swinging the earlier bridge difficult, and it was desired to make the entire mainline twin-track all the way from Jacksonville Terminal to Miami Station.

The first bridge of this type (the New Haven's Cape Cod Canal bridge) had been built in 1910, so this was a very new advance in bridge design. Paul Mallery, in his Bridge and Trestle Handbook, says that the FEC's Bascule Truss was the heaviest yet built for a Strauss Bascule at the time of construction. He also remarks that the bridge is unusual in using Pratt trusses rather than Warren trusses. The design of the counterweights is not one of the four Mallery mentions, being a pair of thin weights which fit inside the tower.

The earlier railroad bridge was used as the foundation for the new one. The original plans called for the bascule span to be hinged on its northern end, but the tower was moved to the south side before construction began. Construction was completed in 1925.

Location

The bridge is located in downtown Jacksonville, about a half-mile south of Jacksonville Terminal. It is situated where the St. Johns River turns sharply from northward flow to eastward to begin its short run through the downtown area. The bridge crosses about half-way through the turn. The channel of the river lies against the northern bank at this point. The St. Elmo Acosta bridge was just downstream from the railroad bridge. Jacksonville is only about 20 miles inland, and the river consequently has tides, which must be accounted for in bridge design.

The northern approaches of the two bridges (illustrated below and at right) lay near where McCoy's Creek debouches into the St. Johns, between the Florida Times-Union building on the west and the Madison Chemical Company (home of "666" cold medicine) on the east. The approach to the Acosta bridge was a T-shaped viaduct, the west arm of which rose over the approach to the FEC bridge. That arm is just visible in the photo below; it is the line just to the right of the copyright notice. At the T was a single stoplight controlling automotive traffic onto the Acosta bridge. The top of Jacksonville Terminal is visible on the horizon, above the roof of the Florida Times-Union, directly above the first "W" in the photo and just to the right of the rightmost airconditioning unit on the roof of the Times-Union; the grey line is two of the three gables in the building, and the two black dots are two of the three great arched windows along the front.

The southern approaches lay between the Prudential Insurance complex to the west and the children's museum to the east. When the bridge was built, the Prudential complex was still marshland, arcing from the FEC mainline down to Gary Street (just south of the freeway feeding Interstate 10 over the Fuller Warren bridge), where the author lived the first six years of his life.

Footings for the Acosta bridge became dangerously washed out, and in the mid-'90s the bridge was replaced with a new structure just downstream of the Acosta bridge; the Acosta bridge was dismantled and removed. The FEC bridge continues in use.

Design

The secret to the operation of the bridge is a gigantic concrete double counterweight, mounted on the south end of a diamond-shaped armature (the Counterweight Truss or "walking beam") on top of the Counterweight Tower (span number 8; the FEC numbers the spans sequentially from north to south). On the north end of the Counterweight Truss is the Counterweight Link, a large trapezoidal truss plane (rather like the top of one of the fixed spans) connecting to one end of the Bascule Truss (i.e.span 7, the moveable span or "leaf"). The bottom of the south end of the Bascule Truss is hinged by a Main (or "heel") Trunnion at the base of the Counterweight Tower.

The two-storey operator's house is located on the downstream side of the pier holding the Main Trunnion. This location permitted the operator to see beneath the Acosta bridge. A set of electric motors in a shack high in the tower turn two pinions, which work a pair of Operating Struts, which in turn pull the top of the Bascule Truss back. While this is going on, the Counterweight Truss rotates backwards and down.

The north portal of the Counterweight Tower is a nearly straight line from the Main Trunnion to the Counterweight Trunnion. The top of the Counterweight Link is hinged to the Counterweight Truss via the First Link Pins. The bottom of the Counterweight Link is hinged to the top of the south portal of the Bascule Truss via the Second Link Pins; the Operating Struts are also attached by the Second Link Pins. A parallelogram is clearly visible, with the north portal of the Counterweight Tower opposing the Counterweight Link, and the south portal of the Bascule Truss opposing the lower northern face of the Counterweight Truss. This permits free motion of all members.

When the bascule span is fully lowered, the concrete weights are at their maximum lateral displacement from the vertical centerline of the trunnion. Thus, gravitational pull is completely tangential to the arc of rotation of the weight, and the weights exert the maximum downward force (torque) on the south end of the Counterweight Truss; this causes the maximum upward force to be exerted at the north end. This force is transmitted from the end of the Counterweight Truss to the top of the southern portal of the Bascule Truss via the Counterweight Link. When the bridge is not completely lowered, the force of the counterweight balances the weight of the Bascule Truss, which is greatest just as the truss begins to rise. The entire weight of the bascule is balanced by the counterweight, and the electric motors have only to overcome inertia in moving the bascule. As the bascule rises, it rotates around the Main Trunnion. As this progresses, more and more of its weight becomes downward thrust on the trunnion, and strain on the Counterweight Link is reduced. At the same time, the concrete counterweights are being lowered until they are underneath the Counterweight Trunnion. More and more of the gravitational pull on the counterweights is radial to the arc of rotation, and thus the torque becomes weaker.

At full travel, the span is elevated to 89 degrees off the horizontal (design angle of opening is 82 degrees 30 minutes), and the counterweights rest inside the tower, barely clearing the tracks. The bridge is normally left at about 45 degrees of elevation, as seen in the photo above. It is lowered to permit trains to pass, and raised to permit ocean-going vessels to pass. Jacksonville is a deep-water port close to the ocean, and large vessels are from time to time anchored in the immense lake upstream of the bridge, near Jacksonville Naval Air Station. Just after World War II, it was common to see combat craft sailing through the bridge. One can still imagine the ghost fleet of destroyers steaming past her bascule as they went to mothballs well upstream.

Below are contrasting detail images of the lowered and maximally raised positions of the moveable span, as seen from upstream.

The astonishing thing about all this is how quiet the bridge is. Standing next to it while it is in motion, all one can hear is the hum of the electric motors.

Central walkways are provided between the ties for the two tracks, beginning at span 1 and continuing up to span 13. A single refuge (illustrated here) is provided on the east (downstream) side of span 20, halfway between the end of the walkway and the southern abutment. There are no obvious fire-fighting provisions at the refuge. The refuge is mounted on a bracket made (presumably) of steel tube.

Guard timbers are visible in photos from the late '20s or early '30s, but they are not found on the bridge in the late '80s. Guard rails are found the full length of the bridge, though they are set back from the breaks in the Bascule Truss, in the direction of travel. That is to say, the downstream (northbound) tracks are toed in to break just before the beginning of the leaf, and don't begin on the leaf itself until about 10 feet or so beyond the break; the same holds true on the next span (6) beyond the leaf. See the photos for spans 6, 7 and 8 for details.

Electricity is carried both north and south. Heavy cables are visible in the photos of spans 8 through 27, presumably carrying current to move the bridge; similar cables run on the upstream side. Other, lighter, cables are visible in the photos of spans 1 through 6, strung on a variety of arms; some are bracketed to the bridge girders, some are on stands on the island, and a special column was poured near span 3 to carry these cables.

Abutments at both ends are of wing design and cast-concrete construction.

Piers are solid and lack clear cutwaters. The pier holding the rest bearings for the leaf is designed for 884 thousand pounds of reaction per truss. The pier holding the main trunnion is designed for 3,018,000 pounds of reaction per truss. The pier holding the south portal of the Counterweight Tower is designed for 2,244,000 pounds of reaction per truss.

General dimensions

The rail bases are 13.2 feet above the mean high water line of 100.6 feet.The tracks are 13 feet apart, center to center. Ties are 8 by 10 inches by 11 feet and are centered at 1 foot intervals. Stringers are 6 feet 6 inches apart, with pairs of stringers set 13 feet apart center to center. Tracks are centered on stringer pairs. The plans call for rivets of 1-inch diameter in the trusses, links, operating struts and cross girders; in the floor system and bracing seven-eighths inch rivets are used. Design dead load is the actual weight of the structure. Design live load is Steinman's M-46 (a Decapod), illustrated below.

The illustration is difficult, offering a five-axle tender. The original drawing is even more troubling, showing the first axle of the tender about 10 feet ahead of the second, though stating the distance to be 5 feet. It appears as though the draftsman became confused and started on the second engine of a double-headed consist.

The deck-girder spans give the appearance of being two parallel bridges, with the downstream spans (closer in the photographs) being left over from the original swing-span bridge. Spans 1, 2, 26 and 27 are unusual, in that the downstream girders are shallower than the upstream girders in the same spans, and shallower than the girders in the other downstream spans. The girders in all of the upstream spans are of the same depth.

Signage

Signage is provided principally for river traffic. Large signs can be seen near shore on the upstream side of the bridge warning boats of underwater cables (I think). A sign (pictured right) on the Bascule Truss advises small boats of the through-girder span (13), which they can pass under even when the leaf is down. In one photograph, a sign on the through-girder advises small boats that the span is closed to river traffic for repairs.

Photographs

All photographs in these pages were taken in the late 1980s and early 1990s. Photos of the piers were taken on a later trip than the photos of the spans, and you can see the growth of junk around the northern approach to the bridge.

The drawing below shows the bridge from the down-stream side, in the same orientation as the photographs. The photos show every inch of rail from the southern abutment to the northern, and all piers. The drawing is a clickable bitmap: click on a span to see a photo of the span, on a pier to see a photo of the pier. Use the BACK button on your browser to return to this page.

Southern abutment
Span 27 Pier 27 Span 26 Pier 26
Span 25 Pier 25 Span 24 Pier 24
Span 23 Pier 23 Span 22 Pier 22
Span 21 Pier 21 Span 20 Pier 20
Span 19 Pier 19 Span 18 Pier 18
Span 17 Pier 17 Span 16 Pier 16
Span 15 Pier 15 Span 14 Pier 14
Through-girder span
Span 12 Pier 12 Span 11 Pier 11
Southern approach span
Through-truss span
Tower span Bascule span
Northern approach span
Span 5 Pier 5 Span 4 Pier 4
Span 3 Pier 3 Span 2 Pier 2
Span 1 Northern abutment

Signals

Entrance to the bridge from the north is controlled by searchlight signals at the northern abutment. Exit from the bridge to the north is controlled by three-indicator signals.

The southern entrance and exit used to be controlled by three-indicator signals at the southern abutment, but those have been removed. Traffic crossing from south to north is controlled by searchlight signals at the southern portal to span 10, the southernmost truss. Thus, trains are prevented from entering any of the trusswork when the leaf is open.

When the bridge is to be lowered, the operator checks for boating traffic, and blows a siren to warn boaters. When the bridge is closed, small boats may divert to the south to pass under span 13, which is a through-girder design. A sign on the Bascule Span indicates this to boaters.

Modeling

Mallery says that the Strauss Heel Trunnion Bridge (as he calls it) was the second-most common bascule design for railroad use. It is probably the least common design in models, owing to the difficulty of modeling it. The bridge should be of strong interest to modelers because it is 1) an unusually grand design and 2) a twin-track version of this design, and 3) it mixes the moveable span with the counterweight tower, three through-trusses, one through-girder (for small boats) and twenty-one deck girder spans. Overall length of the bridge is about 1/2 mile, making modeling of the entire thing difficult, but it would make a fabulous addition to any layout.

Reading

Acknowledgements

The Engineering Department of the Florida East Coast Railway Company of St. Augustine, Florida, has provided generous assistance for producing this article. We gratefully acknowledge their strong contribution to it.

My thanks also to the Jacksonville Fire and Police Departments, for responding quickly when it was erroneously reported that I was trying to jump off the St. Elmo Acosta bridge, while taking these photos.