Engineers on the Rappahannock (Part 3) – How to Build a Pontoon Bridge

So, how do you construct a pontoon bridge? It should be easy, right? After all you just get some pontoons, lay some boards across them, and then let your army march across. Anyone can do that? Or can they?

In this writing, I draw heavily upon the work of Captain James C. Duane, US Army. He wrote the instruction manual, following the testing of pontoon bridge systems conducted by the army in the late 1850’s. This manual was updated several times during the Civil War as the engineer units, regular and volunteer, gained practical experience during various campaigns.  The first was the Peninsula Campaign, followed by bridging the Potomac River during the Antietam Campaign, then the Rappahannock River during the Fredericksburg and Chancellorsville Campaigns, followed by the Overland Campaign and the masterful bridging of the James River, to name a few. In 1870, Duane, then a Lieutenant Colonel in the regular army and brevet Brigadier General, gathered all this experience, publishing his final lessons in a book titled Organization of the Bridge Equipage of the US Army. This was reprinted in 1898. There are four methods of constructing a pontoon bridge – by successive pontoons; by parts; by rafts; and by conversion. In this article, I used pieces from the 1862, 1864 and 1870 manuals, quoting from areas that I felt best explained to me what they were doing 150 plus years ago. Using these, I made a series of sketches showing how a pontoon bridge ‘by successive pontoons’ was constructed (see below). While I have experience as a combat engineer in the 1970’s era, naval aspects were a steep learning curve. I hope I have pieced it together in a way that makes sense to you.

For the remainder of this blog, I will use the original spelling of ponton found in Duane’s writing.

Duane pointed out that; “In constructing a ponton bridge there are two points that require particular attention: the anchorage and the lashing the men who are entrusted with their execution should be selected from the most intelligent and experienced pontoniers in the command.” In other words, insuring proper anchorage so that the bridge was not torn away by river flow and structural integrity through lashing (or tying) the pieces together were vital to the success of a ponton bridge. These men were paid extra and ranked as artificers in recognition of their important skills.

Painting of river crossing

Painting of river crossing – The Upper and Middle Pontoon bridges site came under fire from the 17th Mississippi Volunteer Regiment, Confederate defenders. Brainers’s bridge located at the Upper Crossing which is depicted here was half to two-thirds complete when they were fired upon. Note the layout of material used for construction on the shoreline. The engineers made several attempts to complete the bridge under fire before infantry war ferried across to secure the bridgehead. This painting entitled Essayons is on display in the National Park Service Visitors’ Center at Fredericksburg.

Officer Duties

“The duties of the officer charged with the construction of a bridge are, to discover and collect all material disposable for the purpose; to examine carefully the position chosen for the bridge; and to ascertain the width and velocity of the stream and the nature of the bottom and banks. He can then determine the composition of the bridge, taking care to reserve sufficient material to repair damages, and to lengthen the structure in case of a rise of water. He will then divide his force into detachments, assigning each its duty, and require each to labor at its own task without attempting to interrupt or assist that of any other.”

“For a short bridge, one commissioned officer is sufficient; for a long bridge there should be three—the commanding officer, one at the head of the bridge, and one superintending the anchor sections.”  Fredericksburg’s six bridges were long bridges of 400 to 440 yards (365 to 400 meters).

“The location of the bridge having been selected, the ponton wagons are brought as near the river bank as practicable, with the rear of the carriage towards the stream.”

“The pontoniers are divided into detachments of twenty men each, for the purpose of unloading. A double row of chess is laid from the carriage to the water. The pontons are unleashed and slid from the wagon gently into the water.” Remember, each ponton weighed 1,600 pounds (0.8 tons or 725 kilograms)!

“The balks are piled on the left of the entrance to the bridge… until the pile is five feet [1.5 meters] high.” This was fairly easy because each ponton wagon carried seven balks. Unload the boat then unload the balks that supported the boat during transportation. Balks are the long wooden beams that support the roadway of the bridge.

“Chess are piled on the right of the bridge… Other articles, as trestles, cables, etc., are in separate piles on the right of the chess.” Chess were carried in their own separate wagons. Chess are the planks that form the roadway of the bridge and are laid on the beams or balks.

 “Those pontons which cast up-stream anchors are moored above the approach to the bridge, the others below; cables are attached to the anchors; one of the former is coiled in the bow of each ponton with its anchor on top, the flukes projecting over the gunwale.”

 “The pontoniers are again formed and divided into sections for constructing the bridge, which is executed in the following manner:”

Bridge by Successive Pontons – Plate 6

Bridge by Successive Pontons – Plate 6 of Duane’s 1864 Manual shows a more detailed layout of the placement of the bridge parts. All anchors and trestles are on the up-stream side of the bridge as are the spare lashing ropes, cables, and rack sticks. Tools for preparing the abutment and the approach are on the down-stream side.

Pontoon Bridge Construction teams/sections

“At the command – “Construct the Bridge”—

Abutments

“1st Section, two non-commissioned officers, eight men, whose duty is to prepare a convenient entrance to, and exit from the bridge, and to place the abutment sill. A trench one foot deep [30 cm] is excavated to receive the sill, which is placed in a direction exactly perpendicular to that of the bridge and firmly held into position by pickets eight inches [20 cm] from each end, front and rear; as soon as the balks are in place, a chess is arranged against their ends, its upper surface coinciding with that of the chess forming the roadway, and retained by pickets… If no trestle is used, they then plant pickets thirty paces [or yards][27 meters] above and below, and three and one-half paces [3.2 meters] above and two and one-half paces [2.3 meters] below – the 1st for anchoring [bow and stern] cables and the 2nd for attaching spring lines.”  Because the Rappahannock River at Fredericksburg is tidal, the abutment sill was placed two feet [61 cm] above the high water mark.

“The approach to the bridge is then rendered easy by cutting down and leveling the bank if necessary.”

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Trestle

“2nd Section, two non-commissioned offices, eight men.  Construct raft of two pontons, load it with separate trestles, caps and legs, etc., take raft to its place, put trestle together, right it, put claws of balks to caps, haul on caps, drop legs , disengage raft.

Use of a trestle is designed to bridge the gap in 20 foot (6 meter) increments between the shoreline where the abutment is placed and the closest moored ponton boat. Originally this was thought to be the high-water mark. Trestles were not used at Fredericksburg because the ponton boats could moor adjacent to the river bank thanks to the edge conditions and the tide. In the event, trestles were used on dry land on the approach to the bridge on land that became soggy due to high traffic volume of marching columns, wagons and artillery, rather than a corduroy road.

Up-Stream Anchor

“3nd  Section, two non-commissioned offices, eight men. This section is divided into two half sections, for casting up-stream anchors. Pontons with up-stream anchors are above the abutment. If a ponton is to be anchored down-stream, a second anchor and cable are placed in the stern [of that ponton]. Each half-section embarks in a ponton with an anchor.  Two half-sections enter two boats, the non-commissioned officer steers, they row to place the anchor… The line upon which the anchors are to be cast is marked by two boat hooks, placed about twenty yards [18 meters] apart, on the shore. Arriving on this line, and opposite the position which the ponton is to have on the bridge, the Chief of Section commands – “cast anchor”–  the first man throws over the anchor and plays out the cable until the boat arrives at its place in the bridge, the cable is made fast, and the ponton is brought, with boat-hook and oar, alongside of the bridge-head… and turned over to the cable detachment, the anchor detachment then passes to the shore by the bridge to another ponton.”

“The number of anchors required will depend somewhat on the strength of the current. It is generally sufficient to cast an anchor upstream for every alternate ponton, and half that number downstream; but where the current is very rapid it may be necessary to anchor every upstream boat especially near the middle of the bridge.”

“The distance of the anchors above the bridge should be ten times the depth of the stream, otherwise, witn a less distance the bows of the ponton would sink too deeply in the water.”

“The number of anchors cannot be much diminished, however moderate the current, as the anchorage has a very marked effect in checking the horizontal oscillation to which bridges are subject when troops are marching over them; for this reason it is frequently advisable to increase the number of downstream anchors. A downstream cable is never attached to a ponton that is not anchored upstream.”

Down-stream Anchor

“4th Section, two non-commissioned offices, eight men; [split into] two half sections. First with boat take anchor from boat already in bridge, drop down cast it; second half section bring pontoons with no anchors into bridge and give them to bulk lashers.” Most of the downstream boats required no anchor.

Balk Carriers

“5th Section, one non-commissioned officer, ten men. [Two men each] bring balks by right side of bridge, pass front end to… ponton lashers [in the second ponton], men at front ends take rear ends of balk and assist in placing them for a ponton, balk carriers push it off and give their end to lashers [in the first ponton] and go for more balks… passing down the left side of the bridge.”

“The balks – the long balk which is of white pine 27 feet [8.2 meters] by 5 inches [12.7 cm] by 5 inches, …[by 1864 these were] furnished on each end by a cleat or chaw of oak, the grain of which crosses that of the balk; the distance between claws is 25 feet 8 inches [7.8 meters]. Balks weigh approximately 124 pounds (56 kg). Use of the claw simplified placement of the balks.

Balk Lashers

“6th Section, two non-commissioned officers, seventeen men. Four men alternate, two and two, in fixing the spring lines which connect the bow and stern, and are made fast to the mooring-posts. Three men at the cables — two men for up-stream and one for down-stream cable; ten balk lashers. The first five front men enter the first ponton, station themselves opposite the lashing hooks, facing downstream. They receive ends of the balks from the balk carriers; lay them in place on the outer gunwale, overlapping by six inches [15 cm]. They throw their weight on the balks to keep them in place whilst the boat is being pushed off. In the meanwhile the rear ranks lash the shore ends of the balks, if claw is not used, on the abutment sill and pass over to the second ponton. When the second ponton is pushed off, the lashers in the first ponton receive the ends of the second set of balks  The corresponding balks of adjacent bays lap each other by six feet, and are lashed together and to the gunwales at two points about five feet apart [to each gunwale]. [T]hey then pass onto the third ponton… Thus a strong splice is formed, making five continuous beams running the length of the bridge.

“This method of arranging the balks adds greatly to the strength and stiffness of the bridge….. The stability of the bridge is further increased by the manner of placing and securing the side rails.”

S4-–-Lashing-Diagram-web

Lashing Diagram. A pontoon bridge is held together by one inch thick rope. These diagrams provided on Plate 3 of Duane’s 1864 Manual give us an understanding of their makeup. The top row shows Balk Lashing. Note the use of the hook below the balks. If two balks are used, they are wound twice. If one balk then it is wound three times. The bottom two rows display how the side rail and trestles are attached.

Chess Carriers and Coverers

“7th Section, one non-commissioned officer, twenty four men (twenty carriers and two coverers). Twenty men bring chesses by right side of bridge to another two, who face to the rear and place them. One man stands on the first and second balk, the second stands on the fourth and fifth balks and receives the chess from the chess carriers, and lays them… exactly on the axis of the bridge. Each chess must be pushed firmly against that which precedes it. The covering is carried to within one foot of the ponton.”

“The chess is a white pine plank 13 feet [4 meters] by 12 inches [30.5 cm] by 1½ inch [38 mm]; the width of each end for a distance of 2 feet [61 cm], is reduced to 10 ½ [26.7 cm] inches thus forming a notch on each side for the passage of the side rail lashing…“ The chess weighs approximately 42 pounds or 19 kg. The slot formed between two chess boards allows the side rails to be fastened quickly.

“Both balk and chess are made of the best white pine lumber free from knots, saps and shakes, and perfectly straight grained.”

Side-Railers

“8th Section, two non-commissioned officers, eight men. Bring side rails and lash them down. They are carried in the same manner as the balks and one laid on each bay as soon as the coverers have left it.” The remaining men, two and two, lash the rails.

The side-rails are placed immediately over the outer balks, and lashed by passing the rope around the balk and rail, tying it loosely, and twisting it up tight with a rack-stick; three lashings are placed on each rail.

S5-–-Waud-sketch-of-Middle-Crossing

Waud sketch of Middle Bridge crossing from Library of Congress number 21209u. This is one of the few images of the middle crossing located at Ferry Landing below the railroad bridge. Alfred Waud captures the impact of the Confederate fire upon the engineers while they were about half way across the river. At this location, the engineer casualties were; Company F = 2 killed, 10 wounded of which 2 later died of wounds. Company K = 3 wounded, for a total of 13 casualties at this location.

As you may well imagine, building a ponton bridge is not a simple exercise. Rather it is a highly choreographed undertaking by men trained for specific tasks. Many of the tasks are repetitive with sections alternating to the next boat. As the bridge grows in length, those sections that get fresh supplies; anchors/boats, balks, chess, and side rails, must walk an increasing distance, which impacts the completion time. Typically these men would walk over one mile (3.2 km) to construct a 400 foot (122 meter) pontoon bridge.

Over the course of the Civil War, the engineers made subtle but important Improvements to simplify their work. Importantly, the engineers found the wooden ponton boats were robust enough to withstand resting on the bottom of a river or stream, while the army marched over during periods of low water, such as in low tidal conditions. Secondly they initially believed that they had to use trestles to bridge the gap between dry land and the high-water mark for floating pontoons. While trestles had some importance, they were not an imperative in the construction of a ponton bridge due to the strength of the wooden boats. Third, wooden cleats were added to the ends of the balks making placement of the balk on the gunwale of the ponton boat precise and resistant to slipping out of place, thereby simplifying and easing the workload of the lashers. The duties of the various sections also morphed as they gained experience.

Next we will look at the engineer bridging operation camp sites and their routes of advance to the upper and middle crossing sites.

SOURCES:

Books:

Duane, J.C., Manual for Engineer Troops, New York: Van Nostrand, 1862. Chapter 2, p 17-33. https://babel.hathitrust.org/cgi/pt?id=hvd.hwwlnr&view=1up&seq=11,

Duane, J.C., Manual for Engineer Troops, New York: Van Nostrand, Third Edition, 1864. Chapter 2, p 17-33, and Plates 3 and 6. https://babel.hathitrust.org/cgi/pt?id=uc2.ark:/13960/t6zw1cn8d&view=1up&seq=6,

Duane, James Chatham, Organization of Bridge Equipage of the US Army with directions for the Construction of Military Bridges, Washington 1870, reprinted 1898, chapter 3, p 25-57, chapter 4, p 58-92.  https://books.google.com/books?id=05UDAAAAYAAJ&printsec=frontcover&dq=organization+of+bridge+equipage&source=bl&ots=RqVSKlr97N&sig=o2z49fX3eCCX65quRSD0rKpyL6A&hl=en&sa=X&ei=CvZHUIqoB4Le0QGgyoDYDg&ved=0CC8Q6AEwAA#v=onepage&q=organization%20of%20bridge%20equipage&f=false,

Prints and Painting:

Waud sketch, Building Pontoon Bridges at Fredericksburg Dec. 11th. Library of Congress number 21209u.

Essayons, courtesy of Dale Gallon, on display in NPS Fredericksburg visitors’ center.

About Peter Glyer

I am retired with a lifelong interest in history, primarily the Civil War and WWII - Europe. I was an Army engineer, hence my interest in terrain. I graduated with a degree in City and Regional Planning and a Masters in International Relations.
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