Edwards Ferry – The Physics of Bridging

Thus far with my look at the June 1863 crossing of the Potomac at Edwards Ferry by the Army of the Potomac, I’ve offered an overview of the site mentioning some key terrain from both a historical and present day perspective.  Then I walked through an event time line from the first mentions of crossing the Potomac, to the time the Army was across (part 1), (part 2), (part 3), (part 4).  Now I’m going to examine the pontoon bridges themselves and a bit of the doctrine, under a loose definition, of the time to determine what factors had an impact on the crossing.   In short what were the factors that governed the crossing operations.

In the spectrum of military operations, perhaps the most complicated is a river crossing, even when not directly pressed by the enemy.  The building of the bridge by the engineers is a complex process requiring well rehearsed actions by the team.  And once built, to say the Army simply marches across is over-simplification.  Traffic control, bridge maintenance, site security, communication, and other factors come into play.  I think it important to keep these factors in mind when examining the Edwards Ferry Crossing.

Several references offer the “doctrine” (and again I would use that loosely, as the Army didn’t have a formal Field Manual system as we do today) of bridge building and river crossing.  In 1863, Brigadier General George W. Cullum wrote the System of Military Bridges in Use by the United States Army.  Cullum covered the process for transportation, installation, and removal of pontoon, India-rubber, and trestle bridges.  He is often cited as having adapted the India-rubber pontoon bridge to American use, and thus was an authority on the subject.

Of course from the Civil War perspective, perhaps the most respected “bridge maker” was General Herman Haupt.  His Military Bridges was published in 1864, with a focus, of course, on the construction of railroad bridges to support military operations.  Over 310 pages, Haupt explained practically every facet to building bridges of several types, as well as the use of rafts made of the bridging equipment.

With regard to pontoon bridges, both engineers pointed out the capacity of the bridge is limited by the buoyancy of the pontoon (as opposed to the load bearing capacity of a piling for a conventional bridge).  Furthermore, the calculation of the load capacity is determined for each “bay” or section of the bridge, not as an overall number.  Such capacity is determined by this equation:

bridge capacity formula

Where:

  • W = Weight which can be bore by the bridge section.
  • B = Beam measurement of the pontoon.
  • D = Depth of the pontoon (how low can it set in the water.  The calculations afforded a 3 inch margin of safety to prevent swamping.)
  • L = Length of the pontoon.
  • c = A constant calculated by experimentation expressing the natural buoyancy of the wood.  Spruce and pine offered different qualities.

I could bore you to the point of tears with the resultant calculation, but lets just cut to the chase and say a standard military pontoon could support about 7,000 pounds for all practical purposes across the “bay” without significant worries.

"Bay" of a Pontoon Bridge

"Bay" of a Pontoon Bridge

That calculation is good for bulk cargo, but such bridges were built with a mind to move infantry, cavalry, and artillery.  For the former, the engineers calculated the number of men who would stand on a 18 foot long section of standard bridge decking (in other words what covered the “bay”).  A soldier in formation occupies about two and a half feet, front to back.  The average soldier, they figured, weighed 180 pounds with equipment.  So very simple math here –  18 (length of section) x 180 (pounds per soldier) divided by 2.6 (space between soldiers), gives you 1,246 pounds.  Add to that the weight of the bridging  at 1,560 pounds, for the total weight on the bay of 2,806 pounds.

For the width of a standard bridge decking, up to five abreast would fit.  So multiplying the numbers – a formation two abreast weighed 4,052 pounds ; column of three weighed 5,298 pounds; column of four weighed 6,544 pounds; and that of five weighed 7,790 pounds.  Thus, unless under emergency conditions, four abreast was the formation for crossing infantry.

For cavalry, the engineers figured the horse, rider, and equipment weighed on average 1,300 pounds.  When mounted, the horse took up 10 feet of liner space.  When led, the horse and rider took up 12 (short lead I guess?).    Same formula as above for infantry, with the cavalry numbers translated were, for mounted troopers, 3,900 pounds for a single file; 6,240 pounds double; 8,580 pounds tipple file; and 10,920 in four’s.  If lead, the numbers were 3,510 pounds for single file; 5,460 pounds doubled; 7,410 tippled; and 9,360 in column of fours. Clearly horses crossed in double file mounted or led.

Artillery presented a more complex set of numbers, as the different types of field pieces varied in weight.  Furthermore, the engineers planned for caissons and limbers.  And, consider when teamed up with horses, a field piece with limber was over 40 feet long.  The weight then is distributed across two bays.  The engineer’s calculations were a weight of 6,210 pounds on each bay for a 12-pounder Field Gun (the heavy pre-war type that saw little use, not the Napoleon).  Smaller field guns, the rifled Ordnance rifles and Parrotts, caissons, and battery wagons weighed less, but likely only reduced the weight by a factor of 500 to 600 pounds per bay.  The engineers also indicated supply train wagons easily fit within this weight restriction.

These numbers offer two indicators.  Clearly the passage was well within the capabilities of the equipment.  No brainer there.  The pontoons were designed with a river crossing of an force equipped like the Army of the Potomac in mind.

However, the numbers also tell us a few of the factors governing the movement of forces over the bridge.  As no dispatches or primary accounts indicate a  “forced” crossing or discuss bridge use beyond normal capacity, infantry probably moved across four abreast.   The return for the Army on June 20, 1863 listed 4,770 officers and 67,337 men present for duty as infantry, for a total of  72,107.   The infantry thus theoretically could have formed a mass formation of four ranks, 18027 men in each.  Multiply that by the 2.5 feet “average space” mentioned by the engineers above, and this formation now is 45,067 feet long – or 8.5 miles.  And that is setting aside other factors such as officer’s horses, and other odds and ends.

Add to that length the cavalry.  The last return for the Cavalry Corps, on May 31 (thus not counting losses in the Loudoun Valley engagements), listed 12,162 officers and men.  If double filed (a column of two’s 6082 troopers long) mounted, the column would be 60,810 feet long, or 11.5 miles!

Now add in the artillery.   The return of June 20 tallies 369 pieces.  So the guns with limbers and horses (alone!) would take up about 14,760 feet, or about three miles.  By regulation a six gun battery was issued 12 caissons, one battery wagon, and one traveling forge.  So add on those.  My “guestimate” there is another 2.5, if not 3 miles.

I don’t even want to guess at the number of wagons in the corps, division, and brigade trains.  And also missing from the figures here are the headquarters and support troops.  But counting only the three combat arms here, theoretically if the Army of the Potomac lined up to “parade” non-stop over the Edwards Ferry pontoon bridges, the column would be about 26 miles long.  The column would move at the speed of the infantry (which was in front).  So given perfect traffic control.  No gaps.  Every man moving smartly.  The entire column would pass onto the bridge in a solid day’s marching (in a “vacuum” a formation can march 3 mph, putting the time at about 9 hours).

In reality, the passing took about 65 hours between June 25 – 27.   River crossings are never efficient operations.

About these ads

5 responses to “Edwards Ferry – The Physics of Bridging

  1. Ehhhh! Craig! I’m so confused! You gotta realize, I landed two masters degrees and never… I repeat… NEVER completed college algebra, let alone took a plunge into physics.

  2. While off the subject matter of your post… speaking of being mathematically challenged, and knowing your interests in artillery, by chance, have you heard if anyone has ever found detailed information about the ANV artillery officer boards that were introduced in mid-war? I’ve been trying, on and off, over the years, to find more information about the types of questions asked and have had no luck.

  3. Brian McEnany

    Craig,

    Would be interested in whether you have heard or read about the effects of weather at Edwards Ferry. From Sears, Stackpole, Trudeau and some letters, it started raining on the 24th — I am trying to find any comments about how the troops took the weather while they were waiting to cross — the Chancellorsville crossing was wet — long lines — crowded. did the same thing happen herea//

    Brian

    • Brian,
      The weather was a factor, no doubt. However I have not seen a primary source, specifically the engineers, who seem to have expressed any worries about it. As these postings have been on the long side (1200 to 1400 words!), I’m holding out on the discussion of weather factors until after I’ve set things up with a discussion of the unit crossings.
      Craig.