Blowing up bridges

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Fatboy Coxy
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Blowing up bridges

Post by Fatboy Coxy » 16 Apr 2020 07:23

Apologies, I’m not sure this is the right forum for this question, but I couldn’t see a better one, so working on the hypothetical I’ve put it here

In many WW2 theatres the use of railways was critical to the maintenance of logistics, and thus became a target for the opposing side. A key weakness of the rail network was the rail bridge, if destroyed, it was both time and material costly to repair, and effectively closed a logistical line down.

How to destroy the enemy’s bridges was always the question, and airpower played its part, however bombing wasn’t very accurate, at high altitude it was pure luck than anything else that a bridge was hit, even if carpet bombed, while low level, although it could be more effective, was also much more costly for the air force attacking. The length of bridge also played a part, the bigger the better. So, if you were going to attack a line of communications, ideally you picked the biggest rail bridge, because it would be the most difficult to repair. Knowing this the defending side would ensure it was well defended with AA guns.

However, an alternative way of attacking these lines of communications was by way of the saboteur, who could be a resistance fighter, or a soldier smuggled in by air, sea or torturous land route. However, these forces were constrained by a lack of ability to overcome the forces defending a bridge and their restrictions on the amount of explosive they could carry.

So, their main target was the smaller bridge, a single span, either not defended or just by a watchman, maybe a routine patrol, which would still close down a line of communications, but could be restored quicker. And lastly, their extraction after blowing a bridge up was also problematic. Looking more closely at this option, and expecting most single span bridges of this period to be a truss bridge built of steel, what are the weak points of this bridge, how easy is it to destroy, and what size, weight of explosive might be required.

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Fatboy Coxy

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T. A. Gardner
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Re: Blowing up bridges

Post by T. A. Gardner » 16 Apr 2020 15:38

Actually there are several ways to cripple a rail system.

With bombing, rail yards make an excellent target. These are large enough for high altitude bombing to be effective. Their destruction makes putting trains together difficult. It also takes out critical infrastructure like control stations for signals and switching along the lines, maintenance facilities, and loading platforms.

Another critical item is locomotives. These are complex and expensive to build. There are relatively few of them and they are hard to replace. Low level air attacks on them and their destruction can quickly cripple a rail system. The other way to get them is for partisans and saboteurs to blow the rail line as the train approaches and derail the locomotive (and train) wrecking it. This is more effective than destroying a section of track ahead of time as the damage will normally be discovered before a train derails on it. That is fairly easy to repair and only causes short delays normally.

For bridge bombing to be more effective you either need guided munitions or very big bombs.

Image

That's the Bielefeld Aquaduct, a long rail bridge in Germany that the RAF tried repeatedly to take out. They finally managed it with 22,000 lbs. Grand Slam bombs in early 1945.

The USAAF used the AZON bomb in Burma to improve their chances. It was guided in azimuth so the bomber flew a course along the bridge and the bombardier could correct the bomb left or right to hit the bridge.

Image

And one result by the 458th BG

Image

Carl Schwamberger
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Re: Blowing up bridges

Post by Carl Schwamberger » 16 Apr 2020 20:04

This was something my father was directly involved in 1943-44. He was the ordnance officer in a B26 squadron 9th AF, based in the UK from mid 1943 to Sept 1944 & thereafter in Belgium and France. Picking over his remarks and assorted accounts from historians & other participant testimony I've sifted out the following.

My fathers bombardment group & the 9th Bomber Division staff (originally in the UK a part of 8th AF) came to the UK thinking thy could drop a bridge with a single squadron size attack unit from 'high altitude'. Above 20,000 feet if I am interpreting the testimony correctly. This was based on overconfident misunderstanding of tests and training results in the US the previous few years. They were soon disabused of this idea, having a high failure rate for latter 1943. The RAF advisors dismissed the idea of attacking bridges entirely recommending railway marshaling yards (switch yards in Yank speak), repair buildings, and rolling stock (engines and wagons) as the key targets. Those were also attacked by the 9th Bomber Div in 1943, with mixed results.

One problem was the enemy repair capability was far better than anticipated. The Germans had already established numerous stocks of repair materials, dispersed & camouflaged near key sites. Ballast rock, sleepers or ties, rails, telegraph wire, switch parts, ect... were on the ground, or on rail cars ready for immediate dispatch. Similarly mobile and local repair teams were ready to assemble and start on repairs. Post attack reconnaissance showed breaks in ordinary track functional a day later. On a good day the Germans could have a section with multiple craters restored in hours.

Marshaling yards had through traffic restored in a day or two and train assembly/switching operations back up to 25% or 50% in a week. Repair shops had dispersal underway when the 9th BD came to the battle & damaged work shops were salvaged &/or moved in a few days. Continuing with the current methods meant the 9th BD would have to make repeat attacks on the same targets on a weekly basis to keep ahead of the French/German repair capability. That would have required triple or quadruple the number of medium bombers assigned. This was wholly impractical.

Lt Gen Anderson was a leading expert in ordnance and bombing within the Army Air Force. After reviewing everything done & not done and consulting with the more experienced 15th AF in the MTO some changes were made.

1. Attack groups increased to 36 & preferably 54 planes in a single box.

2. Training for bomb aimers and pilots made serious. Training on bomb runs and aiming redone intensively, with mediocre performers ruthlessly weeded out.

3. Attack altitudes lowered substantially. In the 9th AF attacks were lowered to avg of 15,000 feet and gradually lowered to less. Attack commanders were allowed some initiative on this & its not unusual to find descriptions of bomb runs made from 12,000, 10,000, or less. I've run across descriptions of attack leaders taking it down under overcasts & striking from as low as 2,000 feet. Those may have been rare, but they indicate the flexibility that gradually took hold in the 9th BD. The results were far tighter and more accurate concentrations on the targets. More bombs actually hit something important on the target site. Losses to FLAK defense proved far less than originally predicted. The density of the weapons in France, defending the bridges & related targets appears to have been low enough that attacks at 15,000 or 10,000 feet did not take prohibitive losses.

All this occurred over a year+ tho it looks like the core changes and results were accomplished over a 3-5 month period in late 1943 early 44. Improved ability meant a return to attacking bridges. This was not a abrupt change. The idea bridges should be neglected was held onto by some senior staff in the 9th AF & above. My father referred to them as "Some knothead". Which was a extremely nasty insult in his lexicon. Anderson persisted and was backed up by yet more senior leaders who were after results not proof of theory, so there was a return to attacking bridges.

From November 1943 through February 1944 there was a large diversion of 9th AF sorties to assisting the Brits in attacking V1 launch sites and support sites. The Brits understood the threat & had the RAF with AAF help bombing every suspected bit of rail & concrete, and camouflage net in Flanders & adjacent areas. Eventually SHAEF exerted its weight and from March the 9th BD was increasingly focused on the 'Transportation Plan' & busy dropping bridges, hammering rail yards, and competing with the SS for French railway workers killed.

As with the 15th AF in Italy the 9th AF came to use single engine fighters for attacking rolling stock. The Luftwaffe was a dead letter, a dog that didn't hunt, notable by its absence, & run off the sky of NW France in 1944. The P47s kept busy attacking railway trains and other likely targets. At some point in 1944 there was a effort to use the single engine fighters in glide or dive bombing attacks on bridges & similar precision targets. The results were mixed & the pilots hated it. To execute a successful attack they had to hold the plane steady and straight in the final ten to fifteen seconds of the attack, and release the bomb below 3,000 to 1,500 feet. That allowed every 50mm, 37m, 30mm, & MG caliber weapon a shot or spray of shots at near point blank range. A guy could get away with that once or twice, but it did not look like odds you would take to a poker game.

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Re: Blowing up bridges

Post by Fatboy Coxy » 16 Apr 2020 20:12

Wow thank you guys, wasn't expecting that, but very interesting. However...

Just expanding on the idea of a saboteur type attack, a night attack might be preferable as less chance of discovery, but this must be weighted up against working in darkness. Around the time of a full moon might be the best compromise. Secondly assuming you’re a small unit of 4-6 people, leaving 1-2 on lookout, 3-4 have to carry the explosives onto the bridge, place and wire them up, running the wires back to a detonator. This must be quite time consuming; I wonder how long it might take. And thirdly, the explosive must have been carried by a nondescript car/van, horse cart or pack mule, so weight of explosive will limit the amount available.

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Fatboy Coxy

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Re: Blowing up bridges

Post by Carl Schwamberger » 16 Apr 2020 20:28

T. A. Gardner wrote:
16 Apr 2020 15:38
...

For bridge bombing to be more effective you either need guided munitions or very big bombs.

Image

That's the Bielefeld Aquaduct, a long rail bridge in Germany that the RAF tried repeatedly to take out. They finally managed it with 22,000 lbs. Grand Slam bombs in early 1945.
The 9th BD through rigorous training, lower altitudes and mass attacks managed consistent results. A 54 plane group of B26 twin engined mediums typically used a mix of 500 & 1000 lb bombs. My father described several mixes. One load might be two 1000 & four 500 to reach the nominal B26 payload of 4000 lbs. That allowed a 54 plane attack group to put 324 bombs on target. Odds were one would hit a bridge pier & make the mission a success. Where sorties were available two or more groups would be run in quick succession, a couple minutes apart. If the commander of the second group judged the bridge broken he'd divert to the alternate target. This had the benefit of the FLAK gunners to close to the bridge being suppressed when the following group came in range.

Picking through back issues of the US Field Artillery Journal, the 1920s volumes, I found some summaries of Army Ordnance tests on structures. The specific one I'm thinking of was on a steel reinforced concrete bridge over the Pee Dee River in Georgia. There were photos of 500lb bombs placed on the bridge deck and command detonated. Ditto for 240mm cannon shells detonated on the bridge surface. Those confirm what the 9th BD discovered some 17-18 years later, that the bomb had to explode within a meter or two of a tension or a bearing point. On a bridge deck a 500lb bomb made a hole, or blew away the guard rail. On a steel bridge breaking one or two of the truss or span members often meant nothing more than a skilled repair job for a day. Breaking or undermining a pier, or breaking a key point in a span its what it took for a 500, 1000, or 2000 lb bomb.

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Re: Blowing up bridges

Post by Carl Schwamberger » 16 Apr 2020 20:36

Fatboy Coxy wrote:
16 Apr 2020 20:12
Wow thank you guys, wasn't expecting that, but very interesting. However...

Just expanding on the idea of a saboteur type attack, a night attack might be preferable as less chance of discovery, but this must be weighted up against working in darkness. Around the time of a full moon might be the best compromise. Secondly assuming you’re a small unit of 4-6 people, leaving 1-2 on lookout, 3-4 have to carry the explosives onto the bridge, place and wire them up, running the wires back to a detonator. This must be quite time consuming; I wonder how long it might take. And thirdly, the explosive must have been carried by a nondescript car/van, horse cart or pack mule, so weight of explosive will limit the amount available.

Regards
Fatboy Coxy
This was done. But... one of the additional duties of the FLAK crews was to stand guard on the bridge all night, & all day for that matter. Aside from shooting any saboteurs caught the more bloody mixed Germans would round up several dozen locals and shoot them as well. Units that had served on the Eastern Front were prone to that & the SS seemed to feel a day not rounding up 'saboteurs' & 'Commandos' was a day wasted. I've run across descriptions of SS units shooting the railway workers arriving to fix the damage. With Iron Will like that how could they have lost the war? Anyway there was a degree of difficulty in sabotuer operations due to German security.

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Re: Blowing up bridges

Post by Carl Schwamberger » 16 Apr 2020 20:57

Reading thru the German view of all this two annecdoats pop up. One was discussion of the placement of a armored division in the spring of 1944. The division was to be part of the general Panzer Reserve and be able to respond to either the 7th or the 15th armies area. The low response time required it be place near the coast. Despite that this was only late March the bridges in the area were all broken & the division could only respond to one or the other armies need in the time allowed. The argument went to the top & Hitler arbitrated placing the division on both sides. That meant only half the panzer div could respond in a timely manner.

The other was of a report Rundsteadt read in March 1944. It outlined how only half the railway tonnage delivered that winter was made vs the 1943 average for the same number of weeks. The trend predicted in the report was that by the end of May capacity for northern France would be near 10% of the 1943 average. The report attributed this to numerous disruptions in the tracks, the worst of which were demolished bridges, and destroyed locomotives & wagons. Dead or maimed railway workers & desertions of those were a contributing factor. Rundsteads staff directed some 18,000 laborers be diverted to railway repair from other tasks, including construction teams on the Atlantic Wall. Rommel was frustrated by this diversion of labor from his project, but since the railways could no longer meet the schedule for materials delivery perhaps it did not matter.

Aber
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Re: Blowing up bridges

Post by Aber » 17 Apr 2020 15:15

Fatboy Coxy wrote:
16 Apr 2020 07:23
Apologies, I’m not sure this is the right forum for this question, but I couldn’t see a better one, so working on the hypothetical I’ve put it here

In many WW2 theatres the use of railways was critical to the maintenance of logistics, and thus became a target for the opposing side. A key weakness of the rail network was the rail bridge, if destroyed, it was both time and material costly to repair, and effectively closed a logistical line down.
You could also take out tunnels

https://www.tracesofwar.com/articles/45 ... e-1944.htm

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Re: Blowing up bridges

Post by Carl Schwamberger » 18 Apr 2020 01:24

Love these air strike photos. From the link above scale the TallBoy craters to the railway tracks and buildings.
Saumar Tunnel Entry.png


Compare those to the craters from medium bomber attacks on bridges. I'm unsure if the craters are 1000 lbr or perhaps 2000 lbr
Unknown France copy.jpg


Another view of a medium bomber attack at Trier. Note the multiple small groupings. For this attack the group has been separated into smaller elements attacking in rapid sequence vs a single large box.
Trier Ralway copy.jpg
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Knouterer
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Re: Blowing up bridges

Post by Knouterer » 07 Dec 2020 10:03

To return to the original question, here´s something from a postwar US Army manual (FM 5-25 of 1967). The plan for this (theoretical) demolition of a small road bridge requires six 40 lbs (18 kg) demolition charges plus two cratering charges to blow holes in the abutment, in which demo charges will then be placed to make bigger holes.
Note the reference to the "rotation method": the truss will be cut on only one side. In general, the idea was to blow bridges in such a way that the tangled wreckage remained in place, so that the enemy would have to spend time and effort clearing it before a new bridge could be constructed.
If we imagine a band of partisans on foot, led by SOE or OSS agents, it would take at least a dozen men to transport the charges plus accessories, and at least as many more to provide security on both sides against enemy patrols while the charges are being placed.
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