Note/Dislaimer: the information here is purely hypothetical and educational, discussing the limits of Air Power as a means of severing enemy transportation links. The Golden Gate Bridge is used as a familiar example when discussing the vulnerability of suspension bridges. But the author of this post has zero desire or intention to damage the infrastructure of the US or any other country. Nothing in this post should be taken as a threat or warning of any kind.

Your instincts are correct. The forces are very different. If one of the main cables on a suspension bridge is severed, the entire bridge deck will come down. You would be able to reuse the towers, foundations, and maybe some of the vertical cables supported by the main cable that wasn't hit, but repairing such damage would be similar in difficulty to rebuilding the bridge from scratch. Demolition and restoring that bridge deck probably represents about 50% of the work of building a whole new bridge from scratch.

However, actually severing one of the main cables is lot more difficult than you might think. They're not a giant rod of solid steel; they're thousands of individual pencil-sized wires bundled together. At the anchorages, these individual wires spread out and embed themselves into a giant block of concrete. Here are the Golden Gate Bridge's anchorages under construction.

Those steel elements are only a portion of the total, and after they were built, the entire volume was filled with concrete. The Golden Gate Bridge's anchorages are giant hunks of heavily reinforced concrete. Imagine a cube of super-reinforced concrete 12 stories to a side. That's the scale of what we're talking about.

You can absolutely forget about destroying one of the anchorages directly. They would be trivial to hit. They're giant hunks of concrete. But anything short of an atomic bomb would have trouble doing significant damage to them. While bunker-buster bombs exist, those are made to punch a hole through many feet of concrete and then explode in the air below the concrete. They're not designed to turn a massive 30 meter cube of solid concrete into gravel. It takes a minimum amount of energy to pulverize something that large. And that amount of explosives just can't be delivered by a plane or missile. And even if a bomb takes out a chunk of an anchorage, you've only managed to sever a few dozen of the thousands of wires holding up the bridge. Destroying the anchorages themselves would be even more impractical than destroying regular bridge piers.

So that leaves the cables themselves. You mentioned severing them at the anchorages, but it actually doesn't matter where they're severed. Imagine hanging a rope between two walls in the room you're in now. It doesn't matter where you cut that rope; anything hanging from that rope is going to fall. However, the best point to target would be right where the cables hang from the supporting towers. That is where the tensile forces in the cables are at their maximum.

I'm no expert in military ordnance. I have no doubt the Air Force has studied the problem extensively, but I really don't know how effective bombs are at taking out suspension bridge cables. I don't know exactly how precise the US Air Force is nowadays with their precision bombing, but I'll take it for a given that they have the ability to reliably get a direct hit on one a 1-meter wide bridge cable. Unlike in Vietnam, I'll assume we can actually hit the thing reliably.

You're talking about a cable that is made of thousands of individual wires. The Golden Gate Bridge's main cables are a meter thick. They look like this.

This isn't the type of target most military bombs are designed to destroy. Broadly speaking, there are two main types of bombs. There are area weapons and penetrators. The area bombs subject a large area of ground to heat, shock wave, and high speed debris. Some bombs are designed to say, kill every unarmored person and any lightly armored vehicle within a hundred meter radius. These area weapons would be completely useless on bridge cables. Even with a direct hit, most of the explosive force would bounce off the cable and into the air. You'll probably sever a few of the surface wires, but the vast majority of the cable's capacity will remain intact. And those things are built with a factor of safety. The Golden Gate Bridge was built with a factor of safety of about 2.5. Which means in theory you could cut through more than half of both of each main cable's cross-sectional area, at the point of maximum cable tension, and the thing would still stand.

That leaves the penetrators. Bunker buster bombs have already been mentioned. But what I'm thinking of are the shaped charge weapons designed to punch through things like tank armor. These weapons work by using an explosion to create a pencil-sized jet of superheated molten metal. This instantly drills a small hole through the armor of a tank. This is effective at destroying tanks, as that superheated stream of metal enters the tank cabin, rapidly expands, ricochets off the walls, and pastes everyone inside. And the resulting rapidly expanding gases can blow the turret right off some tanks.

But where my knowledge fails is how effective these shaped charges would be against a meter-thick bundle of steel wires. A shaped charge would certainly be capable of punching a hole, but if you want to sever the cable, you need to sever the entire cross section. A shaped charge may not have enough energy for that, or the stream of vaporized metal might just pass right through the cable, in one side and right out the other. I won't say it's impossible, especially if you designed a bomb specifically for targets like this. But whether a shaped charge that is designed to puncture tank armor can sever a big suspension bridge cable? I'm sure the Air Force knows, but that's beyond my expertise. I'm sure it's been thoroughly studied, and I'm sure those studies are classified to all hell and back. It's possible there may be information on this publicly available, but I feel dicey enough posting about severing suspension bridge cables on a public forum like this. Even if such info is publicly available, I would rather not have some angry men in black suits show up on my front porch asking why I've been reading numerous papers on how to effectively sever the cables of giant suspension bridges. I don't need that in my life right now. 😅

On the plus side, this actually makes big suspension bridges very resilient to terrorist attacks. A specialized shaped charge missile built by Lockheed Martin specifically to sever large suspension bridge cables could do the job. But you can't destroy the Golden Gate Bridge by setting off a home-made truck bomb on it a al Oklahoma City. You would blow a hole in the bridge deck, kill a lot of people, maybe sever some of the support cables, and maybe damage some of the outer layers of the main cables. But if you set of a truck bomb right next one of the main cables, again most of the explosive force would just bounce off. A meter thick steel element is pretty damn resilient. I'm not aware of any incidents of a major long-span suspension bridge being taken down by terrorist bombs like this.

And unfortunately for the US Air Force, planes and bombs are the wrong tool to take down a bridge.