Indonesian Bridge Collapse

I'm sure some of my readers will have heard of the recent collapse of an Indonesian suspension bridge called Kutai Kartanegara. As an armchair civil engineer I wanted to see if I could work out something about why and how the collapse occurred. What failure of engineering could have caused the deaths of 18 people (and counting)?

First, let's take a look at the bridge when it was intact.

Seems to be pretty standard

We can see that it is a fairly standard suspension bridge design, with two towers taking the weight of the roadway in compression (probably made of mild steel), and cables attached to a pair of long catenaries forming the connections with the roadway itself. These would all be high tensile steel cable. To prevent the towers falling over, they have tension on both sides. A large box truss comprised of I-beams encloses the roadway. My guess is that it is there to stop the roadway from buckling in high winds, like what happened to the Tacoma Narrows bridge. A similar box truss runs under the roadway of the Golden Gate.

Close-up of the box truss that provides torsional rigidity for the roadway.

Close up, we can see that instead of more expensive braided cables they have elected to use single strand steel bars connected to the catenary via some sort of clamp. This is quite unlike the Golden Gate, which uses braided cables looped over the catenary.

The use of bolts and brackets to make the connection with the catenary seems to have proved fateful, since photos of the collapsed bridge show these connections are where the failure occurred:

Normally in suspension bridges of this type, the connecting cables, in this case rods, are meant to be redundant; in other words, if one fails completely, there should be enough strength in those around it to take the load while that member is repaired. In fact, the rods may well have been strong enough, but their connections with the catenary evidently were not. Because there was no redundancy in the structure, it was, sadly, a collapse waiting to happen.

On the day it happened there was maintenance going on. Without knowing exactly what they were doing it's hard to guess what the initial cause of the collapse was, but I think it would be something like this: judging by the way one of the towers has been bent, it experienced significant lateral force from the middle span. This is consistent with connections failing either on the main span on the side opposite the bend, or on the ramp side on the same side as the bend. Either way, failed connections overloaded the tower and caused it to lean, destabilising the remaining over-stressed connections and ultimately unzipping the entire bridge span.

So, seeing that the connections were the point of failure, how did they work and how, then, did they fail? From photos it is clear that some snapped right off while most seem to have snapped off one of the hinge plates. They seem to be constructed of two hinge plates held on by bolts to brackets at the top and bottom. these brackets engage with a plate at the bottom, which has a hole through which the connecting rod is bolted into it. At the top there is some sort of clamp that grips the catenary. The most common point of failure seems to be the hinge at the top of one of the plates. These types of plates are made of high tensile steel and cannot sustain much of a crack without failing. I thought we had learned from eyebar bridges not to make things like this on bridges, but apparently not. Without an insanely rigorous inspection and maintenance program these parts fail. Corrosion gets into the steel right next to the bolt and prizes open the part right at the weakest point in the structure. Add a bit of load cycling and one day it just snaps right off. The cracks that can be sustained by these parts are on the order of a few mm, and they are invisible from the outside, so inspection is just about impossible.

Media reports cite poor construction materials as one of the factors. Possibly - this is certainly a bridge built to a tight budget - but ordinarily the safety factor is so high that even rubbish materials will still stand the test of time. This didn't and as a result I find the accountant that selected the connectors at fault. You have to design and build for things like poor maintenance and substandard materials, especially in a place like Indonesia. Failing to take account of those things is as negligent as playing Russian Roulette with someone else's life.

We need a new type of car.

The title says it all, really. I've identified a gap in the motoring psyche that no manufacturer has successfully filled, at least here in Australia. Briefly, there is a problem with most cars on the market today. This is not about CO2, at least not directly - you see, modern cars are far too big and heavy.

Now I do admit, there are some seriously good advantages to this. Big, heavy cars these days are very well built, making them as comfortable to crash into a concrete pylon as laying down on a feather mattress. They also have a lot of space for passengers and luggage, and there's plenty of gadgets to entertain both driver and passenger. There is a place for these cars, yes. I like big, comfortable cars. But I also like small, nimble cars and it's here that manufacters have been forced, by consumers mind you, to be conservative. Even the tiniest hatchback has four seats (for mice, if you want actual humans you really do need a normally sized car, like a Focus). This obsession with seats got me thinking, why do we need them all?

Most of the time, you see, people are only driving themselves. Granted this is probably not the case with families and mum driving the kids to school and various extracurricular goodness, but by and large the commute is conducted solitary. So why do you need four seats? Much of the extra weight that cars carry these days is due to the size of the passenger cabin. If you could trim it down to one seat, with maybe a bit of a shelf for your briefcase, you lose nothing in the usefulness of the car for that particular purpose, and gain in many ways - lower cost of pruduction and of course less fuel are the obvious ones. The economics are fairly sound. You do lose some safety factors of course, but then on the commute you are not travelling at high speed most of the time, and if lots of people did this then collisions would be almost as safe as if everyone drove tanks. Why is the market not cryinge oute for this?

Well, with the theory in hand, let's have a look at some attempts over the last few years to create my vision of the perfect commuter's car.

This yellow electric car gets the concept just fine: a small car only for commutes, usually short distance. But where to begin. Today's car has to look good, and I'm afraid it's total failure for this one. Moreover, I really don't think three wheels is the way of the future. Stability is compromised.

Mmm, this looks nice, but I think that cabin would feel rather a lot like a coffin. Plus we're back to three wheels again. What was the allure of that to begin with? And of course there are any number of similar concepts.

Ah, now that's more like it. The Carver solves the stability issues of 3 wheels by tilting, which is very cool, but the complex engineering involved also makes it quite expensive. Why not just add a fourth wheel? Just a thought ...

VW developed the idea quite a ways in a 2002 concept car (right), and it really showed some of the advantages: 290kg weight would have made it insanely efficient. Unfortunately it was given a 1 cylinder diesel engine with no power so it was never going to go mainstream, and of course it was extremely ugly. I say again, for mass market appeal it has to look good and have a decent engine. 50 kW would get a 500 kg car going quite nicely, potentially as fast as 0-100 in 8 seconds which is right in the sweet spot for a nicely powered family car.

Fortunately for this piece, VW have recently had another go, and I like the look of this version a lot better. Firstly the looks are much improved. They haven't gone for a noisy, rough, stinking pollutin' diesel engine either: this is an electric car as you might have guessed. This makes it short range only but you can't have everything I suppose. At the moment it's just a concept but VW is working hard to get something like it into production in th near future. It probably won't be as nice as the concept, purely for price point reasons of course, but they will be putting something small and electric on the market in the next few years. I do believe there is a market for something this size with a small petrol engine though.

Ah, now, you see, three wheels doesn't cut it. Got that? Doesn't matter if they do tilt in a cool way. The centre of gravity is just too high. Build a proper chassis please, and put away the childish tilting. This is 2011, flying cars are only 4 years away and we have barely got the car safe and efficient for road travel, never mind flying. Go design a new hatchback or something.

Now that we've seen some of the ideas the motoring world has produced, here's my idea:
4 wheels, each driven by its own electric motor. A small petrol engine or gas turbine at the back for range extension, with a good sized fuel tank; this drives an alternator which feeds power to a small bank of batteries and the wheels. The batteries are kept at a constant charge of no less than 50%, thus extending the lifespan of these expensive components. When braking and hill descending the batteries are allowed to charge to maximum extent. All electrical motors and alternator and batteries are water cooled to make them lighter and more efficient. Chassis should be an aluminium monocoque or aluminium/composite. Finally the passenger cabin should have one chair, for larger versions perhaps two. All of this would mave for an efficient, light, relatively fast little car. Only one obstacle remains: cost. I suggest that road tax for single seater or dual seater vehivles be lowered. That would help with the barriers of entry and get more people into them in the first place.