Hard game, Engineering.

Cut slack? Not often!

The structural engineer Bill Harvey, @BillHarvey2 on Twitter followed by The Lad, is one of the few, practising engineers on the social websites. Once, when The Lad made some hard, unjust criticism of a video promoting engineering, he suggested cutting some slack. Slack, though justified there, is seldom available in daily, real engineering and here are a few examples. They are five and random: three small scale, one serious and one lethal. In the wrong place though, even small errors, if not picked up, can create dangerous havoc. Anything ever designed offers chances of error.

Mostly engineers design machines or components for a particular function. Often the upshot is they do something else, such as fail, much better. ” Damn it!” [usually worse]. “Try again” The profession of engineering is vastly important, difficult and usually misunderstood by the media.

All modern engineers know about metal fatigue failure. Loads and low stresses may not break a piece applied once, but if they are often repeated can still cause breakage or cracks. It is a danger always lurking to spring on the unwary design engineer. She designs out sharp internal corners because of the stress concentration can encourage a fatigue crack: so she designs in a nice large radius.

Radiused corner
Radiused corner to minimise metal fatigue failure risk

Ah, but fatigue is pushed only one step further away. For how is the radius to be machined? If the manufacturing process or careless handling allows a scratch to appear: boom, even higher stress concentration and fatigue enters again and takes it’s position centre stage.

Then there used to be this massive pile of print-out. Fifty sheets were common.

pack cover
Cover of a map-folded print-out pack of stress analysis results.

Do you believe what is printed on it?

Arbitrary sample of Fortran results, frequently 50 pages of this.
Arbitrary sample of Fortran results, frequently 50 pages of this.

In those days massive piles of map-folded printout was a product of the batch operation of the Mainframe on the air-conditioned wing of the ground floor. We had to be wary of the ‘never mind the quality: feel the width’ syndrome. Quality of the answer depends on the assumptions fed in, Watch out for GIGO, ‘garbage in: garbage out’.

A colleague was obsessing over the stresses and forces in an hydraulic system and was greatly worried about one static pipe element. In one scenario, a transient increased the compression forces up to enough to buckle the element.

These are tubes or pipes buckling under end loads.

Buckling is bad was our usual rule of thumb. The chief stress engineer asked “Where is it and where is it going?” The eloement was passing through a small, fitting clearance in a hole. If it buckled, all it did was just touch the side. No problem, move on.

Modern gas turbine engines power most of the aircraft in which we are flown about the globe. The gas turbine prime mover is very different to a piston engine. One aspect is the compressor which compresses the gas before fuel is injected and burned.

leaky compressor
The compressor, made up of rotating discs with many blades, is to the left and the combustion chamber is to the right.

This job is also done in a piston engine when, before fuel injection, the piston compresses the air with little or no leakage possible around the piston. In the gas turbine there is a potential convoluted leak path backwards[to the left in the photo] between the blades. In a normally running engine, this leak is only avoided at optimum speed because the rotating compressor with its carefully designed blade shapes pushes air, compressing it as it goes, to the right towards the combustion chamber. Operating the engine at non-optimum speeds (say, start up or acceleration at take-off) can result in airflow slowing and the pressure at the right side growing too high. Then, with a large bang and (if repeated) highly expensive, blade damage, flames rocket out of the front and rear of the engine. Only very careful engineering design based on extensive experiment can stop these explosive forces.

There was a new design of bridge called a Box Girder. Instead of chains, frames or cables, it had a large hollow section to bear most of the loads. Without too much simplification, we can see that this had two advantages. Such a large, closed shell was very strong and stiff which is just what you want for  the deck of a bridge. Secondly, such a smooth design promised low maintenance as a very large part of it, the inside, was protected from the weather. The West Gate Bridge in Australia was such a design.

High in the air supported by piers, the box section can be clearly seen.
High in the air supported by piers, the box section can be clearly seen.

Trouble is, being very stiff (that great advantage), it is difficult to assemble accurately on site and align when installed. Sadly at West Gate in a fatal misjudgement, what were seen as the necessary tweaks to get sections to fit accurately only overloaded the bridge sections. They buckled and crashed to the ground.  Over thirty men lost their lives.

Collapsed West Gate Bridge

Engineering is, and always will be, hard. You want challenges? Be a professional engineer.


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