Is it a fair test

Exercised this am by the photo below of a test in a first year Civil Engineering lab.

When I did that test in 1965 Prof Evans came down for the first few minutes and pointed out why we used a lever mount setup with two gauges set diametrically opposite. Lets start with that now:

1)Concrete is not a homogeneous material the centre of stiffness will vary through the length and may be offset to one side.

2) There is no chance that the geometric centroid is exactly centred between the plattens and pretty much no chance that the plattens are directly aligned. See also the discussion of bearings in https://www.linkedin.com/pulse/cross-report-781-quality-design-construction-major-bridge-mc-nulty/?published=t

A quick calc says that, if this column is 50x50x200mm and E is 15GPa and the load is eccentric by 0.5mm and the gauge by 70mm the error in strain will be +-20%. Do check my sums please.

A comment in the discussion on twitter was that "in a data rich world, may need to encourage use of #goodenough data" but that begs the question of what is good enough and I would argue that this is very far from good enough.

If students are not taught to consider eccentricity at this stage, then when will they "pick it up". We talk of transferable skills but in engineering almost everything you learn impacts on almost everything you do.

I have seen people who should know better installing very expensive equipment to measure strain on the underside of a brick arch. Strain is nearly always measured as a surrogate for stress (except in this test where the result is used to make that strain to stress calculation). Measuring strain on one face when you KNOW the stiffness varies through thickness and you don't know the thickness anyway and you don't have a clear idea of the basic force involved means it is

NOT A FAIR TEST

It doesn't tell you what you think it does and that is the worst sort of test.

Some years ago I was asked to assist in pricing a monitoring system for a metal (possibly wrought iron, possibly steel, possibly a mixture) bridge. The main structure was a set of arches built up from plates by riveting. Above the arches was a system of trusses distributing load across the width and they were built from angles riveted to gusset plates by one leg. The spec required one strain gauge on any measured member. The load was eccentric so the strut or tie would bend under axial load but the eccentricity was unknown and unknowable. It also probably varied through the length of the member so the gauge might chance to be at a point of maximum or minimum curvature. Because the gauges had to be fitted and cabled over a river by roped access, the cost of installation per gauge would have been several thousand pounds and the results could not tell you anything useful.

The results were later presented at an Institution Meeting as providing good agreement with a computer model. Great. What does that tell you? You might as well just save the money and believe the analysis in the first place. But you had better make damn sure that an error of +- 100% or so wouldn't matter.

That is not least because even if the structure itself were well modelled, the boundary conditions are completely unknown and they typically effect the stresses by +-100% at least. And yes, I do mean that the actual force in a member may have the opposite sign to what you believe.

In many places, the wonderful plastic theorems provide us with a remarkable degree of protection against these errors. However, they only apply if the structure is capable of redistributing stress till it finds a working load path, and it says nothing about stiffness or cracking.

Today's teachers of engineering are between a rock and a hard place. Their employers place a very low value on teaching. A prof who took ten minutes out every Thursday afternoon to introduce a first year lab would soon run out of support.

Many work hard against the odds to deliver great teaching but it is so so easy to forget that lessons not learnt can be much more deadly than those that are misunderstood.

And let's be clear, I don't get everything right by any means. For 30 years engineers have been using my (wrong) idea about how masonry viaducts work. There is a paper in the ICE Bridges journal about that but it is behind a pay wall which is a shameful thing. I need it to be there but am not allowed to distribute copies. Time to discuss how I can make it more public.