Strains calculated from Fortran program

[JohnNichols]

1	-1.6	0.79588
2	20	1.30103
3	53	1.724276
4	102	2.0086
5	300	2.477121

Jim:

Table is five equally spaced locations so integers are ok, next column is stress in MPa and next column is log.  (See below about the 0.975 )

I have a steel block and we see  strains and stresses, that vary with the daily temperature - this is about 25 days

I was asked why 

It turns out the manufacture of the block and the rest of the stuff probably allows a rocking mechanism with temperature, the peak stress point migrates back and forward across the block as the bridge warms and we measure strain at one location on the side.  A simple FEM model shows the expected result.  The strain gauge on the other side failed, so if we averaged we would be ok. 

I have estimated the stress in MPA at the five locations across the top for the point load and get a nice set of numbers, if I plot them I get

simple fourth order equation, the lowest number is -1.6 

If I take the log of the last four numbers I get 

So -1.6 plots as 0.8. 

If I take 0.8 from 1 I get 0.2 and that is log of 1.6.  Anything below the one line is negative log calculator so to speak.  It breaks down pretty quickly

Thoughts, PS this is just for fun. 

John

[jimdempseyatthecove]

John,

A lot is not said about your setup.

The object under test... you state you have a steel block. Where do you place your strain gauges? On the side that faces the sun?, shade side?.. Is the block painted?

If in your second chart (blue and orange dots), I assume time is your X axis, and Y reflects a factor of strain. I count ~21 cycles.

It may help you understand what is going on, had you had temperature sensors (of steel) in proximity of the strain gauges. Also to have a measurement of solar radiation in proximity of the strain gauges. As well as ambient temperature indicated on chart.

I find three things of interest:

• The slope up is (generally) not as steep as the slope down.
• There tends to be a jiggle (1) on the slopes up
• There is a change in slope up (2)

Why?

Are there other external factors that might affect thermal radiation? e.g. shadow from trees, road sign, buildings, window reflections from buildings, traffic, ...?

Jim

[JohnNichols]

I have a strain gauge on a steel bar that is the vertical element for a roller bearing on a bridge.  It is a 1927 design from Ketchum, it is actually quite a good idea, except it does not allow for transverse sway.  It was not a problem on the original configuration, with timber decks, but steel decks heat up much faster than the concrete pier, thermal mass is a beast. So transverse sway is a problem we found with the strain gauge.  The client asked about the stresses, which are locally high, but mild steel is forgiving, not like a ____________________. 

I had two gauges, but they are difficult to keep working in the field on a bridge over a river.  The 21 days are the daily cycle, the odd shapes is due to sun movement, bridge shading, clouds and any random flock of geese going south for the winter. 

The interesting question was there a simple relationship between the stress and the load point, answer obviously yes for a symmetric system. 

But the last figure shows the results, but I had a negative -1.6.  If I take the logs of the positive numbers I get a reasonable line, remember I scaled from FEM graphs.  I thought darn it, 1-1.6 plots on the line with a value of 0.8

Turns out 1-0.8 = 0.2 which is the log of 1.6.  So are the negative logs the mirror of the one line.  Is this just a fluke. 

So I thought I would ask you.