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. 

[jimdempseyatthecove]

I didn't factor in the geese.

>>I have a strain gauge on a steel bar that is the vertical element for a roller bearing on a bridge.

Assuming the bridge resembles a truss (from end to end).

Assuming the bridge is supported at three positions. (at the two river banks and in the middle)

Assuming the strain gauge is placed on the steel bar supporting the bridge in the middle.

Then the measurements would reflect thermal warping of the truss (in the vertical direction).

Is the bridge closed to traffic?

Are the geese landing on the bridge?

>>transverse sway is a problem we found with the strain gauge

Multiple strain gauges around the perimeter of the bar could have helped.

Also, if you have a similar steel bar, with same model of strain gauges, unloaded, and thermally cycle it (simulate that at bridge), then you could generate a calibration table per degrees. (with thermocouplers on test bar and bridge bar). 

Jim

[JohnNichols]

Geese are important, trust me.  

Sorry, phone down so I could not log on for a few days. 

You cover the methods perfectly, but this is a engineering project in Northern Iowa, one is allowed minutes for strain gauge placement, you put in 4 and one is still working after 3 days, as you so often show, engineering savvy can make up for missing data.  And being able to ask you helps confirm a thought.  

Also, if you have a similar steel bar, with same model of strain gauges, unloaded, and thermally cycle it (simulate that at bridge), then you could generate a calibration table per degrees. (with thermocouplers on test bar and bridge bar). 

In my wildest dreams would I get the money for this and then finding the time.   At least Intel Fortran makes the coding easy. 

Latest problem, two timber beams, joined by nails in the middle, measurements with an accelerometer shows the main frequency is flipping from 19.5 to 20 Hz on a 6 hour cycle inside a building, the flip is a slow continuum not a delta.  Beams not painted and quite olden.  

It is not temperature, I allowed for that.  

[jimdempseyatthecove]

Frequency will vary with the tension on the beam.

Something must be varying the tension on the beam.

(6 hour cycle inside a building)

ergo, something is happening with the building on a 6 hour cycle.

Possibilities:

6-hour work shift

Heavy mass movement on 6 hour interval

filling a silo-to emptying the silo on a 6 hour cycle

same with bulk liquid tank

or cafeteria meal cycle (6AM, 12AM, 6PM, 12PM)

or something else I am not aware of.

Jim

[JohnNichols]

I think it is more subtle than those things, I will look at adding moisture data, as it happens when there is no one in the building and nothing is moving.  

More Fortran code to compile the data structures. Blast

[jimdempseyatthecove]

Moisture?

Does the A/C cycle on and off on a 6 hour cycle?