Measuring bridge member stresses in-situ
Measuring bridge member stresses in-situ
(OP)
I have to measure the existing stresses (tension or compression) of bridge cross-bracing members on an existing bridge under dead load, preferably in a non-destructive manner. There is a bewildering range of methods and equipment out there to accomplish this. What would be the easiest, cheapest way to accomplish this?






RE: Measuring bridge member stresses in-situ
RE: Measuring bridge member stresses in-situ
RE: Measuring bridge member stresses in-situ
If that is the case, the method selected may depend on your allowable time frame to take the measurements (set up equipment, take readings, breakdown equipment, etc)
In any case, my first thought was to install strain gages
RE: Measuring bridge member stresses in-situ
RE: Measuring bridge member stresses in-situ
maybe install the strain gauge, then add some extra load, get a difference reading, now go to your FEM and see what happens there when you add the same load.
or add some load that represents some proportion of dead load, i'd do several tests.
another day in paradise, or is paradise one day closer ?
RE: Measuring bridge member stresses in-situ
With the "residual" stress gauges, I would think there's potentially a good bit of residual stress in an "unstressed" member, so I'm not sure how you'd separate the residual stresses from dead load stresses.
RE: Measuring bridge member stresses in-situ
RE: Measuring bridge member stresses in-situ
JStephen makes a good point about residual stresses, there are likely a bunch present on an "unloaded" girder just from fabrication. If it's bare steel it shouldn't be too difficult to get an accurate stress due to current dead load.
RE: Measuring bridge member stresses in-situ
i've never heard of a residual stress strain gauge. i looked at the Vishay site, didn't see anything there ? didn't see anything on the micromeasurements site ... a little s/gauge info buried in their "KIS beam", but nothing yelled "residual stresses".
another day in paradise, or is paradise one day closer ?
RE: Measuring bridge member stresses in-situ
RE: Measuring bridge member stresses in-situ
Still doesn't solve the current stresses on the girder, but I would bet that if you removed all of the bracing, that the out of plane bending stresses would then be minimal (provided it never reached any plastic deformation).
RE: Measuring bridge member stresses in-situ
There are strain gauge methods for measuring some residual stresses. Strain gauges normally measure strains/stresses that occur after the gages are applied, zeroed out, temperature compensated, etc. They don’t know or help very much as regards what happened stress wise prior to their installation. I think you would be better off (do better by) studying and trying to understand what the girder jacking/moving process did to the various components. How did that inch or two of girder movement change the geometry, and what does that movement do the various members. You can take a good, sound, stab at analyzing that. To a great extent these will be localized stresses, strains, deformations in the immediate areas of connections, they peter out fairly quickly as you move away from the connections. There will likely be some localized yielding (plastic deformation). They could be ongoing fatigue problems on down the line. This analysis is required to even know where you might want to put strain gauges. It is also required for you to know how to study and monitor this situation. This study might also help you determine/see how you can modify things to minimize the problem. What details can be changed easiest to mitigate the problem.
RE: Measuring bridge member stresses in-situ
Thanks all.
RE: Measuring bridge member stresses in-situ
RE: Measuring bridge member stresses in-situ