Location of load and its effect on bending moment - cantilevered beam

[aa2105]

Hi
I am trying to calibrate strain gauges using two beams, one of which has bend-twist coupling. I have two full wheatstone bridges; one for bending and one for torsion. The dimensions are 3x95x900mm. The strain gauges are placed near the root and I want to correlate the voltage with the bending moment at the gauges. To do this I will apply a tip load at the elastic axis and this will result in bending (and twist for the coupled beam). I can then work out the bending moment at the required location and compare it to the voltage output.

If I now apply the load off the elastic axis, will the bendiing moment at that location still be the same? This time, of course, there will be bending and twist for both beams. If this is not the case, why not? And how would I then compute the bending moment?

Also, how can I calibrate the torsion gauge? Apply a torque (or tip load off the axis) and measure the resulting twist? Or is there something else I can do? Thank you. (Sorry for the novel...)

 

[JoshPlumSE]

What types of beams are you using? I shaped beams, tubes or what?

If you are using I shaped beams, then I would anticipate torsional warping issues will come into play as soon as you start adding in a twist. Therefore, make sure to account for them when comparing your measurements to your expected results.

If warping issues do come into play then something else to think about would be the fact that traditional calculations for warping effects (via the AISC design guide on torsion) only account for the perfectly idealized boundary conditions... And, there can be dramatic differences between a real torsional restraint and one of these idealized cases.