You can calculate the strain from the gauge factor. Load or force is more involved. You'd either need a theoretical model relating the load to the strain or you'd need to calibrate it.

strain > stress > load ?

strain gauge output readily converted to stress.
stress is converted to load by area (usually).
You could use the inner cap area of a longeron for the load in the inner cap.
I would not extrapolate this to the load in the entire longeron, as longerons are usually intended to carry moment.
If you've s/g on both caps then you have a better estimate for axial load and moment in the longeron.
But you'll need to assume some effective skin area.

another day in paradise, or is paradise one day closer ?

Thanks so much for your post. I have some follow up questions.

You could use the inner cap area of a longeron for the load in the inner cap.
how much area to use? i believe it would be area encompassed by the strain gage?

I would not extrapolate this to the load in the entire longeron, as longerons are usually intended to carry moment.
could you please elaborate on this?

If you've s/g on both caps then you have a better estimate for axial load and moment in the longeron.
could you please explain how is this better than having a gage on the web?

But you'll need to assume some effective skin area.
some more explanation on this please

"how much area to use? i believe it would be area encompassed by the strain gage?" ... no, it is not the footprint of the strain gauge. The material around the strain gauge is equally stressed. But if you don't know, then there are more fundamental issues.

"could you please elaborate on this?" ... no, it's too fundamental.

"could you please explain how is this better than having a gage on the web?" ... no, see above.

"could you please explain how is this better than having a gage on the web?" ... sure, a s/g on the web detects shear in the longeron, not bending.

"some more explanation on this please" ... no, see above.

another day in paradise, or is paradise one day closer ?

yea my professors sucked in school.
im here to learn. could you please help?

sorry, but we're not here to teach. I think there are plenty of materials out there on how to use strain gauge data.

another day in paradise, or is paradise one day closer ?

completely understand.
at the moment i dont have the bandwidth (both time & money wise, going thru a divorce) to go investigate this. I looked up some material online and there's isnt much available.
I will greatly appreciate it if you could make an exception and walk me through this. Thank you

I feel your pain, but this is way too involved to resolve in a few posts.

Good Luck.

another day in paradise, or is paradise one day closer ?

No problem

man - do you know how to calculate the stresses in a longeron given axial, shear, moment loads on it (by hand)?

The stress at a strain gage is strain*modulus. To go from stress to loads is more complicated. See question above.

Do you have a copy of Bruhn or Niu?
Do you not have someone else at your company that can help?

Are you interested in loads or strains? I'm most used to getting at the loads so that is what the following is for.

I'd suggest a Free Body Diagram. Model a (maybe) 6" length of the tube (I think that's kind of what a longeron is?) Now in your model place a strain gauge where you think it should be. Then one by one, apply an axial load, a lateral load and a longitudinal load to the ends of the model. Figure out what the stress due to each is (hint - pure axial will cause pure axial stress, lateral and longitudinal will apply bending stress to the tube and gauge). The exact model won't show errors so in your model so move the gauge off center a bit. Recalculate the stresses. That will tell you how accurate you need to be in locating the strain gauge. Also, in the model, put the gauge on at a slight angle (with the axis) to simulate what you think you can achieve when mounting the gauge. Recalculate the stresses. You now have an idea of the sensitivity of the gauge to off-axis loading and mispositioning. If you want to get just axial load then hopefully you are not getting much output from the misaligned or misplaced gauge from lateral and longitudinal loads.

Don't forget Poission's ratio when you are modeling. Gauges are sensitive to that. Also, you'll probably find the at least 2 gauges will give you better performance. For instance, for axial loads mount one on the centerline and another at 180 degrees on the OD. The strains will add for pure axial loads and subtract for bending loads. Make sure to put them in the correct legs of the wheatstone bridge.

Thanks so much!!

You might find these useful... or not...

AIA/NAS NAS942 - STRAIN GAGES, BONDED RESISTANCE

ASTM E251 Standard Test Methods for Performance Characteristics of Metallic Bonded Resistance Strain Gages

IPC IPC/JEDEC-9704 Printed Circuit Assembly Strain Gage Test Guideline

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

Maybe this can help

https://www.youtube.com/watch?v=50GIIkvLEf8

Also... just found it...

NATO AGARD-AG-160-V7 AGARD FLIGHT TEST INSTRUMENTATION SERIES – V7 - STRAIN GAUGE MEASUREMENTS ON AIRCRAFT

PS: GAGE [US] = GAUGE [international]

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]