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Quick question:
Where is the largest load acting on a bike frame during normal riding. I'd guess underneath the downtube at the headtube, or the seattube/toptube junction.
What do you mean by "normal"? Sprinting out of the saddle, puts a lot of torque into the down tube, likewise a lot of compression/bending into the driveside chainstay. Loadings due to impacts/'normal' offroad.... well, the worlds your oyster - get a data logger and find out!
I think you might mean stresses rather than loads. Loads are the forces applied to the bicycle, and many of them change dynamically under different riding conditions.
But like bristolbiker said, what do you define as 'normal conditions'. Sprinting will apply a lot of compressive stress to the drive side chainstay as a result of pedalling, but there will be hardly any at the seat tube junction as the rider will not have any mass on the saddle. Riding down a rocky hill will stress the frame more under the head/downtube junction than riding along a flat road.
e.g here's an FEA of a frame only, showing only the load from a typical front impact. most of the stress is under and around the head tube junction
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I realise the question is badly worded and technically incorrect, just quoting it directly from the questionnaire it was on. So no idea what 'normal riding' is it what they really mean by loads. Just thought I'd stick it out there to see what others made of it.
Oggles, I initially guessed the head/down tube, but then thought that frontal impact isn't 'normal riding'.
Conclusion: very bad question.
Generally the joints are weaker than the tubes.
A properly welded or Brazed joint should be stronger than the material it joins. In fact when you are learning to weld this is a common destructive test to see the quality of your joint. Force a failure and check that it is the material failing and not the join.
In answer to the op if we define normal as the range of riding experienced 99% of the time under normal riding conditions I would guess
1. The head tube /down tube junction. (experiences a lot of torque)
2. The bottom bracket / chainstay junction.
Both areas are area of common frame failure and where gussets are more commonly found so that => higher stress in those areas.