How to measure if there is current between point A and B when source and grounding is unknown
One guess:
There's no connection from your steel frame to the mains AC line, it's connected only to the ground through the concrete which generally tend to be more or less conductive.
But your computer isn't grounded. Modern switchmode power supplies unfortunately have capacitors between the mains AC lines and the metal case of the computer which in addition is connected to the low voltage side GND inside the computer. The capacitors are needed to reduce radio frequency emissions as well to the air and to the mains AC network.
The capacitive mains AC leak onto your hands would be shorted if there were proper grounding. But  without grounding the current through you can really be painful although not as high as something which is considered lethal.
The problem is common, it's presented here in EESE several times and I have seen sometimes it's self-caused when people

break groundings which collect noise their audio systems or
use non-grounded outlets with equipment (like many computers) which are designed to have properly grounded mains AC input.

A competent electrician would check and fix the grounding but that needs real experience, brave soul is not enough to stay alive.
A multimeter in AC voltage range should show if there's a voltage between your computer case and the built steel frame. That's assuming you can use the multimeter and it's not broken. Your &quot;tripped the breaker -incident&quot; hints that your knowledge needs some fixes. Measuring against known ground, other than the unknown would be usefull, but also dangerous for an unexperienced person. Get help!
As said, that was a guess. The other possibility is that there's a live mains AC voltage in your steel frame. Or there were until you tripped the safety breaker somewhere. The voltage returns when the owner of the breaker resets it.
Get an electrician which understands the case, believes you and is free of any general local reasons to stay blind in front of wrong wirings and building errors.