A few back-of-fag-packet scribbles give me a (very) rough estimate of total vehicle mass ~70 secs after launch (ie the time of the explosion) of some 1,300,000kg. The article I linked about talked about the initial catastrophic failure being the hydrogen/oxygen burn in the external fuel tank blasting off its lower dome producing a thrust of "2,800,000lbs" (~1,300,000kgf).
60-70secs after lift-off is close to the point of max dynamic pressure, so the acceleration is throttled down a bit to about 1.5G. At that point the aft dome is blown off and there is a short burst of another 1G, which must have been quite dramatic.
PDR