An 18 Strength is merely upper end for a 150 lb human - medical and size modifiers can produce humans with freakish strengths to about 30 (more of the Conan size).
Though a bit over-thoughtout for most people I made a strength chart equating Strength vs lifting/curl/row/dragging/rolling weights.http://bellsouthpwp.net/g/a/garyashburn ... RENGTH.xls
One of the interesting things on the chart is there is a conversion to/from AD&D strengths included and 18-30 Strength for Harn with lifting weights calibated to Human World records. Harninc 18-30 equates very similarly with 18-18/00 strengths from AD&D and lifting scores proposed by that system.
A little more arbitraily the chart continues to include/equate Hru (Harn Str 50) and D&D goldlike strengths (D&D Str 19-25) with Hru falling at about 23 or 24 on the D&D Strength system. It's always nice to know if the Hru can pick up and smash a vehicle the size of a Toyota or how far it can throw the average 200# man (and how much it hurts when he hits the wall).
Another chart I find useful is:http://bellsouthpwp.net/g/a/garyashburn ... ygiant.xls
which can be used to figure out how heavy various things are.