As far as I know, frequency classifcation and physical demand categorization are based on an what can be accomplished in an hour. Your may want to refer to the US Department of Labor’s Dictionary of Occupational Titles for confirmation.
Your question suggests you are looking for something more comprehensive than what I can offer but Mital et al (1993) A Guide to Manual Materials Handling propose that, for a 12 hour shift, manual weight of lift should be reduced using a multiplier of 0.920. In effect, you determine the weight of lift for an 8 hour shift using (in their case) psychophysical MMH capacity data and then apply the multipler. I hope this is of some small help.
Joe Selan, Advanaced Ergonomics Inc.
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