Slick:
The whole concept of LL reduction was the forerunner of today’s probabilistic and statistical approach to loads in last three or four editions of most codes, particularly ASCE 7.
The thinking went something like this; what is the likelihood that this member is going to see our max. design LL, which is of course a somewhat arbitrary assumed value. In an office space for example, few floors a fully loaded to 50#/s.f., but most likely some areas will be loaded with storage and filing cabinets, so maybe 100#/s.f. is reasonable; and what is a reasonable concentrated load on 2 or 3s.f. area? Engineering judgement used to come into play here, and these three conditions could reasonably occur anyplace on a fl. system, so typical fl. framing, slabs, joists and the like usually got no reduction. But, it was also reasonable to think that it was unlikely that a major beam or girder which had several hundred s.f. of tributary area would all be fully loaded. Also the DL to LL ratio came into play. Generally, LL reductions on fl. members were fairly small, except those that had large tributary areas. Carrying this thinking a step further to columns or walls, after several stories and hundreds of s.f. of area supported, it is very unlikely that max. LL would exist everywhere; and the LL was reduced as the supported area grew, to some max. allowable % reduction. I think it was about 60% for columns and walls. Obviously, this would carry on to the foundation design.
With the latest thinking and machinations they have turned this relatively straight forward thought process into many research projects, much expensive publishing, many expensive software packages, and many pages of calcs. And, they think they can prove that you can save .05% in column and foundation concrete. That translates into 23.93" sq. conc. columns but your contractor doesn’t want to cut down all his 24" sq. column forms, so you should be able to back charge the code writing agencies for the lost savings.
