29 Apr 12 22:28
In building design (and in a different way in bridge design) the building codes require that any type of continuous framing be analyzed with due regard for possible combinations of loaded spans.
For instance, if you have a six span beam system, it is possible in a floor area to have some of the beams loaded and some not. This creates situations where higher moments and shears occur over that found by loading all the spans together.
The combinations required are comprised of six arrangements:
Odd spans loaded
Even spans loaded
All spans loaded
Adjacent span arrangement 1 (load spans 1, 2, 4, 5)-skip 3 and 6
Adjacent span arrangement 2 (load spans 2, 3, 5, 6)-skip 1 and 4
Adjacent span arrangement 3 (load spans 1, 3, 4, 6)-skip 2 and 5
There are no other combinations possible.
For the odd/even spans you will find maximum positive moments.
For the adjacent spans you will find maximum negative moments.
For some of the adjacent and, I think, the total load you will get maximum shears.
The codes don't directly state whether 100% of the live loads must be alternated like this, presuming the engineer will use judgement as to how much of the live load will be semi-permanent so sometimes engineers will skip 75% of the live load and leave the other 25% in place as a total load (all span) condition.