Reason of loading on alternative spans to obtain maximum positive moment in a span of a continuous beam
To attain a maximum sagging moment in a span of a continuous beam, general rule is to load the span under consideration and alternative spans on every side of the span. To account for this rule, let us consider the subsequent example. For illustration loads are applied to the mid-span of a multiple-span continuous beam. It is noticed that this loads induce positive moments near mid-span in all even spans. Thus if all even spans are loaded concurrently this will result in the increase of positive moments in all other loaded spans.
In the same way to obtain maximum negative moment at a support, load adjacent spans of support and then alternative spans on every side.