Structural Analysis
The three moments equation is applicable only when

The spans are equal
There is no settlement of supports
The beam is prismatic
There is no discontinuity such as hinges within the span

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Structural Analysis
The effective length of lacing is the distance

All of these
Between the inner end rivets of the bar for single lacing
Between the inner ends of effective lengths of welds connecting the members
0.7 of the distance between the inner end rivets of the bar for double lacing

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