Continuous RC beam û clear span considerations

[MichSt]

I’d like to know how to determine the design moments in a continuous reinforced concrete beam when the clear span between supports is less than the span used in the beam analysis. For the negative moments, taking the design moment as the moment at the face of the support is typically done to account for this, but what about the positive moment? It seems that if the clear span between supports is less than the span used in your beam analysis the positive moment should also be reduced somehow.

TIA

 

[rapt]

MichSt

No, there is no reduction in positive moments in normal circumstances.

 

[JAE]

rapt is correct that the ACI code provides no reduction in positive moment reduction. That said, the code also allows you to model and analyze the frame such that you approach reality, and the stiffness between the node (at the centerline of the column) and the face of the column, is very stiff and can be modeled as such.

Whether this affects the positive moment to any great extent depends on the span, column width, beam depth, etc.