This appears to be an extension of a previous thread.
My suggestion would be to concrete the central 50% or more of the
span in the first pour, and then the ends. This way, the scaffolding
would see the maximum central loading and achieve most of the
deflection when the central portion of the concrete is green. This part
of the concrete is not stressed. The ends are cast later. The load
at the ends would not result in high central deflection. Since the
concrete is green at the ends, it would take the shape of the
deflected scaffolding without inducing any stress in that region. The
central portion would be subject to only a small additional deflection,
and the stresses would be quite small.
The formula suggested by austim would be valid if the span is
concreted in multiple pours in horizontal layers. For concreting the
way mentioned above, that would not be applicable. Assuming that the
central part would have set by the time the ends are concreted, the
following course of action is to be followed:
(a) Assume that the concrete girder is only supported at the ends of
the first pour. Compute the self-weight deflection at the centre. Compute
the additional deflection of the truss scaffolding due to the weight of the
concrete (relative deflection between support point mentioned above
and centre of the span). If the girder deflection is more than the truss
deflection, the truss supports the girder. Otherwise the girder is
supported only at ends.
(b) Find the compatible deflection pattern of the girder and truss. From
the deflection pattern, the bending moment and stresses can be
computed.
This is a convoluted and iterative procedure. May be one trial would
lead to a conclusion that the additional stresses are not critical.
Good luck,
M. Hariharan