RC beam design (EN1992): Clause 6.2.3(7) Additional tensile force due to shear

Per the "Assumptions and limitations" section of the notes for the RC beam design (EN1992) calculation (and the associated analysis and design calculations):

• The calculation does not determine the additional tensile force, DeltaF_td, in the longitudinal reinforcement due to the design shear force, V_Ed, in accordance with 6.2.3(7) of EN1992-1-1. Users should therefore take account of the 'shift rule' described in 9.2.1.3(2) when determining the curtailments of the longitudinal reinforcement.

Generally speaking the Tedds calculation designs for either the maximum sagging or hogging bending moment. At positions of maximum sagging moment within spans, the shear force is commonly zero and so the additional area of tension reinforcement required for the shear is also zero.

Away from the positions of maximum moment, the shear force does add to the area of reinforcement required to resist the moment.  This is also the case for hogging moments close to supports of cantilevers and continuous beams, where shear is also high.  However per Cl 9.2.1.3 (2) of EN1992-1-1, this can be taken account of by shifting the moment curve by a distance equal to a_l = 0.45d.[cot(theta) - cot(alpha)] - i.e. extending the curtailment lengths to accommodate this shift.   This is termed the "shift rule". 

These points are illustrated in figure 9.2 of EN1992-1-1.  The figure shows that, provided the "shift rule" is used for the full length of the beam, at the support and even a short distance from the support it is not necessary to provide any more reinforcement than that required for the maximum support bending moment (and any externally applied tension (N_Ed)).  Note how the acting tensile force, F_s (curve B), never exceeds the envelope of M_Ed/z + N_Ed (curve A) at the positions of maximum sagging and hogging bending moments.

Section 5.12.6 of the IStructE "Manual for the design of reinforced concrete building structures" (the "green book") also gives a good description of this approach.

Using the "shift rule" then, the additional tension force due to shear then becomes a detailing issue.