We have seen the requirement given by the code that, splices should not be given at sections where the applied bending moment Mu is greater than 50% of the Ultimate moment of resistance MuR . But in the clause in SP24  that gives us the required increase in lap length, the stress in steel fs is used along the Y axis. We  can prove that these two are related.

We know that applied factored bending moment at a section is given by
Mu = fs x Ast x z   … (1)        
also ultimate moment of resistance at that section is given by:
MuR = 0.87 fy x Ast x z … (2)

dividing (1) by (2) we get:
Mu  / MuR = fs / 0.87 fy

Thus we can see that the two methods give the same result.

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