Please explain how the av and shear enhancement factor is calculated in pile-cap design as per IS 456? Also please explain how sometimes shear enhancement factor values are very high which comes in the range of 50?

Below sketch showing the calculation of av value for the pile-cap design.

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  Av          = 290 – 270

                 = 20 mm

  Av is the distance between load location and section considered. Refer below snap from IS 456

   

  So the enhancement factor is     = 2 x deff / av

                                                              = 2 x 540 / 20

                                                              = 54

                         Tce         = Tc x Enhancement factor

                                        = 0.33 x 54

                                         =  17.82 > 3.5 N/sqmm (TcMax) thus restricted to 3.5 N/sqmm

                                         Below snap from IS 456 for TcMax