Solution:

Given,
Pitch (p) = 6 cm
Rivet value (Rv) = 4 tonnes
Eccentric load (P) = 15 tonnes
Distance of load from centre line (e) = 30 cm

To find: Number of rivets required for the riveted connection

Formula used:
For riveted connection carrying eccentric load, the net force on the connection is given by:
Net force = (P * e) / (2 * L * Rv)

Where,
L = Pitch length

Number of rivets required for the connection is given by:
Number of rivets = (Net force * L) / (Rv * Shear strength of one rivet)

Where,
Shear strength of one rivet = 0.5 * π * d^2 * τ

Where,
d = diameter of the rivet
τ = Shear stress

Calculation:

1. Calculation of net force on the connection:
L = 2 * p = 2 * 6 = 12 cm = 0.12 m
Net force = (P * e) / (2 * L * Rv)
Net force = (15 * 0.3) / (2 * 0.12 * 4)
Net force = 3.125 tonnes

2. Calculation of shear strength of one rivet:
d = 1.5 * p = 1.5 * 6 = 9 cm = 0.09 m
τ = 0.4 * Rv / (π * d^2)
τ = 0.4 * 4 / (π * 0.09^2)
τ = 7.97 N/mm^2
Shear strength of one rivet = 0.5 * π * d^2 * τ
Shear strength of one rivet = 0.5 * π * 0.09^2 * 7.97
Shear strength of one rivet = 0.00199 tonnes

3. Calculation of number of rivets required:
Number of rivets = (Net force * L) / (Rv * Shear strength of one rivet)
Number of rivets = (3.125 * 0.12) / (4 * 0.00199)
Number of rivets = 19.7

Therefore, the number of rivets required for the riveted connection carrying an eccentric load of 15 tonnes at a distance of 30 cm from the centre line is 20 (approx).

Note: As the number of rivets required comes in fraction, it is always rounded off to the next integer value.