1. A computer-implemented tire design method comprising the steps of: (a) deciding an initial layout of a first pitch unit of a tire tread pattern; (b) creating a first finite element model of a tire by developing the first pitch unit of the initial layout into a first plurality of pitches in a circumferential direction of the tire; (c) calculating a respective first evaluation index for each element of a ground contact surface in the first finite element model through a structural analysis using the first finite element model of the initial layout; (d) integrating calculated evaluation indices into a tentative pitch unit tantamount to any one among the first plurality of pitches for each element of the tentative pitch unit, where an integrated evaluation index for each element of the tentative pitch unit is expressed by a function A ⁡ ( i , j ) = ∑ k = 1 N ⁢ ⁢ F ⁡ ( i , j , k ) / N ⁢ ⁢ or ⁢ ⁢ A ⁡ ( i , j ) = ∑ k = 1 N ⁢ ⁢ F ⁡ ( i , j , k ) , F(i, j,k) being an evaluation index function for each pitch element, (i, j) representing pitch element indices, k representing a pitch number, and N representing the number of pitches, classifying elements of the tentative pitch unit according to magnitudes of the integrated evaluation indices, respectively, to determine a plurality of classes to which elements of the tentative pitch unit subject to removal belong, and selecting elements of the tentative pitch unit to be removed from each of classes thus determined; (e) selecting elements to be added as restored from among the elements of the tentative pitch unit which have been removed; (f) obtaining a current generation layout through the removal and addition of elements at the steps (d) and (e) to obtain a second pitch unit and to create a second finite element model of the tire by developing the second pitch unit of the current generation layout into a second plurality of pitches in the circumferential direction of the tire; (g) calculating a respective second evaluation index for each element of a ground contact surface of the second finite element model through a structural analysis using the second finite element model of the current generation layout; (h) determining convergence of an objective function from the calculated second evaluation indices, updating the initial layout to the current generation layout and returning to the step (d) when it is determined that convergence has not occurred, and deciding the tire tread pattern using the current generation layout as an optimum layout when it is determined that convergence has occurred; and (i) producing a tire design having the tread pattern decided using the optimum layout.