In Eq. (2a), E is Young’s modulus of the tower material, while n1 and n2 are two non-negative integers whose values are depended on load type. For example, n1 = 0 and n2 = 1 when bending-force loaded, and n1 = 3 and n2 = 0 when distributed-weight loaded [3] and [4]. The theoretical expression of Eq. (1) is modified when a head mass MH is added on the top of the tower, so that the natural frequency becomes: