Comments on ‘‘Modeling of system dynamics of a slewing flexible beam with moving payload pendulum” Nenad Zrnic ´ * , Srd  an Bošnjak University of Belgrade, Faculty of Mechanical Engineering, Kraljice Marije 16, 11120 Belgrade 35, Serbia article info Article history: Received 30 November 2007 Received in revised form 4 May 2008 Available online 6 June 2008 Keywords: Modeling Flexible jib Tower crane abstract This paper comments on the recently published work dealing with the problem of model- ing system dynamics of a slewing flexible beam with moving payload. It also includes remarks on the inadequacy in system modeling and highlights mistakes in the mathemat- ical model. This work points out the practical aspects of tower crane modeling and offers suggestions for appropriately solving the problem in cranes dynamics. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction The recently published paper by Yang et al. (2007) is devoted to the important problem of the vibration of the tower crane slewing flexible jib structure when simultaneously traversed by a moving mass – carriage. The jib structure is modeled as clamped-free Euler–Bernoulli beam attached to a rotating rigid hub (in tower crane glossary known as tower or mast). How- ever, the examination of a model that is not a prototype of some real system is of little interest unless it produces some gen- eral conclusions, which can be applied to other, related configurations. In their references, Yang et al. (2007) have omitted to cite the significant contemporary works dealing with dynamic analysis and response of tower cranes written by Ju and Choo (2005) and Ju et al. (2006). So that is probably the reason of certain inadequacy occurring during the mathematical model set up. The intention of the authors of this contribution is to point out practical aspects of tower crane modeling and to give suggestions for finding suitable solutions of the problem brought up by Yang et al. (2007). 2. Adequacy of mathematical model of jib traversed by a moving mass with payload Yang et al. (2007) are making a mistake by a priori neglecting the flexibility of the tower crane mast. That can be done only after analyzing the dynamic behavior of the whole structure of the tower crane, i.e. after defining natural modes of tower crane structure either by experimental methods or by FEM (Sayer, 2004). Using FEM, Ju and Choo (2005) and Ju et al. (2006) have concluded that the dynamic response of the tower crane structure is dominated by the first and the second natural mode, in addition to the natural frequency of the pendulum. It can be seen that the first mode is dominated by the in- plane deformation of the jib structure that corresponds to the model of the rotating beam given in Fig. 2 by Yang et al. (2007), while the second mode is predominantly the complex bending pattern of the whole crane structure that is not in agreement with the out-of-plane vibration model of the rotating beam given in Fig. 3. It is conclusive that the uncritical a priori 0093-6413/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.mechrescom.2008.05.004 * Corresponding author. Tel.: +381 64 3162 018; fax: +381 11 3370 364. E-mail address: nzrnic@mas.bg.ac.yu (N. Zrnic ´). Mechanics Research Communications 35 (2008) 622–624 Contents lists available at ScienceDirect Mechanics Research Communications journal homepage: www.elsevier.com/locate/mechrescom