Research Article Free and Forced Vibration Analysis of an Infilled Steel Frame: Experimental, Numerical, and Analytical Methods Mohammad Amin Hariri-Ardebili, 1 Hamid Rahmani Samani, 2 and Masoud Mirtaheri 3 1 Department of Civil, Environmental and Architectural Engineering, University of Colorado at Boulder (UCB), Boulder, CO 80309-0428, USA 2 Department of Civil Engineering, Islamic Azad University, Pardis Branch, Tehran 135/16555, Iran 3 Department of Civil Engineering, K.N. Toosi University of Technology, Tehran, Iran Correspondence should be addressed to Mohammad Amin Hariri-Ardebili; mohammad.haririardebili@colorado.edu Received 20 February 2014; Revised 27 July 2014; Accepted 7 August 2014; Published 28 August 2014 Academic Editor: Nuno M. Maia Copyright © 2014 Mohammad Amin Hariri-Ardebili et al. Tis is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Structural frames with masonry infll panels make up a signifcant portion of the buildings constructed in earthquake-prone areas prior to the developing of the seismic design standards. In this paper, the efects of masonry panels on the vibration response of an inflled steel-frame building are investigated. Various ambient and steady state forced vibration tests are carried out to realize the dynamic characteristics of the system. 3D fnite element models of the building with and without infll panels are provided based on marcomodeling theorem. A set of analytical approximate formulas are also derived to estimate the vibrational period. Te natural frequencies of the building are computed using numerical, analytical, and experimental methods. Te results show that neglecting the efect of infll panels leads to considerable error. Moreover, it is shown that there is good agreement among the results obtained by the three methods considering the efect of infll panels. 1. Introduction Steel-frame buildings with masonry infll walls are a type of construction widely used throughout many developing countries. Hollow clay tile blocks, hollow concrete blocks, and normal bricks are used in infll walls (Figure 1(a)). Te infll walls, being traditionally nonengineered, have as-built properties which at the design stage are almost impossible to estimate reliably and/or to specify and at the construction stage are hard to control [1]. However, the efects of infll walls on the structural properties of a building have been recognized by engineers and studied for a long time [2]. Te infll walls are supposed to increase the building stifness. Tey also may have some undesirable efects on the building performance, such as enhancing the sof storey mechanism or causing short column efects. Chaker and Cherifati [3] suggested that plane stress fnite elements provide a better representation of the in-plane initial stifness of the infll panels under the small strain condition. In order to simulate the behavior of the infll wall, diferent types of models can be used [4]: (i) micromodeling in which the efect of mortar joints are considered as a discrete element. In this approach, the brick and mortar are modeled as continuum elements and interface between the brick and mortar is modeled by an interface joint element (Figure 1(b)). (ii) mesomodeling in which the bricks are modeled by continuum elements, but the mortar joint and its interface with bricks are modeled together as an interface element (Figure 1(c)). (iii) macromodeling in which a single numerical model represents the infll panel efect [2]. Macromodeling is considered in two levels, that is, (1) homogenized model in which the brick, mortar, and the interface are modeled as one continuum element (Figure 1(d)), and (2) strut model in which the infll panel is mod- eled by one (Figure 1(e)) or more struts (Figure 1(f)) in each direction. Micromodeling is relatively time-consuming especially for analysis of large structures [5]. Diferent mechanisms have Hindawi Publishing Corporation Shock and Vibration Volume 2014, Article ID 439591, 14 pages http://dx.doi.org/10.1155/2014/439591