DYNAMIC CHARACTERISTICS OF POST-TENSIONED GIRDERS WITH WEB OPENINGS By Nabil F. Grace/ Member, ASCE, and Brian Ross/ Associate Member, ASCE ABSTRACT: Results of an experimental and theoretical investigation on the dynamic characteristics of post- tensioned concrete girders with web openings subjected to repeated cyclic loading are presented. Rectangular, T and I cross-sectional girders were fabricated and tested. Each cross-sectional group included a girder with none, one, and two openings centered about the midspan for a total of nine test girders. Experimental testing included static, dynamic, fatigue, and ultimate loading of each girder. Fatigue loadings of up to 10,000,000 cycles were applied. The finite element method was used to determine the dynamic characteristics of the tested girders and to conduct an extensive parametric study. It is shown that repeated cyclic loading leads to significant cracking at the openings' corners and a deterioration in the dynamic characteristics of such structural members. In addition, the natural frequencies are significantly influenced by the shape of the prestressing strands and magnitude of the prestress force level. INTRODUCTION The provision of large openings in prestressed concrete girders is necessary to accommodate utility services in resi- dential, commercial, and industrial buildings. In residential and commercial construction, air conditioning ducts, electrical conduits, sanitary pipes, and similar utility components can be incorporated within the depth of the supporting girders, as shown in Fig. 1. This in turn creates a significant amount of usable space between the soffit of the girders and the sus- pended ceiling, thereby maintaining a minimum story head- room. In industrial buildings, girders with web openings are used for passage of the same utility components; however, the main concern is providing working clearances for large man- ufacturing machinery and internal mobile cranes. All of the forementioned structural applications of prestressed concrete girders with web openings can result in vibrational excitation and repeated loading of the members. In industrial buildings, heavy manufacturing machinery that transmit considerable vi- bration to the supporting girders is of most concern. Therefore, practicing engineers are usually interested in the dynamic and fatigue responses and characteristics of such members to ac- count for them in the design of economical and safe structures. The trend in recent years toward integration of web openings for service systems into structural components of prestressed concrete industrial building is increasing rapidly. However, it is evident that there is lack of information on the changes in girders' dynamic characteristics with increased damage due to repeated cyclic load produced by heavy manufacturing equip- ment. An exhaustive literature survey revealed little research work of this type. Kennedy et al. (1990) evaluated the dynamic characteristics of rectangular cross-sectional post-tensioned concrete girders prestressed with straight wires and subjected to repeated loads, and a thesis by Abdalla (1993) included experimental and theoretical investigations of the static and dynamic responses of post-tensioned rectangular, T and I cross-sectional girders at transfer, working, and ultimate loads. However, a larger number of studies on the response of rein- forced and prestressed concrete girders with web openings un- 'Prof. and Chmn., Dept. of Civ. Engrg., Lawrence Techno!' Univ., Southfield, MI 48075-1058. 'Res. Asst., Dept. of Civ. Engrg., Lawrence Techno!. Univ., Southfield, MI. Note. Associate Editor: James M. Nau. Discussion open until Novem- ber 1, 1996. To extend the closing date one month, a written request must be filed with the ASCE Manager of Journals. The manuscript for this paper was submitted for review and possible publication on April 3, 1995. This paper is part of the JourlUll of Structural Engineering, Vol. 122, No.6, June, 1996. ©ASCE, ISSN 0733-9445/96/0006-0643-0650/$4.00 + $.50 per page. Paper No. 10454. der static loads have been reported. Of these, the most recent studies include Dinakaran and Sastry (1984), Mansur et al. (1984), Donahey and Darwin (1988), and Kennedy and Ab- dalla (1992). Similar studies on composite beams with open- ings were carried out by Redwood and Poumbouras (1983) and Fahmy (1988). In addition, Mansur et al. (1991) investi- gated the static response of reinforced concrete continuous beams with openings. In this paper, the changes in the dynamic characteristics of post-tensioned girders with web openings due to repeated cy- clic loading are investigated. Results from an analytical inves- tigation including the natural frequencies and associated mode shapes of vibration are discussed. In addition, results from a parametric study examining the effects of prestress force level, shape of the prestressing strands, opening width, opening depth, and number of openings on a girder's dynamic char- acteristics are presented. EXPERIMENTAL INVESTIGATION The main objective of the experimental investigation was to determine the extent of the deterioration in the dynamic char- acteristics of post-tensioned concrete girders with web open- ings due to repeated cyclic loading. Furthermore, the influence of web openings on the dynamic characteristics of such struc- tural members was examined. The investigation included the fabrication, instrumentation and testing of nine simply sup- ported post-tensioned concrete girders. Based on available lit- erature, it is believed that this experimental investigation is the only one of its type. FIG. 1. Utility Services Passing Through an I-Girder JOURNAL OF STRUCTURAL ENGINEERING / JUNE 1996/643 J. Struct. Eng., 1996, 122(6): 643-650 Downloaded from ascelibrary.org by Lawrence Technological University on 09/17/24. Copyright ASCE. For personal use only; all rights reserved.