ارزیابی ضرایب تاثیر برای کامپوزیت پل های سلولی بتن - فولادی

This paper presents a method for determining the dynamic impact factors for straight composite concrete deck–steel girder cellular bridges under AASHTO truck loading. The bridges are modeled as three-dimensional structures using commercially available software. The vehicle is idealized as a pair of concentrated forces, with no mass, travelling across the bridge. An extensive parametric study is conducted, in which 120 composite multi-cell bridge prototypes are analyzed. The key parameters considered in this study are: number of cells, number of lanes, span length, number and area of cross-bracing and top-chord systems, and truck(s) speed and truck(s) positioning. Based on the data generated from the parametric study, expressions for dynamic impact factors for moment, reaction, and deflection for such bridges are proposed. The results from this practical-design-oriented study would enable bridge engineers to design new composite cellular bridges more reliably and economically. Furthermore, the results can be used to evaluate the load-carrying capacity of existing composite cellular bridges since even a small increase in strength for live load can make the difference between closing a bridge and leaving it open.

Composite concrete deck–steel cellular straight bridges are widely used in the highway systems throughout the world. In addition to their lighter weight, shallower depth of cross section, and significant longitudinal bending stiffness, composite cellular straight bridges are favored for their considerable torsional stiffness to resist eccentric load when compared to open sections. The cross section renders an efficient transverse load distribution as well as high resistance to torsional vibration caused by moving vehicles, wind, or seismic conditions. Recent review of available work on the dynamic response of box-girder bridges was presented by Sennah and Kennedy [14]. It was observed that there is lack of information regarding the dynamic response of composite cellular bridges under moving vehicle. The current design practices in North America recommend few analytical methods for the design of straight composite multi-cell box girder bridges to account for the dynamic effect of truck loading. The current design specifications in the United States by the American Association of State Highway and Transportation Officials [1] recommend an impact factor equation as a function of the bridge span only. While, the current design specifications in Canada, the Canadian Highway Bridge Design Code [8], recommend a dynamic load allowance based on the number of truck axles passing over the bridge. The latter recommendation was based on results from the dynamic testing of 27 I-girder bridges in Ontario, Canada [2]. Recently, Sennah and Kennedy [13] presented a simplified design method for straight composite cellular bridges in the form of expressions for moment and shear distributions factors. The objective of this study is to conduct a parametric study to examine the effect of key geometric parameters and loading conditions on straight composite cellular bridges that might influence the impact factors of such bridges. The data generated from the parametric 3D study is used to deduce empirical expressions for impact factors for moments, reactions, and deflections.

An extensive investigation was conducted to determine the effects of key parameters on the impact factors for straight composite concrete–steel cellular bridges. Empirical expressions were deduced for moment, deflection, and reaction impact factors for AASHTO truck loading. Due to the scattered nature of data generated from the parametric study, upper-bound envelopes were adopted to derive these expressions. The expressions were in terms of the fundamental frequency of the bridge as well as in terms of the bridge span. Based on this study, the following conclusions can be drawn: 1. A comparison of the proposed impact factors for longitudinal bending moments and deflections with the available expressions in North American bridge codes show that the latter are over-conservative. 2. Truck speed is an important parameter that affects the impact factors of straight bridges. An increase in the truck speed greatly increases the impact factors for bridges. 3. The impact factors for longitudinal bending moment and deflection are much lower than those for support reactions. This is important in designing bridges for seismic conditions.