Bracing of steel bridges during construction; theory, full-scale tests and simulations

Sammanfattning: A number of steel bridges have suffered lateral-torsional failure during their construction due to their lacking adequate lateral and/or rotational stiffness. In most cases, slight bracing can be of great benefit to the main girders involved through their controlling out-of-plane deformations and enabling the resistance that is needed to be achieved. The present research concerned the performance of different bracing systems, both those of commonly used types and pragmatic alternatives. The methods that were employed include the derivation of analytical solutions, full-scale laboratory testing, and numerical modeling. The results of a part of the study showed that the load-carrying capacity of The Marcy Bridge that collapsed in 2002 could be improved by adding top flange plan bracing at 10-20% of its span near the supports. Theoretically, according to Eurocode 3, providing each bar of an X-type plan bracing having cross-sectional area as small as 8 mm^2 serves to enhance the load-carrying capacity of the bridge by a factor of 1.28, which is sufficient to prevent failure of the bridge during the casting of the deck. The research also included the derivation of a simplified analytical approach for determining the critical moment of the laterally braced steel girders at the level of their compression flange, which otherwise can usually not be predicted without the use of finite element program. The model employed related the buckling length of the compression flange of steel girders in question to their critical moment. An exact solution and a simplified expression were also derived for dealing with the effect of the rotational restraint of the shorter segments on the buckling length of the longer segments in beams having unequally spaced lateral bracings. The effects of this sort are often neglected in practice and the buckling length of compression members in such systems is commonly assumed to be equal to the largest distance between the bracing points. However, the present study showed that this assumption can provide an unsafe prediction of buckling length for relatively soft bracings and can also lead to a significant overdesign in regard to most bracing stiffness values in practice. Full-scale experimental study on a twin-I girder bridge together with numerical works on different bridge dimensions were carried out on the stabilizing performance of a type of scaffolding that is frequently used in the construction of composite bridges. Minor improvements were discussed which found to be needed in the structure of the scaffoldings that were employed. Findings showed the proposed scaffoldings to have a significant stabilizing potential when they were installed on bridges of differing lateral-torsional slenderness ratios. Axial strains in the scaffolding bars were also measured. Indications of the design brace moment involved were also presented which was approximately between 2 and 4% of the maximum in-plane bending moment in the main girders. Three full-scale experimental studies were also performed on a twin I-girder bridge in which the location of the cross-beam across the depth of the main girders was varied. The effects of several different relevant imperfection shapes on the bracing performance of the cross-beams were of interest. It was found that the design recommendations currently employed can provide uncertain and incorrect predictions of the brace forces present in the cross-bracings. Both the tests and FE investigations carried out showed the shape of the geometric imperfections involved to have a major effect on the distortion that occurred in the braced bridge cross-sections. It was also found that significant warping stresses could develop in cross-beams having asymmetric cross-sections, the avoiding of such profiles in the cross-beams being recommended. Finally, seven full-scale laboratory tests of the end-warping restraints of truss-bracings and of corrugated metal sheets when they were installed on a twin I-girder bridge were also performed. The load-carrying capacity of the bridge was found to be enhanced by a factor of 2.5-3.0 when such warping restraints were provided near the support points. Relatively small forces were developed in the truss-bracing bars in order to such significant improvements in the load-carrying capacity of the bridge to be achieved. Moreover, bracing the bridge in question by means of the metal sheets that were employed was found to result in a significantly larger degree of lateral deflection at midspan than use of the utilized truss bracings did.