Trabajo de grado - Doctorado
Effect of longitudinal stiffening on the ultimate resistance of plate girders subjected to patch loading
Loaiza Ramones, Nelson Adolfo
Several investigations have shown that the ultimate resistance of plate girders subjected to patch loading is increased when a longitudinal stiffener is placed at the web panel of the steel girder. However, the influence of factors such as the patch loading length and the configuration of the longitudinal stiffener has received little attention. Therefore, this thesis is aimed at studying the effect of longitudinal stiffening on the ultimate resistance of plate girders subjected to patch loading. At first, the influence of the patch loading length on the critical buckling coefficient is investigated by means of linear finite element analysis. The effect of the panel aspect ratio, the position, and size of the longitudinal stiffener was also addressed. Additionally, the influence of a second stiffener located at the web panel was studied (multiple stiffening). Results indicate that buckling coefficients increment when the patch loading length is increased. Currently, in the Eurocode 3 Part 1-5 the ultimate resistance of girders subjected to patch loading is obtained by calculating the slenderness parameter, a dimensionless factor that depends on the yield resistance and the critical buckling load. The critical load which expression is derived from numerical modelling shows that critical load rises with an increasing distance between the stiffener and the loaded flange. However, at ultimate load level experiments had shown that ultimate resistance rises when the longitudinal stiffener is placed near the loaded flange. With the purpose of obtaining the same behavior described at ultimate load level, new hypothesis of boundary restrictions at the stiffener location are presented to model the elastic buckling behavior of webs subjected to patch loading. From these results a new approach that corrects the calculation of the slenderness parameter and harmonize the behavior between critical load and ultimate load level is proposed. The last edition of the AISC Specifications presents a formulation to calculate the ultimate resistance of unstiffened girders subjected to patch loading. This formulation is based on a four plastic hinges mechanism that considers crippling effect produced in the web panel of slender girders. Considering that the flange-to-web yield strength ratio has no influence on the ultimate resistance for hybrid girders subjected to patch loading, a closed-form solution to predict the ultimate resistance for stiffened girders is proposed by modifying a previously developed mechanism. International code provisions suggest use a longitudinal stiffener at one-fifth of the girder height as the best position for flexural resistance (b1 =0.20hw), nevertheless results indicate that the optimum position to increase the ultimate resistance of a girder subjected to patch loading is different (b1 0.20hw). It has also been demonstrated that when the slenderness of the directly loaded panel b1/tw decreases the ultimate resistance increases. Since bending governs the in-service girders behavior, a check for steel bridges girders erected by incremental launching was performed using nonlinear finite element analysis. In this case two strategies to study the impact of the slenderness ratio b1/tw on the ultimate resistance were evaluated, the first by changing the web thickness tw while maintaining the stiffener at b1 =0.20hw, and the second by changing the position of the stiffener b1 for various web thicknesses. Results show that the contribution of a stiffener placed at b1 =0.20hw is only significant for thinner webs. Finally, the results derived from the theoretical predictions are compared against the test experimental results available in the literature, in order to discuss the goodness and limitations of the approaches developed herein.